Geodetic Slip Research Articles

Robust Imaging of Fault Slip Rates in the Walker Lane and Western Great Basin From GPS Data Using a Multi‐Block Model Approach

AbstractThe Walker Lane (WL) in the western Great Basin (GB) is an active plate boundary system accommodating 10%–20% of the relative tectonic motion between the Pacific and North American plates. Its neotectonic framework is structurally complex, having hundreds of faults with various strikes, rakes, and crustal blocks with vertical axis rotation. Faults slip rates are key parameters needed to quantify seismic hazard in such tectonically active plate boundaries but modeling them in complex areas like the WL and GB is challenging. We present a new modeling strategy for estimating fault slip rates in complex zones of active crustal deformation using data from GPS networks. The technique does not rely on prior estimates of slip rates from geologic studies, and only uses data on the surface trace location, dip, and rake. The iterative framework generates large numbers of block models algorithmically from the fault database to obtain many estimates of slip rates for each fault. This reduces bias from subjective choices about how discontinuous faults connect and interact to accommodate strain. Each model iteration differs slightly in block boundary configuration, but all models honor geodetic and fault data, regularization, and are kinematically self‐consistent. The approach provides several advantages over bespoke models, including insensitivity to outlier data, realistic uncertainties, explicit mapping of off‐fault deformation, and slip rates that are more objective and independent of geologic slip rates. Comparisons to the U.S. National Seismic Hazard Model indicate that ∼80% of our geodetic slip rates agree with their geologic slip rates to within uncertainties.

Inverting Geodetic Strain Rates for Slip Deficit Rate in Complex Deforming Zones: An Application to the New Zealand Plate Boundary

AbstractThe potential for future earthquakes on faults is often inferred from inversions of geodetically derived surface velocities for locking on faults using kinematic models such as block models. This can be challenging in complex deforming zones with many closely spaced faults or where deformation is not readily described with block motions. Furthermore, surface strain rates are more directly related to coupling on faults than surface velocities. We present a methodology for estimating slip deficit rate directly from strain rate and apply it to New Zealand for the purpose of incorporating geodetic data in the 2022 revision of the New Zealand National Seismic Hazard Model. The strain rate inversions imply slightly higher slip deficit rates than the preferred geologic slip rates on sections of the major strike‐slip systems including the Alpine Fault, the Marlborough Fault System and the northern part of the North Island Fault System. Slip deficit rates are significantly lower than even the lowest geologic estimates on some strike‐slip faults in the southern North Island Fault System near Wellington. Over the entire plate boundary, geodetic slip deficit rates are systematically higher than geologic slip rates for faults slipping less than one mm/yr but lower on average for faults with slip rates between about 5 and 25 mm/yr. We show that 70%–80% of the total strain rate field can be attributed to elastic strain due to fault coupling. The remaining 20%–30% shows systematic spatial patterns of strain rate style that is often consistent with local geologic style of faulting.

Geodetic slip rate partitioning in the central Anninghe and Daliangshan faults: Effects of fault geometry, elastic heterogeneity, and viscoelastic rheology

Geodetic measurements can be employed to estimate the secular slip rates and locking depths of crustal faults, two key parameters which influence assessment of long-term seismic hazard presented by a fault. However, this process requires making assumptions about the rheological properties of the Earth's crust and lithosphere, as well as the fault geometry at depth. Previous geodetic studies have inferred widely varying slip rates and locking depths for the central Xianshuihe-Xiaojiang Fault System, located along the southeastern margin of the Tibetan plateau. In this study, we use updated GNSS measurements, including 13 new velocities, to explore the effects of assumed rheology and fault geometry on the inferred slip rates and locking depths of the subparallel Anninghe (ANH) and Daliangshan (DLS) faults in this region. We demonstrate that only models taking into account the viscoelastic relaxation of the lower crust can resolve the apparent discrepancies. Models assuming a fully elastic rheology suggest very deep locking depths (∼35 km) and a lower slip rate on the ANH fault (∼5 mm/yr) than the DLS fault (∼7 mm/yr). Neither the relative slip rates nor the locking depths agree with seismic or geologic observations. We also consider models assuming a heterogeneous elastic rheology, but find they are indistinguishable from the homogeneous models. Modifying the fault geometry can impact the relative slip rates of the two faults but does not produce a reasonable locking depth either. In contrast, viscoelastic models accounting for seismic cycle effects indicate a faster ANH slip rate (∼5–7 mm/yr), a slower DLS rate (∼4–5 mm/yr), and a shallower locking depth for both faults (∼18 km), all of which are more consistent with independent seismic and geological observations. Our study suggests that steady long-term motion on these faults is modulated by long-lasting viscoelastic effects over a relatively broad region.

