Southern California Earthquake Center Research Articles

Validation of the SCEC Broadband Platform V14.3 Simulation Methods Using Pseudospectral Acceleration Data

Online Material: Figures showing bias of PSA between data and simulations and between GMPEs and simulations for validation events and scenarios. This article summarizes the evaluation of ground‐motion simulation methods implemented on the Southern California Earthquake Center (SCEC) Broadband Platform (BBP), version 14.3 (as of March 2014). A seven‐member panel, the authorship of this article, was formed to evaluate those methods for the prediction of pseudospectral accelerations (PSAs) of ground motion. The panel’s mandate was to evaluate the methods using tools developed through the validation exercise (Goulet et al. , 2015) and to define validation metrics for the assessment of the methods’ performance. This article summarizes the evaluation process and conclusions from the panel. The five broadband, finite‐source simulation methods on the BBP include two deterministic approaches herein referred to as CSM (Anderson, 2015) and UCSB (Crempien and Archuleta, 2015); a band‐limited stochastic white noise method called EXSIM (Atkinson and Assatourians, 2015); and two hybrid approaches, referred to as G&P (Graves and Pitarka, 2015) and SDSU (Olsen and Takedatsu, 2015), which utilize a deterministic Green’s function approach for periods longer than 1 s and stochastic methods for periods shorter than 1 s. Two acceptance tests were defined to validate the broadband finite‐source ground‐motion simulation methods (Goulet et al. , 2015). Part A compared observed and simulated PSAs for periods from 0.01 to 10 s for 12 moderate‐to‐large earthquakes located in California, Japan, and the eastern United States. Part B compared the median simulated PSAs with published Next Generation Attenuation‐West 1 (NGA‐West 1) (Abrahamson and Silva, 2008; Boore and Atkinson, 2008; Campbell and Bozorgnia, 2008; Chiou and Youngs, 2008) ground‐motion prediction equations (GMPEs) for specific magnitude and distance cases, using a pass–fail criterion based on a defined acceptable range around the spectral shape of the GMPEs. For the initial part A and part …

Implementation and Validation of EXSIM (A Stochastic Finite-Fault Ground-Motion Simulation Algorithm) on the SCEC Broadband Platform

Online Material: Input files for running EXSIM. This article describes the validation exercise associated with preparing EXSIM, a stochastic extended finite‐fault algorithm for ground‐motion simulation, for use as a time‐history simulation module on the Southern California Earthquake Center (SCEC) Broadband Platform (BBP), version 14.3. This involved importing the algorithm to the SCEC computational platform and standardization and calibration of the input parameters. Herein, we (1) provide a brief overview of the EXSIM method of simulating time histories, (2) summarize its input parameters and their assigned values for generic applications in eastern and western North America (ENA and WNA, respectively), and (3) show comparisons of response spectra from EXSIM simulations with those of selected earthquake suites and ground‐motion prediction equations (GMPEs). EXSIM is an open‐source stochastic finite‐source simulation algorithm, written in FORTRAN, that generates time series of ground motion for earthquakes (Motazedian and Atkinson, 2005; Boore, 2009; Assatourians and Atkinson, 2012). A fault plane, having a specified size according to its seismic moment, is divided into an array of subsources, each of which is treated as a point source. Time series from the subsources are modeled using the point‐source stochastic model developed by Boore (1983, 2003) and popularized by the Stochastic‐Method SIMulation (SMSIM) computer code (Boore, 2003, 2005). The ground motion from each subsource is treated as random Gaussian noise of a specified duration, having an underlying spectrum as given by the Brune (1970, 1971) point‐source model for shear radiation. The Brune model specifies the Fourier spectrum at the source by seismic moment and stress parameter and is attenuated in the frequency domain according to an empirical attenuation model. The duration of motion for each subsource comes from the source duration plus the path duration. The time series from the subsources are summed in the time domain, with appropriate …

Refinements to the Graves and Pitarka (2010) Broadband Ground-Motion Simulation Method

