SCEC Award Number 05149 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Ultimate earthquake triggering: Where and why seismicity migrates along fine-scale lineations in Southern California
Investigator(s)
Name Organization
Heidi Houston University of California, Los Angeles
Other Participants Jelena Tomic
SCEC Priorities A4, n/a, n/a SCEC Groups Source Physics, Fault Systems, Seismology
Report Due Date N/A Date Report Submitted 11/15/2018
Project Abstract
 Improved earthquake relocation techniques have revealed a highly fractured crust in Southern California with many lineaments (or "streaks") of seismicity. In a preliminary survey of about 20 streaks from the new SHLK earthquake catalog (Shearer et al., BSSA, 2005), I discovered two cases of migration of seismicity over distances of up to 8 km and time intervals of 1.5 to 3 years. Both migrating streaks are relatively shallow, at 4 to 5 km depth in the tectonic block between the San Jacinto and Elsinore faults, and are oriented at high angles to major regional trends. The deeper streaks tend to consist of seismicity occurring more uniformly (i.e., randomly) over the 20-year catalog duration. Migrations on this time and space scale are rare in non-volcanic settings, but would be difficult to detect without high-precision relocations. The scale and velocities (1 to 10 km/yr) of the migration suggest an earthquake triggering process driven by migrating fluids.
Intellectual Merit Many preexisting fractures with varied orientations scattered throughout the southern California crust were revealed in catalog SHLK. Slow long-distance migration of seismicity along two of these newly-revealed lineations was detected. This phenomenon has not been seen previously at this space-time scale in non-volcanic crust. Rates of migrations may place constraints on the processes involved. The space-time scale of the two migrating sequences suggests that they are driven by migrating fluids.

The deeper seismicity lineations (below 8 km) are less clustered temporally, appearing randomly distributed in time. This difference could well be key for understanding both the nature of faults and seismic triggering in the complex fault system of Southern California.
Broader Impacts The project supported the development of a foreign female graduate student who has gone on the have a career in geophysics in the Netherlands.
Exemplary Figure Rightmost figure. Time-distance plot of seismicity streaks L5 and L12. The time-distance evolution of L5 and L12 may be interpreted in terms of seismicity being triggered by fluid migration and diffusion, This interpretation yields hydraulic diffusivities of 0.17 m2/s for L5 and 0.015 m2/s for L12. These values are within the range found at similar depths on faults around the KTB borehole.