Paleoseismic Earthquake Recurrence Interval Derivation for the 2022 Revision of the New Zealand National Seismic Hazard Model

Abstract Recurrence intervals of ground-surface rupturing earthquakes are one of numerous datasets used to constrain the rates of fault ruptures in the 2022 revision of the New Zealand National Seismic Hazard Model (NZ NSHM 2022). Paleoearthquake timing and single-event displacement (SED) data in the New Zealand Paleoseismic Site Database version 1.0 alongside geologic and geodetic slip rates from the New Zealand Community Fault Model version 1.0 and NZ NSHM 2022 Geodetic Deformation Model were used to estimate recurrence intervals on faults across New Zealand for inclusion in the NZ NSHM 2022. Past earthquake timings were fit with lognormal, exponential, and Brownian Passage Time recurrence models to derive probability density functions (PDFs) of mean recurrence interval (MRI) in a Bayesian framework. At some sites, SED and slip-rate (SR) data were used to estimate PDFs of MRI; and at sites where timings, slip rate, and displacement data are available, the timings-based and slip-based PDFs were combined to develop tighter constraints on MRI. Using these approaches, we produce a database of maximum-likelihood MRIs and their uncertainties for 80 sites across New Zealand. The resulting recurrence interval dataset is publicly available and is the largest such dataset in New Zealand to date. It provides a valuable resource for future seismic hazard modeling and highlights areas that would benefit from future study.

Slip Deficit Rates on Southern Cascadia Faults Resolved with Viscoelastic Earthquake Cycle Modeling of Geodetic Deformation

ABSTRACT The fore-arc of the southern Cascadia subduction zone (CSZ), north of the Mendocino triple junction (MTJ), is home to a network of Quaternary-active crustal faults that accumulate strain due to the interaction of the North American, Juan de Fuca (Gorda), and Pacific plates. These faults, including the Little Salmon and Mad River fault (LSF and MRF) zones, are located near the most populated parts of California’s north coast and show paleoseismic evidence for three slip events of several-meter scale in the past 1700 yr. However, the geodetic slip rates of these faults are poorly constrained. In this work, we analyze a new compilation of interseismic geodetic velocities from Global Navigation Satellite Systems, leveling, and tide gauge data near the MTJ to constrain present-day slip deficit rates on upper-plate faults and coupling on the megathrust. We construct Green’s functions for interseismic slip deficit for discrete faults embedded in an elastic plate overlying a viscoelastic mantle. We then use a constrained least-squares inversion to determine best-fitting slip rates on the major faults and investigate slip rate trade-offs between faults. Results indicate that the LSF and MRF systems together accumulate 4–5 mm/yr of reverse-slip deficit, although their separate slip rates cannot be determined independently. Modeling of the horizontal and vertical velocities suggests that the southernmost CSZ is coupled interseismically to deeper than 25 km depth. We also find that 6–17 mm/yr of right-lateral slip deficit extends north of the MTJ and into the southern Cascadia fore-arc. These results reinforce the notion that both the southernmost Cascadia megathrust and the smaller fore-arc faults above it contribute to regional seismic hazard.

