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Free, publicly-accessible full text available March 16, 2024
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SUMMARY InSAR displacement time-series are emerging as a valuable product to study a number of Earth processes. One challenge to current time-series processing methods, however, is that when large earthquakes occur, they can leave sharp coseismic steps in the time-series. These discontinuities can cause current atmospheric correction and noise smoothing algorithms to break down, as these algorithms commonly assume that deformation is steady through time. Here, we aim to remedy this by exploring two methods for correcting earthquake offsets in InSAR time-series: a simple difference offset estimate (SDOE) process and a multiparameter offset estimate (MPOE) parametric time-series inversion technique. We apply these methods to a 2-yr time-series of Sentinel-1 interferograms spanning the 2019 Ridgecrest, CA earthquake sequence. Descending track results indicate that the SDOE method precisely corrects for only 20 per cent of the coseismic offsets at 62 study locations included in our scene and only partially corrects or sometimes overcorrects for the rest of our study sites. On the other hand, the MPOE estimate method successfully corrects the coseismic offset for the majority of sites in our analysis. This MPOE method allows us to produce InSAR time-series and data-derived estimates of deformation during each phase of the earthquake cycle.more »
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Abstract The 2019 Ridgecrest conjugate Mw6.4 and Mw7.1 events resulted in several meters of strike‐slip and dip‐slip along an intricate rupture, extending from the surface down to 15 km. Now with >2 years of post‐rupture observations, we utilize these results to better understand vertical postseismic deformation from the Ridgecrest sequence and illuminate the emerging significance of vertical earthquake cycle deformation data. We determine the cumulative vertical displacement observed by the continuous GNSS network since Ridgecrest, which requires additional time series analyses to adequately resolve vertical deformation compared to the horizontal. Using a Maxwell‐type viscoelastic relaxation model, with a best fit time‐averaged asthenosphere viscosity of 4e17 Pa·s and a laterally heterogeneous lithosphere, we find that viscoelastic relaxation accounts for a majority of the cumulative vertical deformation at Ridgecrest and strongly controls far‐field observations in all north‐east‐up components. The viscoelastic model alone generally underpredicts deformation from GNSS and the remaining nonviscoelastic displacement is most prominent in the horizontal near‐field (−16 to 19 mm), revealing a deformation pattern matching the coseismic observations. This suggests that multiple deformation mechanisms are contributing to Ridgecrest's postseismic displacement, where afterslip likely dominates the near‐field while viscoelastic relaxation controls the far‐field. Similar deformation at individual GNSS stations has been observed for pastmore »
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Abstract The La Crucecita earthquake ruptured on the megathrust, generating strong shaking and a modest but long-lived tsunami. This is a significant earthquake that illuminates important aspects of the behavior of the megathrust as well as the potential related hazards. The rupture is contained within 15–30 km depth, ground motions are elevated, and the energy to moment ratio is high. We argue that it represents a deep megathrust earthquake, the 30 km depth is the down-dip edge of slip. The inversion is well constrained, ruling out any shallow slip. It is the narrow seismogenic width and the configuration of the coastline that allow for deformation to occur offshore. The minor tsunamigenesis can be accounted for by the deep slip patch. There is a significant uplift at the coast above it, which leads to negative maximum tsunami amplitudes. Finally, tide-gauge recordings show that edge-wave modes were excited and produce larger amplitudes and durations in the Gulf of Tehuantepec.
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Abstract Interferometric Synthetic Aperture Radar is an important tool for imaging surface deformation from large continental earthquakes. Here, we present maps of coseismic displacement and strain from the 2019 Ridgecrest earthquakes using multiple Sentinel-1 images. We provide three types of interferometric products. (1) Standard interferograms from two look directions provide an overview of the deformation and can be used for modeling coseismic slip. (2) Phase gradient maps from stacks of coseismic interferograms provide high-resolution (∼30 m) images of strain concentration and surface fracturing that can be used to guide field surveys. (3) High-pass filtered, stacked, unwrapped phase is decomposed into east–west and up–down, south–north components and is used to determine the sense of fault slip. The resulting phase gradient maps reveal over 300 surface fractures, including triggered slip on the Garlock fault. The east–west component of high-pass filtered phase reveals the polarity of the strike-slip offset (right lateral or left lateral) for many of the fractures. We find a small number of fractures that have slip polarity that is retrograde to the background tectonic stress. This is similar to observations of retrograde slip observed near the 1999 Mw 7.1 Hector Mine rupture, but the Ridgecrest observations are more completely imaged by the frequent and high-qualitymore »
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Abstract Measuring crustal strain and seismic moment accumulation, is crucial for understanding the growth and distribution of seismic hazards along major fault systems. Here, we develop a methodology to integrate 4.5 years (2015–2019.5) of Sentinel‐1 Interferometric Synthetic Aperture Radar (InSAR) and continuous Global Navigation Satellite System (GNSS) time series to achieve 6 to 12‐day sampling of surface displacements at ∼500 m spatial resolution over the entire San Andreas fault system. Numerous interesting deformation signals are identified with this product (video link:
