Data‐driven two‐fault modelling of the Mw 6.0 2008 Wells, Nevada earthquake suggests a listric fault rupture

Structural fault complexity at depth affects seismic hazard, earthquake physics and regional tectonic behavior, but constraining such complexity is challenging. We present earthquake source models of the 21 February 2008, Mw 6.0 Wells event that occurred in the Basin and Range in the western USA suggesting the rupture of both the shallow and deep parts of a listric fault. We use a large data set including 150 local seismic waveforms from the USArray combined with high‐quality InSAR and teleseismic waveforms. Rather than imposing an a priori fault geometry in the source inversions, as is often done in the literature, we use a data‐driven approach whereby all the faulting parameters and number of faults are determined by the data alone. We find a two‐fault normal faulting solution comprising: (i) a shallow (centroid depth ∼4.6 km) sub‐event with Mw 5.3 and fault dip of ∼ 77°; and (ii) a deeper (centroid depth ∼ 8.8 km), larger Mw 6.0 sub‐event on a fault with shallower dip angle ( ∼ 41°). Our preferred two‐fault model is consistent with aftershocks and with the tectonics of the region. The local USArray waveforms used in the modeling are key to detect the rupture of both shallow and deep parts of the possible listric fault. The lack of such dense and uniform coverage of earthquakes in other regions on Earth may explain why the full seismic rupture of listric faults may have gone undetected in the past. Thus, earthquake slip on whole listric faults may be more common than previously thought

Limitations of rupture forecasting exposed by instantaneously triggered earthquake doublet

Earthquake hazard assessments and rupture forecasts are based on the potential length of seismic rupture and whether or not slip is arrested at fault segment boundaries. Such forecasts do not generally consider that one earthquake can trigger a second large event, near-instantaneously, at distances greater than a few kilometers. Here we present a geodetic and seismological analysis of a magnitude 7.1 intra-continental earthquake that occurred in Pakistan in 1997. We find that the earthquake, rather than a single event as hitherto assumed, was in fact an earthquake doublet: initial rupture on a shallow, blind 2 reverse fault was followed just 19 seconds later by a second rupture on a separate reverse fault 50 km away. Slip on the second fault increased the total seismic moment by half, and doubled both the combined event duration and the area of maximum ground shaking. We infer that static Coulomb stresses at the initiation location of the second earthquake were probably reduced as a result of the first. Instead, we suggest that a dynamic triggering mechanism is likely, although the responsible seismic wave phase is unclear. Our results expose a flaw in earthquake rupture forecasts that disregard cascading, multiple-fault ruptures of this type

Testing the inference of creep on the northern Rodgers Creek fault, California, using ascending and descending persistent scatterer InSAR data

We revisit the question of whether the Rodgers Creek fault in northern California is creeping, a question with implications for seismic hazard. Using imagery acquired by Envisat between 2003 and 2010, we process two persistent scatterer interferometric synthetic aperture radar (InSAR) data sets, one from an ascending track and the other from a descending track, covering the northernmost segment of the Rodgers Creek fault between the cities of Santa Rosa and Healdsburg. The two different viewing geometries provided by the two different tracks allow us to distinguish vertical velocities, which may reflect nontectonic deformation processes, from fault-parallel velocities, which can be used to identify creep. By measuring offsets in InSAR line-of-sight velocity from 12 fault-perpendicular profiles through both data sets, we identify seven locations where we have a high degree of confidence that creep is occurring (estimated creep rate is more than two standard deviations above zero). The preferred creep rates at these locations are in the range 1.9–6.7 mm/yr, consistent within uncertainty with alignment array measurements. Creep is probable (P≥0.70) at another three locations, defining a creeping zone ∼20 km long in total, extending northwest from Santa Rosa. We also estimate the map patterns of fault-parallel and vertical velocities in the region covered by both data sets; these suggest that the Rodgers Creek fault immediately southeast of Santa Rosa remains locked

Continuation of survey GPS measurements and installation of continuous GPS sites at The Geysers, California, for geothermal deformation monitoring

