Re: pinhole RE: Epicenter
From: John or Jan Lahr (JohnJan@lahr.org)
Date: Sat Jan 01 2005 - 12:44:13 PST
Message-Id: <6.2.0.14.2.20050101132616.026113e8@mail.comcast.net>
Date: Sat, 01 Jan 2005 13:44:13 -0700
From: John or Jan Lahr <JohnJan@lahr.org>
Subject: Re: pinhole RE: Epicenter
As you surmise, the epicenter is the point on the surface of the earth
above the
place where the rupture starts at depth (the hypocenter). For very
large events,
this is just the beginning of the rupture process and the place from
which the
very first waves emanate. In the case of the Sumatra event, the
rupture expanded
to the north along the plate boundary, eventually involving more than
1,200 km.
A typical rupture velocity is 3 km/s, so it would have taken at least 400
s (more
than 6 1/2 minutes) for the most northern limit to start rupturing.
Seismic waves
are radiated from all of the portions of the fault that are
rupturing.
Picture a piece of paper that is stretched rapidly until it rips in
two. The tear
will not occur everywhere simultaneously, but will usually start at one
edge
of the paper and propagate rapidly across the paper. Once the paper
starts
tearing at one point, the stress becomes very high at the neighboring
un-torn
location, which rips next... and so forth across the paper.
In a similar way, once a fault zone starts rupturing, the adjacent
portion of
the fault zone becomes rapidly stressed, slips, and so forth as the
rupture propagates, usually at about 80% of the S-wave velocity.
As another example, the 1964 Alaska earthquake (M 9.2) started
rupturing
at a point beneath Prince William Sound. The rupture continued to
expand
until it was about 600 km long and 200 km wide. The average slip
was about
20 meters.
Hope this helps.
John
At 12:16 PM 1/1/2005, you wrote:
I always thought of the
epicenter as the place from which the shock waves are radiating, so I
thought that even during large earthquakes the length of a rupture
couldn't be very much. From your comments I guess the epicenter is
the ppoint above where the first rupture happens but the if the plates
were ruptured along a stretch of 1500 km, wouldn't the source of the
shock waves also be 1500 km? Does anyone have information on the
length of ruptures for other quakes?
##################################/
John C. Lahr
#################################/ Emeritus Seismologist
################################/ U.S. Geological Survey
===========================/ Geologic Hazards Team, MS966
##############################/ PO Box 25046
#############################//##############################
############################//###############################
Denver, Colorado 80225-0046
/################################
Phone: (303) 215-9913 /=============================
Fax: (303) 273-8540 /##################################
lahr@usgs.gov /###################################
/####################################
http://jclahr.com/science/
This archive was generated by hypermail 2.1.3
: Mon Aug 01 2005 - 16:06:46 PDT