Separate lines of research in seismology, deformation and rock mechanics indicate that classification of faults and fault segments as either aseismic or seismic is too restrictive. More often, active faulting involves some combination of the two. For example, episodic slow slip and tremor involve large scale aseismic slip that can be accompanied by much smaller scale earthquakes. Other examples include foreshocks occurring on oceanic transform faults as well as some megathrust earthquakes (i.e., prior to the 2011 M9 Tohoku-oki earthquake) that apparently are driven by aseismic slip. In the case of foreshocks, the aseismic slip is detectable at the earth's surface and is arguably the immediate cause of much of the concurrent seismicity. In such regions aseismic deformation may be useful in monitoring earthquake precursory activity.
Other work suggests that even at a single point on a fault in the earth's crust slip may be either seismic or aseismic depending on slip speed, time within the seismic cycle and other factors. For instance, near surface portions of some faults in the Bay Area of California such as the Hayward slip aseismically at a significant fraction of the plate motion rate and also are believed to slip rapidly co-seismically during large earthquakes. Such mixed mode behavior has been applied to the creeping segment of the San Andreas Fault in the 2014 Uniform California Earthquake Rupture Forecast (UCERF3). Similarly portions of some faults in this region have slipped both co-seismically during recent earthquakes (2004 Parkfield, 2014 South Napa) and also subsequently undergone significant aseismic afterslip. Mixed mode slip is also known to occur near the base of the seismogenic zone where background on-fault seismicity that follows large earthquakes is often observed to deepen transiently into the otherwise ductile root of the fault.
At present little is known about the particular conditions, rheological properties and environmental factors that control transitions between aseismic and seismic deformation and that control the spatial and temporal extent of one behavior or the other. Many weak silicates, including natural fault gouges, show either stable or unstable mechanical behavior depending on pressure, temperature, strain rate and pore fluid composition. Effects of pore fluid diffusion and fault valving on crustal faults continues to be debated. Insufficient observational evidence has been collected to fully incorporate aseismic deformation into hazard assessments and to use detectable aseismic deformation reliably in monitoring earthquake occurrence.
We seek a postdoctoral fellow for this Mendenhall Research Opportunity who can make fundamental contributions in rock physics and fault mechanics with a focus on the conditions leading to seismic versus aseismic faulting as well as the spatial and temporal interaction of fault slip modes to advance our understanding of earthquake occurrence and earthquake hazard. Experimental research is expected to be conducted in the Rock Physics Laboratory of the Earthquake Science Center of the USGS. This laboratory has been devoted to the study of earthquake processes for nearly 50 years and continues to be a leader in the study of high pressure rock mechanics related to earthquakes. Strong candidates are expected to possess demonstrated abilities in developing fundamental theory as well as numerical modeling of experimental results and applications to natural systems.
Proposed Duty Station: Menlo Park, California
Areas of Ph.D.: Geophysics, seismology, geology, volcanology, rock mechanics or related fields (candidates holding a Ph.D. in other disciplines, but with extensive knowledge and skills relevant to the Research Opportunity may be considered).
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.)
