25. Cascadia Subduction Zone Seismic and Aseismic Slip Scenarios: Implications for Public Policy

The Cascadia subduction zone, extending from northern California to Vancouver Island, Canada, has a 10,000 year record of producing earthquakes of M8.5 or greater at intervals of several hundred years, with the last major event (likely a magnitude 9) in 1700.  However, the subduction interface has remained seismically quiet at about the magnitude 5 level for the duration of modern seismic recording. As public agencies in California, Oregon, Washington, and British Columbia work to raise public awareness of the hazards posed by a repeat Cascadia earthquake and its ensuing tsunami, there is a need to anticipate a wider variety of possible scenarios than just “The Big One” (Petersen and others, 2008).  More specifically, paleoseismic data indicate that past great Cascadia earthquakes have had a range of magnitudes (Nelson and others, 2006).  The entire zone could rupture sequentially in a series of M8-8.5 earthquakes; immediately after the first such shock, seismologists would be expected to intensively investigate the expected time frame of the next earthquake in the sequence.   Any seismicity increase on the subduction interface, say at the M6 level, would come under scrutiny as a possible foreshock; indeed, if such an earthquake were to occur now in the California reach of the megathrust, that state’s Office of Emergency Services would issue a public earthquake advisory.  The relationship of aseismic slip events in Cascadia with the time-dependent earthquake hazard is an active topic of research, now facilitated by a large supply of data from continuous GPS stations and strainmeters of the Plate Boundary Observatory. 

We seek an individual with experience in seismology, and ideally, an interest in public policy applications of earthquake science, to pursue research that could help provide guidance during possible episodes of increased seismicity, or anomalous aseismic slip, in Cascadia. The postdoctoral researcher will find this to be a very rich area of study. A few of the numerous possible research topics include: studies of earthquake dynamic or static stress triggering in subduction zones; foreshock probabilities (e.g., updating the global synthesis of Reasenberg, 1999); forecasting Cascadia post-seismic slip and its hypothetical implications for evolution of a Cascadia megathrust earthquake sequence; risk communication and social implications of possible “Earthquake Advisory” announcements; relationship of slow slip events to major earthquakes(e.g.,  Mazzotti and Adams, 2004); optimal utilization of seismic data to rapidly constrain depths and focal mechanisms of earthquakes offshore Cascadia (e.g., Trehu et al., 2008); studying the feasibility of implementing a Tokai-like warning scheme for Cascadia.  

Research under this Opportunity has the potential to contribute critical findings about giant subduction earthquakes to improve public safety in the next major Cascadia event.  For example, recognition of a foreshock could save lives in Cascadia coastal communities, who will generally face tsunami impact 10-20 minutes after the seismic rupture, and in some localities, coastal coseismic downdrop on an even shorter time scale.  Communities must plan for major Cascadia earthquakes to occur without warning, but even in a sequence that begins with a main shock, seismology may be able to provide critical guidance as to the likely evolution of the sequence.  Translating any of these findings into public policy will require involving, as equal partners, state governments, and the emergency response community.  The Mendenhall project will build on the experience with issuing Earthquake Advisories in California, and on the experience with identifying sub-populations at high risk in Cascadia tsunami inundation zones (Wood and Soulard, 2008).

References

Mazzotti, S., and J. Adams,  2004, Variability of near-term probability for the next great earthquake on the Cascadia subduction zone: Bulletin of the Seismological Society of America, v. 94, p. 1954–1959.

Nelson, A.R., H.M. Kelsey, and R.C. Witter,  2006, Great earthquakes of variable magnitude at the Cascadia subduction zone: Quaternary Research, v. 65, p. 354-365.

Petersen, M.D., A.D. Frankel, and others, 2008, Documentation for the 2008 update of the United States National Seismic Hazard Maps: U.S. Geological Survey Open-File Report 2008–1128.

Reasenberg, P.A., 1999, Foreshock occurrence before large earthquakes: Journal of Geophysical Research, v. 104, p. 4755–4768.

Trehu, A.M., J. Braunmiller, and J. L. Nabelek, 2008, Probable low-angle thrust earthquakes on the Juan de Fuca-North America plate boundary: Geology,  v. 36, p. 127–130.

Wood, N., and C. Soulard, 2008, Variations in community exposure and sensitivity to tsunami hazards on the open-ocean and Strait of Juan de Fuca coasts of Washington: U.S. Geological Survey Scientific Investigations Report 2008-504, 34 p.

Proposed Duty Station: Vancouver, WA; Menlo Park, CA; Seattle, WA

Areas of Ph.D.: Seismology, geophysics, geodesy

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): Evelyn Roeloffs, (360) 993-8937, evelynr@usgs.gov; Nick Beeler, (650) 329-4863, nbeeler@usgs.gov; Craig Weaver, (206) 553-0627, craig@usgs.gov; Nathan Wood, (360) 993-8951, nwood@usgs.gov; James Goltz (California Office of Emergency Services), (626) 356-3810, jim.goltz@oes.ca.gov

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

Summary of Opportunities