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Intellectual Merit
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We had a number of illuminating presentations and discussions on the day of the workshop, as well as a follow-up discussion on Tuesday evening of the regular SCEC meeting. Our discussions revolved around the following four questions:
1. What is the probability of a through-going San Andreas rupture?
A definitive answer to this question has fundamental implications for forecasting the earthquake hazard in southern California. To answer this question, we require answers to the two following questions.
2. What is the subsurface geometry of active faulting through the San Gorgonio Pass?
To best predict potential rupture through the San Gorgonio “knot” and ground motions related to such an event, additional information is needed about the active fault configuration to 25 km depth. A data gap between 0-7 km depths hinders linking the very complex surface mapped fault traces with the deeper, potentially through-going fault structures defined by seismicity in the region.
• A number of alternative active fault models link the surface and subsurface; some incorporate structures that steepen with depth and others use structures that become less steep with depth. Many possibilities and alternative 3D fault representations are parameterized in the SCEC CFM 5.0.
• The seismicity below 7 km near the possible location of a through-going fault tends to have a very high stress drop. Does this seismicity reflect volumetric strain on a family of small faults, or slip on a single master structure?
• Some earthquakes in this volumetric strain zone appear to define discrete faults with multiple orientations. Which of these faults may represent a through-going master fault remains unclear, because some – if not many – of these deep faults may be second-order structures.
• How does the oblique collision between the San Jacinto and San Bernardino mountains relate to the zone of high stress drop and fault complexity near the surface?
• Along with the fault structure, the state of stress along active faults is also not well characterized, but is necessary for understanding the propagation of earthquake rupture and slip in the Pass.
3. What is the earthquake potential in the San Gorgonio Pass region?
If fault geometry, stressing rates and slip rates are known, then we will be able to develop more accurate earthquake models, which will in turn produce better estimates of earthquake size, earthquake interactions, ground motion, and seismic hazard.
• We have a number of new estimations of slip rates on various strands of faults in the SGP.
o Roughly ten different on-going geologic investigations are providing much needed new insight on the slip history of the SGP. These new results support a preliminary model with steep along-strike gradients in slip rate as well as abrupt transfer of slip between overlapping fault strands. There are still some inconsistencies between strike-slip rate estimates, so the slip rate model for active faults in the SGP is still a work in progress.
o Mechanical models testing two often hypothesized active fault configurations show that all of the observed slip rates cannot be matched with any one active fault configuration. The existing distribution of slip rate results could be further tested with measurements at additional sites, comparison within forward models that can test for consistency of self-consistency of the strike-slip rates, and ongoing geodetic inversions for slip rate distribution.
• We also have new information on paleoseismic ruptures in the SGP. The most geometrically complex region appears to have infrequent earthquakes.
o These results support a model with infrequent, very large (M 7.5+) events in the SGP, with more smaller-sized events that tend to terminate on either side of the pass.
o There may exist a >700 year recurrence of events with >4-7 m of slip, which would be almost impossible to be confined only to the pass and suggests large rupture through the SGP.
• We have filled in a number of data gaps in the past few years, but much strategic work still needs to be done. |