We present updated time-series and velocities based on survey (episodic) Global Positioning System (GPS) measurements across the geothermal field at The Geysers, California. These surveys show that a slowing of the rapid rate of subsidence and contraction seen across The Geysers has occurred since the start of the Southeast Geysers Effluent Pipeline (SEGEP) in October 1997 and the Santa Rosa Geysers Recharge Pipeline (SRGRP) in September 2003, most significantly in areas close to high volume injector wells, although subsidence is still occurring. We also introduce the first continuous GPS sites in The Geysers, which were installed in December 2012 and January 2013. These continuous sites are intended to capture and monitor transient deformation associated with Calpine's Enhanced Geothermal Systems demonstration projects

Continuation of survey GPS measurements and installation of continuous GPS sites at The Geysers, California, for geothermal deformation monitoring

We present updated time-series and velocities based on survey (episodic) Global Positioning System (GPS) measurements across the geothermal field at The Geysers, California. These surveys show that a slowing of the rapid rate of subsidence and contraction seen across The Geysers has occurred since the start of the Southeast Geysers Effluent Pipeline (SEGEP) in October 1997 and the Santa Rosa Geysers Recharge Pipeline (SRGRP) in September 2003, most significantly in areas close to high volume injector wells, although subsidence is still occurring. We also introduce the first continuous GPS sites in The Geysers, which were installed in December 2012 and January 2013. These continuous sites are intended to capture and monitor transient deformation associated with Calpine's Enhanced Geothermal Systems demonstration projects

Joint earthquake source inversions using seismo-geodesy and 3-D earth models

A joint earthquake source inversion technique is presented that uses InSAR and long-period teleseismic data, and, for the first time, takes 3-D Earth structure into account when modelling seismic surface and body waves. Ten average source parameters (Moment, latitude, longitude, depth, strike, dip, rake, length, width and slip) are estimated; hence, the technique is potentially useful for rapid source inversions of moderate magnitude earthquakes using multiple data sets. Unwrapped interferograms and long-period seismic data are jointly inverted for the location, fault geometry and seismic moment, using a hybrid downhill Powell-Monte Carlo algorithm. While the InSAR data are modelled assuming a rectangular dislocation in a homogeneous halfspace, seismic data are modelled using the spectral element method for a 3-D earth model. The effect of noise and lateral heterogeneity on the inversions is investigated by carrying out realistic synthetic tests for various earthquakes with different faulting mechanisms and magnitude (Mw 6.0-6.6). Synthetic tests highlight the improvement in the constraint of fault geometry (strike, dip and rake) and moment when InSAR and seismic data are combined. Tests comparing the effect of using a 1-D or 3-D earth model show that long-period surface waves are more sensitive than long-period body waves to the change in earth model. Incorrect source parameters, particularly incorrect fault dip angles, can compensate for systematic errors in the assumed Earth structure, leading to an acceptable data fit despite large discrepancies in source parameters. Three real earthquakes are also investigated: Eureka Valley, California (1993 May 17, Mw 6.0), Aiquile, Bolivia (1998 February 22, Mw 6.6) and Zarand, Iran (2005 May 22, Mw 6.5). These events are located in different tectonic environments and show large discrepancies between InSAR and seismically determined source models. Despite the 40-50 km discrepancies in location between previous geodetic and seismic estimates for the Eureka Valley and Aiquile earthquakes, the seismic data are found to be compatible with the InSAR location. A 30° difference in strike between InSAR and seismic-derived source models is also resolved when taking 3-D Earth structure into account in the analysis of the Eureka Valley earthquake. The combination of both InSAR and seismic data further constrains the dip for the Zarand earthquake, and in all cases the seismic moment is more robustly constrained in the joint inversions than in the individual data set inversions. Unmodelled lateral heterogeneities in Earth and the models could partly explain some of the observed source parameter discrepancies related to the seismic data. © The Authors 2014. Published by Oxford University Press on behalf of The Royal Astronomical Society

Widespread Fault Creep in the Northern San Francisco Bay Area Revealed by Multistation Cluster Detection of Repeating Earthquakes

We search for repeating earthquakes (REs) in the northern San Francisco Bay Area in 1984–2016. By comparing over 670,000 waveforms from ∼75,000 events, we identify candidate clusters of events whose waveforms have high cross-correlation coefficients at multiple stations. A key difference with our approach is that these “multistation clusters” do not require each event in a family be recorded at multiple common stations. We validate these candidate REs by estimating precise relative relocations for the events in each cluster. We identify 59 RE families whose relocated hypocenters are separated by less than one source radius. These are distributed throughout the Maacama fault zone and along the northern Rodgers Creek and central Bartlett Springs faults, implying that widespread, pervasive creep occurs on those faults, at rates of 1–6 mm/year. At either end of the Maacama fault, the RE pattern highlights structural complexity, suggesting that multiple subparallel strands may be active and creeping

