Seismology - David M. Boore

1 See AlsoAnalytical Geology: Seismology ; Natural and Anthropogenic Geohazards;Site and Ground Investigation; Site ReadingEnvironmental and Engineering Geophysical Society. and Safety Executive (UK) (2000)Avoiding Dangerfrom Underground and SafetyExecutive, DM, Eddleston M, Fenning PJ, and Reeves GM(eds.) (1997)Modern Geophysics in Geology Special Publication : Geological PW, Barker RD, and Butcher AP (eds.)(2002)Geophysics in Engineering Geology Special Publication 19. J (2003)Field Geophysics,3rd edn., The Geo-logical Field Guide Series. Chichester: John Wiley & JM (1997)An introduction to Applied andEnvironmental : John Wiley & WM, Geldart LP, and Sheriff RE (1990)AppliedGeophysics,2nd edn. Cambridge: Cambridge Geophysical Method Descriptons.

2 J Bommer, Imperial College London, London, UKD M Boore , United States Geological Survey, MenloPark, CA, USA 2005, Elsevier Ltd. All Rights Seismology is an integral part of earth-quake engineering, a specialized branch of civil en-gineering concerned with the protection of the builtenvironment against the potentially destructive effectsof earthquakes. The objective of earthquake engineer-ing can be stated as the reduction or mitigation ofseismic risk, which is understood as the possibility oflosses human, social or economic being caused byearthquakes. Seismic risk exists because of the convo-lution of three factors: seismic hazard, exposure, andvulnerability. Seismic hazard refers to the effects ofearthquakes that can cause damage in the built envir-onment, such as the primary effects of ground shakingor ground rupture or secondary effects such as soilliquefaction or landslides.

3 Exposure refers to the pop-ulation, buildings, installations, and infrastructure en-countered at the location where earthquake effectscould occur. Vulnerability represents the likelihoodof damage being sustained by a structure when it isexposed to a particular earthquake Hazards andSeismic RiskRisk can be reduced in two main ways, the first beingto avoid exposure where seismic hazard is , human settlement, the construction ofindustrial facilities, and the routing of lifelines suchas roads, bridges, pipelines, telecommunications, andenergy distribution systems are often governed byother factors that are of greater importance. Indeed,for lifelines, which may have total lengths of hun-dreds of kilometres, it will often be impossible toavoid seismic hazards by relocation. Millions ofpeople are living in areas of the world where there isappreciable seismic hazard.

4 The key to mitigatingseismic risk therefore lies in control of vulnerabilityin the built environment, by designing and buildingstructures and facilities with sufficient resistanceto withstand the effects of earthquakes; this is theessence of earthquake is not to say, however, that the aim is toconstruct an earthquake-proof built environmentthat will suffer no damage in the case of a strongearthquake. The cost of such levels of protectionwould be extremely high, and, moreover, it mightmean protecting the built environment against eventsthat may not occur within the useful life of a particu-lar building. It is generally not possible to justify suchinvestments, especially when there are many compet-ing demands on resources. Objectives of earthquake-resistant design are often stated in terms that relatedifferent performance objectives to different levels ofearthquake motion, such as no damage being sus-tained due to mild levels of shaking that may occurfrequently, damage being limited to non-structuralelements or to easily repairable levels in struc-tural elements in moderate shaking that occurs occa-sionally, and collapse being avoided under severeENGINEERING GEOLOGY/Seismology499ground shaking that is only expected to occur objectives are adjusted according to the conse-quences of damage to the structure; for rare occur-rences of intense shaking, the performance target fora single-family dwelling will be to avoid collapse ofstructural elements, so that the occupants may escapefrom the building without injury.

5 For a hospital or firestation, the performance target will be to remain fullyoperational under the same level of shaking, becausethe services provided will be particularly important inthe aftermath of an earthquake. For a nuclear powerplant or radioactive waste repository, the perform-ance objective will be to maintain structural integrityeven under extreme levels of shaking that may beexpected to occur very planners and developers have taken decisionsregarding the location of civil engineering projects,or once people have begun to settle in an area, thelevel of exposure is determined. In general, seismichazard cannot be altered, hence the key to mitigatingseismic risk levels lies in the reduction of vulnerabi-lity or, stated another way, depends on the provisionof earthquake resistance. In order to provide effectiveearthquake protection, the civil engineer requiresquantitative information on the nature and likeli-hood of the expected earthquake hazards.

