Research Objectives

The objectives of the Seismology group are to gather data on the range of seismic phenomena observed in southern California and to integrate these data into models of fault slip. Of particular interest are proposals that foster innovations in network deployments, data collection, real-time research tools, and data processing. Proposals that provide community products that support one or more of the SCEC4 goals or those that include collaboration with network operators in Southern California are especially encouraged. Proposers should consider the SCEC resources available including the Southern California Earthquake Data Center (SCEDC) that provides extensive data on Southern California earthquakes as well as crustal and fault structure, the network of SCEC funded borehole instruments that record high quality reference ground motions, and the pool of portable instruments that is operated in support of targeted deployments or aftershock response.

Research Strategies

• Enhancement and continued operation of the SCEDC and other existing SCEC facilities particularly the near-real-time availability of earthquake data from SCEDC and automated access.
• Real-time processing of network data such as improving the estimation of source parameters in relation to faults, especially evaluation of the short-term evolution of earthquake sequences and real-time stress perturbations on major fault segments.
• Enhance or add new capabilities to existing earthquake early warning (EEW) systems or develop new EEW algorithms. Develop real-time finite source models constrained by seismic and GPS data to estimate evolution of rupture and potentially damaging ground shaking; develop strategies for robust uncertainty quantification in finite-fault rupture models.
• Advance innovative and practical strategies for densification of seismic instrumentation, including borehole instrumentation, in Southern California and develop innovative algorithms to utilize data from these networks. Develop metadata, archival and distribution models for these semi-mobile networks.
• Develop innovative methods to search for unusual signals using combined seismic, GPS, and borehole strainmeter data; collaborations with EarthScope or other network operators are encouraged.
• Investigate near-fault crustal properties, evaluate fault structural complexity, and develop constraints on crustal structure and state of stress.
• Collaborations, for instance with ANSS that would augment existing and planned network stations with downhole and surface instrumentation to assess site response, nonlinear effects, and the ground coupling of built structures.
• Preliminary design and data collection to seed future passive and active experiments such as dense array measurements of basin structure and large earthquake properties, OBS deployments, and deep basement borehole studies.
• Improve locations of important historical earthquakes.

Research Priorities

• Tremor. Tremor has been observed on several faults in California, yet it does not appear to be ubiquitous. We seek proposals that explore the distribution and source characteristics of tremor in California and those that explore the conditions necessary for the generation of seismically observable tremor.
• Low-cost seismic network data utilization and archiving. Several groups are developing seismic networks that use low-cost MEMS accelerometers. We seek proposals that would address development of seismological algorithms to utilize data from these networks in innovative ways. We also seek proposals that would develop metadata and archiving models for these new semi-mobile networks, as well as archive and serve these data to the SCEC user community.
• Short-Term Earthquake Predictability. We seek proposals that develop new methods in earthquake statistics or analyze seismicity catalogs to develop methods for determining short-term (hours to days) earthquake probability gain.
• Seismicity studies in the two SFSA; Ventura and San Gorgonio. We seek proposals that use earthquake data to map the structure and seismotectonics of these regions as part of the SFSA community effort.

Recent Results

The Seismology Group gathers data on the range of seismic phenomena observed in southern California and integrates these data into seismotectonic interpretations as well as physics-based models of fault slip. Resources include the Southern California Earthquake Data Center (SCEDC) that provides extensive data on Southern California earthquakes as well as crustal and fault structure, the network of SCEC funded borehole instruments that record high quality reference ground motions, and the pool of portable instruments that is operated in support of targeted deployments or aftershock response.

Figure 1. About 3 weeks of tremor activity along the San Andreas Fault as detected by the array analyses. Note the streaking nature of tremor propagation. Azimuth is with respect to the array center.

Figure 2. An example of tremor waveform detection along the San Jacinto Fault using a LFE template (red). In the left panel, continuous data is shown in black. Time is set in reference to the origin time of the 2002 Denali Fault earthquake. On the right panel, a zoom-in plot of the template (red) and detected event (blue).