A fault-based probabilistic seismic hazard model for Lebanon, controlling parameters and hazard levels

The present work develops a comprehensive probabilistic seismic hazard study for Lebanon, a country prone to a high seismic hazard since it is located along the Levant fault system. The historical seismicity has documented devastating earthquakes which have struck this area. Contrarily, the instrumental period is typical of a low-to-moderate seismicity region. The source model built is made of a smoothed seismicity earthquake forecast based on the Lebanese instrumental catalog, combined with a fault model including major and best-characterized faults in the area. Earthquake frequencies on faults are inferred from geological as well as geodetic slip rates. Uncertainties at every step are tracked and a sensitivity study is led to identify which parameters and decisions most influence hazard estimates. The results demonstrate that the choice of the recurrence model, exponential or characteristic, impacts the most the hazard, followed by the uncertainty on the slip rate, on the maximum magnitude that may break faults, and on the minimum magnitude applied to faults. At return periods larger than or equal to 475 years, the hazard in Lebanon is fully controlled by the sources on faults, and the off-fault model has a negligible contribution. We establish a source model logic tree populated with the key parameters, and combine this logic tree with three ground-motion models (GMMs) potentially adapted to the Levant region. A specific study is led in Beirut, located on the hanging-wall of the Mount Lebanon fault to understand where the contributions come from in terms of magnitudes, distances and sources. Running hazard calculations based on the logic tree, distributions of hazard estimates are obtained for selected sites, as well as seismic hazard maps at the scale of the country. Considering the PGA at 475 years of return period, mean hazard values found are larger than 0.3 g for sites within a distance of 20–30 km from the main strand of the Levant Fault, as well as in the coastal region in-between Saida and Tripoli (≥ 0.4 g considering the 84th percentile). The study provides detailed information on the hazard levels to expect in Lebanon, with the associated uncertainties, constituting a solid basis that may help taking decisions in the perspective of future updates of the Lebanese building code.

Spatiotemporal variation of crustal deformation in northeastern Tibet following the 2008Mw 7.9 Wenchuan earthquake and its impact on fault activity

SUMMARYThe 2008 Wenchuan earthquake, which occurred on the eastern edge of the Tibetan Plateau, produced significant time-dependent post-seismic deformation in the northeastern Tibetan Plateau. To explore the spatio-temporal evolution of crustal deformation and its impact on nearby active faults due to the Wenchuan earthquake, we first solve the velocity fields, strain rates and slip rates of the major faults at different stages of the earthquake cycle, based on GPS observations spanning approximately 10 yr before and after the Wenchuan earthquake. The results show that: (a) during the late interseismic phase, the GPS velocity fields relative to the Sichuan basin and strain rates near the Longmenshan fault (the seismogenic fault of the Wenchuan earthquake) are insignificant in magnitude, and the geodetic slip rates of the major faults are in good agreement with geological investigations; (b) After the Wenchuan earthquake, post-seismic deformation on both flanks of the causative Longmenshan fault is asymmetrically distributed, with significant deformation distributed between the Longmenshan fault and the Longriba fault. The post-seismic deformation decreases with increasing distance away from the Longmenshan fault. Furthermore, geodetic slip rates for the major active faults at the early post-seismic stage differ significantly from those at the late interseismic stage; (c) The crustal deformation in the northeastern Tibetan Plateau is generally small during the late interseismic phase, but it increases significantly during the early post-seismic stage and begins to decrease again as it progresses to the second post-seismic stage. We speculate that this kinematic evolution of crustal deformation is ascribed to that the Longmenshan fault was in a locking state during the late interseismic period, whereas the Longmenshan fault is no longer locked after the Wenchuan earthquake, and thus the marked post-seismic deformation occurred in the epicentral and its surrounding area. Then, the post-seismic deformation becomes weaker with time lapse, and generally the crustal deformation begins to slowly recover to the pre-seismic level. Using a forward model, we test whether post-seismic deformation caused by viscoelastic relaxation of the lower crust 4 yr after the Wenchuan earthquake can explain the observed spatio-temporal pattern of the crustal deformation. We find that this mechanism can account for the evolutionary pattern of crustal motion in the Songpan-Ganzi block east of the Longriba fault in central-east Tibet and central-eastern and southern regions of the West Qinling-Songpan Tectonic Syntaxis in east Tibet 4 yr after the Wenchuan earthquake. Moreover, the post-seismic viscoelastic relaxation of the Wenchuan earthquake is the main cause of loading rate variations of primary faults in the early post-seismic stage in the northeastern Tibetan Plateau.

