GPS Displacement Research Articles

Postseismic uplift of the Andes following the 2010 Maule earthquake: Implications for mantle rheology

AbstractPostseismic surface deformation associated with great subduction earthquakes is controlled by asthenosphere rheology, frictional properties of the fault, and structural complexity. Here by modeling GPS displacements in the 6 years following the 2010 Mw 8.8 Maule earthquake in Chile, we investigate the impact of heterogeneous viscosity distribution in the South American subcontinental asthenosphere on the 3‐D postseismic deformation pattern. The observed postseismic deformation is characterized by flexure of the South America plate with peak uplift in the Andean mountain range and subsidence in the hinterland. We find that, at the time scale of observation, over 2 orders of magnitude gradual increase in asthenosphere viscosity from the arc area toward the cratonic hinterland is needed to jointly explain horizontal and vertical displacements. Our findings present an efficient method to estimate spatial variations of viscosity, which clearly improves the fitting to the vertical signal of deformation. Lateral changes in asthenosphere viscosity can be correlated with the thermomechanical transition from weak subvolcanic arc mantle to strong subcratonic mantle, thus suggesting a stationary heterogeneous viscosity structure. However, we cannot rule out a transient viscosity structure (e.g., power law rheology) with the short time span of observation.

Rupture features of the 2016 Mw 6.2 Norcia earthquake and its possible relationship with strong seismic hazards

AbstractFor analyzing possible reasons for the heavy damage and seismogenic features of the 24 August 2016 Norcia earthquake, we constructed and analyzed its rupture process by incorporating data sets of near‐field strong‐motion, teleseismic and static GPS displacements. The optimized model revealed a relatively compact slip pattern with mainly normal fault components. The maximum slip was around 0.9 m, while the rupture areas extended ~11 km and ~20 km along dip and strike, respectively. The total seismic moment was 2.3 × 1018 Nm, equivalent to Mw 6.2. Most seismic moments were released within 10 s, radiating 3.5 × 1013 J of seismic energy. The rupture history showed asymmetric propagation and is characterized by a relatively high rupture velocity within the first 6 s with a maximum of ~3.2 km/s. The main shock slip pattern correlated well with the aftershocks distribution, and most of the accumulated strain was released in the east of seismic gap between the nearby 1997 and 2009 earthquake sequences.

2011年東北地方太平洋沖地震（Mw9.0）に関連する地磁気日変化異常の時空間的特徴

Recent studies have reported unusual behaviors of geomagnetic diurnal variation (GDV) in the vertical component prior to the 2011 off the Pacific coast of Tohoku earthquake (Mw 9.0). To make a better understanding of this phenomenon, time-spatial analysis of GDV has been applied in this study. Geomagnetic data of long term observations at 17 stations in Japan have been analyzed using the same method in Han et al. 2015. Ratios of diurnal variation range between the target station and the reference station KAK have been computed. After removing seasonal variations revealed by wavelet transform analysis, the 15-day mean values of the ratios in the vertical component shows a clear anomaly exceeding the statistical threshold about 2 months before the mega event in both ESA and MIZ stations in the Tohoku Region. Similar results could not be found in other regions of Japan. Spatial distributions of the ratios show a good agreement between the location of the anomalies and the epicenter of Mw 9.0 earthquake. These time-spatial results are consistent with independent results obtained from other observations such as radon density, seismicity, and GPS displacements, which suggest the geomagnetic data might be useful in earthquake monitoring and disaster mitigation.

Single‐station automated detection of transient deformation in GPS time series with the relative strength index: A case study of Cascadian slow slip

AbstractThe discovery of slow‐slip events over the past decades has changed our understanding of tectonic hazards and the earthquake cycle. Proper geodetic characterization of slow‐slip events is necessary for studies of regional interseismic, coseismic and postseismic deformation, and miscalculations can affect our understanding of the regional stress field and tectonic hazard. Because of the proliferation of GPS data over the last two decades, an automated algorithm is required to analyze the signals and model the deformation on a station by station basis. Using the relative strength index (RSI), a financial momentum oscillator, we test the ability to detect events of various sizes and durations. We first determine the statistics of the RSI under different noise conditions and then use this information as the basis for the automated transient detection algorithm by testing different synthetic signals. We then apply the technique to daily GPS displacement time series from 213 stations along the Cascadia subduction zone to form a record of transient deformation between 2005 and 2016. Our estimates of the spatial extent, duration, and propagation of major episodic tremor and slip (ETS) events are consistent with previous studies. We use the automated detections to remodel the displacement time series and obtain transient deformation rates over the past decade and discuss the tectonic implications. Finally, we analyze the correlation between transient detections and tremor showing good agreement between the two at slab depths commonly associated with ETS events.

