A First Observation of Fault Guided PSV-Waves at SAFOD and its Implications for Fault Characteristics
Abstract
A new type of seismic wave has been observed on a 3-component downhole seismograph installed in the main borehole of San Andreas Fault Observatory at Depth (SAFOD MH). This phase arrives between the P- and S- waves, is normally dispersed and has a clear Airy phase. It was recorded by 15 Hz seismometers located 2650 m below ground, 3270 m along the inclined borehole, and approximately 40 m from the center of a major fault crossed by it. This fault has caused deformation of the borehole casing, indicating that it is one of the active stands of the San Andreas system. Sonic log velocities in the core of the fault are very low, with Vp and Vs averaging as little as 3.2 km/s and 1.8 km/s within a ~20-to-30 m-wide zone. This 20%-25% reduction from surrounding velocities forms a channel that is embedded in a broader ~200-m-wide low velocity zone, albeit with smaller velocity reductions from its surrounding rocks. The velocity reductions of both zones are large enough that different types of fault zone guided waves should propagate along them. In fact, the seismograms with the new wave type also contain the well-known, SH-type, "FL" fault guided waves. Further, not all nearby earthquakes generated either the new or SH-type signals. Given its arrival time and other characteristics, we suggest that the new phase is a "leaky mode" PSV, "Fφ" fault guided wave. The existence of Fφ depends on unique relationships between the P and S velocities of the fault and surrounding rocks, as well as their thicknesses. The presence of Fφ on a seismogram and its propagation speed, low frequency cut-off, and Airy phase frequency thus give special insights into the structure and multi-stranding of the San Andreas Fault zone. Analytical and numerical acoustical models of the observed fault core suggest that its velocity reductions and thickness must extend along the entire 2-to-6 km source-receiver paths of the Fφ waves. If born out by more complete modeling, our observations provide strong support for the hypothesis that multiple, narrow, low velocity fault zones can extend deep into the seismogenic crust.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.T23E..02E
- Keywords:
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- 1219 Gravity anomalies and Earth structure (0920;
- 7205;
- 7240);
- 5144 Wave attenuation;
- 7215 Earthquake source observations (1240);
- 8010 Fractures and faults;
- 8163 Rheology and friction of fault zones (8034)