Chapter 5 INTRODUCTION 5.1 This chapter reviews the assessment of seismic hazards, their different aspects and principle causes, and the methods and data required. Mainly standard methods and techniques are described that have been applied in many countries around the world and have produced reliable results. The primary hazard results from the direct cllects of earthquake motion. Earthquake triggered sea waves, avalanches, rockfalls and landslides are considered to be secondary hazards, which may be important in certain areas. These hazards, their resulting risks and how to deal with them, are not covered in this chapter A comprehensive description can be found in Horhck-Jones ec reiief SEISMIC HAZARDS al. (1995). Earthquake hazard evaluation is the initial step in the general strategy of risk assessment and disastcr mitigation measures in seismically active areas. Seismic risk is thereby assumed to be composed of: ( 1) seismic hazard; (2) vulner- ability; and (3) exposure of persons and goods to primary (and secondary) hazards. The complete disastcr manage- ment and risk reduction plan comprises the following actions and professionals (SEISXIED, 1990) Professionals involved are Seismic hnznrd assesjment. essentially seismologists, geologists and geotechnical engi- neers. Their activities are devoted to the production of various types oE technical maps with site-specific hazard figures. Earthquake hazard is usually expressed in probabil- ities of occurrence of a certain natural earthquake effect (e.g., level of strong ground shaking) in a given time frame. VuZnerabiiiry analysis. Professionals involved are mainly civil and geotechnical engineers and architects investigating the interaction between soil and structures under seismic load and the susceptibility of structures to damage. Typical vulnerability figures are presented as the percentage of a building type showing damage of a certain degree due to a selcctcd seismic ground motion level. Exposure evdiintron. The socio-geographical and eco- nomical aspects of an environment prone to earthquakes are evaluated by planners, engineers, economists and administrators. The results of these investigations will ultimately be the guide to adequate actions (Hays, 1990), such as. Planning: The evaluation of the expected losses due to strong earthquakes should Lead to a revision in urban and regional planning, as well as to procedures for limiting dam- age to buildings (e.g. building codes and regulatlons). Administration: The earthquake-resistant design speci- fications (e.g., zoning maps) that have been studied and produced by the scientific and engineering communities become instruments for disaster mitigation. DisnsteTpTrpurr!dnL.~r- The !ogistical and dmii:istrative authorities prepare plans, measures and training facilities in anticipation of earthquake t.mergencies, which include res- cue, and rehabilitation. International organizations compile databases containing ready-to-use scicnkilic, tech- nical and educational tools (STEND, 1996). Public aw~~renew Programmes to inform the public on earthquake risk are prepared with the participation of gov- ernments. local authoritres, and the mass media including scenarios and disaster simulation 5.2 * DESCRIPTION OF EARTHQUAKE HAZARDS An earthquake is caused by the abrupt release of gradually accumulated strain energy along a fault or zone of fractur- ing within the earth's crust. When a fault ruptures seismic waves are propagated in all directions from the source. As the waves hit the surface of the earth, they can cause a var- ier y of physical phenomena and associated hazards. Each of these hazards can cause damage to buildings, facilities and lifelines systems. Table 5 1 Lists the major earthquakes since 1990 that have resulted in more than one thousand deaths. In general, rhe effects of earthquakes at the ground surface may be classified into the following domains- - permanent rupturing (faults, fissures, etc.); - transient shaking (frequency, amplitude, duration, etc.); - permanent deformacion (folds, settlements, etc.); - induced movement (liquefaction, landslides, etc ). Other common effects of earthquakes are fires and floods. Aftershocks, usually following an already disastrous earthquake. d t c n cause additional damage by reactivating any or all of thcse physical phenomena. As a consequence of the intensity, spectral content and duration of the ground shaking, buildings and lifeline sys- tems (depending on their geometry) are forced to vibrate in the vertical and horizontal directions. Extensive damage takes place if the structures are not designed and built to withstand the permanent displacements and dynamic forces resulting from earrhquake motions. Evaluation of earthquake hazards and associated risks is a complex task (Hays, 1990) Scientists and engineers must perform a wide range of technical analyses that are conducted on different scales. Regional studies esrabIish the physical parameters needed to define the earthquake poten- tial of a region. Local studies define the dominant physical parameters that control the site-specific characteristics of the hazard. In principle, all of the studies seek answers to the following tec.hnica1 questions- Wherc arc the earthquakes occurring now? Where did they occur in the past? Why are they occurring? How orten do earthquakes of a certain size (magnitude) occur? How big (severe) have the physical effects been in the past? How big fclture! by magnitude In two well-established can they be the How do thc physical effects vary in space and time7 The size or severity of an earthquake is usually expressed ities: and (epiccntral) intensity. Magnitudes at-c determined from instrumental recordings (seismograms), scaled logarithmically - - eluant 08!09/1905 Cornprehensn~ risk assessment for natural hazards 000 or more Q a m n Table 5.1 - Earthquakes with I 1611 2/1902 04/04/1905 Turltestan India, Kangra Italy, Calabria Colomba Formosa Chde, Santiago lamaica Central Asia 31 i011'1906 17/03/1906 17/08/1906 14/01/1907 21 11 0/1907 28/12/1908 Italy, Messina Marmara Sea 09/0aii 91 z 13/01 /I91 5 16/12/1920 01 /09/1923 16/03/1925 07/03/1927 22/05/1927 01 105l1929 Italy, Avezzano Tango China, Cansu japan, Kwanto China, Yunnan lapan, China, Xining Islamic Republic of Iran Italy China, Gansu Japan, Sannku Bihar-Nepal Formosa Palustan, Quetta Chile, Chillan Emncan Tutkey, lapan, Tottori Japan, Tonankar Japan, Mikawa Turkey !ran Peru, Ancash Japan, Tonankai Japan, Fukui Ecuador, Arnbato &am, Tibet PJgena Islamic Republic of Iran khmic Repubk of Iran Morocco, Agadir Chile Islamic Republic of Iran, Yugoslavia, Skopje Idamic Republic of Turkey, Varto Eastern China China, Yunnan Turkey, Cediz Pew Islamic Republic of Iran Nkaragua, Managua Turkey Guatemala northeastern Itdly, Papua New Guinea China, Tangshan Phihpphes, Mindan Islamic Republic of Iran, northwest Romania Islamic Republic of iran Algetia, El k n a m Italy, southern lslamic Republic of Iran, southern Islamic RepuMlc of Iran, southern W. Arabian Penrnsula Turkey Mexico, Michoacan El Salvador Colomba-Ecuador 23/07/1930 2511 Zil932 0210311 933 15/01 /1934 20/04/1935 30/05/1935 25/01 /1939 26/12/1939 1 0/09/1943 07/12/1944 12iOli1945 31 /05/1946 lOil1/1946 20/1211946 28/06/1948 05/08/1949 15/08/1950 09/09/1954 02/07/1957 1 311 2/1957 29i02/1960 22!0Sil960 01 :'0911962 26107/1963 19/08/1966 31 /OW1968 25/07/1969 04/01 /I 970 28/03/1970 31 /05/1970 10/04/1972 