SEISMIC WAVE BASICS

Finding an Epicenter

What is an Epicenter? A point on the surface of the Earth directly above the FOCUS of the earthquake. The point within the Earth from which earthquake waves originate.

Use the data from the recording stations: Station A: San Francisco, California P-Wave arrival 3:02:20 S-Wave arrival 3:06:30 What is the time difference between P and S wave arrivals? 4:10

Use the data from the recording stations: Station B: Denver, Colorado P-Wave arrival 3:01:40 S-Wave arrival 3:05:00 What is the time difference between P and S wave arrivals? 3:20

Use the data from the recording stations: Station C: Missoula, Montana P-Wave arrival 3:01:00 S-Wave arrival 3:03:00 What is the time difference between P and S wave arrivals? 2:00

Difference in arrival times: San Francisco: 4:10 Denver, Colorado: 3:20 Missoula, Montana: 2:00

TAKE A PIECE OF PAPER, AND MARK OFF THE DIFFERENCE IN ARRIVAL TIME 4:10 2800Km

MOVE THE PAPER UNTIL THE TWO TICK MARKS LINE UP WITH THE P AND S CURVES WHEN TICK MARKS LINE UP, GO STRAIGHT DOWN AND READ THE EPICENTER DISTANCE EPICENTER DISTANCE OF 2800 KM

EPICENTER DISTANCES San Francisco: 4:10 Denver, Colorado: 3:20 2,800km Denver, Colorado: 3:20 2,000km Missoula, Montana 2:00 1100km

Recording Board Difference in arrival times: San Francisco: 41:0 2,800km Open your compass to the EXACT distance on the scale. 1,000 2,000 3,000 4,000 5,000

. . . Use your carefully set compass to draw a circle around each seismic station. . . . You've found a possible epicenter! Does this seem like a likely location for an earthquake to occur?

Compare your answer to the historical record

Need a closer view to be sure. Go to http://earthquake. usgs Need a closer view to be sure? Go to http://earthquake.usgs.gov/regional/states.php and retrieve the seismicity map (like the one on the left) for the state containing your possible solution. Based on this map, earthquakes have previously occurred in the region where our epicenter fell. Thus our answer seems likely USGS

What if our epicenter had fallen here instead What if our epicenter had fallen here instead? Does this mean our solution is wrong or less likely? What could help us find out? Check our work to see if we made a mistake Additional circles created from other stations e.g. the more stations, the more accurate the location. A longer historical seismicity record. This record is from 1990-2001 Perhaps of map of historic faults in the region.

Credits: This file was originally created by Robert M Credits: This file was originally created by Robert M. DeMarco of Marlboro Central High School Edits have been added by: Chris Visco - Sachem High School Eric Cohen- Westhampton Beach High School Michael Hubenthal - IRIS Consortium