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Activity4:LocatinganEarthquakeEpicenter
ExaminingSeismograms
ThethreebasictypesofseismicwavegeneratedbyanearthquakeatitsfocusareP‐waves, S‐waves, and Surface waves. P and S‐waves are body waves and travelthroughtheinterioroftheearth.P‐waveshavethegreatestvelocityandreachtheseismic station first. S‐waves arrive at the seismic station after the P‐waves. Theamount of time that passes between the P‐wave arrival and the S‐wave arrival isimportantinhelpingseismologistsdeterminetheepicenteroftheearthquake.
Todaywewillbeusingatravel‐timegraph,wheretheverticalseparationbetweenthePandScurvesisequaltothedifferenceinthearrivaltimesbetweentheP‐waveand S‐wave. To accurately locate an earthquake epicenter, records from threeseismogramsarenecessary.
Objectives:Ifyoucompletealltheproblems,youshouldbeableto: IdentifyP,S,andsurfacewavesonasimpleseismogram.
Locatetheepicenterofanearthquakeusingseismogramsandtravel‐timescurves.
Completeanearthquakeintensitymaybydrawingisoseismalsfromintensitydata.
Describehowthegeologyofaregioncanaffecttheintensityofanearthquake.
Figure1.Atypicalseismogram.
Usingatraveltimegraph
1. Use Figure 2 to determine the difference in arrival times (inminutes) between the first P‐wave and first S‐wave for stationsthatarethefollowingdistancesfromanepicenter.
700miles:__________________minutesdifference
450miles:__________________minutesdifference
2500miles:__________________minutesdifference
2. FromtheseismograminFigure1,thedifferencesinarrivaltimesbetween the first P‐wave and the first S‐waves equal (3, 7, 10)minutes.Circleyouranswer:
3. Refer to the travel‐time graph. What is the distance from theepicentertothestationthatrecordedtheearthquakeinFigure1?
_____________miles
TravelTimeGraph(Figure2)
Directions:
Figure3illustratesseismogramsfromthesameearthquakerecordedatLosAngeles,CA,St.Louis,MOandHouston,TX.Usethisinformationtoanswerthefollowingquestions.
1. Usethetravel‐timegraph,Figure2,todeterminethedistancethateachstationinFigure3isfromtheepicenter.Writeyouranswersintheepicenterdatatable,Table1.
2. Afteryouhavedeterminedthedistancethateachstationisfromtheepicenter,usethedrawingcompassprovidedtodrawacirclearoundeachofthethreestationswitharadius,inmiles,equaltoitsdistancefromtheepicenter.UsethemapoftheUnitedStates.
(Note:Usethedistancescaleprovidedonthemaptosetthedistanceonthedrawingcompassforeachstation.)
Figure3.
DetermininganEarthquakeEpicenter
1. Didallthreeofthecirclesdrawnwiththecompassoverlapinonespot?
2. Thisearthquakeoccurredclosesttowhichcity,inwhichstate?
3. Thisearthquakeoccurred inawell‐knownseismiczone.What isthenameofthisseismiczone?
4. To thebestyouofyourability,useapencil,orcoloredpencil todraw the approximate location of the seismic zone named inquestion3.Besuretolabeltheseismiczoneonthemap.
Brainstorming:
Assume that the sameearthquakeoccurred, but the seismic station inLos Angeles malfunctioned, and did not record the earthquake. Withonly the information from seismic stations in St. Louis and Houston,brainstorm how a geologist might determine the location of theearthquakeepicenter?