Interpreting the Pakistan earthquake seismogram

I'll try to work through this one step at a time, in order to allow our many readers to catch up.

Let's consider when waves from the recent Pakistan earthquake should have arrived at the Lamont-Doherty seismograph, and compare that to when they actually did.

The NEIC information says that this quake (the second of two similarly sized events) occurred at Oct. 29 at 11:32:43 UTC (coordinated universal time).

Let's solve this today by using the theoretical P-wave arrival times tabulated and graphed by the NEIC. According to the table, the P (diffracted wave) should arrive in New York at 11:46:30, 14 minutes after the earthquake occurred.

Look at the seismogram below. There seems to be a small blip of energy at about 11:51, 18minutes and 20 seconds after the earthquake occurred.

Is this the wave arrival referred to above? Why is this blip so small compared to what follows? Can we get a more detailed look at this seismogram? How is this approach related to the use of travel-time curves? And what is Pdiffracted anyway?

You'll have to come back for that.

NEIC - Your best source of earthquake information

If you hear about an earthquake on the news, where can you go for more information? I assume people would search their favorite journalistic source - Google, Yahoo, New York Times, BBC, Reuters, NPR - for the newsy stuff. But what about the basic science of the event?

I'd suggest for large teleseismic (see NEIC glossary) quakes that you visit the U.S. Geological Survey Earthquake Hazards Program/National Earthquake Information Center, at http://earthquake.usgs.gov/ .

Each significant event has its own listing, including the Pakistan quake.

For the geographically faint-of-heart, you can get a general global location:

You can look at historic earthquake patterns in the area (seismicity), and the relationships to plate boundaries:

You can read about the geology and tectonic environment that are related to this event:

Earthquakes and active faults in western and northern Pakistan and adjacent parts of Afghanistan are the result of the India plate moving northward at a rate of about 40 mm/yr (1.6 inches/yr) and colliding with the Eurasia plate. Along the northern edge of the Indian subcontinent, the India plate is subducting beneath the Eurasia plate, causing uplift that produces the highest mountain peaks in the world, including the Himalayan, the Karakoram, the Pamir and the Hindu Kush ranges. West and south of the Himalayan front, the relative motion between the two plates is oblique, which results in strike-slip, reverse-slip, and oblique-slip earthquakes. The pattern of elastic waves that were radiated by the October 28 and 29, 2008, earthquakes implies that each earthquake was the result of predominantly strike-slip faulting. Seismographically recorded waveforms imply that the shocks were caused by either left-lateral slip on a northeast-striking fault or right-lateral slip on a northwest-striking fault.

For this major event, you can even download a poster suitable for display.

And, in what will become useful for a subsequent lesson on interpreting seismograms, you can find out how long it should have taken the P-waves from this earthquake to reach any point on Earth:

(click above for more detail)

You can be sure that we'll return to the NEIC web site in the future.

Pakistan earthquake - a teachable moment

(click above for more detail)

For better of for worse, major earthquakes make us wonder about this geophysical phenomenon. Educators can use these events in the news to help their students think about science. So let's try a few entries focused on the Pakistani earthquake.

First, a seismogram (the record) from the Lamont-Doherty seismograph (the instrument) in Palisades, NY.

Where can you get this seismogram? What can you do with it? What can you learn from it? Stay tuned for subsequent posts.