19-2. Earthquake rupture and the brittle-ductile transition: A Subduction Zone Science Team Project

This opportunity focuses on improved estimates of the physical properties and position of the brittle ductile transition, with emphasis on research in U.S. plate boundary settings such as the Cascadia subduction zone and the San Andreas strike-slip system. The opportunity is intended to provide a better understanding of the seismic to aseismic transition, thereby producing a better understanding of large to great earthquake ruptures, leading to reductions in uncertainty in earthquake hazards estimates such as the National Seismic Hazard Model (NSHM).  

Earthquake ruptures are limited to the brittle portion of the crust and cannot extend significantly into the ductile lower crust where rapid slip produces no stress drop. Thus, the depth location of the rheological transition from brittle to ductile deformation (BDT) determines the depth extent of coseismic rupture. Intermediate and larger earthquakes rupture the crust from the surface all the way down to the BDT defining the 'width' of the seismogenic zone (W). Applying static, uniform, constant stress drop models to these events predicts that: slip increases linearly with W, slip is independent along-strike length (L), that moment increases linearly with L and with fault area (Scholz, 1982).  

However, in practice the depth of coseismic rupture can’t be well-determined from slip and along-strike rupture length (Hanks and Bakun, 2014). There is no clear evidence for saturation of slip at rupture lengths greater than W and if it saturates at all it does so much more gradually and at much larger L. For constant stress drop, the lack of saturation with L implies that W increases with magnitude instead of being fixed, i.e., that dynamically the BDT (W) extends deeper for progressively larger events. Alternatively, assuming fixed W instead of constant stress drop, the observations can be interpreted as requiring stress drop to increase with magnitude. Unfortunately, there is little evidence from earthquake source studies or observed ground motions to support either increasing stress drop or increasing co-seismic depth extent with earthquake magnitude.  

Despite these important issues of scaling in earthquake hazards estimates (e.g., magnitude-area and slip-fault length relations), the position of the BDT is not well-known or well-determined by observations. Resolution is commonly limited by the large distance between surface stations and W. Another problem is that ruptures of the entire seismogenic depth occur infrequently and most hazard analysis relies on historical events or paleoseismic records of faulting that lack instrumental records. As a result, the BDT is often inferred rather than measured; for example, in Cascadia it is often assumed that the downdip limit of the locked portion of the subducting slab marks the location of the depth extent of large earthquake rupture. However, the position of the locked extent itself is not well constrained by geodetic observations, due in part to the network configuration that lacks offshore instruments, and perhaps to a gradual transition of coupling with depth. Additional concerns arise from the unknown material properties in the ‘gap’, the portion of the interface between the estimated downdip extent of the locked zone and the up-dip extent of tremor and slow slip. A similar gap exists on the central San Andreas below the creeping section and above the region hosting low frequency earthquakes. Whether these regions are actively slipping, and if so, why, is not known. Of greatest concern is whether the gap in Cascadia fails during megathrust events, as the gap extends approximately 60 km eastward, towards the population centers of the region.  

Improved hazards estimates might arise from including rheological constraints on the depth extent of seismic rupture and the locking depth. For example, in laboratory based estimates, the BDT depends on temperature (depth in the crust) and the ductile rheologies that control it are strongly temperature weakening (Goetz and Evans, 1979). Consequently, regions of elevated temperature like volcanoes and geothermal fields exhibit a shallower BDT. Were slip in these regions coupled to W, it would be small and in plate boundary settings such as in southern California where there are active geothermal fields, the elevated BDT may significantly influence along-strike rupture propagation and nucleation of large earthquakes. Additionally, the BDT depends on strain-rate. Since estimates that rely on the geotherm require a choice of strain rate, the positions of W and locking depth are expected to be quite different. For instance, the appropriate strain rate to estimate locking depth would be the plate rate, whereas for large earthquake W the estimate would be a higher dynamic rate, corresponding to a greater depth.  

The Fellow will seek to understand the influence of the brittle ductile transition on the depth extent of seismic rupture, earthquake magnitude and size scaling relations, earthquake source properties and hazard. This may be through connections with known thermal structure, crustal properties as measured using geophysical techniques (magnetotellurics, compressive and shear wave speeds), seismological studies of earthquake occurrence or earthquake source properties, geodetic investigations of locking depth, laboratory experiments on natural or analog materials, or numerical modeling studies of deformation and rupture propagation. The opportunity involves any appropriate earth-science discipline necessary to accomplish the proposed research (e.g., seismology, geodesy, rock physics, crustal hydrology, theory and modeling). Some ideas of research topics include, but are not limited to:  

• Differences in rock properties, porosity, and pore pressure are implied between dilatant brittle and non-dilatant ductile deformation. These may be apparent in electrical conductivity, Vp-Vs ratios and other observable properties. In Cascadia and the central San Andreas, differences may also be expected between the shallow brittle portion of the fault interface and the tremor zone. How does the nature of the BDT differ in these regions? 

• Does the depth to BDT vary significantly along well-instrumented strike-slips faults such as the San Jacinto in California? Does it vary significantly between faults? Do any variations correlate with lithology, stress orientation, temperature or other factors? 

• Are observed earthquake magnitude to fault area relations controlled by the depth of the BDT? Does the transition depth increase at larger magnitudes? 

• Do small earthquake focal depths increase following a large mainshock? And, if so, is this due to the strain rate dependence of the BDT? 

• Pore pressure is thought to play a significant role in the depth of the BDT, with elevated pore pressure increasing its depth. For well instrumented recent large earthquake ruptures, such as Ridgecrest, is the depth extent of rupture well-constrained? Is it consistent with simple thermal models, or are non-hydrostatic fluid pressures also required? Can knowledge of stress orientation constrain brittle fault strength and in-situ pore pressure? 

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

References:  

Goetze, C., and B. Evans (1979), Stress and temperature in the bending lithosphere as constrained by experimental rock mechanics, Geophys. J. R. Astron. Soc., 59, 463–478. 

Hanks, T.C.  and W.H. Bakun, (2014), M–logA Models and Other Curiosities, Bulletin of the Seismological Society of America, 104, 2604-2610 

Scholz, C. H. (1982). Scaling laws for large earthquakes: consequences for physical models, Bull. Seismol. Soc. Am., 72, 1–14. 

Proposed Duty Station: Menlo Park, CA or Moffett Field, CA 

Areas of PhD: Geophysics, seismology, geology, engineering geology 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 one of the following qualifications:  Research Geologist, Research Geophysicist, Research Engineer, Research Geodesist.  

(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: Beverly Ledbetter, 916-278-9396, bledbetter@usgs.gov 

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