Figure 3. From Jolivet et al. (2015). Seismic and aseismic asperities along the central San Andreas Fault. Color represents the mode of the a posteriori PDF of slip in the along-strike direction. Semi-transparent areas marked with red dashed lines correspond to asperities where significant earthquakes are known to have occurred, including the 1857 M7.9 Fort Tejon, 1906 M7.9 San Francisco and 1966 and 2004 M6.0 Parkfield earthquakes. white transparent areas with question marks are zones that are inferred to be coupled and the potential source for future earthquakes.

Figure 4. (Left) The cross-section view of the SJF around the hypocenters of moderate earthquakes. Dots denote aftershocks, which are color coded by their origin times and scaled by their magnitudes. The horizontal bars denote the β-values (black β<2; red β>2). The vertical grey lines denote the defined aftershock zone. (Right) The histogram of depth distribution of aftershocks. The horizontal red line denotes the depth of the mainshock.

Tremor Detection Analysis

Deep fault slip can manifest in the form of tremor and has been observed on several faults in California, yet is far from ubiquitous. Several studies explore tremor along the select set of faults in California where it occurs, notably the San Andreas fault, San Jacinto fault. Tremor along the Parkfield segment of the San Andreas was the first to be identified outside of a subduction zone, but the factors that control tremor activity are still not well understood. Ghosh has been operating a temporary seismic array in the region and finds that tremor occurs almost daily. They use backprojection to locate the tremor and determine that it occurs in distinct patches along the fault (Figure 1). Additionally, they find that tremor rates increase dramatically in the hours following the South Napa earthquake. Peng and Yang conducted a systematic search for tremor in California, focusing on a region below the San Gabriel Mountains and along the San Jacinto fault. They find no evidence for tremor beneath the San Gabriel Mountains despite near lithostatic pore pressures (Yang and Peng, 2013). And, in an extensive search for tremor along the San Jacinto fault that utilized matched filter techniques they only find one instance of clear tremor; this tremor was previously reported and occurs during passing surface waves of the 2002 Denali earthquake (Figure 2). These results confirm that a unique set of conditions are needed for tremor to occur.

Fault Coupling, Slip Behavior, and Source Properties

Both seismic and aseismic slip distribution and source properties can vary significantly along strike and with depth with changes in fault coupling, frictional properties, pore-fluid pressures, and/or fault structure. Ampuero conducted the first probabilistic estimate of of fault coupling along the Parkfield-Cholame section of the San Andreas fault. Fault coupling is estimated from high-resolution SAR- and GPS-derived observations of surface displacements. The results show that locked asperities are consistent with the inferred locations of M > 6 earthquakes, including patches possibly associated with two foreshocks of the 1857 M7.9 Fort Tejon earthquake (Figure 3). A study by Peng examined the variation in aftershock distributions for a set of 10 M>4 mainshocks along the San Jacinto fault (SJF) near Anza. They find that all aftershock distributions are extended in the along-strike direction. Additionally, deeper mainshocks have abnormally long aftershock zones suggesting that they are modulated by changes in fault frictional properties as depth increases (Figure 4). Further, Peng postulates that the deep aftershocks zones may be driven by deep creep along the SJF (Meng and Peng, 2015). McGuire and Ben-Zion explore rupture velocity and directivity for M>3 earthquakes along the SJF to determine how fault structure and damage zones can affect these source properties and use second moment estimates and measurements of peak ground motions to estimate the directivity. They observe a clear correlation between Peak Spectral Accelerations (PSAs) near the corner frequency and the expected directivity from second moment estimates for the 2013 M5.1 earthquake on the SJF (Figure 5).

Estimating Stress from Anisotropy

Anisotropy can be used to estimate crustal stress and mantle flow and provide a better understanding of tectonic forcing that drives deformation. Miller and Becker are collecting disparate anisotropy datasets to develop a 3D model of anisotropy for southern California. They conduct a number of comparisons between different inferences of crustal stress and strain-rates. For example, they compare coseismic stress estimates using a focal mechanism inversion and compare to Kostrov summed strain-rates. They find that throughout much of southern California these two estimates are closely aligned (Figure 6); however, they do find some deviation in the estimates near the Transverse Ranges and near the southern segment of the San Andreas. The origin of the differences is being investigated but may be caused by heterogeneous rock rheology or time-dependent alignment of stress and strain through the seismic cycle.

