-----Original Message-----
From: Larson, Timothy
Sent: Thursday, January 13, 2005 10:59 PM
Subject: recording of Sumatra earthquake
Here, by popular demand, is the ISGS's record of the great Sumatra
earthquake of December 26, 2004.
Some comments about the record:
The record spans nearly three hours, so it is very compressed. Yet the
surface waves are still very large. This earthquake occurred nearly on
the opposite side of the globe (136.64 degrees to be exact) so we don't
have typical distinct P S and surface wave arrivals. To compensate, I
added an inset showing the first few minutes in detail so you can see
the first arrivals. The very first arrival, about 17 minutes after the
origin is the p-wave that diffracted at the mantle-core boundary. It is
very weak. A few minutes later is a strong arrival of the p-wave that
traveled through the outer core, then through the mantle again (it is
called pkp). The next strong arrival is the p-wave that bounced off the
underside of the earth's surface and then continued to us (it is called
pp). The last easily recognized arrival is the start of the s-waves (SKP
and PKS). These can be seen in the lower trace which shows horizontal
movement.
Some other comments about the earthquake:
Final calculations are not yet available, but initial calculations
suggest:
Rupture length: 1200 km (estimated)
Maximum offset: 20 m (this is at a depth of several kilometers within
the crust, offset at seafloor was probably less than this)
Seismic moment: 3 x 10E21 Newton-meters (USGS "Fast" solution)
converting this to energy is difficult, but it is approximately 10E19
joules
For perspective, the total annual US energy consumption is about 10E20
joules.
This is a type of earthquake called a mega-thrust, which are known to
occur in subduction zones. This particular earthquake occurred as a
result of the subduction of the Indo-australian plate beneath the Burma
microplate. The Burma microplate includes the northern tip of Sumatra,
the Nicobar Island group and the Andaman Island group. It is about
1200-1500 km long. The rupture and the aftershock sequence involved
nearly the entire microplate.
A very similar tectonic geometry exists off the coast of the Pacific
northwest where the Juan de Fuca micro-plate is being subducted beneath
North America. The Juan de Fuca plate is nearly the same size as the
Burma micro-plate. A similar rupture on the Juan de Fuca Plate is
theoretically possible. The comparable dimensions would be a rupture
from offshore Cape Mendocino (northern California) all the way to the
north end of Vancouver Island.
I have been gathering other facts and images as I am sure that many of
you are doing as well.
Is there any interest in pooling resources for a collaborative
PowerPoint (or similar) document that could be used for future
educational purposes?
Regards,
Tim Larson
Timothy H. Larson
Applied Geophysics Section
Illinois State Geological Survey
615 E. Peabody Drive
Champaign, IL 61820
217-244-2775 (voice)
217-244-2785 (fax)
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