23/12/1972 06/09/1975 04/02/1976 06/05/1976 25,'06/1976 27/07/1976 16i08/1974 24/11 /1 976 04/03/1977 16/09/1978 10/10/1980 2311 111 980 11 /OW 981 zaio7ii 981 13ilZi1982 30il011983 1910911 985 1011 O/1986 06/03/1987 2010811 988 07/12/1988 2010611 990 16/07/1990 Nepal to India Turkey-USSR, Islamic Republic of Iran, western Phili ppines, tuzon 35.0 N 25.5 N deuthsfiom 1900 to 1990 (Source: NEIC, 1990) 4 0 8 N 33 0 N 39.4 1 72.6 E 76.0 E 16.4 E 5w - N N81 - N 33 s 18.2 38 N 38 N 40.5 N 42 N 35.8 N 35.8 N N 36.8 38 N 41.1 72 W 76.7 W E 69 € 15.5 E 27 E 13.5 105 7E 1395E 1003E 134.8 E 102.8 E 58 E 15.4 E 97.0 143.0 86.8 E 121 O E 66 5 E 4 500 19 000 2 500 6.4 8.6 7 9 8 9 7 1 8 6 1 MI0 1 300 20 OOO 1600 12000 6 5 8.1 7.5 > 70 000 1950 29 9aa 200 000 143 000 N 39.7 N 3 9 0 N 26 6 N 24.0 N 29.6 N 36 2 5 39.6 N 35.6 N 33.7 N 34.8 N 39.5 N 8 3 5 32SN 36 1 N 1 2 5 2 8 7 N 36 N 36.2 N 34.4 N 30 N 5 39.5 35 6 N 421 N 39 2 N 34 0 N 21.6 N 24.1 N 39.2 N 9 2 5 28.4 N 12.4 38.5 15 3 N 46 4 N 4.6 E E W 72.2 38 E 134.2 E 136.2 E 137.0 41.5 E E 77.8 w 1345 E 136.2 E 78 5 E 966E l.6E 52.7 E 47.6 E 9 w w 74 49.9 E 21.4 E 41.7 E 59.0 E 111 9E 5 102.5 E 29 5 E 78aw N N S 39.6 N 6.3 N 391 N 45 8 N N 36.1 N 40.9 29.9 N 30.0 N 14.7 N 40.3 N 1 8 2 N 33.2 13.8 E f 57.8 44.4 E 42 2 E 102.5 w 89.2 W 52.8 E 86.1 W 4 0 7 W 89.1 W 13.3 f 140.1 E l l 8 O E 124OE 44.0 E 26.8 E 574E 1.4 E 15.3 E 57.7 77.8 w N N 0.2 N 26 8 N 41 .O lu 37.0 N 15.7N E 86.6 E 44 2 E 49.4 121.2E 7.8 7.5 5 000 3 020 200 3 300 1430 000 70 000 > 2990 10 700 3 280 30 000 Do0 8.6 8.3 7.1 7.9 8 3 7.4 6.5 7.6 8.9 8.4 7.1 7.5 8.3 8.0 7 4 a 3 7.1 6 0 7.3 8.4 7.3 6.8 8.7 6 8 7 4 28 OOO 30 1190 t 000 1900 1 300 1400 1330 5 390 6 000 1530 1250 1 200 1130 s l o o o o >4 000 12 230 1100 2 520 > 12 000 3 000 10000 1100 66 000 5 054 5 000 2 300 23 000 1 000 422 255 000 8 000 5 000 1500 15 000 3 500 3 000 3 000 1500 2 800 1 342 ooo+ 9 500 1 1 OOOL 1450 25 000 > 40 000 1621 7.3 5 9 9.5 7.3 6.0 7.1 7.3 5.9 7.5 7.3 7 8 7.1 6.2 6.7 7.5 6.5 7.5 8.0 7.9 7.3 7 2 7.8 7.7 7.2 6.9 7.3 6.0 6.9 8.1 5 5 7 0 6.6 7 0 7 7 7 R . - Major fractures, landslides Great Tokyo fire large fractures completely Quetta almost Landslides, great desmcaon Large larrdslides. topagraphtcal 47 . - and presumed changes Great topographical changes. landslides, Boo& Occurred at shallow depth Tsunami, volcanic aaivlty, floods Occurred at shallow depth Great rock slide, floods > 9 000 rnrswng &ad - For earthquakes with relatively shallow focal depths km), the following approximate empirical rela- intensity 48 to represent the total energy release in the earthquake focus. In general, this scale is called Richter scale, but it should be noted that different magnitude scales are in use by special- ists. if not stated otherwise, the term magnitude simply refers to the Richter scale throughout this chapter. On the other hand, the felt or damaging effects of an earthquake can also be used for scaling the size of an earth- quake. This scale is called seismic intensity scale, sometimes also referred to as the Mercalli scale aEter one of the early authors. It is common in most countries of the world to use 12 grades of intensity, expressed with the Roman numerals I to XI. Japan is an exception and uses a 7-grade intensity scale. The maximum intensity of an earthquake, usualIy found in the epicentral areas, is called the epicentral inten- sity, and replaces or complements the magnitude when describing the size of historical earthquakes in catalogues. (about 10 tionship holds: Magnitude (Richter) 3 3.5 Epicentral V intensify I11 IV VI 4 4.5 Magnitude Epicentral (Richter) VII VIII IX X 5 5.5 6 (>6) I :I 5.3 5.3.1 The concept of large Lthospheric plates migrating on the earth‘s surface allows deep insight into earthquake genera- tion on a global scale. This has become known as plate tectonics. Three plate boundary related mechanisms can be identified through which more than 90 per cent of the ( u ) earth’s seismicity is produced These are: Subduction zones in which deep focus earthquakes are produced in addition to the shallow ones coast of South America, Japan); Capacity CAUSES OF EARTHQUAKE HAZARDS Natural seismicity (e.g., west Year of [km3) tmpounding ’I 936 1959 Dom (m) Height 221 105 130 128 38.3 11.5 0.3 160 0.1 2.8 locution Hoover, USA Hsinfengkiang, Chlna Monteynard, France Kariba, Zambia/Zimbabwe Contra, Switzerland Koyna, India Zealand Benmore, New 230 103 110 2.1 4.8 1962 I 958 1964 1962 1965 1965 1972 160 300 Kremasta, Greece Nurek, Tajikistan 10.5 ( c ) west coast of North America, northern Turkey). In general, shallow-focus earthquake activity contributes much more to the earthquake hazard in an area than the less frequently occurring deep earthquake activity. However, deep strong earthquakes should not be neglected in compIete seis- mic hazard calculations Sometmes they even may dominate the seismic hazard at intermediate and larger distances from the active zone, as found, for example, in Vrancea, Romania. A smaller portion of the world’s earthquakes occur within the lithosphere plates away from the boundaries. These “intra-plate” earthquakes are important and some- times occur with devastating effects. They are found in the eastern USA, northern China, central Europe and western Australia 5.3.2 Year of Induced seismicity Reservoir-induced seismicity is observed during periods when hydroelectric reservoirs are bang filled (Gough, 1978). About 10-20 per cenr of all large dams in rhe world showed some kind oE seismicity either during the first filling cycle or later when the change of the water level exceeded a certain rate. A significant number of prominent cases are described in the lireracure. Table 5.2 provides a sampling of such occurrences However, many other large reservoirs similar in size and geologic setting to the ones listed in Table 5.1 have never shown any noticeable seismic activity other than normal natural seismiciry. Mining-induced seismicity is usualIy observed in places with quickly progressing and substantial underground mining activity. The magnitudes oEsome events have been remarkable (kchter rnagnicude > 5), resulring in substantial damage in the epicentral area This type of seismicity is usually very shallow and the damaging effect is rather local. Examples of regions with well-known induced seismic activity are in South Africa ( Winvarersrrand) and central Germany (Ruhrgebiet). Explosion-induced seismic activity of the chemical or nuclear type is reported in the literature, but this type of seismicity is not taken into account for standard seismic hazard assessment. Strangest event Mognltude (Richter) 5 .O 6.1 4.9 5 8 /urgest event 1939 1961 1963 1963 1965 1967 5.0 6.5 Chapter 5 - Seismic hazards ( b ) Midocean ridges with mainly shallow earthquakes, which are often connected with magmatic activities (e g , Iceland, Azores); and Transform faults with mainly shaliow seismicity (e.g., Table 5.2 - Selected cases 5.0 6.2 at sersmmty 1966 1966 1972 of induced hydroelectric reservoirs 4.5 Comprehensive risk assessment for natural hazards CHARACTERISTICS OF EARTHQUAKE HAZARDS 5.4.1 Application Dynamic ground shaking and permanent ground movement are the two most important effects considered in the analysis of seismic hazard, at Ieast with respect to buildings and lifelines. Dynamic ground shaking is the important factor for buildings. Permanent ground movements such as surface fault rupture, liquefaction, landslide, lateral