Hazard Workshop

September 19, 2016 at USGS in Menlo Park, CA


Agenda for Hazard Workshop


The goal of the workshop was to develop a workflow for contributing geodetic information to a future iteration of the National Seismic Hazard Map (NSHM) in a way that we as a community find acceptable. Below, I have summarized a few key points of the workshop.
  1. Deliverables: In the first step towards the above goal, we will produce a combined geodetic dataset of the western United States. Rob McCaffrey will perform the combination. Workshop participants commit to using this dataset in future modeling efforts towards the national seismic hazard map, to eliminate a common source of inconsistency in geodetic models. A velocity field for Alaska will be available through Jeff Freymueller’s current NEHRP project.
  2. Future deliverables and deadlines: Possible future steps include a coordinated modeling effort among workshop participants (and open to additional participants). In order to be used in the National Seismic Hazard Map, work must be published by a deadline prior to the next update of the NSHM. This deadline is March 2017 for the 2018 update, and June 2018 for the 2020 map.
  3. Past challenges to incorporating geodesy: Previous efforts to incorporate geodetic observations into seismic hazard have been a challenge due to discrepancies between geodetically estimated fault slip rates and those estimated from geology. Furthermore, differences in geodetic modeling approaches and assumptions lead to differences among geodetic models. However, there are clear advantages for incorporating geodesy into hazard: there is a clear correlation between seismic hazard and regions of high geodetically observed strain rate, and geodetic strain rate observations do not depend on the presence of mapped surface structures. How can we improve the combined geologic and geodetic inversion models so that we have more confidence in the results?
  4. Geodetic modeling uncertainties: The challenge in estimating geodetic fault slip rates is that geodetic observations of the interseismic phase of the earthquake cycle, in which seismogenic faults are locked, must be interpreted in the context of a fault model. Subduction zone coupling, in addition to transient deformation from post-seismic relaxation, GIA, and slow slip events, adds additional modeling complexity. Therefore, in addition to aleatory uncertainty of the geodetic observations and modeling machinery, geodetic slip rates also have high epistemic uncertainties, or uncertainties as to the most appropriate model choices. Assessing and minimizing model uncertainty requires systematic compilation of existing models, and/or a coordinated effort to use consistent modeling strategies.
  5. Regional distribution of geodetic models: Individual geodetic slip rates studies are dense in central and southern California. Fewer studies have been dedicated to Northern California, Walker Lane, the Pacific Northwest, and Alaska. Geodetic information is especially valuable in remote regions with few geologic slip rates or other indicators of seismic hazard, such as Alaska. Because observations of geodetic strain rate do not rely on individual mapped structures, geodesy provides essential constraints on deformation in the Pacific Northwest, and in low strain rate regions like the Basin and Range, although converting low strain rates to slip rates remains a challenge.
  6. Strain rate and earthquake hazard: Converting strain rates to slip rates in many places remains a challenge, as creeping faults produce high geodetic strain rates, but result in lower seismic hazard. Directly estimating fault coupling/locking may be an important component of geodetic slip rate models. Alternatively, moment accumulation rate may be a more appropriate value to estimate and report. Similarly, tradeoffs between subduction zone coupling/locking and slip on hanging wall faults often means that both must be estimated simultaneously, which is an important consideration in Alaska and Cascadia.
  7. Off-fault deformation: Geodetic strain rates do not differentiate between on-fault and off-fault strain rate (off-fault here defined to be deformation not accommodated by modeled faults). Estimates of off fault strain rate, including the results of UCERF3, find that ~30% of plate boundary deformation is accommodated by deformation away from major faults. How can we use off-fault strain rates to augment the smoothed seismicity model?


Eileen Evans USGS Menlo Park
Jeff Freymueller University of Alaska, Fairbanks
Ryan Gold USGS Golden
Bill Hammond University of Nevada, Reno
Liz Hearn Capstone Geophysics
Steve Hickman USGS Menlo Park
Kaj Johnson Indiana University
Keith Knudsen USGS Menlo Park
Rob McCaffrey Portland State University
Jessica Murray USGS Menlo Park
Tom Parsons USGS Menlo Park
Mark Petersen USGS Golden
Fred Pollitz USGS Menlo Park
Paul Segall Stanford University
Zeng-Kang Shen UCLA
Bob Simpson USGS Menlo Park
Wayne Thatcher USGS Menlo Park
Yuehua Zeng USGS Golden