This brief article describes refinements to the Graves and Pitarka (2010) broadband ground‐motion simulation methodology (GP2010 hereafter) that have been implemented in version 14.3 of the Southern California Earthquake Center (SCEC) Broadband Platform (BBP). The updated version of our method on the current SCEC BBP is referred to as GP14.3. Our simulation technique is a hybrid approach that combines low‐ and high‐frequency motions computed with different methods into a single broadband response. The separate low‐ and high‐frequency components have traditionally been called deterministic and stochastic, respectively; however, this nomenclature is an oversimplification. In reality, the low‐frequency approach includes many stochastic elements, and likewise, the high‐frequency approach includes many deterministic elements (e.g., Pulido and Kubo, 2004; Hartzell et al. , 2005; Liu et al. , 2006; Frankel, 2009; Graves and Pitarka, 2010; Mai et al. , 2010). Although the traditional terminology will likely remain in use by the broader modeling community, in this paper we will refer to these using the generic terminology low‐ and high‐frequency approaches. Furthermore, one of the primary goals in refining our methodology is to provide a smoother and more consistent transition between the low‐ and high‐frequency calculations, with the ultimate objective being the development of a single unified modeling approach that can be applied over a broad frequency band. GP2010 was validated by modeling recorded strong motions from four California earthquakes. Although the method performed well overall, several issues were identified including the tendency to overpredict the level of longer period (2–5 s) motions and the effects of rupture directivity. The refinements incorporated in GP14.3 are aimed at addressing these issues with application to the simulation of earthquakes in western United States (WUS). These refinements include the addition of a deep weak zone (details in following section) to the rupture characterization and allowing perturbations in …

The Composite Source Model for Broadband Simulations of Strong Ground Motions

My goal is to generate synthetic strong ground motions that are sufficiently realistic to be useful for engineering applications. The composite source model (CSM) uses a kinematic source model for rupture on a finite fault. This source is propagated to the station using a flat‐layered velocity model, scattering, and attenuation that can be measured from independent seismological observations. A key objective is to reproduce the wave propagation within the constraints of the measured velocity and Q structure. This article summarizes the CSM as implemented for the Southern California Earthquake Center (SCEC) Broadband Platform validation exercise (Goulet et al. , 2015). The CSM was first described by Yu (1994) and Zeng et al. (1994). Motivated by Frankel (1991), the model describes the source slip function as a superposition of overlapping circular subevents of random sizes, located randomly on the fault. Multiple source realizations naturally result in different seismograms. Zeng and Anderson (1996) and Zeng and Chen (2001) demonstrated that it is possible to find a specific realization of the CSM that reproduces the low‐frequency accelerograms in Northridge and Chi‐Chi earthquakes and that the slip functions resemble the slip function determined by other methods. Additional applications of the model include Khattri et al. (1994), Yu et al. (1995), Anderson and Yu (1996), and Su, Anderson, Ni, et al. (1998), Su, Anderson, Zeng (1998), and Hartzell et al. (2011). Variations of the original model have been proposed (e.g., Zeng et al. , 1991, 1995). For simplicity, the current implementation is close to the basic version, although some aspects of the Broadband platform implementation have not been described elsewhere. For this reason, this article gives a brief summary of the model on the Broadband Platform. ### Representation Theorem The theoretical basis for understanding ground motion from a finite fault is the representation theorem (e.g., …