spreading, compacting and regional tectonic deformation are typically more important than ground shaking with regard to extended lifeline systems. In summary, the following effects of strong earthquakes must be quantitatively investigated for standard seismic hazard and risk evaluations 5.4 5.4.2 Ground shaking Ground shaking refers to the amplitude, Erequency content and duration of the horizontal and vertical components of the vibration of the ground produced by seismic waves arriving at a site, irrespective of the structure or lifeline systems at that site. The frequency range of interest for buildings and engineered structures is generally 0.1-20 Hertz, although higher frequencies may be important for components of lifelines such as switches and distribution nodes In electrical power stations. Ground shaking will cause damage to structures, facilities and lifeline systems unless they are designed and constructed to withstand the vibrations that coincide with their natural frequencies. The damages or other significant effects observed either at the epicentre (usually the location of maximum effects for that earthquake) or at locations distant to the epicentre are often used for the specification of ground motion in terms of seismic intensity grades. This IS the preferred procedure for areas where no instrumental data are indicated in the catalogues of historical earthquakes. Caution in comparing intensity data of different origin has to be exercised, as various intensity scales are currently in use in different parts of the worId (see 5.1 1 ) . The spatial, horizontal and vertical distribution of ground motions are very important considerations for extended li€eIine systems. Spectral velocity and displacement are more significant values than peak acceleration for some structures such as bridges and pipelines. Ground shaking can also trigger permanent ground deformation. Buried pipelines are especially sensitive to these displacement-controlled processes rather than to the force-controlled process of ground shaking, which have the most pronounced effect on buildings. The estimation of ground motion and ground shaking is sometimes considered important for the design of underground structures. However, seismological measurements show that the intensity of the ground shaking decreases with increasing depth from the surface, while permanent ground motion is the dominating parameter of concern. Surface faulting 5.4.3 mapping due to its very local nature. Liquefaction 5.4.4 49 Surface faulting is the offset or rupturing of the ground surface by differential movement across a fault during an earthquake. This phenomenon IS typically limited to a linear zone along the surface. Only a small fraction of earthquakes cause surface faulting Faulting tends to occur when the earthquake has a shallow focus (5-lokmdepth) andisrelativelystrong(magni- tude larger than Richter 6).Although a spectacular feature, the direct effect of faulting does not play a major role in hazard Liquefaction is a physical process generated by vibration during strong earthquakes and is generally restricted to dis- tinct localities leading to ground failure. Liquefaction normally occurs in areas predominated by clay to sand sized particles and high groundwater levels. Persistant shaking increases pore water pressure and decreases the shear strength of the material, resulting in rapid fluidization of the soil. Liquefaction causes lateral spreads, flow failures and loss of bearing strength. Although uncommon, liquefaction can occur at distances of up to 150 km from the epicentre of an earthquake and may be triggered by levels of ground shaking as low as intensity V or VI (12-grade intensity scale). A recent example of strong liquefaction was observed in the Kobe (Japan) earthquake of 1995 (EERI, 1995). 5.4.5 Landslides Landslides can be triggered by fairly low levels of ground motion during an earthquake if the slope is initially unstable. The most abundant types of earthquake-induced landslides are rock falls and slides of rock fragments that form on steep slopes. The lateral extent of earthquake induced landslides reaches from a few metres to a few kilometres depending on the local geological and meteorological conditions. Landslides may produce large water waves if they slump into filed reser- voirs, which may result in the overtopping of the dam. Although not as a result of an earthquake, a landslide on 9 October 1963 caused the overtopping of the Valont dam, flooding Longarone and other villages in Italy. The flooding resulted in appromately 2 000 deaths. Large earthquake-induced rock avalanches, soil avalanches and underwater landslides can be very destruc- tive. One of the most spectacular examples occurred during the 1970 Peruvian earthquake when a single rock avalanche triggered by the earthquake killed more than 18 000 people The 1959 Hebgen Lake, Montana, earthquake triggered a similar but less spectacular landslide that formed a lake and killed 26 people. Tectonic deformation 5.4.6 Deformation over a broad geographic area covering thou- sands of square kilometres IS a characteristic feature of 50 (c) earthquakes having large magnitudes. In general, the fol- lowing effects can be observed in principle and have to be recognized in seismic hazard assessment for specific sires: (a) tilting, uplifting, and down warping; ( b ) fracturing, cracking, and fissuring, compacting and subsidence; (4 creeping in fault zones. 5.5 TECHNIQUES FOR EARTHQUAKE HAZARD ASSESSMENT Principles 5.5.1 Objective of earthquake hazard assessment The objective of a statistical earthquake hazard analysis is to assess the probability that a particular level of ground motion (e.g., peak acceleration) at a site is reached or exceeded during a specified time interval (such as 100 years). An alternative approach is to consider the evaluation of the ground motion produced by the maximum conceiv- able earthquake in the most unfavourable distance to a specific site. Limits of earthquake hazard assessment Earthquake hazard assessment in areas of low seismicity is much more subject to large errors than in areas with high earthquake activity. This is especially the case if the time span of the avaiIable data is considerably smaller than the mean return interval of large events, for which the hazard has to be calculated. Incorporation of uncertainties Uncertainties resuit from Iack of data or/and Iack of knowl- edge, In seismic hazard computations, the uncertainties of the basic input data must be taken into account (McGuire, 1993). This task is accompfished by developing alternative strategies and models in the interpretation of those input data, for which significant uncertainties are known to exist. This applies in particular for: ( a ) the size, location, and time of occurrence of future earthquakes; and ( b ) the attenuation of seismic waves as they propagate from all possible seismic sources in the region to a11 possible sites. 