Slow slip hidden in the noise: The intermittence of tectonic release

AbstractReferred to as slow slip events, the transient aseismic slip that occurs along plate boundaries can be indirectly characterized through colocated seismicity, such as tectonic tremor and low‐frequency earthquakes (LFEs). Using the timing of cataloged LFE and tremor activity in Guerrero, Mexico, and northern Cascadia, I decompose the interaseismic GPS displacement, defined as the surface deformation between previously detected slow slip events, into separate regimes of tectonic loading and release. In such a way, previously undetected slow slip events that produce less than a millimeter of surface deformation are extracted from the geodetic noise. These new observations demonstrate that the interaseismic period is not quiescent and that slow slip occurs much more often than previously thought. This suggests that the plate interface where slow slip and tremor occur is in fact strongly coupled and that slow aseismic release occurs over a wide spectrum of time scales.

Slip model of the 17 November 2015 Mw = 6.5 Lefkada earthquake from the joint inversion of geodetic and seismic data

AbstractWe inferred the rupture process of the 2015 Lefkada earthquake by combining teleseismic and near‐field strong motion data, together with static and dynamic GPS displacements. The joint inversion of all data sets revealed a relatively complex slip pattern with a heterogeneous distribution. Slip is confined in the upper 10 km. Two principal asperities with peak slip amplitude of 2.35 m, which released 65% of the total seismic moment, were recovered southwest of the epicenter. Remarkably, the 2015 earthquake ruptured the part of the Cephalonia Transform Fault Zone that remained unbroken during the 2003 event, revealing a pattern of rupture segmentation along the fault zone. In this context the Lefkada segment can be divided into a northern and southern part. Preseismic deformation onshore southern Lefkada revealed that since the 2003 rupture, the northern part of the Lefkada segment continuously loaded the southern part, which eventually broke during the 2015 event.

Spatiotemporal characteristics of the geomagnetic diurnal variation anomalies prior to the 2011 Tohoku earthquake (Mw 9.0) and the possible coupling of multiple pre-earthquake phenomena

Xu et al. (2013) and Han et al. (2015) have reported unusual behaviors of geomagnetic diurnal variation (GDV) in the vertical component prior to the 2011 off the Pacific coast of Tohoku earthquake (Mw 9.0). To make a better understanding of this phenomenon, temporal-spatial analyses of GDV have been applied in this study. Geomagnetic data of long-term observations at 17 stations in Japan have been analyzed using the same method in Han et al. (2015). Ratios of diurnal variation range between the reference station KAK and the target stations have been computed. After removing seasonal variations, the 15-day backward running mean values of the ratios in the vertical component shows a clear anomaly exceeding the statistical threshold about 2months before the mega event at both ESA and MIZ stations in the Tohoku Region. Locations of anomalies in spatial distribution show a good correlation with the epicenter of the Mw 9.0 earthquake. These spatiotemporal results are consistent with those obtained from other independent observations such as groundwater level and GPS displacements. The coupling of multiple pre-earthquake phenomena may help to understand the preparation process of a mega earthquake in the subduction zone.