Geodetic modelling of the 2022Mw 6.6 Menyuan earthquake: insight into the strain-partitioned northern Qilian Shan fault system and implications for regional tectonics and seismic hazards

SUMMARYStrain partitioning between strike-slip faults in mountains and thrust faults in the foreland is a typical mountain building process to accommodate oblique plate convergence. Studying the geometry and movement of such strain-partitioned fault systems is key to understanding the mountain building process and related seismic hazards. The 2022 Mw 6.6 Menyuan earthquake is the largest strike-slip earthquake to have ruptured the northern Qilian Shan fault system in the modern geodetic era. We combined satellite and field observations to determine the fault geometry and coseismic slip distribution in the Menyuan earthquake, and link the distribution of coseismic slip with the pattern of interseismic strain accumulation within the northern Qilian Shan from our geodetic slip model. We find that the Menyuan earthquake ruptured a 25 km-long section of the left-lateral Longlongling Fault between the surface and 7 km depth. The maximum slip was 4 m at 3–4 km depth. Damage to a high-speed railway tunnel recorded a fault offset of 2.7 m at a depth of 200 m compared to 2.5–3.0 m on the surface, suggesting that dispersion of the rupture through unconsolidated shallow sediments was limited, at least at the tunnel site. We also determined the pattern of interseismic deformation prior to the earthquake using Interferometric Synthetic Aperture Radar and Global Navigation Satellite System data. We found the interseismic geodetic data can be explained by the oblique movement of a low-angle décollement beneath the Qilian Shan rather than a strain partitioning fault system. We suggested that the strike-slip faults and foreland thrusts are separated by a creeping décollement, which would act as a barrier to stop the cascading rupture of the strike-slip and thrust fault.

Updated concepts of seismic gaps and asperities to assess great earthquake hazard along South America

So far in this century, six very large-magnitude earthquakes (MW ≥ 7.8) have ruptured separate portions of the subduction zone plate boundary of western South America along Ecuador, Peru, and Chile. Each source region had last experienced a very large earthquake from 74 to 261 y earlier. This history led to their designation in advance as seismic gaps with potential to host future large earthquakes. Deployments of geodetic and seismic monitoring instruments in several of the seismic gaps enhanced resolution of the subsequent faulting processes, revealing preevent patterns of geodetic slip deficit accumulation and heterogeneous coseismic slip on the megathrust fault. Localized regions of large slip, or asperities, appear to have influenced variability in how each source region ruptured relative to prior events, as repeated ruptures have had similar, but not identical slip distributions. We consider updated perspectives of seismic gaps, asperities, and geodetic locking to assess current very large earthquake hazard along the South American subduction zone, noting regions of particular concern in northern Ecuador and Colombia (1958/1906 rupture zone), southeastern Peru (southeasternmost 1868 rupture zone), north Chile (1877 rupture zone), and north-central Chile (1922 rupture zone) that have large geodetic slip deficit measurements and long intervals (from 64 to 154 y) since prior large events have struck those regions. Expanded geophysical measurements onshore and offshore in these seismic gaps may provide critical information about the strain cycle and fault stress buildup late in the seismic cycle in advance of the future great earthquakes that will eventually strike each region.

Western U.S. Deformation Models for the 2023 Update to the U.S. National Seismic Hazard Model

Abstract This report describes geodetic and geologic information used to constrain deformation models of the 2023 update to the National Seismic Hazard Model (NSHM), a set of deformation models to interpret these data, and their implications for earthquake rates in the western United States. Recent updates provide a much larger data set of Global Positioning System crustal velocities than used in the 2014 NSHM, as well as hundreds of new faults considered as active sources for the 2023 NSHM. These data are interpreted by four geodetic models of deformation that estimate fault slip rates and their uncertainties together with off-fault moment release rates. Key innovations in the 2023 NSHM relative to past practice include (1) the addition of two new (in addition to two existing) deformation models, (2) the revision and expansion of the geologic slip rate database, (3) accounting for fault creep through development of a creep-rate model that is employed by the four deformation models, and (4) accounting for time-dependent earthquake-cycle effects through development of viscoelastic models of the earthquake cycle along the San Andreas fault and the Cascadia subduction zone. The effort includes development of a geologic deformation model that complements the four geodetic models. The current deformation models provide a new assessment of outstanding discrepancies between geologic and geodetic slip rates, at the same time highlighting the need for both geologic and geodetic slip rates to robustly inform the earthquake rate model.

