22. Improving Ground Motion Forecasts for Future Earthquakes

Studies over the last decade have shown that the spatial variation and character of ground shaking involves a complex interaction between the causative fault and surrounding geologic structures, particularly sedimentary basins. Accurately forecasting the expected shaking from future large earthquakes requires simulations with 3-D structural models that account for these complex interactions. These structural models describe the geologic structure (geometry of faults and lithologic boundaries) and the physical properties (seismic velocities).

In 2006 the U.S. Geological Survey (USGS) developed a 3-D structural model of the San Francisco Bay area filling a 290 km-long, 140 km-wide, and 45 km-deep volume. This structural model includes a 3-D geologic model composed of 34 geologic units and 25 fault surfaces, and a 3-D seismic velocity model, constructed from the geologic model using empirical relations among Vp, Vs, density, rock type, and depth. A coarser resolution structural model extends the coverage to a 650 km-long by 330 km-wide region around the extent of the 1906 earthquake rupture. Although simulations using this structural model reproduce the basic features of recorded ground motions (amplitude and duration) at frequencies up to 0.25 to 0.5 Hz, the 3-D ground motion simulations fail to predict more complex features of the waveforms. Additionally, modeling of recordings of moderate earthquakes shows complex relationships between misfits in the travel times and waveform amplitudes.

Additional datasets (such as broadband ambient noise recorded by Earthscope's USArray) and recently developed analytic techniques (such as adjoint tomography) present opportunities to improve the accuracy and resolution of the 3-D structure. Providing better characterization of the structure -- through additional constraints on the physical properties and geometry of fault surfaces, lithologic boundaries, and internal subdivisions of geologic units -- will improve the accuracy of ground motion modeling of scenario events and probabilistic seismic hazard maps. Refining the model will expand its usefulness into new areas such as fault kinematics and interactions, dynamic fault rupture, tectonic strain buildup, and rupture segment definition.

This Research Opportunity focuses on developing methods to improve our understanding of 3-D and 4-D earth structure (physical properties and fault evolution) and applying them to the San Francisco Bay area. The applicant must have a background in earthquake seismology, computational elastodynamics, tectonophysics, or a similar relevant field. The research under this Opportunity should involve use of the improved structural model(s) in simulating ground motions from moderate or large earthquakes, or long-term fault kinematics while focusing on one or more of the following areas:

1. Improving the characterization of the crustal structure in the San Francisco Bay area using adjoint methods with earthquake and active source data. For example, adjoint tomography, which is beginning to be applied in southern California and Japan to improve seismic velocity models, could be further developed and extended to include active source data while being applied in the San Francisco Bay area.
2. Joint inversion of two or more geophysical datasets to determine physical properties and geologic structure. Potential datasets for joint inversion include regional recordings of moderate earthquakes, USArray recordings of ambient noise and teleseismic wavefields, and gravity and magnetic fields. Research in this area could include development of methods to characterize geologic structure and variations in physical properties based on the evolution of fault systems with application to the San Andreas fault system in the San Francisco Bay area.
3. Characterization of short-length scale variations in physical properties within geologic units, such as those related to scattering of seismic waves. This work would involve development of methods to define the short-length scale variations of geophysical fields using, for example, fractal and/or stochastic fields. Potential datasets include borehole seismic logs and recordings of small earthquakes on seismic arrays in the Santa Clara Valley.
4. Characterization of the uppermost 100 m of the 3-D geological structure for improved estimates of site response caused by local conditions. This research could examine the relative contributions of near-surface sediments and geologic structure to ground motions as a function of frequency and level of shaking in regions of the San Francisco Bay area where 3-D geologic structure and/or site response are important. Areas of particular interest include the Santa Clara Valley, Santa Rosa, and regions with levees within the San Joaquin-Sacramento delta and artificial fills around the perimeter of the San Francisco Bay. Potential datasets include borehole sonic logs and isopach maps of Quaternary and Holocene deposits.

Proposed Duty Station: Menlo Park, CA

Areas of Ph.D.: Geophysics, seismology, tectonophysics

Qualifications: Applicants must meet one of the following qualifications: Research Geologist, 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): Brad Aagaard, (650) 329-4789, baagaard@usgs.gov; Thomas Brocher, (650) 329-4737, brocher@usgs.gov; Robert Jachens, (650) 329-5300, jachens@usgs.gov; Robert Simpson, (650) 329-4865, simpson@usgs.gov; Tom Parsons, (650) 329-5074, tparsons@usgs.gov; Carl Wentworth, (650) 329-4950, cwent@usgs.gov

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

Summary of Opportunities