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 coin...how 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 hernlund@............ __________________________________________________________ Public Seismic Network Mailing List (PSN-L)
Larry Cochrane <cochrane@..............>