Fault Characteristics, Energy Estimates, and Earthquake Recurrence: What One Seismologist Wants from Fault Drilling

Abstract

There are some significant differences between the way seismologists and geologists typically look at faulting from large earthquakes. Seismologists tend to analyze waveform data which can usually resolve structures on the order of tens of kilometers; possibly down to one kilometers there is very good near-field data. Also, the derived physically properties are averaged over this scale length. Although it is difficult to resolve small scale features, seismologic studies have the advantage of seeing all depths of the fault. In contrast fault zone geologists can make direct observations of the fault zone structures and properties, which are quite different from the remote sensing techniques of seismologists. Geologists tend to look at structures on the scale of microns to meters when examining the cores obtained from drilling into faults. These observations are spot measurements and it is often difficult to assess how representative they are of the entire fault. Also, fault zone sample studies are usually limited to a few kilometers depth, which may barely be in the range of the seismogenic zone that produces the seismic waves. These differences mean that seismologists and geologists are often looking at quite different aspects of the earthquake process. The approaches are different, but can also be complementary. Drilling DepthThe depth of drilling is an important point for fault zone investigations. To compare physical properties obtained in cores with seismological results obtained from waveform analyses, it is necessary to reach depths of the seismogenic zone, that is the regions of fault slip that have significant stress change to produce seismic waves. The shallow regions of faults, where materials have low rigidity, likely slip sympathetically with the seismogenic zone but do not produce seismic waves. The depth of the beginning seismogenic zone depends on local geology and probably is a transition zone. Waveform inversion studies suggest that the seismogenic zone may be as shallow as 1 km for the Chi-Chi, Taiwan earthquakes (Ji et al., 2001). It would be interesting if geologic studies could distinguish between seismic and nonseismic slip of the fault. Temperature MeasurementsOne of the fundamental issues in understanding the physical mechanisms of earthquakes is clarifying the level of friction on the fault. One way to estimate the frictional levels during the faulting is to measure the heat produced. Measurements of the heat flow associated with the San Andreas fault have long been discussed over the past decades (e.g., summarized in Scholz, 2002, section 3.4.4). A more direct estimate would be to measure the fault zone temperatures immediately after a large earthquake. There were informal discussions to measure fault-zone temperatures following the large 1992 Landers and 2001 Denali earthquakes, although measurements were not done. Currently, the only available data of fault-zone temperatures following an earthquake are for the 1999 Chi-Chi, Taiwan earthquake (Kano et al., 2006). This result infers a ve...