5.5.2 Standard techniques Input models for probabilistic seismic hazard anaIysis: (a) Earthquake source models The identification and delineation of seismogenic sources in the region is an important step in preparing input para- meters for hazard calculation. Depending on the quality and completeness of the basic data available for this task, these sources may have different shapes and characteristics. Faults are line sources specified by their three-dimen- sional geometry - slip direction, segmentation and possible rupture length. A line source model is used when Chapter 5 - Seismic hazards earthquake locations are constrained along an identified fault or fault zone. All future earthquakes along this fault are expected to have the same characteristics. A set of line sources is used to model a large zone of deformation where earthquake rupture has a preferred orientation but a ran- dom occurrence. Area sources must be defined, if faults cannot be identi- fied or associated to epicentres. Seismicity is assumed to occur uniformly throughout an area. An area source encompassing a collection of line sources is used when large events are assumed to occur only on identified active faults and smaller events are assumed to occur randomly within the region. The existing distribution of earthquakes and the seis- rnotectonic features can be represented by more than one possible set of source zones leading to quite different hazard maps for the same region (Mayer-Rosa and Schenk, 1989; EPRI, 1986). Occurrence modeZs (b) For each seismic source (fault or area), an earthquake occur- rence model must be specified. It is usually a simple cumdative magnitude (or intensity) versus frequency dis- tribution characterized by a source-specific b-value and an associated activity rate. Different time of occurrence models such as Poissonian, time-predictable, slip-predictable and renewal have been used in the calculation process. Poissonian models are easy to handle but do not always rep- resent correctly the behaviour of earthquake occurrence in a region. For the more general application, especially where area sources are used, the simple exponential magnitude model and average rate of occurrence are adequate to specify seismicity (McGuire, 1993). It must be recognized that the largest earthquakes in such distributions sometimes occur at a rate per unit time that is larger than predicted by the model. A “characteristic” earthquake distribution is added to the exponential model to account for these large events. (c) Ground motion models The ground motion model relates a ground motion para- meter to the distance from the sourcetsl and to the size of the earthquake. The choice of the type of ground motion parameter depends on the desired seismic hazard output. Usual parameters of interest are peak ground acceleration (PGA), peak ground velocity (PGV) and spectral velocity for a specified damping and frequency. Effective maximum acceleration is used as a parameter when large scatter of peak values is a problem. All these parameters can be extracted from accekerograms, which are records produced by specific instruments (accelerometers) in the field. In cases where the primary collection of earthquakes consists of pre-instrumental events for which seismic inten- sities have been evaluated (see section 5.4.2), the site intensity (specified for example either by the EMS or MMI scale) is the parameter of choice for the representation of the ground motion Ievel However, this method includes high uncertainties and bias due to the subjectiveness of intensity estimation in general. Furthermore, information on ground motion frequency is not explicitly considered within such models. - - Conrprehensive risk assessment for natural hazards A preferred procedure in many countries to predict physical ground motion parameters at sites of interest is to convert the original intensity information into magnitude values and to use deterministic attenuation relations for acceleration and distance. [r) arid magnitude the deductive method ofseimogenic sources ni arc calculated, i v i t h hypocrntral distance log,, n (m) = a - bm Seismic hazard calculation The following two approaches of probabilistic hazard calcu- lation are frequently applied: (1 j The deductive method uses statistical interpretations (or extrapolations) of the original data to describe the occurrence of earthquakes in time and space and their general characteristics. and Cornell (1968) developed the method, while Algermissen and Perkins (1976) and McGuire ( lcI76) wrote its computer codes. Hays (1980) Bashani and Giardini (1993) describe the proce- dure. The handling of uncertainties is contained in (1993). All steps from the collection of the McGuire basic data to the application of the method is shown schematicalty in Figure 5.1. (2) The historic method directly uses the historical record of earthquakes and does not involve the definition of distinct seismic sources in form of faults and areas (Veneziano et a/., 1984). Each historical event is treat- ed as a source for which the effect on the site is calculated individually. Seismic hazard is assessed by summation of the effects of all historical events on the site. In both approaches, the probability of exceedance or non-exceedance of a certain level of ground motion for a given exposure time is the targct result, considering earth- quakes of all possible magnitudes and distances having an influence on the site. Application of gr o met r ic Stcp I : Definition Faults and area sources have to be delineated describing the ( 3 - d i ens io na I) d i s t r ibu t i on o f earthquake occurrence in the investigated area. Then distancc and mag- nitude distributions ( m ) . S ~ r p 2: Definition ofseismicity partrrneters It is assumed that the rate of recurrence of earthquakes in general follows the Gutenberg-Kichter ( GK) relation where (5.2) n(m) IS the mean number of events per year having magnitudes greater than rn, while a and b are constants defined by regression analysis as described in 5.5.2b above. For a single souIce, the modified G-K relation for the annuai mean rate o f occurrence is 51 where mu and ml are the upper- and lower-bound magni- tudes, and aN is the number of events per year in the source having magnitudes m equal to or greater than m!. (5.4) and a' are ground motion values (acceleration). T ) Step 3: Establishing theground motion model The ground motion equation is calculated for the condi- tional probability of h exceeding a' giyen an earthquake of magnitude m occurring at a distance r from a site G(A > a* Irn. r) wheru A Step 4: Probability analysis The contribution of each source to the seismic hazard at the site is calculated from the distributions of magnitude, dis- tance and ground motion amplitude. Following equations 5.1 to 5.4, the probability that the value A of ground motion at the site exceeds a specified level a+ is; A >a* Im, r ) fm(rn) C lJG( P( A > a * ) = no 1 where source . 