Use of a GPS-Derived Troposphere Model to Improve InSAR Deformation Estimates in the San Gabriel Valley, California

We evaluate the potential of troposphere models derived from ground meteorological data (pressure, temperature, and relative humidity) and Global Positioning System (GPS) data to improve InSAR measurements and models derived from them. We test this approach on an ERS-2/Envisat data set collected during a transient surface deformation episode that occurred from January to July 2005 in the San Gabriel Valley, southern California, USA. We find that the interferometric phase change observed over the corresponding period cannot be solely attributed to hydrological uplift associated with rising groundwater levels but also includes a significant contribution from differential tropospheric delay due to differing quantities of water vapor in the troposphere on the two SAR observation dates. We show that, if the tropospheric phase contribution is mistakenly interpreted as the range change associated with changes in groundwater storage, both the surface displacement and the groundwater storage coefficient may be overestimated by up to 30%. This method could be applied in real time where meteorological measurements are available near one or more GPS permanent site(s)

Fault slip rates and interseismic deformation in the western Transverse Ranges, California

To better constrain fault slip rates and patterns of interseismic deformation in the western Transverse Ranges of southern California, we present results from analysis of GPS and interferometric synthetic aperture radar (InSAR) data and three-dimensional mechanical and kinematic models of active faulting. Anthropogenic motions are detected in several localized zones but do not significantly affect the vast majority of continuous GPS site locations. GPS measures contraction rates across the Ventura Basin of ~7 mm/yr oriented west-northwest with rates decreasing to the west and east. The Santa Barbara channel is accommodating ~6.5 mm/yr in the east and ~2.5 mm/yr in the western portions of N/S contraction. Inversion of horizontal GPS velocities highlights a zone of localized fast contraction rates following the Ventura Basin. Using a mechanical model driven by geodetically calculated strain rates, we show that there are no significant discrepancies between short-term slip rates captured by geodesy and longer-term slip rates measured by geology. Mechanical models reproduce the first-order interseismic velocity and strain rate patterns but fail to reproduce strongly localized contraction in the Ventura Basin due to the inadequate homogeneous elastic properties of the model. Existing two-dimensional models match horizontal rates but predict significant uplift gradients that are not observed in the GPS data. Mechanical models predict zones of fast contraction in the Santa Barbara channel and offshore near Malibu, suggesting that offshore faults represent a significant seismic hazard to the region. Furthermore, many active faults throughout the region may produce little to no interseismic deformation, making accurate seismic hazard assessment challenging. © 2013. American Geophysical Union. All Rights Reserved

Fault slip in the 1997 Manyi, Tibet earthquake from linear elastic modelling of InSAR displacements

The Mw 7.6 1997 Manyi earthquake occurred in an area of central northern Tibet where sparse vegetation coverage and a lack of human habitation provide excellent conditions for Interferometric Synthetic Aperture Radar (InSAR) studies. We use coseismic pairs of radar images acquired by the ESA ERS-2 satellite to construct interferograms of the surface displacement field due to the earthquake. The location and extent of the coseismic fault rupture are mapped using a combination of optical satellite imagery, high-resolution digital topography, interferometric correlation and azimuth offset measurements; in so doing, we are able to relate prominent geomorphic features in the fault zone to bends in the fault. Using elastic dislocation models consistent with this mapped fault trace, we then test a range of fault geometries and slip conditions to find the combination which best explains the InSAR displacements. Our favoured model contains a reversal in fault dip, approximately halfway along its length, occurring at the location of a restraining bend. Slip on this model fault is heterogeneous, with two areas of peak slip of 7 m or greater, and components of dip-slip displacement which vary significantly along-strike. The success of this model in fitting the data implies that an observed asymmetry in the coseismic interferograms can be explained in terms of the local fault geometry, rather than by using non-linear elastic rheologies as suggested by earlier authors. © 2007 The Authors Journal compilation © 2007 RAS