The Sentinel-1 mission for the improvement of the scientific understanding and the operational monitoring of the seismic cycle

We describe the state of the art of scientific research on the earthquake cycle based on the analysis of Synthetic Aperture Radar (SAR) data acquired from satellite platforms. We examine the achievements and the main limitations of present SAR systems for the measurement and analysis of crustal deformation, and envision the foreseeable advances that the Sentinel-1 data will generate in the fields of geophysics and tectonics. We also review the technological and scientific issues which have limited so far the operational use of satellite data in seismic hazard assessment and crisis management, and show the improvements expected from Sentinel-1 dat

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

Towards coordinated regional multi-satellite InSAR volcano observations:results from the Latin America pilot project

Within Latin America, about 319 volcanoes have been active in the Holocene, but 202 of these volcanoes have no seismic, deformation or gas monitoring. Following the 2012 Santorini Report on satellite Earth Observation and Geohazards, the Committee on Earth Observation Satellites (CEOS) developed a 4-year pilot project (2013-2017) to demonstrate how satellite observations can be used to monitor large numbers of volcanoes cost-effectively, particularly in areas with scarce instrumentation and/or difficult access. The pilot aims to improve disaster risk management (DRM) by working directly with the volcano observatories that are governmentally responsible for volcano monitoring as well as with the international space agencies (ESA, CSA, ASI, DLR, JAXA, NASA, CNES). The goal is to make sure that the most useful data are collected at each volcano following the guidelines of the Santorini report that observation frequency is related to volcano activity, and to communicate the results to the local institutions in a timely fashion. Here we highlight how coordinated multi-satellite observations have been used by volcano observatories to monitor volcanoes and respond to crises. Our primary tool is measurements of ground deformation made by Interferometric Synthetic Aperture Radar (InSAR), which have been used in conjunction with other observations to determine the alert level at these volcanoes, served as an independent check on ground sensors, guided the deployment of ground instruments, and aided situational awareness. During this time period, we find 26 volcanoes deforming, including 18 of the 28 volcanoes that erupted – those eruptions without deformation were less than 2 on the VEI scale. Another 7 volcanoes were restless and the volcano observatories requested satellite observations, but no deformation was detected. We describe the lessons learned about the data products and information that are most needed by the volcano observatories in the different countries using information collected by questionnaires. We propose a practical strategy for regional to global satellite volcano monitoring for use by volcano observatories in Latin America and elsewhere to realize the vision of the Santorini report

Surface displacements and source parameters of the 2003 Bam (Iran) earthquake from Envisat advanced synthetic aperture radar imagery

The M w 6.6, 26 December 2003 Bam (Iran) earthquake was one of the first earthquakes for which Envisat advanced synthetic aperture radar (ASAR) data were available. Using interferograms and azimuth offsets from ascending and descending tracks, we construct a three-dimensional displacement field of the deformation due to the earthquake. Elastic dislocation modeling shows that the observed deformation pattern cannot be explained by slip on a single planar fault, which significantly underestimates eastward and upward motions SE of Bam. We find that the deformation pattern observed can be best explained by slip on two subparallel faults. Eighty-five percent of moment release occurred on a previously unknown strike-slip fault running into the center of Bam, with peak slip of over 2 m occurring at a depth of ∼5 km. The remainder occurred as a combination of strike-slip and thrusting motion on a southward extension of the previously mapped Bam Fault ∼5 km to the east

Data-Driven Two-Fault Modeling 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 February 21, 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 Interferometric Synthetic Aperture Radar 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

Systematic comparisons of earthquake source models determined using InSAR and seismic data

Robust earthquake source parameters (e.g., location, seismic moment, fault geometry) are essential for reliable seismic hazard assessment and the investigation of large-scale tectonics. They are routinely estimated using a variety of data and techniques, such as seismic data and, more recently, Interferometric Synthetic Aperture Radar (InSAR). Comparisons between these two datasets are frequently made although not usually in a comprehensive way. This review compares source parameters from global and regional seismic catalogues with those from a recent database of InSAR parameters, which has been expanded with 18 additional source models for this study. We show that moment magnitude (Mw) estimates agree well between the two datasets, with a trend for thrust events modelled using InSAR to have slightly larger Mw estimates. Earthquake locations determined using InSAR agree well with those reported in regional catalogues, with a median difference of 6.3 km between them, which is smaller than for global seismic catalogues. We also investigate the consistency of source parameters and source directivity by comparing ISC hypocentres with GCMT and ICMT centroid locations for earthquakes with Mw = 6.5. In some cases the source directivity is qualitatively comparable with previous studies, especially when comparing ISC and ICMT locations. The average difference between InSAR-determined depths and those in the EHB catalogue is reduced if a layered half-space is used in the inversion of InSAR data. Overall, faulting geometry (strike, dip and rake angles) remain in good agreement with values from the GCMT catalogue, and any large discrepancies can be attributed to tradeoffs between parameters. With continued investment in satellites for radar interferometry, InSAR is a valuable technique for the estimation of earthquake source parameters. The observed trends and discrepancies between InSAR and seismically determined source parameters are the result of issues with the data, different inversion techniques and the assumed Earth structure model