Detection of Aseismic Slip and Poroelastic Reservoir Deformation at the North Brawley Geothermal Field From 2009 to 2019

AbstractThe North Brawley Geothermal Field, located within the Brawley Seismic Zone of Southern California, presents a case study for understanding seismic hazards linked to fluid injection and geothermal energy extraction. An earthquake swarm near the geothermal field in 2012 included two earthquakes with magnitudes greater than 5 and was potentially preceded by a years‐long aseismic slip transient. To better understand ground deformation around the geothermal field, including its evolution with time and its physical mechanisms, we analyze deformation before, during, and after the swarm using ground‐ and satellite‐based geodetic techniques between 2009 and 2019. We integrate observations from GNSS, Sentinel‐1, TerraSAR‐X, UAVSAR, and leveling surveys into a single deformation history. Modeling of this new collection of observations at the North Brawley Geothermal Field provides evidence for 80% more pre‐swarm aseismic slip than previously recognized from 2009 to 2012. During the 2012 Brawley swarm, our geodetic slip inversions closely match the results of seismic waveform inversions from the swarm events. After the 2012 swarm, surface deformation is dominated by poroelastic deformation of a shallow fluid reservoir at <1 km depth rather than fault slip. The deformation history and seismicity catalogs at North Brawley suggest a cessation of fault‐related slip during the ∼7 years after the 2012 earthquake swarm.

Source Process Featuring Asymmetric Rupture Velocities of the 2021 Mw 7.4 Maduo, China, Earthquake from Teleseismic and Geodetic Data

AbstractOn 21 May 2021, an Mw 7.4 left-lateral strike-slip earthquake occurred within the Bayan Har block in the Tibetan plateau. To learn about the source rupture process, we collected the teleseismic waveforms and utilized the backprojection method to investigate the rupture kinematics of the earthquake. The results indicate that the earthquake was a bilateral rupture event with asymmetric rupture velocities. The rupture velocity in the east of the epicenter was uniform and in the range of 2.72–3.67 km/s, whereas, in the west, it was in the range of 1.39–1.78 km/s in the first 20 km and then increased to 2.82–3.17 km/s. The slip distribution constrained by the Interferometric Synthetic Aperture Radar and Global Positioning System displacements clearly reveals kinematic coseismic slip in greater detail, which makes up for the limitations of the backprojection method. Two main asperities in the east verify the results of the backprojection method. The rupture depth in the west was slightly shallower than that in the east, which may be the reason for the asymmetry of rupture velocities. The initial rupture point was updated based on the asymmetric velocities and geodetic slip distribution. The multiple-point-source moment tensors based on the rupture velocities and new initial rupture point not only match the fault geometries determined by relocated aftershocks but also fit well with the released energy distribution, which proves the asymmetry of rupture velocities.

The 29 July 2021 MW 8.2 Chignik, Alaska Peninsula Earthquake Rupture Inferred From Seismic and Geodetic Observations: Re‐Rupture of the Western 2/3 of the 1938 Rupture Zone

AbstractOn 29 July 2021, an MW 8.2 thrust‐faulting earthquake ruptured offshore of the Alaska Peninsula within the rupture zone of the 1938 MW 8.2 earthquake. The spatiotemporal distribution of megathrust slip is resolved by jointly inverting regional and teleseismic broadband waveforms along with co‐seismic static and high‐rate GNSS displacements. The primarily unilateral rupture expanded northeastward, away from the rupture zone of the 22 July 2020 MW 7.8 Shumagin earthquake. Large slip extends along approximately 175 km, spanning about two third of the estimated 1938 aftershock zone, with well‐bounded depth from 20 to 40 km, and up to 8.6 m slip near the hypocenter. The rupture terminated in the eastern portion of the 1938 aftershock zone in a region of very large geodetic slip deficit where peak slip appears to have occurred in the 1938 rupture. The 2021 and 1938 events do not have similar slip distributions and do not indicate persistent asperities.

