Introduction

During SCEC4 (2012-2016) Special Fault Study Areas (SFSAs) were established to focus interdisciplinary research on geographically-targeted problems of fault system behavior. The SFSA concept will evolve in SCEC5 (2017-2022) into the Earthquake Gates Area (EGA) initiative (see SCEC5 proposal). An “earthquake gate” describes a region of fault complexity that can halt earthquake ruptures conditional on proximal fault geometry, rupture direction, and prior earthquake history. These as well as other factors, such as near-fault rheology, may determine whether and how often an earthquake gate is open or closed to a propagating rupture. For example, rupture termination at fault complexities can store significant strain energy, suggesting that loading by prior ruptures can increase the probability of a future event breaking through in a larger earthquake. However, this effect is sensitive to the dynamics of the rupture process and cumulative elastic deformation. Therefore, these fault subsystems must be modeled dynamically over multiple cycles to assess the relative importance of competing forces and to calibrate them with geologic and geophysical observations. Our working hypothesis is that, within the San Andreas fault system, the dynamics of earthquake gates control the time-dependent probability of large, multi-segment (and multi-fault) ruptures.

An interdisciplinary approach that combines field studies and structural modeling with dynamic rupture modeling has proven to be very successful in our two initial SFSAs: San Gorgonio Pass and Ventura Area. The choice of San Gorgonio Pass as an SCEC4 SFSA was motivated by the importance of southern San Andreas ruptures to seismic hazards in Southern California. SCEC4 focused both field-based and model-based research on the region, leading to a consensus that the region’s fault complexity impedes, but does not prohibit, through-going ruptures. Similar interdisciplinary studies within the Ventura SFSA indicated that the thrust systems bounding the northern Ventura Basin tend to link together, generating very large (M > 7.5) ruptures, and they may produce tsunamis and subsidence events in the heavily populated footwall. The distinct outcomes of the two SCEC4 SFSAs highlight how interdisciplinary research that is geographically focused on fault complexities can significantly revise our understanding of seismic hazards.

SCEC5 Earthquake Gates Areas

The SCEC Earthquake Gates Area initiative in 2017-2022 will focus interdisciplinary research on targeted problems of fault-system behavior. The new initiative is similar to the Special Fault Study Areas of SCEC4 (2012-2016), but shifts the investigation to factors that conditionally halt or pass earthquake ruptures, and thus control the probability of large, multi-segment or multi-fault ruptures. Earthquake gates areas arise where fault complexity, rupture direction, near fault rheology, and earthquake history may lead to an earthquake gate being open or closed to a through-going rupture.

The SCEC Earthquake Gates Area Incubator Workshop was held in March 2017 in order (1) to solicit and discuss ideas from the SCEC5 community for EGA locations or topics, (2) to facilitate communication and collaboration between potential EGA investigators, and (3) to bring forward coordinators of potential EGA projects. Investigators interested in leading an EGA and those keen to participate in potential projects were encouraged to attend. Presentations were given that outlined the scope, purpose, and community involvement of potential Earthquake Gates Area. Possible locations or ideas were introduced, looking for cross-over and leveraging opportunities, and garnering interest in common problems.

Following the SCEC Earthquake Gates Area Incubator Workshop, proposals were solicited for targeted earthquake gates areas research during SCEC5 (2017-2022). These proposals were evaluated separately from the standard SCEC Science Plan process. To be considered for a SCEC5 Earthquake Gates Area, interested groups had to submit a science proposal (maximum 5 pages including figures) that included:

1. The location or central topic of the earthquake gate area (EGA) to be investigated.
2. The fundamental problems in earthquake science and seismic hazard reduction the proposed EGA could address.
3. The potential contributions by various research disciplines to the proposed EGA research.
4. A four-year science plan and estimated timeline for research activies (to span 2018-2022).

The SCEC Planning Committee evaluated the proposals and passed their recommendation to the SCEC Board of Directors. The earthquake gates area selected in SCEC5 was prioritized according to (a) prospects for reducing seismic hazard uncertainties associated with conditional propagation of earthquakes, (b) the opportunities for obtaining new hazard information from a broad range of studies and (c) availability of data to constrain conditional propagation of earthquakes through the region.

Several proposals supporting different portions of the southern California fault system were submitted in 2017. The juncture between the San Andreas and San Jacinto faults (Cajon Pass region) was chosen as the first and only Earthquake Gate for the SCEC5 Earthquake Area Initiative. Funding for specific research projects and workshops related to the Cajon Pass Earthquake Gate Initiative will be awarded through the standard yearly SCEC Science Plan process. You may coordinate with the people listed below and/or the SCEC leaders Michele Cooke and Ramon Arrowsmith if interested in contributing to the earthquake area.