The SDSU Broadband Ground-Motion Generation Module BBtoolbox Version 1.5

Research Article| December 17, 2014 The SDSU Broadband Ground‐Motion Generation Module BBtoolbox Version 1.5 Kim Olsen; Kim Olsen Department of Geological Sciences, GMCS 231A, San Diego State University, 5500 Campanile Drive, San Diego, California 92182 U.S.A.kbolsen@mail.sdsu.edu Search for other works by this author on: GSW Google Scholar Rumi Takedatsu Rumi Takedatsu Department of Geological Sciences, GMCS 231A, San Diego State University, 5500 Campanile Drive, San Diego, California 92182 U.S.A.kbolsen@mail.sdsu.edu Search for other works by this author on: GSW Google Scholar Seismological Research Letters (2015) 86 (1): 81–88. https://doi.org/10.1785/0220140102 Article history first online: 14 Jul 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Kim Olsen, Rumi Takedatsu; The SDSU Broadband Ground‐Motion Generation Module BBtoolbox Version 1.5. Seismological Research Letters 2014;; 86 (1): 81–88. doi: https://doi.org/10.1785/0220140102 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySeismological Research Letters Search Advanced Search The Southern California Earthquake Center (SCEC) has completed phase 1 of its Broadband Platform (BBP) ground‐motion simulation results, evaluating the potential applications for engineering of the resulting 0.01–10 s pseudospectral accelerations (PSAs) generated by five different methods. The exercise included part A, in which the methods were evaluated based on the bias of simulation results to observations for 12 well‐recorded historical earthquakes: 7 in the western United States, 2 in Japan, and 3 in the eastern United States/Canada. In addition, part B evaluated simulation results for Mw 5.5, 6.2, and 6.6 scenarios at 20 and 50 km from the... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Metrics for Comparing Dynamic Earthquake Rupture Simulations

Online Material: Explanation of the concept of a dynamic rupture simulation. Earthquakes are complex events that involve a myriad of interactions among multiple geologic features and processes. One of the tools that is available to assist with their study is computer simulation, particularly dynamic rupture simulation. A dynamic rupture simulation is a numerical model of the physical processes that occur during an earthquake. Starting with the fault geometry, friction constitutive law, initial stress conditions, and assumptions about the condition and response of the near‐fault rocks, a dynamic earthquake rupture simulation calculates the evolution of fault slip and stress over time as part of the elastodynamic numerical solution (![Graphic][1] see the simulation description in the electronic supplement to this article). The complexity of the computations in a dynamic rupture simulation make it challenging to verify that the computer code is operating as intended, because there are no exact analytic solutions against which these codes’ results can be directly compared. One approach for checking if dynamic rupture computer codes are working satisfactorily is to compare each code’s results with the results of other dynamic rupture codes running the same earthquake simulation benchmark. To perform such a comparison consistently, it is necessary to have quantitative metrics. In this paper, we present a new method for quantitatively comparing the results of dynamic earthquake rupture computer simulation codes. Over the past decade, the Southern California Earthquake Center–U.S. Geological Survey (SCEC‐USGS) Spontaneous Rupture Code Verification Project has published a series of computational dynamic rupture benchmark exercises (Harris and Archuleta, 2004; Harris et al. , 2009, 2011). The benchmarks, a subset of which of are shown in Figure 1, are listed on the project’s website (http://scecdata.usc.edu/cvws, last accessed November 2014) and cover many earthquake settings, including a range of fault geometries (strike‐slip faults, normal faults, … [1]: /embed/inline-graphic-1.gif

Sensitivity of uplift patterns to dip of the San Andreas fault in the Coachella Valley, California

Three-dimensional mechanical simulations of the San Andreas fault system within the Coachella Valley in southern California produce deformation that matches geologic observations and demonstrate the first-order impact of fault geometry on uplift patterns. To date, most models that include the Coachella Valley segment of the San Andreas fault have assumed a vertical orientation for this fault, but recent studies of seismicity and geodetically observed strain suggest that this segment of the fault may dip 60°–70° to the northeast. We compare models with varied geometry along this segment of the fault and evaluate how well they reproduce observed uplift patterns in the Mecca Hills and Coachella Valley. Incorporating well-constrained fault geometry in regional models will provide a more accurate understanding of active faulting in southern California, which is critical for rupture and hazard modeling that is used to identify regions most susceptible to earthquake damage. We have tested three boundary-element method models for the active geometry of the Coachella Valley segment of the San Andreas fault: one contains a vertical Coachella segment, the second contains a northeast ∼65° dipping Coachella segment, and the final alternative contains a vertical Coachella segment plus a subparallel northeast-dipping fault at depth. This final model honors the geometric interpretation of seismicity from the Southern California Earthquake Center Community Fault Model version 4.0. The models containing vertical Coachella Valley segments both produce uplift between the San Andreas and San Jacinto faults that is more uniformly distributed than geologic observations suggest, and these models fail to produce uplift in the Mecca Hills. The dipping model produces tilting of the Coachella Valley consistent with geologic observations of tilting between the San Jacinto and San Andreas faults. The dipping model also produces relative subsidence southwest of the fault and localized uplift in the Mecca Hills that better match the geologic observations. These results suggest that the active Coachella Valley segment of the San Andreas fault dips 60°–70° to the northeast.

