PSN-L Email List Message

Subject: Re: Ground water anomalies
From: John Hernlund hernlund@............
Date: Fri, 09 Feb 2001 12:31:22 -0800

In several cases water table levels were observed to change prior to
earthquakes, but I don't know that there is a high correlation in this.
The water table always wants to achieve an equipotential surface, as
well as fill in any pore space it can with the volume of water in the
ground.  Perhaps prior to earthquakes the pores change their shape due
to strain, and this will cause a change in the water table level.  If
the strain is extensional, pore space will increase and the water table
will drop, if it is contractional then pore space will decrease and the
water table will rise.  However, there is a flip side to the
changes in water table levels might affect the frictional behavior and
state of stress on a fault.  So there is give and take here...pretty
complicated like everything else having to do with quakes.

The presence of even a small amount of water is known to drastically
alter the behavior of faults and of materials as a whole.  The tendency
is towards lower friction and softer rheology.  Some people (like
myself) think that the abundance of water is the only reason the Earth
has plate tectonics in the first place...without it the crust should
freeze up and have a rigid lid similar to Mars.  But water reduces the
viscosity and strength in a focused manner, which creates weak zones
(plate boundaries) along which plates can move relative to one another.
In mantle convection, the idea is that the top is cooled by being near
the surface, which makes it more dense and it will have a tendency to
sink if given the opportunity...and water provides this opportunity by
weakening certain zones (subduction zones) where crust is returned to
the mantle.

At any rate, I hope people continue researching water tables and
earthquakes as well as any other data that can be collected.  The more
data we have, the better the picture gets.

John Hernlund
Department of Earth and Space Sciences
University of California, Los Angeles


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Larry Cochrane <cochrane@..............>