Induced Seismicity

The identification of induced seismicity and its impact on seismic hazard are of growing interest to scientists and the public alike. Chen and McGuire examine how earthquake source properties vary near geothermal operations in the Salton Sea region. They find that stress drops correlate with distance from geothermal wells, such that stress drops are lowest within 300 m of injection wells. Additionally, they also find low stress drops on a nearby fault that hosted a series of earthquake swarms in 2005, 2009, and 2010. Their results show that geothermal operations can locally change the source properties of earthquakes and provide new insights into the interaction between faults and fluids in a geothermal field. In 2014, a flurry of moderate earthquakes in the Los Angeles region raised concern as to whether some of the seismicity was of anthropogenic origin rather than tectonic origin. Hauksson et al. (2015) searched for evidence of induced earthquakes associated with oilfield operations in the seismically active Los Angeles basin (LA basin) (Figure 7). Such anthropogenic earthquakes can be caused by changes in loading on the adjacent crust as well as inflation or collapse of an oilfield reservoir when large volumes of fluids are injected or extracted. Overall, they found no obvious previously unidentified induced earthquakes, and that the management of balanced production and injection of fluids appears to reduce the risk of induced earthquake activity in the oilfields. To quantify the relationship between oil field activities and potential induced seismicity, Goebel et al. (2015) developed a novel method to identify likely induced seismicity in tectonically active regions based on short-range spatio-temporal correlations between changes in fluid injection and seismicity rates. They applied this method to Kern County, central California, and found that most earthquakes within the region are tectonic in origin, except for four different possible cases of induced seismicity.

Figure 5. Apparent source time functions (ASTF) resulting from Empirical Green's Function deconvolutions at stations in the SJF array as a result of the March 2013 M5.1 earthquake (red triangle). Each ASTF is plotted at the location of the station denoted by the circle. The color scale of the circles denotes the characteristic duration of that moment-rate function, τc(s), in seconds. The earthquake lasted about 0.3 seconds but appears longer to the SE and shorter to the NW.	Figure 6. (Left) Comparison of coseismic “stress” from Michael (1984) type inversion (green, from Yang and Hauksson, 2013) and Kostrov summed strain-rates (yellow) based on the same focal mechanisms (Yang et al., 2012) (compressive axes show). (Right) Angular difference between the two axes, with sign determined as indicated in the legend, along with histogram (y axis showing frequency percent) over all sampled regions (sub plot), with legend stating the mean ± standard deviation of angular difference.	Figure 7. Relocated seismicity 1981-2014/06 recorded by SCSN and oilfields shown as irregular light blue areas (DOGGR web site). Symbol sizes are scaled with the earthquake magnitude with Mw≥5 shown as octagons (see scale in upper right corner), and color-coded by date. LB – Long Beach oilfield; MB – Montebello oilfield; MDR – Marina Del Rey; N-I-Fault: Newport-Inglewood Fault; WC – West-Coyote;

Select Publications

• Goebel, T., H. W., E. Hauksson, F. Aminzadeh, J.-P. Ampuero (2015), An objective method for the assessment of waste water injection induced seismicity and application to tectonically active regions in central California, J. Geophys. Res., (in press). SCEC Contribution 6112
• Hauksson, E., T. Goebel, and J.-P. Ampuero, Cochran, E. (2015), A century of oilfield operations and earthquakes in the greater Los Angeles basin, southern California, The Leading Edge (SEG), 650-656, doi: 10.1190/tle34060650.1. SCEC Contribution 2072
• Jolivet, R., Simons, M., Agram, P. S., Duputel Z., Shen, Z.-K. (2015), Aseismic slip and seismogenic coupling along the central San Andreas Fault, Geophysical Research Letters, v. 42, pp. 1-10. SCEC Contribution 2084
• Meng, X. and Z. Peng (2015), Improved understanding of moderate-size earthquake sequences on the San Jacinto Fault and their relationship with deep creep, Seismol. Res. Lett., 86(2B), 659. SCEC Contribution 6035
• Yang, H., and Z. Peng (2013), Lack of additional triggered tectonic tremor around the Simi Valley and the San Gabriel Mountain in southern California, Bull. Seismol. Soc. Am., 103(6), 3372-3378, doi: 10.1785/0120130117. SCEC Contribution 1813