The Establishment of an Operational Earthquake Forecasting System in Italy

On 6 April 2009, an M w 6.2 earthquake struck beneath the city of L’Aquila, central Italy. The shock created significant damage and caused more than 300 deaths in the city and environs. The event followed a seismic sequence that started at the beginning of the year, with its largest shock of M w 4.2 occurring on 30 March. The 6 April earthquake became infamous worldwide because seven experts, who attended a Grandi Rischi Commission meeting on 31 March, were convicted of failing to properly warn the public about the possibility of the mainshock and were sentenced to six years in jail. A second trial is in process. We do not wish to further discuss this important case here (instead see Marzocchi, 2012 and the website http://processoaquila.wordpress.com/; last accessed June 2014); however, it illustrates the importance of providing authoritative scientific information about earthquake probabilities to the public and other users and serves as a catalyst for the scientific developments now underway in Italy. Just after the event, the Italian Civil Protection nominated an International Commission on Earthquake Forecasting (ICEF) chaired by Thomas H. Jordan, director of the Southern California Earthquake Center; the other participants were Yun‐Tai Chen (China), Paolo Gasparini (Italy), Raul Madariaga (France), Ian Main (United Kingdom), Warner Marzocchi (Italy), Gerassimos Papadopoulos (Greece), Gennady Sobolev (Russia), Koshun Yamaoka (Japan), and Jochen Zschau (Germany). The commission had two main duties: (1) to report on the current state of knowledge of short‐term prediction and forecasting of tectonic earthquakes and (2) to indicate guidelines for utilization of possible forerunners of large earthquakes to drive civil protection actions, including the use of probabilistic seismic‐hazard analysis in the wake of a large earthquake. The report of the commission (Jordan et al. , 2011) is available for free downloading (http://www.annalsofgeophysics.eu/index.php/annals/article/view/5350/5371; last accessed June 2014). In addition …

Full‐3‐D tomography for crustal structure in Southern California based on the scattering‐integral and the adjoint‐wavefield methods

AbstractWe have successfully applied full‐3‐D tomography (F3DT) based on a combination of the scattering‐integral method (SI‐F3DT) and the adjoint‐wavefield method (AW‐F3DT) to iteratively improve a 3‐D starting model, the Southern California Earthquake Center (SCEC) Community Velocity Model version 4.0 (CVM‐S4). In F3DT, the sensitivity (Fréchet) kernels are computed using numerical solutions of the 3‐D elastodynamic equation and the nonlinearity of the structural inversion problem is accounted for through an iterative tomographic navigation process. More than half‐a‐million misfit measurements made on about 38,000 earthquake seismograms and 12,000 ambient‐noise correlagrams have been assimilated into our inversion. After 26 F3DT iterations, synthetic seismograms computed using our latest model, CVM‐S4.26, show substantially better fit to observed seismograms at frequencies below 0.2 Hz than those computed using our 3‐D starting model CVM‐S4 and the other SCEC CVM, CVM‐H11.9, which was improved through 16 iterations of AW‐F3DT. CVM‐S4.26 has revealed strong crustal heterogeneities throughout Southern California, some of which are completely missing in CVM‐S4 and CVM‐H11.9 but exist in models obtained from previous crustal‐scale 2‐D active‐source refraction tomography models. At shallow depths, our model shows strong correlation with sedimentary basins and reveals velocity contrasts across major mapped strike‐slip and dip‐slip faults. At middle to lower crustal depths, structural features in our model may provide new insights into regional tectonics. When combined with physics‐based seismic hazard analysis tools, we expect our model to provide more accurate estimates of seismic hazards in Southern California.