6 As alreadyindicated, this may mean defining the hazard, notin terms of the effects that a single earthquake eventmay produce at the site, but as a synthesis of thepotential effects of many possible earthquake scen-arios and quantitative definitions of the particulareffects that may be expected to occur with differentspecified frequencies. This is the essence of engineer-ing Seismology : to provide quantitative assessmentsof earthquake generation creates a number of effectsthat are potentially threatening to the built envir-onment (Figure 1). These effects are earthquakehazards, and engineering Seismology , in the broadestsense, is concerned with assessing the likelihood andcharacteristics of each of these hazards and their pos-sible impact in a given region or at a given site ofinterest. Earthquakes are caused by sudden ruptureon geological faults, the slip on the fault ruptureranging from a few to tens of centimetres for moder-ate earthquakes (magnitudes from 5 to about ) tomany metres for large events (seeTectonics:Earth-quakes).

7 In those cases where the fault extends to theground surface (which is often not the case), therelative displacement of the two sides of the faultpresents an obvious hazard to any structure crossingthe fault trace. In the 17 August 1999 Kocaeli earth-quake in Turkey, several hundred houses and at leastone industrial facility were severely damaged by thesurface deformations associated with the slip on theNorth Anatolian fault. However, buildings must besituated within a few metres of the fault trace forsurface rupture to be a hazard, whereas groundshaking effects can present a serious hazard even attens of kilometres from the fault trace; hence, even ifthe fault rupture is tens or even hundreds of kilo-metres in length, the relative importance of surfacerupture hazard is low. An exception to this is wherelifelines, particularly pipelines, cross fault traces, al-though identification and quantification of the hazardallows the design to take into account the expectedslip in the event of an earthquake: in the vicinity of theDenali Fault, the trans-Alaskan oil pipeline is mountedon sleepers that allow lateral movement, whichpermitted the pipeline to remain functional despitemore than 4 m of lateral displacement on the fault inthe magnitude earthquake of 3 November fault ruptures in the ocean floor cangive rise to tsunamis, seismic sea waves that aregenerated by the sudden displacement of the surfaceof the sea and travel with very high speeds.

8 As thewaves approach the shore and the water depthdecreases, the amplitude of the waves increases tomaintain the momentum, reaching heights of up to30 m. When the waves impact on low-lying coastalareas, the destruction can be almost of the other hazards generated by earthquakesare directly related to the shaking of the ground caus-ed by the passage of seismic waves. The rapid move-ment of building foundations during an earthquakeFigure 1 Potentially destructive effects of earthquakes, show-ing the elements o fthe earthquake generation process and thenatural environment (ovals) and the resulting seismic hazards(rectangles).500 ENGINEERING GEOLOGY/Seismologygenerates inertial loads that can lead to damage andcollapse, which is the cause of the vast majority offatalities due to earthquakes. For this reason, themain focus of engineering Seismology , and also ofthis article, is the assessment of the hazard of groundshaking.

9 Earthquake ground motion can be amplifiedby features of the natural environment, increasingthe hazard to the built environment. Topographicfeatures such as ridges can cause amplification ofthe shaking, and soft soil deposits also tend to in-crease the amplitude of the shaking with respect torock sites. At the same time, the shaking can inducesecondary geotechnical hazards by causing failure ofthe ground. In mountainous or hilly areas, earth-quakes frequently trigger landslides, which cansignificantly compound the losses: the 6 March1987 earthquake in Ecuador triggered landslidesthat interrupted a 40-km segment of the pipelinecarrying oil from the production fields in the Amazonbasin to the coast, thereby cutting one of the majorexports of the country; the earthquake that struck ElSalvador on 13 January 2001 killed about 850people, and nearly all of them were buried bylandslides.

10 In areas where saturated sandy soils areencountered, the ground shaking can induce lique-faction (seeEngineering Geology:Liquefaction)through the generation of high pore-water pressures,leading to reduced effective stress and a significantloss of shear strength, which in turns leads to thesinking of buildings into the ground and lateralspreading on river banks and along coasts. Extensivedamage in the 17 January 1994 Kobe earthquake wascaused by liquefaction of reclaimed land, leavingJapan s second port out of operation for 3 assessment of landslide and liquefactionhazard involves evaluating the susceptibility of slopesand soil deposits, and determining the expected levelof earthquake ground motion. The basis for earth-quake-resistant design of buildings and bridges alsorequires quantitative assessment of the groundmotion that may be expected at the location of theproject during its design life.