Seismogenic, Electrically Conductive, and Fluid Zones at Plate Boundaries in New Zealand, Himalaya, and California-USA Anza, California, ML 5.1 Mainshock and the First Aftershocks Recorded by the Anza Seismic Networ

Abstract

We explore the idea that fluid occurrence below the seismogenic zone plays an active role in the rupture process by examining how fluids spatially relate to seismicity at three continental plate boundaries: South Island of New Zealand, the Himalaya, and San Andreas fault, USA. With this objective, we project earthquake hypocenters onto magnetotelluric (MT) electrical resistivity cross-sections. MT detection of conductive zones in the crust containing low fractions of fluids (<1%) requires an interconnected network of fluid-filled porosity facilitated by shearing, fracturing, and/or grain-edge wetting. Mechanisms promoting fluid reservoirs in the ductile crust include: 1) stalling of upward propagating porosity waves, 2) tectonically induced neutral buoyancy, and 3) development of ductile shear zones. Distinct conductive horizons are detected at depth in the ductile crust in New Zealand and the Himalaya where the tectonic convergence is high. In the Parkfield segment of the San Andreas fault, where convergence is low, there is high conductivity in the ductile crust but it forms a sub-vertical corridor to the surface with no distinct top. The tops of sub-horizontal conductive zones are ~20 km depth in New Zealand and ~25-40 km in the Himalaya where the seismogenic crust extends only to 12 and 25 km depth, respectively. The deep conductive layer in New Zealand may have originated as a "water sill" facilitating water-weakening, localized deformation, and eventually becoming a water-rich, anisotropic, mylonized, ductile shear zone. Fluid exchange through the active Alpine fault may initiate or be initiated by fault rupture. Localized, unstable flow in deep fluidized zones detected by MT may trigger earthquakes above.

Electronic Supplement to

Seismogenic, Electrically Conductive, and Fluid Zones at Continental Plate Boundaries in New Zealand, Himalaya, and California-USA

George R. Jiracek, Victor M. Gonzalez, T. Grant Caldwell, Philip E. Wannamaker & Debi Kilb

Abstract

Interactive Visualization

3-D interactive Visualization (Requires iView3D freeware, see below)

• Scene file [Download the scene file Jiracek_etal_2006.v1.scene (~163 MB)]

• iView3D freeware [iView3D; viewable on any platform -- Mac OSX, Windows, Linux, Sun, SGI ]

QuickTime Movies: A 3D flight through the data