A Chicken-Or-Egg Riddle: Why Do Many Large Basin and Range Ruptures Terminate in Seismically Active Geothermal Zones?
Abstract
The 26 March 1872 M 7.6 Owens Valley, CA, rupture extended between two seismically active geothermal areas: Ridgecrest-Coso to the south and Mammoth-Long Valley to the north. Is this a coincidence, or is the rupture extent related to the geothermal and seismic activity? The 1872 rupture might have been confined between these two shattered, fluid-saturated zones if, because of very low friction, they do not accumulate stress. Alternatively, the 1872 earthquake could have activated these zones, as they are located where the rupture is calculated to have increased the Coulomb stress by 1-2 bars. Cause or effect, this phenomenon may be common in trans-tensional regimes: There are geothermal areas near or just beyond the ends of the 1954 M=7.1 Fairview Peak and M=6.8 Dixie Valley, NV, ruptures, with some seismicity clusters beyond the rupture tips. The 2008 Mw=6.0 South Iceland earthquake, another site of trans-tensional faulting and geothermal activity, shows intense aftershock clusters off the ends of the main rupture, resembling the 1872 earthquake. This chicken-or-egg riddle would be easy to solve if it were known whether the earthquake rate in the geothermal areas increased as a result of the 1872 shock. There is only one recorded quake before the mainshock, a July 1871 M 5.5 event at the southern tip of the 1872 rupture; this suggests the clusters might have been active before the mainshock. On the other hand, one of the two largest aftershocks of the 1872 event, a M 6.8 near Bishop two weeks after the mainshock, stuck in the center of the northern cluster, which supports the clusters were activated by the mainshock. We walk away with a tie. If the 1872 rupture did trigger the activity in the clusters, it begs a harder question: Could aftershocks or triggered seismicity have continued almost 150 years after the mainshock? The Owens Valley Fault slips 2-3 mm/yr, about a tenth of the San Andreas Fault rate. Rate/state theory and observations from the central US and Japan indicate that an Owens Valley aftershock sequence should last about ten times longer than one on the San Andreas. The Owens Valley Fault appears to produce M 7.5 shocks roughly every 3,500 yr, and so 150 yr is 4% of the interevent time, equivalent to about 10 yr on the San Andreas. Therefore, one cannot exclude the possibility that these earthquakes are aftershocks.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFM.S21D..03S
- Keywords:
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- 7209 Earthquake dynamics;
- SEISMOLOGYDE: 7230 Seismicity and tectonics;
- SEISMOLOGYDE: 8168 Stresses: general;
- TECTONOPHYSICSDE: 8488 Volcanic hazards and risks;
- VOLCANOLOGY