SCEC Award Number 14113 View PDF
Proposal Category Collaborative Proposal (Integration and Theory)
Proposal Title Estimating Fault Zone Properties From Seismic Fault Zone Trapped Noise
Investigator(s)
Name Organization
Gregor Hillers Université Joseph Fourier, Grenoble (France) Yehuda Ben-Zion University of Southern California
Other Participants
SCEC Priorities 1a, 4b, 6a SCEC Groups Seismology, FARM, USR
Report Due Date 03/15/2015 Date Report Submitted N/A
Project Abstract
 We discovered a new type of seismological signature, fault zone trapped noise (Hillers et al. 2014), that extends the range of previously known ballistic fault zone waves. Analyzing data from the Calico field experiment we studied the interaction of the ambient seismic wavefield with a fault zone environment; we showed that the high-frequency wavefield in the fault differs from the field in the adjacent crust. Noise within the fault is characterized by significantly increased isotropy, which indicates that randomization of propagation directions is more efficient, and energy can be considered as trapped. We showed that conditions controlling the emergence of seismic fault zone trapped noise have less limitations compared to trapped ballistic waves. We proposed to investigate the potential of the correlation field associated with trapped noise to provide a high resolution tool for imaging the internal structure (dimensions, seismic velocity, attenuation, continuity) of fault damage zones, in analogy to properties of fault zone trapped waves (e.g., Li et al., 1990, 1994; Ben-Zion et al., 2003; Lewis et al., 2005). We construct noise correlation functions from the scattered seismic wavefield recorded by a spatially dense array with >1100 vertical-component nodes centered on the Clark segment of the San Jacinto Fault Zone southeast of Anza (BenZion et al., 2015). Based on estimates of the similarity of causal and acausal waveforms we assess the noise correlation symmetry, which is an indicator of noise wavefield isotropy. Spatial distributions of similarity and hence isotropy markers can be indicative of trapping and waveguiding structures.
Intellectual Merit Fault zone trapped noise has only recently been discovered and described. Additional analyses targeting manifestations of this signature help to assess the generality of its occurrence, and its applicability for fault zone imaging.
Broader Impacts The study constitutes a novel approach to analyze noise correlation functions constructed from data collected by a very dense array, a configuration that many regard as a blueprint for future seismological investigations. Our symmetry analysis targets systematic wavefield variations occurring in fault zone environments. Results can yield images of mechanically important waveguide units associated with low velocity fault damage zones.
Exemplary Figure Figure 4. Frequency dependence (top to bottom) of the cross-correlation waveform similarity estimator based on coherent energy in the spectral domain. The x-axis corresponds to the fault-normal position of the reference station. The y-axis is the absolute distance between a correlation station pair. The z-axis is the magnitude of the estimator. At high frequencies (bottom row) low far field values indicate a propagation barrier around fault normal distance 0.2 and 0.4 km.