Adding Fling Effects to Processed Ground-Motion Time Histories

Fling is the engineering term for the effects of the permanent tectonic offset, caused by a rupturing fault in the recorded ground motions near the fault. It is expressed by a one‐sided pulse in ground velocity and a nonzero final displacement at the end of shaking. Standard processing of earthquake time histories removes some of the fling effects that may be required for engineering applications. A method to parameterize the fling‐step time history and to superimpose it onto traditionally processed time histories has been developed by Abrahamson (2002). In this paper, we first present an update to the Abrahamson (2002) fling‐step models, in which the fling step is parameterized as a single cycle of a sine wave. Parametric models are presented for the sine‐wave amplitude ( D site) and period ( T f ). The expressions for D site and T f are derived from an extensive set of finite‐fault simulations conducted on the Southern California Earthquake Center broadband platform (see Data and Resources). The simulations were run with the Graves and Pitarka (2010) hybrid simulation method and included strike‐slip and reverse scenarios for magnitudes of 6.0–8.2 and dips of 30 through 90. Next, an improved approach for developing design ground motions with fling effects is presented, which deals with the problem of double‐counting intermediate period components that were not removed by the standard ground‐motion processing. Finally, the results are validated against a set of 84 empirical recordings containing fling.

The Prediction Problems of Earthquake System Science

Editor’s note: The following is the text of the SSA Presidential Address presented at the Annual Luncheon of the Seismological Society of America (SSA) Annual Meeting on 30 April 2014. The Seismological Society of America (SSA) has always been dedicated to understanding and reducing the earthquake threat. The Society was founded in 1906 “for the acquisition and diffusion of knowledge concerning earthquakes and allied phenomena.” According to our new strategic plan, approved by the Board in 2012, the core purpose of SSA is to “advance seismology and the understanding of earthquakes for the benefit of society.” This plan lays out the vision for SSA to be “the primary forum for the assembly, exchange, and dissemination of scientific knowledge essential for an earthquake‐aware and safer world.” In the past twenty years or so, the study of earthquakes has become a true system science, offering new pathways for the advancement of seismology. Today I would like to explore what the rise of earthquake system science might imply for the future of our field and for SSA’s mission in earthquake research. System science seeks to explain phenomena that emerge from nature at the system scale, such as global climate change or earthquake activity in California or Alaska. The “system” is not a physical reality, but a hypothetical representation of nature, typically a numerical model that replicates an emergent behavior and predicts its future course. The choice of target behavior determines the system model, as can be illustrated by two representations of earthquake activity in California. One is UCERF3, the latest uniform California earthquake rupture forecast of the Working Group on California Earthquake Probabilities, which represents future earthquake activity in terms of time‐dependent fault‐rupture probabilities. Another is the Southern California Earthquake Center (SCEC)’s CyberShake ground‐motion model, which uses simulations to represent the probability of …

Ground-Motion Simulation and Validation of the 2008 Chino Hills, California, Earthquake Using Different Velocity Models

Abstract We present a set of ground‐motion simulations of the 2008 M w 5.4 Chino Hills earthquake using different seismic‐velocity models available for southern California. The simulations are tailored for a maximum frequency of 4 Hz and a minimum shear‐wave velocity of 200 m/s. We use the community models developed by the Southern California Earthquake Center, CVM‐S and CVM‐H. In the case of CVM‐H, we use two available alternatives, with and without considering the geotechnical layers (GTL). We compare the simulation results obtained with the models against each other and validate the synthetics with recordings obtained at more than 300 stations. The comparisons are used to gain insight into the sensitivity of the results to differences in the models and to make conclusions about the strengths of each model’s description of the crustal structure and the GTL in the region. The validation is done using a goodness‐of‐fit (GOF) criterion. We find the GOF values show a better match at frequencies below 1 Hz, within the basins, and below 0.5 Hz in general. The models, however, do not converge at frequencies lower than 0.5 Hz, and the areas where each model leads to better results do not always coincide. The synthetics start to deviate significantly from observations at frequencies above 1 and 2 Hz, although very good agreement with data can still be found at individual locations, even at the higher frequencies. CVM‐S yields the best results at frequencies up to 0.25 Hz. Our results reveal significant sensitivity of the ground motion to the differences in the structural representation of basins and shallow layers in the models. This indicates the models need to be cross referenced. We close by offering suggestions on the type of improvements that could be adopted in the future.

