SCEC Award Number 12150 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Spatio-temporal Patterns of Tectonic Tremor Activity
Investigator(s)
Name Organization
Jean-Paul Ampuero California Institute of Technology
Other Participants One graduate student
SCEC Priorities 1b, 5d, 5c SCEC Groups FARM, Seismology
Report Due Date 03/15/2013 Date Report Submitted N/A
Project Abstract
 We studied tectonic tremors from two perspectives: observational and theoretical. We conducted a search for spontaneous tremors near the Anza gap of the San Jacinto fault. Analysis of waveform data from the Anza and PBO seismic networks was complemented with field experiments based on small-aperture seismic arrays. We detected several periods of transients containing repeating waveforms with spectra peaked around 4 Hz. We determined that these transients are associated with cultural activity, most likely train traffic along the Coachella Valley. Our observations place bounds on the capability to detect spontaneous tremors in southern California with current instrumentation and techniques, and warrant the development of tremor discriminators based on spectra. We investigated the mechanical relations between spatio-temporal patterns of tectonic tremor activity and fault heterogeneities by developing a rate-and-state model of the collective behavior of fault asperities mediated by transient creep. We conducted integrated simulations of slow slip and tremor swarms that reproduce the observed diversity of tremor migration patterns. Our model provides a mechanical connection between speed and orientation of tremor swarms and the distribution of slip velocity within the underlying slow slip pulse. We identified tremor properties that are robust to uncertainties in the characteristics of fault heterogeneities. We developed relations between slow slip rate, friction parameters and the response of tremors to oscillatory loadings, which provide a framework to interpret the variable sensitivity of tremors to tides and surface waves. Our modeling results highlight the potential role of tremors as a monitor for aseismic transients.
Intellectual Merit This research addresses a priority objective of FARM to understand the implications of slow slip events and non-volcanic tremors on the constitutive properties of faults and overall seismic behavior, and that of CDM to investigate possible causes and effects of transient slip. Slow slip and tremors, one of the most intriguing discoveries of the last decade in Earth sciences, has been the target of many observational studies but modeling studies are still needed. Spontaneous tremor has not been documented yet in southern California. The data analysis developed here will guide future efforts to detect tremor activity in southern California. The model developed here provides a framework to unify a diversity of tremor observations and to relate them to the mechanical properties of faults and their heterogeneity near brittle-ductile transition zones. Our models contribute to the physical basis of using tremors as a monitor for aseismic transients. They also contribute to the design and interpretation of laboratory observations (Lengline et al, 2011a, 2011b, 2012; Yamaguchi et al, 2012).
Our research directly addressed the following SCEC4 research priorities and requirements.
• 1b: We conducted a “focused numerical study of the character of the lower crust, its rheology, stress state”. Our overarching goal is to extract information about the rheology and heterogeneity of the brittle-ductile transition region of active faults from the study of tremor activity patterns.
• 5d: Our research includes a “development of physics-based models of slow slip and tectonic tremor” constrained by observed features of tremor occurrence.
• 5c: Our work includes “collaboration with rock mechanics laboratories on laboratory experiments to understand the mechanisms of slow slip and tremor”. Our focus is currently on acoustic emission data from analog experiments (not rocks) that capture the main ingredients of our proposed models (Lengline et al, 2011a, 2011b, 2012).
Broader Impacts The project provided training and research opportunities for three graduate students in Caltech. The observational activities involved other SCEC researchers (Jamie Steidl, UCSB) and SCEC PBIC instrumentation. Open source software for quasi-dynamic earthquake simulation was produced and was made available online (http://code.google.com/p/qdyn/). A workshop on its usage was provided to Caltech graduate students and to researchers in a European university (Utrecht). This research has implications on earthquake nucleation and predictability.
Exemplary Figure Figure 4
A 3D integrated simulation of slow slip and tremor with the software QDYN. Left: spatial distribution of the logarithm of slip velocity, normalized by the plate rate, at 5 different times during a slow slip event. The simulation includes tremors triggered on small velocity-weakening asperities by slow slip, and interacting with each other via local creep transients. Right: tremor activity indicated by peaks of the maximum slip velocity as a function of time (Luo and Ampuero, 2012c).
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