Name: ______Period: ______

Locating an Earthquake Epicenter

Examining Seismograms

The three basic typesofseismicwavegeneratedbyanearthquakeatitsfocusareP‐ waves, S‐waves, andSurfacewaves.PandS‐wavesarebodywavesandtravelthrough the interioroftheearth.P‐waveshavethegreatestvelocityandreachtheseismicstationfirst.S‐wavesarriveattheseismicstationaftertheP‐waves.TheamountoftimethatpassesbetweentheP‐wavearrivalandtheS‐wavearrival is importantinhelping seismologistsdetermine the epicenteroftheearthquake.Todaywewillbeusingatravel‐timegraph,wherethe verticalseparationbetweenthePandScurvesisequaltothedifferenceinthearrivaltimesbetweentheP‐waveandS‐wave.Toaccuratelylocateanearthquakeepicenter, recordsfromthree seismogramsarenecessary.

  1. FromtheseismograminFigure1, does thedifferencesin arrivaltimesbetweenthefirstP‐waveandthe first S‐wavesequal 3, 7, or 10 minutes?

UseFigure 2 (on the last page) todetermine the difference in arrival times (in minutes) between the first P‐wave and first S‐wave for stations thatarethefollowingdistancesfromanepicenter.

  1. 700 miles: ______minutesdifference
  1. 450 miles: ______minutes difference
  1. 2500 miles: ______minutes difference
  1. 1350 miles: ______minutes difference
  1. Refer to the travel‐time graph. What is the distance fromthe epicentertothestationthatrecordedtheearthquakeinFigure1? ______miles.

Directions:

Figure3illustratesseismogramsfromthesameearthquakerecordedat Los Angeles, CA, St.Louis,MOand Houston,TX.Usethisinformationtoanswerthefollowingquestions.


  1. Usethetravel‐timegraph,Figure2,todeterminethedistancethateachstationinFigure3isfromtheepicenter.Writeyouranswersintheepicenterdatatable,Table 1.

  1. Afteryouhavedeterminedthedistancethateachstationisfromtheepicenter,usethedrawingcompassprovidedtodrawacirclearoundeachofthethreestationswitharadius,inmiles,equaltoitsdistancefromtheepicenter.UsethemapoftheUnitedStates.


(Note:Usethedistancescaleprovidedonthemaptosetthedistanceonthedrawingcompassforeachstation.)

Determining an Earthquake Epicenter

  1. Didallthreeofthecirclesdrawnwiththecompassoverlapinonespot?
  1. This earthquake occurred closest to which city,in which state?
  1. Brainstorming: Assume that the same earthquake occurred, but the seismic station in Los Angeles malfunctioned, and did not record the earthquake. With only the information from seismic stations in St. Louis and Houston, brainstorm how a geologist might determine the location of the earthquake epicenter?