18-6. Using converted wave imaging to probe the Cascadia and Alaska megathrusts

Estimating the onshore extent of megathrust earthquake rupture in the Cascadia subduction zone represents a key uncertainty in earthquake hazard for the Pacific Northwest that is governed by the physical state and mechanical properties of the plate interface.  Geodetic observations demonstrate that the megathrust is spatially segmented into an interseismically locked zone that is likely confined to depths shallower than ~20 km, a region of episodic tremor and slow slip (ETS) at depths between ~30-50 km, with an intervening transition zone. Current USGS hazard models implement three possible scenarios where earthquakes are either contained largely offshore, propagate far onshore to reach the ETS zone, or terminate within the transition zone. Each of these scenarios predicts significantly different impacts on the overlying regions.  While the physical properties of the ETS zone including very high fluid content have been clearly established by receiver function imaging, the physical properties of the locked zone in the 10-20 km depth range, which is located primarily offshore, and the conditions that govern the downdip edge of interseismic locking are poorly imaged. This key depth range is expected to undergo large coseismic slip in future great earthquakes and cause strong onshore shaking but it remains difficult to image with many seismic techniques including offshore active source experiments, onshore teleseismic receiver function studies, and local earthquake tomography.  Hence, we know relatively little about the physical properties of the portion of the megathrust that constitutes both a large seismic hazard and a significant source of uncertainty in Cascadia. 

Similarly, in the Alaska subduction zone, there is considerable along-strike segmentation in interseismic locking, with the Semidi segment currently locked and expected to rupture in a future great earthquake, while the neighboring Shumagin segment appears to accommodate a large fraction of plate motion aseismically.  Because this along-strike transition in megathrust behavior happens offshore it has been difficult to pinpoint how rapidly it occurs and to unravel the physical conditions responsible for the change in earthquake rupture potential. 

This research opportunity seeks a candidate to apply advanced seismic imaging techniques based on converted seismic phases from local earthquakes to image the material properties in the locked zones of the Cascadia and Alaska subduction zones.  Recent deployments of ocean bottom seismometers (OBSs) in Cascadia and Alaska have opened a new opportunity for imaging the depth range from the seismogenic zone through the downdip transition in interseismic locking (Toomey et al., 2014; Abers et al., 2019). The abundant seismicity in the downgoing plate in both the southernmost Cascadia subduction zone produces high-frequency (10 Hz) seismic waves that convert from P to S or S to P as they cross the plate boundary zone. Similar phases are expected to be common in the Alaska OBS dataset. Exploiting the considerable information in these phases to infer details of the fine structure and material properties of the plate boundary zone requires utilizing advanced imaging techniques.  

Applicants to this opportunity are encouraged to apply recently developed waveform based imaging techniques, which may include reverse time migration or wave equation velocity analysis, that take advantage of the unique features of converted phases to constrain the material property contrasts at discontinuities in the vicinity of the plate boundary faults.  

While considerable data from onshore-offshore seismic experiments is already in hand or soon will be, there is also the possibility of collecting a custom-tailored dataset using the recently acquired USGS pool of ‘nodal’ seismometers. Candidates could propose the possibility of collecting datasets specifically designed for applying array-based methods to converted phase imaging. 

Interested applicants are strongly encouraged to contact the Research Advisor(s) early in the application process to discuss project ideas.

Proposed Duty Station: Moffett Field, CA

Areas of PhD: Seismology 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: Applicants must meet the qualifications for:  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.)

Human Resources Office Contact: Audrey Tsujita, 916-278-9395, atsujita@usgs.gov