Investigating the Brittle Ductile Transition Zone Using Southern California Seismicity and the SCEC Community Fault Model
Abstract
The depth to the brittle-ductile transition zone depends on crustal parameters such as rock type, temperature, fluid pressure, and strain rate. We analyze the relocated background seismicity (1981-2005) in southern California to identify features that may be associated with rupture patterns near the brittle-ductile transition zone in past major earthquakes. The bulk of this seismicity is aftershocks that decay with time since the occurrence of the mainshock. We focus our analysis on the seismicity associated with the 1987 Mw6.6 Superstition Hills, 1992 Mw6.1 Joshua Tree, 1992 Mw7.3 Landers, 1994 Mw6.7 Northridge, and 1999 Mw7.1 Hector Mine earthquakes. Both observational and modeling studies suggest that the coseismic stress changes from major earthquakes should cause a sudden increase in the strain rate in the vicinity of the brittle-ductile transition zone at the time of the mainshock. For instance, the average depth of seismicity after the 1992 Mw7.3 Landers earthquake became ~2 km deeper as compared to before the mainshock. Apparently, the stress redistribution due to the mainshock faulting caused a strain-rate dependent downward displacement of the brittle-ductile transition. We have also determined the changes in the mean and maximum depths of seismicity for these southern California mainshocks. At the time of the mainshocks, the mean depth of seismicity exhibits a transient change of about 1 to 2 km by decaying to a new deeper depth, except for the Superstition Hills earthquake, where the seismicity depth remained unchanged. Similarly, the D95% (the depth above which 95% of the earthquakes occur) and D5% (the average depth of the deepest 5% of the earthquakes) behave like the mean depth, with a rapid transient and subsequent returning to the same depth or a slightly shallower depth. Only the thrust-faulting Northridge sequence exhibits a deep cluster that is separate from the main aftershock zone, which is predicted by geodynamic the thrust-fault models. We will also use focal mechanisms to investigate the state of stress during these strain transients. Although immediate aftershocks are often deeper than the pre-mainshock seismicity, the background seismicity is a good predictor of depth of faulting in major earthquakes. Across southern California, the seismogenic thickness is highly variable, ranging from less than 10 km in the Salton Trough and less than 5 km beneath the Coso Range, to greater than 25 km near the southwestern edge of the San Joaquin Valley, with deep seismicity beneath the Ventura basin and Banning Pass, which extends south across the Peninsular Ranges. The seismogenic thickness has an inverse spatial correlation with heat flow. In addition, in areas of high heat flow a denser network of faults appear to accommodate the regional seismicity, while in regions of low heat flow and thick crust the seismicity is confined to a few major faults.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.T51D..03H
- Keywords:
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- 7209 Earthquake dynamics (1242);
- 7215 Earthquake source observations (1240);
- 8118 Dynamics and mechanics of faulting (8004);
- 8123 Dynamics: seismotectonics;
- 8159 Rheology: crust and lithosphere (8031)