Time-dependent seismology

Abstract

The time variation of crustal velocities in tectonic regions is most reasonably attributed to stressinduced variations in crack porosity. The decrease in V•,/Vs before earthquakes is due primarily to a large decrease in Vp. This supports the Nur dilatancy hypothesis but not the effective stress hypothesis. New data from the San Fernando region verify the Vp drop, show that this drop cannot be entirely due to source depth effects, and give strong support to the explanation of material property, or path effect, rather than source effect variations. Calculations show that the crack-widening model works even for midcrustal depths in saturated rock. Narrow cracks of low aspect ratio are required to satisfy the velocity and uplift constraints. The recovery of velocity prior to fracture can be due to fluid flow or crack closure. The t • L •' relation does not require diffusion. Diffusion of groundwater or crack closure leads to increased pore pressure and rock weakening. Observations of gravity, conductivity, and crustal distortions along with velocities should narrow the choice of models. The crust in regions of thrust tectonics is probably always dilatant to some degree. The aftershock region is smaller than tl•e anomalous velocity region, which in turn must be smaller than the dilatant region. A simple relationship is derived for the relative sizes of the anomalous and aftershock regions.

A new interpretation of magnetic field variation associated with the Matsushiro earthquakes.

“…Three theoretical curves for spherical diffusion are also shown. dimension of earthquakes obtained by SCHOLZ et al (1973) and ANDERSON and WHITCOMB (1975). From the proportional constant, a, we estimate the bulk permeability, k, of this region to be k=107 nano-darcy, which agrees well with that estimated from the time-delay of the seismic activity accompanied with the water injection into a deep well drilled in the Matsushiro area (OHTAKE, 1974).…”

Depth Constraints on Dilatancy Induced Velocity Anomalies

“…However, this additional porosity at depth will significantly increase the dilatant porosity increase required for undersaturation. Therefore, while the affects of pore porosity are somewhat variable with depth, it appears that the net effect will reinforce the results arrived at above, namely, that the potential for velocity anomalies decreases with depth.While our assumption of a hydrostatic pore pressure gradient seems reasonable, it conflicts somewhat with the conclusions of HANKS (1974, BOOKER (1975) and ANDERSON and WHITCOMB (1975), who suggest that substantially lower pore pressures may exist at depth. If this is so, then the pore fluid at depth will be closer to the liquid-vapor transition pressure than we have assumed, and undersaturation will be achieved with smaller porosity increases than we have calculated.…”

“…While our assumption of a hydrostatic pore pressure gradient seems reasonable, it conflicts somewhat with the conclusions of HANKS (1974, BOOKER (1975) and ANDERSON and WHITCOMB (1975), who suggest that substantially lower pore pressures may exist at depth. If this is so, then the pore fluid at depth will be closer to the liquid-vapor transition pressure than we have assumed, and undersaturation will be achieved with smaller porosity increases than we have calculated.…”

Geomagnetic induction study of the seismically active fault along the southwestern coast of the Sea of Japan.

“…3). It was pointed out, recently, that the groundwater plays an important role in the occurrence of shallow earthquakes (ANDERSON and WHITCOMB, 1975). Meanwhile, the water content of the rocks is recognized to have a remarkable effect to their electrical conductivity (PARKHOMENKO, 1967).…”