Comparison of Probabilistic Seismic-Hazard Models Using Averaging-Based Factorization

Abstract We generalize the formulation of probabilistic seismic hazard analysis to accommodate simulation‐based hazard models by expressing the joint probability distribution among the parameters of a kinematically complete earthquake rupture forecast in terms of a conditional hypocenter distribution and a conditional slip distribution. The seismological hierarchy implied by these dependencies allows the logarithmic excitation functional to be exactly and uniquely decomposed into a series of uncorrelated terms that include zero‐mean averages of the site, source, hypocenter, and source‐complexity effects. We use this averaging‐based factorization to compare the CyberShake prototype hazard model developed by the Southern California Earthquake Center, CS11, with the empirical ground‐motion prediction equations (GMPEs) of the 2008 Next Generation Attenuation (NGA08) project. For horizontal‐response spectral accelerations at long periods (2–10 s), the basin and directivity effects of CS11 are substantially larger than those of the NGA08 GMPEs. Directivity–basin coupling and other 3D wave propagation effects not represented in the GMPEs contribute significantly to the excitation patterns in CS11. The total variance of the CS11 excitations is about 60% higher than the NGA root mean square (rms) at the 2 s period but almost 30% lower at 10 s. Relative to the NGA rms, the residual variance in CS11 at 2 s is larger than the aleatory variability in the NGA08 database by a factor of nearly 1.6. Recent CyberShake experiments with alternative source and structural models suggest that the high CS11 variances are due to an overestimation of the basin and directivity effects at short periods. The CyberShake site and path effects unexplained by the NGA08 models account for 40%–50% of total residual variance, suggesting that improvements to the simulation‐based hazard models could reduce the aleatory variability intrinsic to the current GMPEs by as much as 25%.

Southern San Andreas Fault Evaluation Field Activity: Approaches to Measuring Small Geomorphic Offsets--Challenges and Recommendations for Active Fault Studies

In southern California, where fast slip rates and sparse vegetation contribute to crisp expression of faults and microtopography, field and high‐resolution topographic data (<1 m/pixel) increasingly are used to investigate the mark left by large earthquakes on the landscape (e.g., Zielke et al. , 2010; Zielke et al. , 2012; Salisbury, Rockwell, et al. , 2012, Madden et al. , 2013). These studies measure offset streams or other geomorphic features along a stretch of a fault, analyze the offset values for concentrations or trends along strike, and infer that the common magnitudes reflect successive surface‐rupturing earthquakes along that fault section. Wallace (1968) introduced the use of such offsets, and the challenges in interpreting their “unique complex history” with offsets on the Carrizo section of the San Andreas fault; these were more fully mapped by Sieh (1978) and followed by similar field studies along other faults (e.g., Lindvall et al. , 1989; McGill and Sieh, 1991). Results from such compilations spurred the development of classic fault behavior models, notably the characteristic earthquake and slip‐patch models, and thus constitute an important component of the long‐standing contrast between magnitude–frequency models (Schwartz and Coppersmith, 1984; Sieh, 1996; Hecker et al. , 2013). The proliferation of offset datasets has led earthquake geologists to examine the methods and approaches for measuring these offsets, uncertainties associated with measurement of such features, and quality ranking schemes (Arrowsmith and Rockwell, 2012; Salisbury, Arrowsmith, et al. , 2012; Gold et al. , 2013; Madden et al. , 2013). In light of this, the Southern San Andreas Fault Evaluation (SoSAFE) project at the Southern California Earthquake Center (SCEC) organized a combined field activity and workshop (the “Fieldshop”) to measure offsets, compare techniques, and explore differences in interpretation. A thorough analysis of the measurements from the field activity will be provided separately; this …

How much can off-fault deformation contribute to the slip rate discrepancy within the eastern California shear zone?