Foreshock triggering of the 1 April 2014 Mw 8.2 Iquique, Chile, earthquake

Abstract On April 1st, 2014, a Mw 8.2 (U.S. Geological Survey moment magnitude) earthquake occurred in the subduction zone offshore northern Chile. In the two weeks leading up to the earthquake, a sequence of foreshocks, starting with a Mw 6.7 earthquake on March 16th and including three more Mw 6.0+ events, occurred predominantly south of the April 1st mainshock epicenter and up-dip of the area of significant slip during the mainshock. Using earthquake locations and source parameters derived in a previous study ( Hayes et al., 2014 ) and a Coulomb failure stress change analysis of these events, we assess in detail the hypothesis that the earthquakes occurred as a cascading sequence, each event successively triggering the next, ultimately triggering the rupture of the mainshock. Following the initial Mw 6.7 event, each of the three largest foreshocks (Mw 6.4, 6.2 and 6.3), as well as the hypocenter of the mainshock, occurred in a region of positive Coulomb stress change produced by the preceding events, indicating these events were brought closer to failure by the prior seismicity. In addition, we reexamine the possibility that aseismic slip occurred and what role it may have played in loading the plate boundary. Using horizontal GPS displacements from along the northern Chile coast prior to the mainshock, we find that the foreshock seismicity alone likely does not account for the observed signals. We perform a grid search for the location and magnitude of an aseismic slip patch that can account for the difference between observed signals and foreshock-related displacement, and find that a slow slip region with slip corresponding to a Mw ∼ 6.8 earthquake located coincident with or up-dip of the foreshock seismicity can best explain this discrepancy. Additionally, such a slow slip region positively loads the mainshock hypocentral area, enhancing the positive loading produced by the foreshock seismicity.

Along-strike variations of the partitioning of convergence across the Haiyuan fault system detected by InSAR

Oblique convergence across Tibet leads to slip partitioning with the coexistence of strike-slip, normal and thrust motion on major fault systems. A key point is to understand and model how faults interact and accumulate strain at depth. Here, we extract ground deformation across the Haiyuan Fault restraining bend, at the northeastern boundary of the Tibetan plateau, from Envisat radar data spanning the 2001\textendash2011 period. We show that the complexity of the surface displacement field can be explained by the partitioning of a uniform deep-seated convergence. Mountains and sand dunes in the study area make the radar data processing challenging and require the latest developments in processing procedures for Synthetic Aperture Radar interferometry. The processing strategy is based on a small baseline approach. Before unwrapping, we correct for atmospheric phase delays from global atmospheric models and digital elevation model errors. A series of filtering steps is applied to improve the signal-to-noise ratio across high ranges of the Tibetan plateau and the phase unwrapping capability across the fault, required for reliable estimate of fault movement. We then jointly invert our InSAR time-series together with published GPS displacements to test a proposed long-term slip-partitioning model between the Haiyuan and Gulang left-lateral Faults and the Qilian Shan thrusts. We explore the geometry of the fault system at depth and associated slip rates using a Bayesian approach and test the consistency of present-day geodetic surface displacements with a long-term tectonic model. We determine a uniform convergence rate of 10\,[8.6\textendash11.5]\,mm yr-1 with an N89\,[81\textendash97]°E across the whole fault system, with a variable partitioning west and east of a major extensional fault-jog (the Tianzhu pull-apart basin). Our 2-D model of two profiles perpendicular to the fault system gives a quantitative understanding of how crustal deformation is accommodated by the various branches of this thrust/strike-slip fault system and demonstrates how the geometry of the Haiyuan fault system controls the partitioning of the deep secular motion.

Monsoonal loading in Ethiopia and Eritrea from vertical GPS displacement time series

AbstractVertical GPS displacement time series from 16 continuous sites over a period from 2007 to 2014 are compared to time series of monthly averages of liquid water equivalent thickness from the Gravity Recovery and Climate Experiment and precipitation from the Climate Research Unit to investigate hydrologic loading in Ethiopia and Eritrea. The GPS vertical time series record the presence of one or two rainy seasons, the amplitude surface displacements in response to monsoon water load, and phases consistent with a purely elastic response to a water load that accumulates throughout the rainy period. Comparison of observed amplitudes to those calculated for an average Earth model shows no systematic weakness related to the rift.

Rupture process of 2011 Mw7.1 Van, Eastern Turkey earthquake from joint inversion of strong-motion, high-rate GPS, teleseismic, and GPS data