5.5.3 (5.5) the Study 10 f~ ( dm dr in which the summation is performed over a11 sources i. no is the mean annual rate of occurrence for a Refinements to standard techniques of paieoseismicity Field techniqucs have been dcveloped to determine dates of prehistoric earthquakes on a given fault and to extend the historical seismicity back in time as much as years or more. These techniques involve trenching and age dating of buried strata that immediately pre-date post-date a historic earthquake. The application of 000 And these techniques is called a"pa1eoseismicity" study (Pantosti and Yeats, 1993). Study of site amplification These studies help to quantify thc spatial variation of ground shaking susceptibility and, thus, more precisely define the engineering design parameters. Experience and data have shown that strong contrasts in the shear-wave selocity between the near-surface soil layers and underlying bedrock can cause the ground motion to be amplified in a narrow range of frequencies, deterrnincd by the thickness of the soft laycrs. All relevant parameters, such as peak amplitudes, spec- tral composition and duration of shaking, are significantly changed when the velocity contrast exceeds a factor of about 2 and the thickness of the soil layer is between 10 and 200 m. Microzonation studies have been performed for a number of large cities in the world (Petrovski, 1978). Study of the potential for liquefactiorr and landslides Liquefaction is restricted to cert'iin geologic and hydrologic conditions. It is mainly found in areas where sands and silts were deposited in the last 10 000 years and the ground water levels arc within the uppermost 10 :n of the ground. As a general rule, the younger and looser the sediment and the Figure 5. I 1 - Schematic diagram uf the steps (I-IV) and basic components of probubilisric earthquake hazard assessment Epicentre and energy release maps, magnitude-frequency distributions Zoning maps: expected intensity, acceleration, velocity for specific return periods Engineering: soil conditions I 1 identification of areas with systematic intensity ana rnalies identification of areas with high potential for damage Disaster management: emergency measures L 1. BASIC GEOSCIENCE DATA time-series 1 II. DATA PROCESSING Strong motion attenuation models, spectral acceleration Devefopment of design spectra and relevant ground motion 1 tV. APPLICATION earthauake resistant design 1 ltl. ZONING 1 L (mechanisms of fracturing, simulation of earthquake occurrence) I Definition of source regions and earlhquake occurrence models national economic figures and development models 1 development, land use Comprehensive risk assessment for natural hazards higher the water table, the more susceptible a clay to sandy soil will be to liquefaction. Liquefaction causes three types of ground failures: lat- eral spreads, flow failures and loss of bearing strength. Liquefaction also enhances ground settlement. Lateral spreads generally develop on gentle slopes (< 3 degrees) and typically have horizontal movements of 3-5 m. In slope ter- rain and under extended duration of ground shaking the lateral spreads can be as much as 30-50 m. Alternative techniques 5.5.4 Although the deductive methods in seismic hazard assess- ment are well established other methods may also give useful results under special conditions. These include his- torical and determinate approaches, described below. The historical methods In contrast to the deductive seismic source methods, non- parametric methods are often employed when the process of earthquake generation is not well known, or the distribu- tion of historical earthquakes do not show any correlation with mapped geological features. A historical method (Veneziano e t al., 1984) is based only on historical earthquake occurrence and does not make use of interpretations of seismogenic sources, seismic- ity parameters and tectonics. The method has limitations when seismic hazard for large mean return periods, i.e. larger than the time span of the catalogues, are of interest. The results have large uncertainties. In general, to apply the his- torical method, the following steps have to be taken: ( a ) Comprlation of a complete catalogue with all historic events including date, location. magnitude, andlor intensity and uncertainties (Stucchi and Albini, 1991; Postpischl, 1985). ( b ) Development of an attenuation model that predicts ground motion intensity as a function of distance for a complete range of epicentral intensities or magnitudes. Uncertainties are introduced in the form of distribu- trons representing the dispersion of the data. ( c ) Calculation of the ground motion produced by each historical earthquake at the site of interest. The summation of all effects IS finally represented in a function relating the frequency of occurrence with all ground motion levels. (d) Specification of the annual rate ofexceedance by divid- ing this function through the time-span of the catalogue. For small values of ground motion the annual rate is a good approximation to the annual probability of exceedance. The deterministic approach Deterministic approaches are often used to evaluate the ground-shaking hazard for a selected site. The seismic design parameters are resolved for an a priori fixed earth quake that is transposed onto a nearby tectonic structure, nearest to the building, site or lifeline system An often- applied procedure includes the following sseps: (a) Choosing the largest earthquake that has occurred in history or a hypothetical large earthquake whose (c) based on local data, or at least taken from another seismotectonically similar region. Calculation of the ground motion at the site of interest for this largest earthquake at the closest possible location. (e) Repetition for all seismotectonic zones in the neigh- bourhood of the site and choice of the largest calculated 53 occurrence would be considered plausible in a seisrno- genic zone in the neighbourhood of the site. (b) Locating this earthquake at the nearest possible point within the zone, or on a fault. Adoption of an empirical attenuation function for the desired ground motion parameter, preferably one d e h e the (a) ground motion value. Estimations of seismic hazard using this method usually are rather conservative. The biggest problem in this relativepj simple procedure is the definition of those critical source boundaries that are closest to the site and, thus, distance of the maximum earthquake. Deterministic methods deliver meaningful results if alI critical parameters describing the source-path-site-system are sufficientfywell known. DATA REQUIREMENTS AND SOURCES The ideal database, which is never complete andlor avail- able for all geographic regions in the world. should contain the information for the area under investigation (Hays, 1980) as outlined rn this section. This database corresponds to the components under “Sasic Geoscience Data” in Figure 5.1. Seismicity data These data include complete and homogeneous earthquake catalogues, contaning all locations, times of occurrence, and size measurements of earthquakes with fore- and after- shocks identified. Uniform magnitude and intensity definitions should be used throughout the entire catalogue (Gruenthal, 1993), and uncertainties should be indicated for each of the parameters. Seismotectonic data 5.6 5.6.1 5.6.2 The data include maps showing the seismotectonic provinces and active faults with information about the earthquake potential of each seismotectonic province, including information about the geometry, amount and sense of movement, temporal history of each fault, and the correlation with historical and instrumental earthquake epicentres. The delineation of seismogenic source zones depends strongly on these data. 5.6.3 Strong ground motion data These data include acceleration recordings of significant earthquakes that occurred in the region or have influence on the site. Scaling relations and their statistical distribution for 54 ground-motion parameters as a function of distance have to be developed for attenuation models. 