https://www.youtube.com/watch?v=SxNLQKmHWpY ). We decompose the line‐of‐sight InSAR displacements into three dimensions by combining the deformation azimuth from a GNSS‐derived interseismic fault model. We then construct strain rate maps using a smoothing interpolator with constraints from elasticity. The resulting deformation field reveals a wide array of crustal deformation processes including, on‐ and off‐fault secular and transient tectonic deformation, creep rates on all the major faults, and vertical signals associated with hydrological processes. The strain rate maps show significant off‐fault components that were not captured by GNSS‐only models. These results are important in assessing the seismic hazard in the region. -
Abstract Rheologic variations in the Earth's crust (like elastic plate thickness [EPT] or crustal rigidity) modulate the rate at which seismic moment accumulates for potentially hazardous faults of the San Andreas Fault System (SAFS). To quantify rates of seismic moment accumulation, Global Navigation Satellite Systems, and Interferometric Synthetic Aperture Radar data were used to constrain surface deformation rates of a four‐dimensional viscoelastic deformation model that incorporates rheological variations spanning a 900 km section of the SAFS. Lateral variations in EPT, estimated from surface heat flow and seismic depth to the lithosphere‐asthenosphere boundary, were converted to lateral variations in rigidity and then used to solve for seismic moment accumulation rates on 32 fault segments. We find a cluster of elevated seismic moment rates (11–20 × 1015 Nm year−1km−1) along the main SAFS trace spanning the historical
M w 7.9 1857 Fort Tejon earthquake rupture length; present‐day seismic moment magnitude on these segments ranges fromM w 7.2–7.6. We also find that the average plate thickness in the Salton Trough is reduced to only 60% of the regional average, which results in a ∼60% decrease in moment accumulation rate along the Imperial fault. Likewise, a 30% increase of average plate thickness results in at least a ∼30% increase in moment ratemore » -
Contemporary earthquake hazard models hinge on an understanding of how strain is distributed in the crust and the ability to precisely detect millimeter-scale deformation over broad regions of active faulting. Satellite radar observations revealed hundreds of previously unmapped linear strain concentrations (or fractures) surrounding the 2019 Ridgecrest earthquake sequence. We documented and analyzed displacements and widths of 169 of these fractures. Although most fractures are displaced in the direction of the prevailing tectonic stress (prograde), a large number of them are displaced in the opposite (retrograde) direction. We developed a model to explain the existence and behavior of these displacements. A major implication is that much of the prograde tectonic strain is accommodated by frictional slip on many preexisting faults.
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Abstract The 2018 Palu tsunami contributed significantly to the devastation caused by the associated
7.5 earthquake. This began a debate about how the moderate size earthquake triggered such a large tsunami within Palu Bay, with runups of more than 10 m. The possibility of a large component of vertical coseismic deformation and submarine landslides have been considered as potential explanations. However, scarce instrumental data have made it difficult to resolve the potential contributions from either type of source. We use tsunami waveforms derived from social media videos in Palu Bay to model the possible sources of the tsunami. We invert InSAR data with different fault geometries and use the resulting seafloor displacements to simulate tsunamis. The coseismic sources alone cannot match both the video‐derived time histories and surveyed runups. Then we conduct a tsunami source inversion using the video‐derived time histories and a tide gauge record as inputs. We specify hypothetical landslide locations and solve for initial tsunami elevation. Our results, validated with surveyed runups, show that a limited number of landslides in southern Palu Bay are sufficient to explain the tsunami data. The Palu tsunami highlights the difficulty in accurately capturing with tide gauges the amplitude and timing ofmore » -
Abstract Our understanding of plate boundary deformation has been enhanced by transient signals observed against the backdrop of time‐independent secular motions. We make use of a new analysis of displacement time series from about 1,000 continuous Global Positioning System (GPS) stations in California from 1999 to 2018 to distinguish tectonic and nontectonic transients from secular motion. A primary objective is to define a high‐resolution three‐dimensional reference frame (datum) for California that can be rapidly maintained with geodetic data to accommodate both secular and time‐dependent motions. To this end, we compare the displacements to those predicted by a horizontal secular fault slip model for the region and construct displacement and strain rate fields. Over the past 19 years, California has experienced 19 geodetically detectable earthquakes and widespread postseismic deformation. We observe postseismic strain rate variations as large as 1,000 nstrain/year with moment releases equivalent up to an Mw6.8 earthquake. We find significant secular differences up to 10 mm/year with the fault slip model, from the Mendocino Triple Junction to the southern Cascadia subduction zone, the northern Basin and Range, and the Santa Barbara channel. Secular vertical uplift is observed across the Transverse Ranges, Coastal Ranges, Sierra Nevada, as well as large‐scalemore »