Slip Rates Are Dead. Long Live Slip Rates.

Date: 4/24/2019

Time: 03:15 PM

Room: Vashon

Fault slip rate is a fundamental parameter for earthquake rupture forecasts. However, the path from geologic slip rates to earthquake rates in seismic hazard models has many potential pitfalls that deserve close examination. It has long been recognized that magnitude-frequency relations/recurrence models for individual faults are highly uncertain and hard to test. But it is also increasingly clear that the theoretical relationship between fault slip rates and moment release rates (Brune, 1968) presents serious challenges at the temporal and spatial scales relevant for seismic hazard models. On- versus off-fault deformation, aseismic slip, and potential rapid variation in loading or release rates are only a few of the issues that complicate the conversion of slip rates to coseismic moment release. Detailed recurrence information from site-specific paleoseismic studies is usually considered the best way to augment and strengthen slip rate information, yet paleoseismic data are often highly uncertain themselves and can be very difficult to obtain.

Can we develop more useful hazard models with a different approach to collecting, reporting, and using geologic slip rates? We’d like to spark a discussion about how we collect and treat geologic slip rates by envisioning earthquakes as ruptures propagating along fault networks, rather than events ‘caused’ by faults with fixed, prescribed slip rates. From the fault-network hosted-rupture perspective, a few issues emerge. 1) No slip rate, no problem? The 3D character of the potentially active fault network is perhaps more important than modeling only faults with known slip rates; 2) Slip rates are functions: Characterizing slip rates as smooth functions that vary along network faults is key – how do we get there?; 3) Honesty in reporting: Slip rates should be reported as probability density functions that fully account for time and displacement history uncertainties; 4) Recognize brick walls: Where do we acknowledge the limitations of geologic slip rates and promote other approaches, like geodesy?

Presenting Author: Richard W. Briggs

Authors