5.6.4 Macroseismic data These data include macroseismic observations and isoseis- ma1 maps of all significant historical earthquakes that have affected the site. Relationships between macroseismic observations (intensities) and physical ground motion mea- surements (accelerations) have to be established 5.6.5 5.7 5.8 Spectral data Adequate ensembles of spectra are required for“ca1ibratin~ the near field, the transmlssion path,and the local-ground response. Thus, frequency dependent anomahes m the spatial distribution of ground motions can be identified and modelled 5.6.6 Local amplification data These data describe seismic wave transmission characteris- tics (amptification or damping) of the unconsolidated materials overlying bedrock and their correlation with physical properties including seismic shear wave velocities, densities, shear module and water content. With these data, microzonation maps can be developed in local areas identi- fying and delineating anomalous amplification behaviour and higher seismic hazards. ANTHROPOGENIC FACTORS The factors that continue to put the world’s population cen- tres at risk from earthquake hazards are: - rapid population growth in earthquake-prone areas; - growing urban sprawl as a worldwide phenomenon, - existence of large numbers of unsafe buildings, vulner- able critical facilities and fragile lifelines; and - interdependence of people in Iocal, regional, national and global communities. PRACTICAL ASPECTS The earthquake database used in seismic hazard assessment as a basic input usually consists of an instrurnentatly deter- mined part and a normally much larger historical time span with macroseismically determinzd earthquake source data. It is essential to evaluate the historical (macroseismic) part of the data by using uniform scales and methods. For the strongest events, well-established standard methods must be applied (Stucchi and hlbini, 199L; Guidoboni and Stucchi, 1993). Special care must be taken whenever cata- logues of historical earthquakes of different origin are merged, e g , across national borders The total time span of earthquake catalogues can vary from some tens to some thousands of years. In general, the Chapter 5 - Sersrnrc hazards earthquake database is never homogeneous with respect to completeness, uniform magnitude values or location accu- racy. The completeness of catalogues must be assessed in each case and used accordingly to derive statistical parame- ters such as the gradient of fog frequency-magnitude relations. It is inevitable that one has to extrapolate hazard from a more or less limited database. The results of hazard calcula- tions are, therefore increasingIy uncertain as larger mean recurrence periods come into the picture. This is especially so, if these periods exceed the entire time window of the underlying earthquake catalogue. The user of the output of seismic hazard assessments should be advised about the error range involved in order to make optimal use of this information. Different physical parameters for ground shaking may be used to describe seismic hazard These include peak acceleration, effective (average) acceleration, ground vel- ocity, and the spectral values of these parameters. However, for practical and traditional reasons, the parameter selected most often for mapping purposes is horizontal peak acceler- ation (Hays, 1980) 5.9 5.9.1 PRESENTATION OF HAZARD ASSESSMENTS Probability terms With respect to hazard parameters, two equivalent results are typically calculated These are the peak acceleration cor- responding to a specified interval of time, which is known as exposure time, or the peak acceleration having a specified average recurrence interval. Table 5 3 provides a few examples of these two methods of expressing hazard. While recurrence intervals in the order of 100 to 500 years are considered mainly for standard building code applications, larger recurrence intervals of 1 000 years or more are chosen for the construction of dams and critical lifeline systems. Even lower probabilities of exceedance (e.g., 10 000 year recurrence interval or more or 1 per cent in 100 years or smaller) have to be raken into accounr for nuclear installations, although the life span of such structures may only be 30 to 50 years. Use is made of equation 6.1 to obtain the recurrence internal as listed in Table 5 3 5.9.2 Hazard maps In order to show the spatial distribution of a specific hazard parameter, usually contoured maps of different scaIes are prepared and plotted. These maps may be classified into dif- ferent levels of importance, depending on the required detail of information, as hsted in Table 5.4. These scales are oniy approximate and may vary in other fields of natural hazards. Seismic hazard assessment on the local and project level usually incorporates the influence of the local geologi- cal conditions. The resulting hazard maps are presented then in the form of so-called “Microzoning” maps showing mainly the different susceptibility to ground shaking in the range of metres to kilometres. Cornprehensive risk assessmentfrit nuturd hazards ~~~ , ~_._I_- - - ., . equivalent Table 5.3 - h m p k s of haznrdj5gures Figure 5.2 shows a composite of different national seis- mic hazard maps. The parameter representing ground motion is intensity defined according to the new European Macroseismic Scale (Gruenthal, 1998). The probability of non-exceedance of the indicated intensities in this map is 90 per cent in 50 years. equivalent to a recurrence interval of exactly 475 years, which corresponds to the level required for the new European building codes-ECS. This map in an example for international normalization of procedures, since it was uniformly computed with the same criteria and assumptions for the three countries - Austria, Germany and Switzerland. 5.9.3 d ministic hazard assessment; ( b ) maps of engineering coefficients and design para- meters, mainly used for national building code (c) recurrence different ground conditions (rnicrozondtion and (d) maps of zones where different political and/or admin- istrative regulations have to be applied with respecr to Seismic zoning Zoning maps are prepared on the basis of seismic hazard assessment for providing information on expected earth- quake effects in different areas. The zoned parameters are of different nature according to their foreseen use. The fol- lowing four types of zoning maps may serve as examples: (a) maps of rnammurn seismic intensity, depicting the spa- tial distribution of the maximum observed damage during a uniform time period, mainly used for deter- specificat ions; maps of maximum expected ground motion (accelera- tion, velocity, displacement, etc.) for different intervdls, including amplification factors for Probability of exceedance for a given exposure time 10 per cent in 10 years 10 per cent in 50 years 10 per cent in 100 years 1 per cent in 100 years maps); earthquakes, mainly used for economic and/or logistic purposes. zoning maps dre those used in earthquake- (Basham et two mays for Canada shown in Figure 5.3 1985). Shown are the peak ground motion values with 10 Typical budding codes (Sachanski, 1978) The specification and quantification of the different zones in terms of design para- meters and engineering coefficients is demonstrated in the al., Level lbble 5 4 level National - i$importance and scales in seismic hazard mapping Level scok scole 1 25000 Local Proiect 1 1 000 000 250 000 1 Regional 1 5 000 55 I .. - Equrvutent approximate averoge recurrence Probabilrty or for a given exposure time non-exceedonce rntervol 100 years 500 years 1 000 years 10 000 years 90 per cent in 10 years 90 per cent in SO years 90 per cent in 100 years 99 per cent in 100 years per cent probability of exceedance in 50 years, together with the zoning specifications. They differ in terms of the con- toured type of the ground motion parameter, horizontal acceleration and horizontal velocity, respectively. 