SCEC Award Number 12040 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Fault Zone Widths as a Parameter in Seismic Hazard Estimates
Investigator(s)
 Name Organization Bruce Shaw Columbia University
Other Participants NA
SCEC Priorities 4b, 4c, 4e SCEC Groups WGCEP, FARM, Seismology
Report Due Date 03/15/2013 Date Report Submitted N/A
 Project Abstract This project proposal arose from an effort to contribute further to not only an understanding of fault system geometry, but to address issues of importance to the UCERF3 project. In the end, contributing to UCERF3 ended up driving the effort on this project, with multiple contributions to UCERF3 being the main results presented below. Two main publications were supported by this project, which are discussed below. These publications contributed directly to a key part of the UCERF3 project concerning scaling laws in the longer term hazard estimates. Fault system geometry relationships relevant to testing the model,and updating it in time dependent manner, was deemed a less urgent priority by the Working Group, and so results relevant to that were bumped down in priority. On that front, work examining the role that complex stranded geometries play in fault system dynamics was initiated, with interesting preliminary results obtained. This last work is presented at the end of this report.
 Intellectual Merit We show surface slip-length data is well fit by a new scaling relation we developed based on constant stress drop. The constant stress drop model provides a geometrical explanation for a longstanding puzzle of why slip only begins to saturate at large aspect ratios. The good fit of the constant stress drop model to the slip-length data lends further support to the observations of constant stress drop scaling across the whole range of magnitudes of earthquakes, from small to great earthquakes. New methodologies were introduced into seismic hazard analysis whereby moment balancing to match event rates to fault slip rates was replaced with slip-rate balancing, and a new data source based on surface slip data was applied. In this way two branches for estimating slip, a more traditional implied slip based on magnitude-area scaling and a new surface slip based on surface slip-length scaling was implemented. Broader Impacts Results from this work are being incorporated into the UCERF3 hazard analysis, which impacts seismic hazard estimates and building codes, helping society better mitigate earthquake hazards.
 Exemplary Figure Figure 2. Constant stress drop scaling does a good job fitting surface slip data for different focal mechanisms. Curves show dip effects on expected scaling. Data points are geological surface slip observations of average slip versus length. Symbol type indicates focal mechanism: strike-slip (circle), normal (diamond), thrust (square). Curves show scaling expected for fixed seismogenic depth, but changing dip, and thus downdip width and changing modulus effects. All curves have the same stress drop, a value for best fit stress drop to strike-slip data of $3.91~MPa$ . Different curves use same seismogenic depth of $15km$ and different dips $90^{\circ}$ (solid line) strike-slip %Equation~(\ref{bssl2}), and $60^{\circ}$ (short-dashed line) and $30^{\circ}$ (long-dashed line) dip-slip. %Equation~(\ref{dipping}). Note only relatively small differences expected in scaling of events with different focal mechanisms, which appears consistent with data. ~~From [Shaw, 2013a]