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?