We analyzed the rupture process of the 2011 Mw7.1 Van, Eastern Turkey earthquake using teleseismic, strong-motion, 1-Hz GPS waveforms and static GPS displacement measurements. We performed data sensitivity analyses using four different rupture scenarios. Overall, when geodetic and seismic datasets are modeled jointly, slip distribution is well-constrained and rupture velocity can be obtained. The 2011 Van earthquake is a case where none of the available datasets are sufficient to constrain the slip distribution and the rupture kinematics on their own. This study confirms that rather than fitting one dataset perfectly, using multiple datasets jointly leads to a better-constrained slip distribution. The kinematic model obtained from the joint inversion of all the available data shows a 45-km-long bilateral rupture with two sub-events; one larger slip patch propagating up-dip and toward west of the hypocenter and a smaller second slip patch toward the east. The highest slip is to the west of the hypocenter with a peak value of 4.5 m. The slip is confined to the depths of 7.5 to 20 km and the shallower part of the fault remains unbroken. The average rupture velocity is around 3 km/s, close the Rayleigh wave velocity. The rupture is faster with shorter rise times for the larger sub-event toward the west, while the rise times are longer for the smaller sub-event to the east. This difference in seismic behavior might be related to the segmentation of the fault.

The 2012 Mw 8.6 Wharton Basin sequence: A cascade of great earthquakes generated by near‐orthogonal, young, oceanic mantle faults

AbstractWe improve constraints on the slip distribution and geometry of faults involved in the complex, multisegment, Mw 8.6 April 2012 Wharton Basin earthquake sequence by joint inversion of high‐rate GPS data from the Sumatran GPS Array (SuGAr), teleseismic observations, source time functions from broadband surface waves, and far‐field static GPS displacements. This sequence occurred under the Indian Ocean, ∼400 km offshore Sumatra. The events are extraordinary for their unprecedented rupture of multiple cross faults, deep slip, large strike‐slip magnitude, and potential role in the formation of a discrete plate boundary between the Indian and Australian plates. The SuGAr recorded static displacements of up to ∼22 cm, along with time‐varying arrivals from the complex faulting, which indicate that the majority of moment release was on young, WNW trending, right‐lateral faults, counter to initial expectations that an old, lithospheric, NNE trending fracture zone played the primary role. The new faults are optimally oriented to accommodate the present‐day stress field. Not only was the greatest moment released on the younger faults, but it was these that sustained very deep slip and high stress drop (>20 MPa). The rupture may have extended to depths of up to 60 km, suggesting that the oceanic lithosphere in the northern Wharton Basin may be cold and strong enough to sustain brittle failure at such depths. Alternatively, the rupture may have occurred with an alternative weakening mechanism, such as thermal runaway.

Very shallow dyke intrusion and potential slope failure imaged by ground deformation: The 28 December 2014 eruption on Mount Etna

AbstractOn 28 December 2014, eruptive activity resumed at Mount Etna with fire fountain activity feeding two lava flows on the eastern and southwestern upper flanks of the volcano. Unlike all previous summit activity, this eruption produced clear deformation at the summit of the volcano. GPS displacements and Sentinel‐1A ascending interferograms were calculated in order to image the ground deformation pattern accompanying the eruption. The displacements observed by GPS depict a local ground deformation pattern, affecting only the upper part of the volcano. Despite snow cover on the summit, differential interferometry synthetic aperture radar (DInSAR) data allowed obtaining more detail on the ground deformation pattern on the upper eastern side of the volcano. Three‐dimensional GPS displacements inversion located a very shallow NE‐SW intrusion just beneath the New Southeast Crater. However, this model cannot justify all the deformation observed by DInSAR thus revealing a gravitational failure of the lava flow field.

Postseismic Deformation Following the 2010 $$M = 7.2$$ M = 7.2 El Mayor-Cucapah Earthquake: Observations, Kinematic Inversions, and Dynamic Models

