Research Opportunity 32

We know that earthquakes and earthquake clusters are “normal” for many areas of the country, but when earthquakes occur, and especially when multiple earthquakes occur, we are often faced with the question “is the current activity normal.” As in medicine, where an abnormal or nonsteady-state finding can be diagnostic of a new process underway in the body, we want to make better use of the signals and signs that the Earth is giving us. The implication is that, if the current activity is abnormal, then we need to develop and test methods that can be used to reassess our seismic hazards assessments.

Our long-term seismic hazards assessments and short-term earthquake warnings based on earthquake clusters of foreshocks and aftershocks rely on models in which the underlying strain rates from plate tectonics are constant. In these models, temporal changes of the earthquake probabilities are due to the time since the last event on a fault and interactions between earthquakes. However, the strain rate can change due to slow slip events or the migration of pore fluids within the crust. These transient events can also generate temporary departures from long-term strain accumulation, as measured on geodetic instruments. In some cases they are interpreted to transfer strain from deeper, more ductile parts of fault zones to the shallower, brittler parts, where damaging large earthquakes will eventually occur.

Identifying and tracking earthquake behavior influenced by slow slip events or pore fluid changes may help identify time periods when the likelihood of large earthquakes is heightened by mechanisms that are not currently included in the statistical models currently used to model earthquake clustering for short-term warnings. Because these slow changes in stressing rates may take place over weeks to years, modeling them may provide a route to intermediate-term earthquake forecasts that bridge the gap between our current short-term warnings and long-term hazards assessments.

Identification and tracking of these events requires continuously recording seismic and geodetic instruments in the right places. The U.S. Geological Survey (USGS) and its academic partners have networks of seismic and geodetic instruments throughout the Western United States, and these have been recently augmented by the wealth of instrumentation installed through the National Science Foundation-funded EarthScope program. Detecting transient strain signals from the seismicity is particularly challenging because the underlying changes in earthquake rates are often masked by the larger rate changes due to earthquake clustering. Focused research programs to understand earthquake rates and clustering, to detect and analyze transient slip events and to study the migration of pore fluids are on the frontier of earthquake geophysics.

Recent cases of strain transients that have affected seismicity rates have occurred at the southern end of the San Andreas fault where a slow slip event may have affected earthquake rates and in the Reno, Nev., area where a vigorous swarm of earthquakes was likely caused by changes in pore fluids and resulted in damage to property. A variety of transient slip behavior has been observed in subduction zones and along creeping segments of the San Andreas Fault, and such events have also been inferred to have triggered seismicity in Hawaii and Japan. Successful research on this topic will likely require utilizing data from a range of regions and tectonic settings.

Research under this opportunity will address the challenges of differentiating normal from abnormal earthquake activity, identifying and characterizing the underlying changes in strain rate, and (or) integrating strain rate changes into seismic hazard assessment. Research to be performed under this opportunity may focus on one or both of the following:

A background in earthquake seismology, geodesy, fault mechanics, or tectonophysics is required, depending on the research problem. This opportunity may target any region or regions with adequate data and evidence for past changes in strain rates including, but not limited to, the southern end of the San Andreas fault, creeping segments of the San Andreas fault, swarms such as the 2008 Mogul-Somersett Earthquake Sequence in Reno, Nev., Hawaii, or Japan.

Proposed Duty Station: Menlo Park, CA

Areas of Ph.D.: Geophysics, seismology, geodesy, tectonophysics (candidates holding a Ph.D. in other disciplines but with knowledge and skills relevant to the Research Opportunity may be considered).

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): Andrew Michael, (650) 329-4777, michael@usgs.gov; Jeanne Hardebeck, (650) 329-4711, jhardebeck@usgs.gov; Jessica Murray-Moraleda, (650) 329-4864, jrmurray@usgs.gov; Thomas Parsons, (650) 329-5074, tparsons@usgs.gov; Fred Pollitz, (650) 329-4821, fpollitz@usgs.gov

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

32. Detecting the Causes of Earthquake Rate Changes