Complex 3‐D Surface Deformation in the 1971 San Fernando, California Earthquake Reveals Static and Dynamic Controls on Off‐Fault Deformation

International audienceAbstract The shallow 1971 M W 6.6 San Fernando, California earthquake involved a complex rupture process on an immature thrust fault with a non‐planar geometry, and is notable for having a higher component of left‐lateral surface slip than expected from seismic source models. We extract its 3‐D coseismic surface displacement field from aerial stereo photographs and document the amount and width of the vertical and fault trace‐parallel components of distributed deformation along strike. The results confirm the significant left‐lateral surface offsets, suggesting a slip vector rotation at shallow depths. Comparing our offsets against field measurements of fault slip, we observe that most of the offset was accommodated in the damage zone, with off‐fault deformation averaging 69% in both the fault trace‐parallel and vertical components. However, the magnitude and width of off‐fault deformation behave differently between the vertical and fault trace‐parallel components, which, along with the rotation in rake near the surface, can be explained by dynamic rupture effects

The Sentinel-1 mission for the improvement of the scientific understanding and the operational monitoring of the seismic cycle

We describe the state of the art of scientific research on the earthquake cycle based on the analysis of Synthetic Aperture Radar (SAR) data acquired from satellite platforms. We examine the achievements and the main limitations of present SAR systems for the measurement and analysis of crustal deformation, and envision the foreseeable advances that the Sentinel-1 data will generate in the fields of geophysics and tectonics. We also review the technological and scientific issues which have limited so far the operational use of satellite data in seismic hazard assessment and crisis management, and show the improvements expected from Sentinel-1 dataPublished164-1741.10. TTC - TelerilevamentoJCR Journalope

Fault identification for buried strike-slip earthquakes using InSAR: The 1994 and 2004 Al Hoceima, Morocco earthquakes

The 1994 Mw 6.0 and 2004 Mw 6.5 Al Hoceima earthquakes are the largest to have occurred in Morocco for 100 yr, and give valuable insight into the poorly understood tectonics of the area. Bodywave modelling indicates the earthquakes occurred on near-vertical, strike-slip faults with the nodal planes oriented NW–SE and NE–SW. Distinguishing between the primary fault plane and auxiliary planes, using either geodetic or seismic data, is difficult due to the spatial symmetry in deformation fields and radiation pattern of moderately sized, buried, strike-slip earthquakes. Preliminary studies, using aftershock locations and surface observations, have been unable to identify the orientation of the primary fault plane for either earthquake conclusively. We use radar interferometry and aftershock relocation of the earthquake sequence to resolve the ambiguity. For the 2004 earthquake, inverting the interferograms for a uniform slip model based either of the two potential nodal planes results in similar misfits to the data. However, the NE–SW best-fit fault plane has an unrealistically high fault slip-to-length ratio and we conclude the NW–SE striking nodal plane is the primary fault plane and slip was right lateral. We carry out tests on synthetic data for a buried strike-slip earthquake in which the orientation of the fault plane is known a priori. Independent of geometry, missing data, and correlated noise, models produced assuming the auxiliary plane to be the fault plane have very high fault slip-to-length ratios. The 1994 earthquake had a smaller magnitude and comparisons of model misfits and slip-to-length ratios do not conclusively indicate which of the nodal planes is the primary fault plane. Nonetheless, the InSAR data provides valuable information by improving the accuracy of the earthquake location by an order of magnitude. We carry out a multiple event relocation of the entire earthquake sequence, including aftershocks, making use of the absolute locations for the 1994 and 2004 main shocks from our InSAR study. The aftershock locations are consistent with a NW–SE orientated fault plane in 2004 and suggests that the 1994 earthquake occurred on a NE–SW fault; perpendicular to the fault which ruptured in 2004. Previous tectonic models of the area proposed a bookshelf model of block rotation with NNE–SSW left-lateral faults. This model requires modification to accommodate the observation of right-lateral slip on a NW–SE fault plane for the 2004 earthquake and we prefer to interpret the fault orientations as due to a zone of distributed shear with a right-lateral fault striking at ∼115° and conjugate, clockwise rotating, left-lateral faults striking at ∼25°