>Kinematic assumptions of geodetic inversions for fault slip require that the slip sums to the (plate) boundary velocity. This assumption neglects permanent off-fault deformation, which could account for discrepancies between geologic and geodetic estimates. We use three-dimensional mechanical models to assess if unaccounted permanent strain surrounding faults could contribute to slip rate discrepancies across disconnected faults within the Mojave Desert (California, USA) portion of the eastern California shear zone (ECSZ). We modified fault configurations derived from the Southern California Earthquake Center Community Fault Model to better represent the disconnected nature of active faults in the ECSZ south of the Garlock fault. The models with revised fault geometry produce slip rates that better match geologic strike-slip rates, thus validating the revisions. Within these models, off-fault deformation accounts for 40% ± 23% of the total strain across the ECSZ. This suggests that a significant portion of the discrepancy between the geologic and geodetically modeled slip rates in the ECSZ could be due to the geodetic inversion model assumption of zero permanent off-fault deformation.

Change of apparent segmentation of the San Andreas fault around Parkfield from space geodetic observations across multiple periods

Sequences of earthquakes are commonly represented as a succession of periods of interseismic stress accumulation followed by coseismic and postseismic phases of stress release. Because the recurrence time of large earthquakes is often greater than the available span of space geodetic data, it has been challenging to monitor the evolution of interseismic loading in its entire duration. Here we analyze large data sets of surface deformation at different key episodes around the Cholame, Parkfield and creeping segments of the San Andreas Fault that show evidence of significant deceleration of fault slip during the interseismic period. We compare the average fault slip rates before and after the 2004 Mw6 Parkfield earthquake, in the 1986–2004 and 2006–2012 periods, respectively, avoiding 2 years of postseismic deformation after 2004. Using a combination of GPS data from the Plate Boundary Observatory, the Southern California Earthquake Center Crustal Motion Map and the Bay Area Velocity Unification networks and interferometric synthetic aperture radar from the Advanced Land Observing Satellite (ALOS) and Envisat satellites, we show that the area of coupling at the transition between the Parkfield and Cholame segments appears larger later in the interseismic period than it does earlier on. While strong plate coupling is uniform across the Parkfield and Cholame segments in the 1986–2004 period, creep occurs south of the 2004 epicenter after 2006, making segmentation of the San Andreas Fault south of Parkfield more clearly apparent. These observations indicate that analyses of surface deformation late in the earthquake cycle may overestimate the area of plate coupling. A fault surface creeping much below plate rate may in some case be a region that does not promote earthquake nucleation but rather just be at a slower stage of its evolution. Our analysis also shows signs of large variation of slip velocity above and below plate rate in the creeping segment indicating that cycles of weakening and hardening can also be at play in dominantly aseismic areas.

Combining GPS and Remotely Sensed Data to Characterize Time‐Varying Crustal Motion

Development of more physically realistic models of interseismic crustal strain and stressing rates, postseismic effects over multiple time scales, transient deformation, and lithospheric rheology are examples of research applications requiring measurements of Earth surface motion at high spatio­temporal resolution. To address this need, the Southern California Earthquake Center (SCEC) has initiated a 5‐year project to create a Southern California community geodetic model (CGM) that will leverage the complementary strengths of temporally dense GPS and spatially dense interferometric synthetic aperture radar (InSAR) data.

