SCEC Award Number 11008 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Connections Across the Base of the Seismogenic Zone: Correlated Tremor and Seismicity on the Creeping and Locked San Andreas Fault
Name Organization
Robert Nadeau University of California, Berkeley Roland Bürgmann University of California, Berkeley
Other Participants 1
SCEC Priorities A11, A9, A4 SCEC Groups FARM, EFP, Seismology
Report Due Date 02/29/2012 Date Report Submitted N/A
Project Abstract
Repeating earthquakes and tremors represent the seismic signature of dominantly aseismic slip at shallow and deep crustal levels, respectively. Repeating micro-earthquakes and LFE making up tremor likely take up only << 1% of the fault surface and total moment release. Thus, we interpret changes in occurrence rate of repeating earthquakes and LFE to represent transient changes in aseismic slip rate. The strongly episodic nature of deep-seated tremor suggests that shallow seismicity may also correlate with periods of accelerated tremor and slow slip. For a 9+ year period (2001-2011) we investigated the distribution of deep tremor and shallow repeating and overall seismicity along the creeping section of the SAF NW of Parkfield, as well as along the NW extent of the Cholame segment SE of Parkfield and directly above the tremor, which last ruptured in the 1857 ~M7.9 Fort Tejon earthquake. Over a 90 km long segment of the SAF above the Cholame tremor, the accumulation of seismicity decreases significantly at the initiation time of the tremor episodes, remains relatively low during the 5 to 10 day duration of the episodes, and shows a marked acceleration after the episodes. Both the tremor and repeating earthquake activity show a clear co- and post- seismic response to the 2004 Parkfield earthquake, but their responses differ in the degree of response, depending on their distance from the 2004 event. To the NW in the SAF creeping section, 3-year quasi-periodic slip pulses from repeating earthquakes correspond with delayed 3-year pulses of LFE activity.
Intellectual Merit Intellectual Merit: An improved understanding of the spatially and temporally varying deformation field of fault zones to great depth is critically important for understanding active tectonics, fault-fault interaction and the occurrence of large earthquakes. Unique to the Parkfield region of California is the combination of a rich historic data set, the recent deployment of EarthScope instrumentation, fault complexity, and a variety of natural transient phenomena occurring throughout the crust that, in effect, make this section of the San Andreas fault system a natural laboratory for attaining this objective. The proposed research analyzes, integrates and interprets tremor and seismicity data in this region to investigate the underlying architecture and mechanics of the faulting process. Modeling of repeating earthquake data is used to resolve the 4-dimensional distribution of slip in the upper crust. We conducted a systematic and detailed study of the spatial and temporal association of seismic and tremor events. Non-volcanic tremor episodes provide unique information on transient activity in the deepest reaches of the Earth’s crust. We are particularly interested in exploring the nature of interactions between seismic and aseismic deformation processes that occur in various depth ranges of the fault zone. Fault slip is tied to the mechanical properties of the fault zone rocks and adjoining crustal blocks and the long baseline of preexisting data will be used to investigate possible relationships among cumulative, long-term, and short-term transient slip behavior.
Broader Impacts Broader Impacts: Results from this work have allow us to assess the role of aseismic fault slip transients in earthquake occurrence and clustering. Long-term societal benefits might ultimately arise from improved understanding of aseismic slip transients, their relation to regional strain anomalies, and improved models of the earthquake cycle that should improve earthquake forecasts and intermediate to longer-term predictions.
This project has provided partial support for an undergraduate research assistant (Ryan Turner). For this purpose we developed suitable research “sub-projects”. The projects will be developed as honors theses for the student, which will strongly increase his prospects for graduate school applications. Bürgmann will incorporate issues and results from this work in his undergraduate teaching, including lecture material in EPS150 Case Studies in Earth Sciences, EPS116 Structural Geology and Tectonics (this course includes sections on fault mechanics and seismicity), as well as a graduate course EPS216 Active Tectonics which includes numerous case examples from the San Andreas fault zone in lectures on crustal deformation and seismicity.
Exemplary Figure Figure 2. Fifty-day period stacks of local seismicity (M >= 1.4) occurring 25 days before through 25 days after the peak activity of 44 of tremor episodes (excluding episodes whose 50-day periods overlap with the 2004 Parkfield mainshock and 12 mo. aftershock period). 2860 days of the entire 3295 day study period are represented. Seismicity is within a box 90 km along SAF strike and 15 km wide centered above the Cholame tremor zone. (Top) cumulaive seismicity over the 50 days (top) as a function of minimum magnitude. Dashed lines are typical 10-day duration periods of the tremor episodes. (Bottom) Seismicity as a function of time, along fault position and magnitude (symbol size). White are seismicity between M 1.4 and 2.7. Colored circles are >= M2.8 seismicity with red indicating seismicity >= 4.0. Orange bar is position of 25 km segment where episodic component of tremor occurs. Green is stacked moment rate function of the tremor episodes.

Credit: Guilhem and Nadeau, Episodic tremors and deep slow-slip events in Central California, (in prep).