The electrical conductivity structure between the transitional (near SAFOD) and locked (SE of Cholame) segments of the San Andreas Fault, including the source region of non-volcanic tremors

摘要

The San Andreas Fault (SAF) Zone changes its mechanical behavior near Parkfield, CA, from being locked (to the SE) to creeping (to the NE). Within the northeastern branch of the transitional zone (i.e. near the creeping segment), our MT data suggest the existence of a fluid channel that allows fluid migration from the upper mantle/lower crust into the upper crustal, brittle deformation zone of the SAF. Towards the SW (i.e. near the locked segment), ~ 30-50 km deep non-volcanic tremors (NVT) have been observed. Previous MT work suggests an impermeable mid- to lower crustal barrier near the source-region of NVTs and hence a completely different situation than further to the NE.Over the past 3 years, we have collected MT data at more than 250 sites to image the entire transitional segment of the San Andreas Fault (SAF) near Parkfield. The MT sites cover an area of 130 km x 60 km and are arranged on seven 130 km long profiles perpendicular to the SAF,extending from the Pacific coast into the Central Valley and crossing the San Andreas Fault Observatory (SAFOD) near Parkfield and the source region of NVT deep beneath the SAF near Cholame. With this huge array of MT data, we want to study the crustal and upper mantle fluid distribution near the SAF and changes in the deep hydraulic system to develop constraints for understanding the locking/creeping phenomena and the localized occurrence of NVT.