Testing Kalman Smoothing/PCA Transient Signal Detection Using Synthetic Data

The Southern California Earthquake Center (SCEC), a community of researchers from over 60 institutions worldwide, has made efforts to understand earthquakes and mitigate earthquake risk in southern California and elsewhere. The broad objective of SCEC’s tectonic geodesy disciplinary activities is to make available a variety of geodetic data collected in southern California and use these data for addressing problems associated with deformation processes. One of the topics of interest for fulfilling this objective is to “develop a geodetic network‐processing system that will detect anomalous strain transients” (Murray‐Moraleda and Lohman, 2010). Our detection method consists of smoothing based on Kalman filtering and principal component analysis (PCA) (Ji and Herring, 2013). Smoothing improves the signal‐to‐noise ratio (SNR) in the time domain by reducing the white noise with adjustments for steady motions and temporally correlated noise, whereas PCA improves the SNR in the space domain by accounting for the spatial coherence of transient signals. This method is based purely on data without any source‐specific model, in contrast to the Network Inversion Filter (Segall and Matthews, 1997; McGuire and Segall, 2003) that requires definition of fault geometry and typically assumes elastic deformation. Inaccurate fault geometry or inelastic deformation may lead to unreliable fault‐slip estimates and limit the detection capability of the filter. The Network Strain Filter (NSF) proposed by Ohtani et al. (2010) is similar to our method in that it avoids source‐specific models and uses Kalman filtering. However, for spatially coherent transient deformation the NSF uses predefined wavelets that are included in the Kalman filter, whereas our method uses data‐based eigenvectors from PCA after Kalman filtering. SCEC has supported a project, the transient‐detection exercise, in which participants applied their detection methods to synthetic Global Positioning System (GPS) daily‐position time series and reported back on any transient signals they detected. Because the data …

Realistic Simulations of Geodetic Network Data: The Fakenet Package

Online Material: Program manual, source code, and associated files. The Southern California Earthquake Center (SCEC) Transient Detection intercomparison required that all participants apply their methods to the same datasets, each dataset containing some tectonic signal (or none), plus realistic noise. One way to produce these would be to add tectonic signals to actual data from the California continuous GPS network (CCGPSN), but there are two problems with this: using the same background series for all datasets makes it easy to isolate signals by differencing, and we would not know if there was a transient already present in the data. Instead, fully synthetic datasets were used, each of which had a different realization of the noise added to a different signal known only to the creator of the dataset. The Fakenet package was written to provide software for creating these datasets. In developing Fakenet, the goals were to be able to: This paper describes the procedures used in this package, as well as outlining its structure. A user’s manual is available in ![Graphic][1] the electronic supplement to this paper. It describes the details of operation, installation procedures, and the file formats. The supplement also includes the software itself, … [1]: /embed/inline-graphic-1.gif

Statistical Approaches to Detecting Transient Signals in GPS: Results from the 2009-2011 Transient Detection Exercise

We present the results of our participation in four phases of the Southern California Earthquake Center (SCEC) transient detection exercise (Lohman and Murray, 2013). In each phase, a blind test was conducted in which sets of synthetic Global Positioning Systems (GPS) data were released and a deadline set for submission of detection results. For each data set, the presence or absence of transient events was to be determined, and the location and time of each specified. After all submissions were received, the ground‐truth information about any transient signals in the data was released. The synthetic data sets were generated by FAKENET, a software package that produces realistic GPS time series that include secular motion and seasonal signals as well as realistic noise and distributions of missing data (Agnew, 2013). Station locations in the synthetic data set were a subset of GPS installations in the western United States. In this work, we pursue a purely data‐driven approach to transient detection, rather than one based on an assumption of an underlying physical model. We view this approach as having two important advantages. First, it facilitates the detection of events, which might happen through a previously unknown, but geophysically interesting, physical process or on a previously unknown fault or structure. Second, it enables the detection of events, which have nothing to do with the solid earth but, although not the subject of the SCEC exercise, have scientific or practical merit. These include signals such as those due to atmospheric phenomena as well as signals, which result from hardware faults or failures and software processing glitches. The four phases of the exercise were conducted over approximately two years. As a result, our approach evolved as we learned from experiences in the initial phases as well as from other ongoing work during that period. The …