5.10 cost discontinuities in borders. For Europe a new uniform code, Eurocode 8, is rhe level of prorecrion across national expected to improve this situation in Future. One way to also reduce the financial consequences for the individual is by insurance (Perrenoud and Straub, 1978). However, the integral costs of a disastrous earthquake in a densely populated and industrialized area may well exceed the insured property value and may severely affect the eco- nomic health of a region. This is beyond aspects dealing with the human losses. Disaster response planning and increasing prepared- ness for strong earthquakes may also reduce considerably the extreme effects of earthquakes. Preparedness on farniiy, community, urban and national levels is cruciat in earth- quake-prone countries. Such preparedness plans have been developed in many countries. Public education and increased awareness, through at times the involvement of the m a s media, are very efficient tools in reducing educational efforts in schools and universities provide a solid basis for the transfer of knowledge (Jackson and Burton, 1978). 11 PREPAREDNESS AND MITIGATION There are basically three ways to reduce the risk imposed by earthquakes (Hays, 1990). They are: ( a ) to reduce vulnerability of structures, ( b ) to avoid high hazard zones; and ( c ) to increase the awareness and improve the prepared- ness of the population. The reduction of vulnerability is achieved most econom- ically by applying established building codes. With the proven measures Iisted in such codes and developed for engineering practice, the desired level of protection can be achieved with a good benefit-cost ratio or the minimum expected life-cycle (Pires eta!., 1996; Ang and De Leon, 1996). Such building codes, eirher in rigid legal form or as a more flexible profes- sional norm, are available in almost every civilized country in the world. However, the national codes of even neighbouring countries are often found to differ considerably, leading to mks on a personal level. Professional and GLOSSARY OF TERMS Accelerngram: The recording of an instrument called accelerometer showing ground motion acceleration as 5 57 Figure 5 3 - Typicalpeak horizon t d acceleration zoning map (above) and peak horizontal velocity zoning map (below) for rhe probability of exceedance of 10 per cent In 50 years, wed in the new building code of Canada. Seven zones, Za and Zv, are conloured with units in fractions ofgrmity, g = 981 ds2, and m/s, respectively (afier Basham et al., 1985) phenomena accompanying an earthquake such as ground shaking,ground failure, surface faulting, tecton- ic deformation, and inundation which may cause damage and loss of life during a specified exposure time having a broad frequency content. The design spectrum Earthquake hazards: Probability of occurrence of natural can be either site-independent or site-dependent. The site-dependent spectrum tends to be less broad band as it depends also on (narrow band) local site conditions. Durarion: A description of the length of time during which g) Earthquaka- Sudden reIease of previousfy accumulated stresses in the earth’s crust and thereby producing seis- Comprehensive risk assessment for natural hazards ground motion at a site exhibits certain characteristics such as being equal to or exceeding a specified level of acceleration (e.g., 0.05 mic waves. (see also earthquake risk). Earthquake risk: The social or economic consequences of earthquakes expressed in money or casualties. Risk is composed from hazard, vulnerabjljty and exposure. In more general terms, it is understood as the probability of a loss due to earthquakes. .. .- 58 Earthquake WCIYM: Elastic waves (body and surface waves) propagating in the earth, set in motion by faulting of a portion of the earth. EMS-Scale 1998: Short form: - Notfelt. I 11 I11 - Weak, felt indoors by a few people, light trem- - Scarcelyfelt, only by very few individuals at rest. bling. - Largely abserved, felt indoors by many people, outdoors by very few. A few people are awak- ened. Windows, doors and dishes rattle. - Stsong, felt indoors by most, olitdoors by few. .Many sleeping people are wokcn up. A few arc Eright- ened Buildings tremble throughout. Hanging objects swing considerably. Small objects are - Slrghtl;l damaging, many people are frightened shifted. Doors and windows swing open or shut. and run outdoors. Some objects fall. Many from houses suffer slight non-structural damage. - Damaging, most people are frightened and run outdoors. Furniture is shifted and objects fall sheIves in large numbers. Many well-built ordinary buildings suffer moderate damage Older buildings may show large cracks in walls W V VI VII and failure of fill-in walls. WII- Heav~Iy damaging, many people find it difficutt to stand. Many houses have large cracks in walls. A few well-buih ordinary buildings show ous failure of walls. Wedk old structures may h e a y damage, e g , seri- collapse IX - Destruc~ive, general panic. Many weak con- structions collapse Even well-built ordinary buildings show very structural failux X - Very dzstructiw, many ordinary well-built partral - Devastating, most ordinary well-built buildings buildings collapse. collapse, even some with good earthquake resis- XI XI1 at tant design. - Completeb devncming, almost all buildings are destroyed. The point on tho earth's surface vertically above the point where the first fault rupture and the first earthquake motion occur. Excredunce pm-obnbrliry: The probability (for example 10 per cent) over some exposure time that an earthquake will Epicrntre generate a value of ground shaking greater than a spec- ified level. time: The period ot time (for example, 50 years) the earth's surface after Exposure that a structure or faciliiy is exposed to earthquake haz- ards. The exposure time is sometimes related to the design lifetime of the structure and is used in seismic risk calculations. Fault A fracture or fracture zone in the earth along which displacement of the two sides relative to one another has occurred parallel to the fracture. Oftcn visible as fresh ground displacement strong, shallow events. Focnl d e p k Thr vertical distance b e w w n the tarthyuakt hypocentre and the earth's surface. Chapter 5 - Seismic hazards Ground motion: A general term including all aspects of motion; for example particle acceleration (usually given in fractions oE the earth's gravitation (g) or in percent- it), velocity or displacement. Duration oE the rnotion and spectral contents are further specifications of ground motion. Ground acceleration, response age of spectra (spectral acceleration, veIocity and displace- ment) and duration are the paramerers used most frequently in earthquake- resis t an t design to character - ize ground motion. Design spectra are broad-band and can be either site-independent (applicable for sites hav- ing a wide range of local geologic and seismologrc conditions) or site-dependent (applicablc to a partlcu- lar per second. Induced seismicity Generated by human activities mainly in mining and reservoir areas. Can produce considcrable or even dominating hazards. There are two likely causes for the triggering effect of a Iarge reservoir. The