Archive for May, 2013

How deep was that earthquake?

May 26, 2013

The seismogram shown here of a magnitude 6.8 aftershock of the magnitude 8.3 Sea of Okhotsk earthquake (see previous post) has two spikes that  tell us we recorded a deep earthquake.

The quake is very deep (>600 km), and when a deep earthquake occurs there are seismic “P waves” that go directly from the quake to a seismograph, and also P waves that go from the quake to the surface, bounce off the surface, and then go to the seismograph (pP). The difference in timing of those two waves (pP-P) gives an estimate of the depth. For this case, based on this one seismogram, the difference between the times of the two spikes is about 2 minutes, which we calculated from this BC-ESP seismogram to correspond to about 620 km. The official USGS depth, based on many seismograms all around the world is 623 km.

So, this one observation, on this one seismogram, gives a pretty good estimate of the depth of the quake.

Okhotsk_052413_Aft_Depth

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Magnitude 8.3 – Sea of Okhotsk – May 24, 2013

May 24, 2013

WO_3quakes_052413(1) Sea of Okhotsk, Magnitude 8.3 (Strong shaking near bottom of seismogram, blue arrow)
(2) Northern California, Magnitude 5.7 (Smaller signal just above Sea of Okhotsk signal, green arrow)
(3) Tonga, Magnitude 7.4 (Smaller signal near top of seismogram, red arrow)

5.7-Magnitude Earthquake Shakes Northern California (USA Today) –
http://www.usatoday.com/story/news/nation/2013/05/24/california-earthquake/2356997/

Magnitude 7.4 – 282km SW of Vaini, Tonga – May 23, 2013

May 24, 2013

IPL_Tonga_052313

USGS Earthquake Summary

Location:  23.025°S 177.109°W depth=171.4km (106.5mi)

Event Time:  2013-05-23 17:19:04 UTC

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