Showing papers by "Mark Simons published in 2020"

Surface Deformation Related to the 2019 Mw 7.1 and 6.4 Ridgecrest Earthquakes in California from GPS, SAR Interferometry, and SAR Pixel Offsets

Abstract: We analyzed Synthetic Aperture Radar (SAR) images from Copernicus Sentinel‐1A and 1B satellites operated by the European Space Agency and the Advanced Land Observation Satellite‐2 (ALOS‐2) satellite operated by the Japan Aerospace Exploration Agency and Global Navigation Satellite System (GNSS) data from the Network of the Americas for the 4 July 2019 M_w 6.4 and 5 July (local; 6 July UTC) M_w 7.1 Ridgecrest earthquakes. We integrated geodetic measurements for the 3D vector field of coseismic surface deformation for the two events, using SAR data from Sentinel‐1 and ALOS‐2 satellites. We combined less precise large‐scale displacements from SAR images by pixel offset tracking or matching, including the along‐track component, with the more precise SAR interferometry (Interferometric Synthetic Aperture Radar [InSAR]) measurements in the radar line of sight (LoS) direction and intermediate‐precision along‐track InSAR to estimate all three components of the surface displacement for the two events together. We also estimated the coseismic deformation for the two earthquakes from time‐series processing of continuous Global Navigation Satellite System data stations in the area. InSAR coherence and coherence change maps the surface disruptions due to fault ruptures reaching the surface. Large slip in the M_w 6.4 earthquake was on a NE‐striking fault that intersects with the NW‐striking fault that was the main rupture in the M_w 7.1 earthquake. The main fault bifurcates towards the southeast ending 3 km from the Garlock Fault. The Garlock fault had triggered slip of about 20 mm in the radar LoS along a short section directly south of the main rupture. About 3 km northwest of the M_w 7.1 epicenter, the surface fault separates into two strands that form a pull‐apart with about 1 m of down‐drop. Further northwest is a wide zone of complex deformation.

Interseismic loading of subduction megathrust drives long term uplift in northern Chile

Abstract: Large earthquakes are the product of elastic stress that has accumulated over decades to centuries along segments of active faults. Assuming an elastic crust, one can roughly estimate the location and rate of accumulation of elastic stress. However, this general framework does not account for inelastic, irrecoverable deformation, which results in large scale topography. We do not know over which part of the earthquake cycle such deformation occurs. Using InSAR and GNSS measurements, we report on a potential correlation between long‐term, inelastic and short‐term, interseismic vertical rates in northern Chile. Approximately 4 to 8% of the geodetically‐derived interseismic vertical rates translates into permanent deformation, suggesting topography of the forearc builds up during the interseismic period. This observation provides a quantitative basis for an improved understanding of the interplay between short‐term and long‐term dynamics along convergent plate boundaries.

Accounting for uncertain 3-D elastic structure in fault slip estimates

Abstract: Earthquake source estimates are affected by many types of uncertainties, deriving from observational errors, modelling choices and our simplified description of the Earth’s interior. While observational errors are often accounted for, epistemic uncertainties, which stem from our imperfect description of the forward model, are usually neglected. In particular, 3-D variations in crustal properties are rarely considered. 3-D crustal heterogeneity is known to largely affect estimates of the seismic source, using either geodetic or seismic data. Here, we use a perturbation approach to investigate, and account for, the impact of epistemic uncertainties related to 3-D variations of the mechanical properties of the crust. We validate our approach using a Bayesian sampling procedure applied to synthetic geodetic data generated from 2-D and 3-D finite-fault models. We show that accounting for uncertainties in crustal structure systematically increases the reliability of source estimates.

A comparison of predicted and observed ocean tidal loading in Alaska

Abstract: We investigate the elastic and anelastic response of the crust and upper mantle across Alaska to mass loading by ocean tides GPS-inferred surface displacements recorded by the Plate Boundary Observatory network are compared with predictions of deformation associated with the redistribution of ocean water due to the tides We process more than 5 yr of GPS data from 131 stations using a kinematic precise point positioning algorithm and estimate tidal contributions using harmonic analysis We also forward calculate load-induced surface displacements by convolving ocean-tide models with load Green’s functions derived from spherically symmetric Earth models We make the comparisons for dominant tidal harmonics in three frequency bands: semidiurnal (M₂), diurnal (O₁) and fortnightly (M_f) Vector differences between predicted and observed ocean tidal loading (OTL) displacements are predominantly sub-mm in magnitude in all three frequency bands and spatial components across the network, with larger residuals of up to several mm in some coastal areas Accounting for the effects of anelastic dispersion in the upper mantle using estimates of Q from standard Earth models reduces the residuals for the M₂ harmonic by an average of 01–02 mm across the network and by more than 1 mm at some individual stations For the relatively small M_f tide, the effects of anelastic dispersion (<003 mm) are undetectable within current measurement error Incorporating a local ocean-tide model for the northeastern Pacific Ocean reduces the M₂ vertical residuals by an average of 02 mm, with improvements of up to 5 mm at some coastal stations Estimated RMS observational uncertainties in the vertical component for the M₂ and O₁ tides are approximately ±008 mm at the two-sigma level (±003 mm in the horizontal components), and ±021 mm for the M_f harmonic (±007 mm in the horizontal components) For the M₂ harmonic, discrepancies between predicted and observed OTL displacements exceed observational uncertainties by about one order of magnitude None of the ocean tide and Earth model combinations is found to reduce the M₂ residuals below the observational uncertainty, and no single forward model provides a best fit to the observed displacements across all tidal harmonics and spatial components For the O₁ harmonic, discrepancies between predicted and observed displacements are generally several-fold larger than the observational uncertainties For the M_f harmonic, the discrepancies are roughly within a factor of two of the observational uncertainties We find that discrepancies between predicted and observed OTL displacements can be significantly reduced by removing a network-uniform tidal-harmonic displacement, and that the remaining discrepancies exhibit some regional-scale spatial coherency, particularly for the M₂ harmonic We suggest that the remaining discrepancies for the M₂, O₁ and M_f tides cannot be fully explained by measurement error and instead convey information about deficiencies in ocean-tide models and deviations from spherically symmetric Earth structure

A stochastic view of the 2020 Elazığ Mw 6.8 earthquake

Abstract: Until the Mw 6.8 Elazig earthquake ruptured the central portion of the East Anatolian Fault (EAF) on January 24, 2020, the region had only experienced moderate magnitude (Mw 6.2) earthquakes over t...