strain in the rock is increased by the extra load of the reservoir ing. However, this theory IS physically not as acceptable as the second one, which involves increased pore pres- site having spectfic geologic and seismological conditions). Hertz. Unit of the frequency of a vibration, given in cycles fill, and reaches the condition for local fault- sure due to infiltrated water, thereby lowering the shear strength of the rocks along existing fractures and trig- gering seismtcity. The focal depths of reservoir-induced earthquakes are usually shallower than Intensity A numerical index describing the effects of an earth- IMedvedc?;-Sponheuer-h'arnik (MSK) 10 km. quake on the earth's surface, on people and on structures. The scale in common use in the USA today is the Modified Mercalh Intensity (MMI) Scale of 193 I . The Scale of 1964 is widely used in Europe and was recently updated to the new European Macroseismic (EMS) Scale in 1998. These scales have Intensity values indicated by Roman numerals from I to XII. The narrative descriptions of the intensity values of the different scales are comparable and there€ore the three scales roughly correspond. In Japan the 7-degree scale of the Japan MeteoroIogicaI Agency (JMA) is used. Its total range of effects is the same as in the 12-degree scales, but its lower resolution allows for an easier scpara- tion of the effects. Refer to downward and outward movement on thosc that Lontlslrdes: slopes oi rock, sail, artificial fill and similar rnalcrials. The factors that controt landsliding are increase the shearing stress on the slope and decrease the shearing strength of the earth materials. The latter is likely to happen in periods with large rainfalls. Lzquejurtion: The primary factors used to judge the poten- tial for liqucfactian, the transformation of unconsolidated materials into a fluid mass, are: grain size, soil density, sod structure, age of soil deposit and depth to ground water. Fine sands tend to be more sus- ceptible to liquefaction than silts and gravef. Behaviour of soil deposits during historical earthquakes in many parts of the wortd show that, in general, liquefaction susceptibiky of sandy sods decreases with increasing age of the soil deposit and increasing depth to ground the average period of time - or recurrence interval - Basham, P., D. Weichert, F.Anglin and M. Berry, 1985: New probabilistic strong motion maps of Canada, Bulktin of 36, pp. 15-26 76-0416,45 pp target reliabdrtres and upgrudirig ofmuctures. Proceedings of the 11th World Conference on Comprehensive risk assrssment for natural hazards water. Liquefaction has the potential of occurring when seismic shear waves having high acceleration and long duration pass through a saturated sandy soil, distorting its granular structure and causing some of the void spaces to collapse. The pressure of the pore water between and around the grains increases until it equals or exceeds the confining pressure. At this point, the moves upward and may emerge at the surface. a fluid €or a short water The liquefied soil then behaves like time rather than as a solid. Mugnitude. A quantity characteristic of the total energy released by an earthquake, as contrasted to intensity that describes its effects at a particular place. C.F. Richter devised the logarithmic scale for local magni- tude (ML) Ln 1935. Magnitude is expressed in terms of the motion that would be measured by a standard type of seismograph Iocared 100 km from the epicentre of an earthquakc. Several other magnitude scales in addition to ML are in use; for exampke, body-wave magnitude (mb) and surface-wave magnitude (MS). The scale is theoretically open ended, but the iargest known earth- quakss have MS magnitudes slight!y over 9. accelemlron: The value of the absolutely highest acceler- Peuk ation in a certain frequency range taken from strong-motion recordings. Effective m a m u m accelera- tion (EMA) is the value of maximum ground acceleration considered to be of engineering significance. EMA is usually 20-50 per cent lower than the peak value in the same record. It can be used to scale design spectra and is often determined by filtering the ground-motion record to remove the very h g h frequencies that may have little or no influence on structural response. Plate tectonics: Considered as the overall governing process responsible for the worldwide generation of earthquake activity. Earthquakes occur predominantly along plate boundaries and to a lesser extent within the plates. Intra- plate activity indicates that lithospheric plates are not rigid or free from internal deformation. Response spectrum: The peak response of a series of simple graphical form the variations of the peak spectral accel- eration, velocity and displacement of the oscillators as a function of vibration period and damping. Return period For ground shaking, return period denotes Recurrence interval (see return period). harmonic oscillators having different natural periods when subjec.ted mathematically to a particular earth- quake ground motion The rcsponse spectrum shows in betwezn events causing ground shaking that exceeds a particular level at a site; the reciprocal of annual proba- bility of exceedance. A return period of 475 years means that, on the average, a particular lcvel of ground motion will be equalled or exceeded once in 475 years Risk: Lives lost, persons injured, property damaged and eco- nomic activity disrupted due to a particular hazard. Risk is the product of hazard and vulnerahiliry. men: Total subsurface area that is supposed to be Rupture sheared by the earthquakc mechanism Seisniic nzicrozonrng The division of a region into geographic areas having a similar relative response to a particular 59 _- earthquake hazard (for example, ground shalung, surface fault rupture, etc.). hlicrozoning requires an integrated study oE I ) the frequency of earthquake occurrence in the region; 2 ) the source parameters and mechanics of fault- ing for historical and recent earthquakes affecting the region; 3) the filtering characteristics of the crust and mantle along the regional paths along which the seismic waves are travelling; and 4) the filtering characteristics of the near-surface column of rock and soil. zoning: The subdivision of a large region (e.g., d Seismic earthquake activity with approximately the same characteristics, Seismorecrorric province: Area demarcating the location of city) into areas within which have uniform seismic parameters to be used as design input for structures. Seismogenzc source: Area with historical or/and potential historic or/and potential earthquake activity with sim- ilar seismic and tectonic characteristics. The tectonic processes causing earthquakes are believed to be sirni- lar in a gwen seismotectonic province. Source: The source of energy release causing an earthquake. The source is characterized by one or more variables, for example, magnitude, stress drop, seismic moment. Regions can be divided into areas having spatially homogeneous source characteristics. Strong motion. Ground motion of sufficient amplitude to be of engineering inreresr in the evaluation of damage resulting from earthquakes or in earthquake-resistant travelliiig design of structures. Tsunami: Large sea waves caused by submarine earthquakes over long disrances and thereby forming dis- 5.12 astrous waves on shallow-water seashores. 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