Chapter 24 - Active Seismic Monitoring of the San Andreas Fault at Parkfield

Abstract

Abstract A unique data set of seismograms for 720 source-receiver paths has been collected as part of a controlled source Vibroseis experiment at the San Andreas Fault (SAF) in Parkfield, California. In the experiment, seismic waves repeatedly illuminated the epicentral region of the expected M6 event at Parkfield, from June 1987 until November 1996. For this effort, a large shear-wave vibrator was interfaced with a three-component (3-C) borehole High-Resolution Seismic Network (HRSN), providing precisely timed collection of data for detailed studies of changes in wave propagation associated with stress and strain accumulation in the fault zone (FZ). Data collected by the borehole network were examined for evidence of changes associated with the nucleation process of the anticipated M6 earthquake at Parkfield. These investigations reported significant travel-time changes in the S coda for paths crossing the fault zone southeast of the epicenter and above the rupture zone of the 1966 M6 earthquake. Analysis and modeling of these data—and comparison with observed changes in creep, water level, microseismicity, slip-at-depth and propagation from characteristic repeating microearthquakes—showed temporal variations in a variety of wave propagation attributes, variations that were synchronous with changes in deformation and local seismicity patterns. The main lesson learned from the Vibroseis experiment is that changes were clearly observable in the locked part of the SAF, which has relatively little natural seismicity and could hardly be used for monitoring of travel-time and attenuation changes. The heavily instrumented, creeping part of the SAF northwest of Parkfield is not expected to accumulate stress. Monitoring of this region revealed no significant changes in seismic signatures. Remarkably, in 2004, the expected M6 earthquake at Parkfield occurred and nucleated well into the locked SAF section, well to the southeast of the Vibroseis/HRSN monitoring experiment primarily centered on Middle Mountain. This result suggests that active seismic monitoring can be a useful tool for detecting stress changes associated with the nucleation of larger earthquakes, even when event observations are made over nucleation zones with low natural seismicity. Numerical modeling studies and a growing number of observations have argued for the propagation of fault-zone guided waves (FZGW) within the SAF zone that is 100 to 200 m wide at seismogenic depths and with 20% to 40% lower shear-wave velocity than the adjacent unfaulted rock. FZGW are also capable of assessing the degree of fault continuity and other complex FZ geometries, such as fault jogs. The SAF in the Cholame Valley, where a 2004 M6 earthquake nucleated, is characterized by such complexity. Because FZGW also primarily propagate within the core of fault zones, active continuous seismic monitoring, using guided waves, is our proposed solution for earthquake studies in the Parkfield area.