Due to its location on a transtensional section of the Pacific-North American plate boundary, the Salton Trough is a region featuring large strike-slip earthquakes within a regime of shallow asthenosphere, high heat flow, and complex faulting, and so postseismic deformation there may feature enhanced viscoelastic relaxation and afterslip that is particularly detectable at the surface. The 2010 M=7.2 El Mayor-Cucapah earthquake was the largest shock in the Salton Trough since 1892 and occurred close to the US-Mexico border, and so the postseismic deformation recorded by the continuous GPS network of southern California provides an opportunity to study the rheology of this region. Three-year postseismic transients extracted from GPS displacement time-series show four key features: (1) 1–2 cm of cumulative uplift in the Imperial Valley and ∼ 1 cm of subsidence in the Peninsular Ranges, (2) relatively large cumulative horizontal displacements > 150 km from the rupture in the Peninsular Ranges, (3) rapidly decaying horizontal displacement rates in the first few months after the earthquake in the Imperial Valley, and (4) sustained horizontal velocities, following the rapid early motions, that were still visibly ongoing 3 years after the earthquake. Kinematic inversions show that the cumulative 3-year postseismic displacement field can be well fit by afterslip on and below the coseismic rupture, though these solutions require afterslip with a total moment equivalent to at least a M=7.2 earthquake and higher slip magnitudes than those predicted by coseismic stress changes. Forward modeling shows that stress-driven afterslip and viscoelastic relaxation in various configurations within the lithosphere can reproduce the early and later horizontal velocities in the Imperial Valley, while Newtonian viscoelastic relaxation in the asthenosphere can reproduce the uplift in the Imperial Valley and the subsidence and large westward displacements in the Peninsular Ranges. We present two forward models of dynamically coupled deformation mechanisms that fit the postseismic transient well: a model combining afterslip in the lower crust, Newtonian viscoelastic relaxation in a localized zone in the lower crust beneath areas of high heat flow and geothermal activity, and Newtonian viscoelastic relaxation in the asthenosphere; and a second model that replaces the afterslip in the first model with viscoelastic relaxation with a stress-dependent viscosity in the mantle. The rheology of this high-heat-flow, high-strain-rate region may incorporate elements of both these models and may well be more complex than either of them.

Rupture Process of the 23 October 2011 Mw7.1 Van Earthquake in Eastern Turkey by Joint Inversion of Teleseismic, GPS and Strong-Motion Data

On 23 October 2011, a strong earthquake (Mw = 7.1) struck the Van (Eastern Turkey) region and its surrounding areas at 10:41:22 UTC (hereafter referred to as the Van earthquake), causing severe damage to the source region. Several studies have been carried out on the focal mechanism, seismic moment, focal depth and rupture model of this earthquake. However, there are still significant differences in the moment release, focal depth and slip asperities among these models. Since most of these models only used a single data set to investigate the source parameters, the reliability of these results is still a concern. In order to make clear the uncertainties and provide a reliable detailed rupture model for the mainshock, we investigated the rupture process of the Van earthquake by a joint inversion of teleseismic broadband seismograms, near-field static GPS displacement records and strong-motion data. The inversion results indicate that the mainshock was dominated by a thrust slip with a small part of a left-lateral strike-slip component below the hypocenter. The rupture initiated at a focal depth of 16 km and propagated to the surface with a relatively low average rupture velocity of ~1.8 km/s, suggesting that the major energy of the earthquake was released in a long-period band, which is the main reason why the seismic moments inverted by a long period or static signals are higher than those obtained by short-period data. Most of the slip occurred around the hypocenter with a maximum slip of more than 3.3 m, and the associated static stress drop was ~3 MPa. The total seismic moment of the whole fault was 5.76 × 1019 N·m, and most energy was released in the first 20 s, which is in the variation range of the released rupture models. The major slip was concentrated at deeper depth and extended to a depth of around 25 km. Meanwhile, the surface rupture was quite small, which explains why only weak ruptures were observed at the surface although caused by such a strong earthquake.

An adjoint-based FEM optimization of coseismic displacements following the 2011 Tohoku earthquake: new insights for the limits of the upper plate rebound

The 11 March 2011 Tohoku earthquake was the strongest event recorded in recent historic seismicity in Japan. Several researchers reported the deformation and possible mechanism as triggered by a mega thrust fault located offshore at the interface between the Pacific and the Okhotsk Plate. The studies to estimate the deformation in detail and the dynamics involved are still in progress. In this paper, coseismic GPS displacements associated with Tohoku earthquake are used to infer the amount of slip on the fault plane. Starting from the fault displacements configuration proposed by Caltech-JPL ARIA group and Geoazur CNRS, an optimization of these displacements is performed by developing a 3D finite element method (FEM) model, including the data of GPS-acoustic stations located offshore. The optimization is performed for different scenarios which include the presence of topography and bathymetry (DEM) as well as medium heterogeneities. By mean of the optimized displacement distribution for the most complete case (heterogeneous with DEM), a broad slip distribution, not narrowly centered east of hypocenter, is inferred. The resulting displacement map suggests that the beginning of the area of subsidence is not at east of MYGW GPS-acoustic station, as some researchers have suggested, and that the area of polar reversal of the vertical displacement is rather located at west of MYGW. The new fault slip distribution fits well for all the stations at ground and offshore and provides new information on the earthquake generation process and on the kinematics of Northern Japan area.

