Using Seismograms

Earthquake Terms•Focus: (a.k.a.: hypocenter)–the point at which the first plate movement occurs and the energy is released (deep in the earth).

•Epicenter: –the point on the surface directly above the focus. (This is where we state the earthquake occurred, although it can obviously effect places far away.)

Earthquake Terms•Fault:–the area that serves as the interface between two distinct land masses. The focus of earthquakes occur along faults.

Locating Earthquakes

The time between the arrivals of the p-waves and the s-waves (from a seismogram) is related to the distance of the seismograph from the epicenter.

There is no way to tell which direction it came from.

Locating Earthquakes

Find the p-s lag for the seismogram below.

Use the time-lag/distance chart to find the distance between the station that recorded the data and the epicenter of the earthquake.

35 s

Locating Earthquakes

300 km

Locating Earthquakes

Locating Earthquakes

•Locating the Epicenter–One seismogram from one station will give you a radius from that station where the epicenter might be located.–Two seismograms from two separate stations will give two points (where the circles intersect) as possible epicenters.–Three seismograms from three separate stations are needed to find the epicenter of an earthquake; however, with every additional station that reports a p-s lag, the epicenter is located with increasing accuracy.

Locating Earthquakes

•Each seismograph station gives a different epicenter distance through the p-s lag that it records.•The epicenter is at the point at which all those distances intersect.

Locating Earthquakes

Find the p-s lag for the Las Vegas and Phoenix stations. Use the times to estimate the distances.

39 s

60 s

400 km

600 km

Locating Earthquakes