23. Time-Dependent earthquake Probabilities

The U.S. National Hazards Mapping program produces seismic hazard maps for the country, including the greater Los Angeles metropolitan region. Two known limitations of these maps are that they do include microzonation (i.e., the effect of local geology), and they are currently based on time-independent earthquake probabilities. That is, the forecasted rates of earthquakes on different faults are based only on an assessment of the long-term geological rates of activity. Yet we know that future earthquake probabilities will depend on past earthquakes. Once a large earthquake occurs on a given fault, a repeat of that particular earthquake is thought to be unlikely. On the other hand, a large earthquake can increase the stress on a neighboring fault, increasing the likelihood of a large earthquake. The importance of the latter effect is highlighted by any number of recent large earthquakes, including the 1971 Sylmar and 1994 Northridge earthquakes as well as the 1992 Landers and 1999 Hector Mine earthquakes. Current hazard mapping methodologies do not allow for an incorporation of so-called earthquake interactions. To move forward to a consideration of time-dependent earthquake hazard will require extending existing hazard assessment methodologies to include a physics-based consideration of earthquake interactions. Given the recent development of detailed, three-dimensional models of the complex faults in the greater Los Angeles region, a key step will be to develop a computational framework to assess how a past or future earthquake on any one fault will affect the stress on neighboring faults. This will also allow us to significantly improve real-time forecasting of aftershocks, as well as to assess the probability of subsequent large events after a moderate/large earthquake strikes the region. That is, rather than simply forecasting the expected overall rate of aftershocks, one could identify regions where potentially damaging earthquakes—such as the M6.5 Big Bear aftershock in 1992—are likely to occur. To generate true urban seismic hazard maps, it will further be necessary to consider the effect of local geology on expected shaking during large earthquakes.

This Opportunity focuses on developing and applying methods that will improve seismic hazard assessment in the densely populated urban regions of southern California. The applicant must have a background in earthquake seismology, tectonophysics, or a similar relevant field. The research under this Opportunity will focus on one or more of the following areas:

1. Development of a computational framework to quantify the effect of a large (past or future) earthquake in the greater Los Angeles metropolitan region on other faults in the region. The framework will cover the greater Los Angeles metropolitan region, including the growing “inland empire” communities (San Bernardino, etc) as well as Orange County. A longer-term goal will involve a regional extension of this framework, in particular with consideration of the southern San Andreas-San Jacinto system. To carry earthquake-interaction results through to hazard implications, the results will be incorporated into hazard-mapping tools developed as part of the OpenSHA project (Field and others, 2005.) Although the research will focus on southern California, we expect the methodology developed to be applicable to other regions.
   
   Development of a first-generation urban hazard map for the greater Los Angeles region using OpenSHA, incorporating abundant site characterization results for the greater Los Angeles region (for example,, Tinsley and others, 2004).
2. Use of the growing collection of modern broadband data and remote-sensing imagery to better quantify local site geology on the scale needed for microzonation, and better understand the effect of shallow geological structure on earthquake shaking.

References

Field, E.H., Gupta, V., Gupta, N., Maechling, P., and Jordan, T.H., 2005c, Hazard map calculations using GRID Computing: Seismological Research Letters, v. 76, p. 565–573.

Tinsley, J., Hough, S.E., Yong, A., Kanamori, H.,Yu, E., Appel, V., and Wills, C., 2004, Geotechnical characterization of TriNet sites: Seismological Research Letters, v. 75, p. 505–529.

Proposed Duty Station: Pasadena, CA

Areas of Ph.D.: Geophysics, seismology

Qualifications: Applicants must meet one of the following qualifications: Research Geophysicist

(This type of research is performed by those who have backgrounds for the occupations stated above. However, other titles may be applicable depending on the applicant's background, education, and research proposal. The final classification of the position will be made by the Human Resources specialist.)

Research Advisor(s): Susan Hough, (626) 583-7224, hough@usgs.gov; Ned Field, (626) 583-7814, field@usgs.gov

Human Resources Office contact: Candace Azevedo, (916) 278-9393, caazevedo@usgs.gov

Summary of Opportunities