Locating the Epicenter The method of triangulation

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The method of triangulation by knowing the distance to the epicenter of at least three seismic stations we can determine, or triangulate, its location. 3 seismic stations St. Louis Ellensburg San Diego The relative distance of the quake from each station The intersection of the three circles gives the location of the epicenter

How do we go from this…. …to that? St. Louis Ellensburg San Diego

We will exploit the fact that S-Waves travel a little slower than P-Waves P S

between the P and S waves We will exploit the fact that S-Waves travel a little slower than P-Waves With time the distance between the P and S waves increases! Start Time A Time B The greater the distance between the P and S waves the farther we are from the epicenter! P S P S P S S

The time between the arrival of the P-Wave and the S-Wave The Distance between the P and S Wave The time between the arrival of the P-Wave and the S-Wave

Three seismograms of the same quake Station 1 P-Wave S-Wave Time in seconds 9.2 sec Station 2 Step 1: Identify P and S-Wave Step 2: Measure the time between their arrivals Step 3: Do this for all three station seismograms Station 3

Three seismograms of the same quake Station 1 P-Wave S-Wave Time in minutes 9.2 seconds Station 2 P-Wave S-Wave 16.5 seconds Station 3 P-Wave S-Wave 21.2 seconds

The Travel Time Graph Helps us convert the time interval between arrival times of P and S waves into distances from the epicenter for each station S Wave P Wave 2,000 4,000 6,000 8,000 10,000 Distance between epicenter and seismograph (km) 10 20 30 40 50 Time (seconds) Station #1 9.2 seconds 3,000 km 6,500 km 8,750 km Station #2 16.5 seconds Station #3 21.2 seconds _,___ km _,___ km _,___ km

Finding the epicenter using triangulation This circle was drawn using a compass set at a radius scaled to 3000km using the map scale. Station #1: (Seattle) 9.2 seconds 3,000 km The earthquake occurred along the Alaskan Aleutian Island Chain – an active subduction zone! Station #2 (Sudbury) 16.5 seconds 6,500 km Notice the scale bar! Station #3 (St. John’s) 21.2 seconds 8,750 km 1,000 km

Determining the magnitude of the quake We need to have two pieces of data: 1) The distance to the earthquake 2) The amplitude of the biggest recorded wave 20 mm Station 1: Seattle P-Wave S-Wave Highest Amplitude R & L-Waves

The Distance-Amplitude Graph Helps us determine the magnitude of an earthquake based on the distance to the epicenter and the amplitude of the highest wave Distance (km) Magnitude (M) Amplitude (mm) 50 200 400 600 1000 2000 3000 4000 5000 1 2 3 4 5 6 0.1 0.2 0.5 10 20 100 Station #1: (Seattle) 9 min, 20 sec 3,000 km Magnitude 5.2 20 mm Here we used only one seismogram to find magnitude – you must plot all three in your assignment!