Geodetic evidence for tectonic activity on the Strymon Fault System, northeast Greece

The current kinematic patterns of northeast Greece remain largely unknown. This is mainly because this region is characterised by low seismicity rates and is poorly covered by GPS stations. Here, we analyse new homogeneous GPS data collected over a period of 5.5years from the first permanent network in northeast Greece (HEPOS) to shed light in the kinematics of this region. We find that GPS displacement vectors that derive from either side of the natural depression of the Strymon Valley differ significantly in orientation and magnitude. Specifically, we find that across a distance of ca. 30km the GPS displacement vectors change orientation by >130° (from NNW to SSW), producing a mean horizontal strain rate of 3.3±0.3mm/yr. We attribute this kinematic translation to result from movement along the Strymon Fault System (SFS), a >200km-long structure that strikes NW-SE and extends from SW Bulgaria to North Aegean (where possibly abuts against the North Anatolian Fault). Up to date, the SFS was considered to be a major Alpine and Neogene geotectonic boundary of unclear Holocene activity. If the interseismic strain stored currently across the fault system is released seismically, large earthquakes (M>7) and associated coseismic lateral displacements should be expected in the area. These results call for a better assessment of the earthquake risk in northeast Greece, which is currently evaluated as a low seismic hazard region.

Mathematical Modeling for Lateral Displacement Induced by Wind Velocity Using Monitoring Data Obtained from Main Girder of Sutong Cable-Stayed Bridge

Based on the health monitoring system installed on the main span of Sutong Cable-Stayed Bridge, GPS displacement and wind field are real-time monitored and analyzed. According to analytical results, apparent nonlinear correlation with certain discreteness exists between lateral static girder displacement and lateral static wind velocity; thus time series of lateral static girder displacement are decomposed into nonlinear correlation term and discreteness term, nonlinear correlation term of which is mathematically modeled by third-order Fourier series with intervention of lateral static wind velocity and discreteness term of which is mathematically modeled by the combined models of ARMA(7,4)and EGARCH(2,1). Additionally, stable power spectrum density exists in time series of lateral dynamic girder displacement, which can be well described by the fourth-order Gaussian series; thus time series of lateral dynamic girder displacement are mathematically modeled by harmonic superposition function. By comparison and verification between simulative and monitoring lateral girder displacements from September 1 to September 3, the presented mathematical models are effective to simulate time series of lateral girder displacement from main girder of Sutong Cable-Stayed Bridge.

高低差の大きい長大斜面のGPS変位計測における不動点計測値を用いた対流圏遅延補正法の提案と効果の検証

高低差の大きい長大斜面のGPS変位計測における不動点計測値を用いた対流圏遅延補正法の提案と効果の検証

A framework for fast probabilistic centroid-moment-tensor determination—inversion of regional static displacement measurements

The determination of earthquake source parameters is an important task in seismology. For many applications, it is also valuable to understand the uncertainties associated with these determinations, and this is particularly true in the context of earthquake early warning (EEW) and hazard mitigation. In this paper, we develop a framework for probabilistic moment tensor point source inversions in near real time. Our methodology allows us to find an approximation to p(m|d), the conditional probability of source models (m) given observations (d). This is obtained by smoothly interpolating a set of random prior samples, using Mixture Density Networks (MDNs)—a class of neural networks which output the parameters of a Gaussian mixture model. By combining multiple networks as ‘committees’, we are able to obtain a significant improvement in performance over that of a single MDN. Once a committee has been constructed, new observations can be inverted within milliseconds on a standard desktop computer. The method is therefore well suited for use in situations such as EEW, where inversions must be performed routinely and rapidly for a fixed station geometry. To demonstrate the method, we invert regional static GPS displacement data for the 2010 MW 7.2 El Mayor Cucapah earthquake in Baja California to obtain estimates of magnitude, centroid location and depth and focal mechanism. We investigate the extent to which we can constrain moment tensor point sources with static displacement observations under realistic conditions. Our inversion results agree well with published point source solutions for this event, once the uncertainty bounds of each are taken into account.