Co-seismic and post-seismic deformation for the 1995 Colima–Jalisco and 2003 Tecomán thrust earthquakes, Mexico subduction zone, from modelling of GPS data

SUMMARYWe invert ∼25 yr of campaign and continuous Global Positioning System daily positions at 62 sites in southwestern Mexico to estimate co-seismic and post-seismic afterslip solutions for the 1995 Mw = 8.0 Colima–Jalisco and the 2003 Mw = 7.5 Tecomán earthquakes, and the long-term velocity of each GPS site. Estimates of the viscoelastic effects of both earthquakes from a 3-D model with an elastic crust and subducting slab, and linear Maxwell viscoelastic mantle are used to correct the GPS position time-series prior to our time-dependent inversions. The preferred model, which optimizes the fit to data from several years of rapid post-seismic deformation after the larger 1995 earthquake, has a mantle Maxwell time of 15 yr (viscosity of 2 × 1019 Pa s), although upper-mantle viscosities as low as 5 × 1018 Pa s cannot be excluded. Our geodetic slip solutions for both earthquakes agree well with previous estimates derived from seismic data or via static co-seismic offset modelling. The afterslip solutions for both earthquakes suggest that most afterslip coincided with the rupture areas or occurred farther downdip and had cumulative moments similar to or larger than the co-seismic moments. Afterslip thus appears to relieve significant stress along the Rivera plate subduction interface, including the area of the interface between a region of deep non-volcanic tremor and the shallower seismogenic zone. We compare the locations of the seismogenic zone, afterslip and tremor in our study area to those of the neighbouring Guerrero and Oaxaca segments of the Mexico subduction zone. Our newly derived interseismic GPS site velocities, the first for western Mexico that are corrected for the co-seismic and post-seismic effects of the 1995 and 2003 earthquakes, are essential for future estimates of the interseismic subduction interface locking and hence the associated seismic hazard.

GEODETIC SLIP RATE ESTIMATES FOR THE KUMERING AND SEMANGKO SEGMENTS OF THE SUMATERA FAULT

<p>The Sumatran fault is a right lateral active inland fault in southern Sumatra, Indonesia. Although historical earthquake records have shown that magnitude 7 class earthquakes have occurred during the last century, the slip rates along the Sumatran fault have not been studied in detail. This is the first research using a new dense GPS array, in which stations are orthogonal to the fault, to analyze the fault slip rates along the Kumering and Semangko segments in southern Sumatra. In this study, we process GPS data from 14 campaign and continuous GPS points. The results show velocities of 14 mm/yr and 15 mm/yr for these two fault segments, respectively. Our estimated geodetic slip rate suggests that the Sumatran fault has a relatively homogeneous slip rate from southern to northern Sumatra.</p>

Observations noise and stability of geodetic variable slip fault models

SUMMARY Variable slip models of seismic faults represent ill-posed (or underdetermined) problems with infinity of solutions; a single solution is typically obtained using constraints imposed by the L-curve theory, through an experimentally derived coefficient which optimizes the trade-off between fault roughness (smoothness, mean slip gradient) and mean misfit of observations to the fault model. However, in some cases, diverse variable slip fault models (VSFM) have been presented, even using the same data sets. In this paper, we investigate the problem of stability (sensitivity) of VSFM to input geodetic (GNSS/GPS, INSAR) ground slip measurements, based on statistics of synthetic data: statistically similar sets of synthetic ground displacements were the only degree of freedom in an analysis leading to comparable VSFM, regarded as images of slip amplitude and of rake. Differences of synthetic models from the reference fault model for different levels of noise, both for filtered and for PCA-filtered were examined, and results were summarized in images depicting the differences and the variability (standard deviation) of each fault slip patch, both for slip and rake. From this ‘truth’-based approach, it was found that an increase in observations noise is reflected in increase of variability (instability) of fault models, especially away from the fault centre, higher for slip than in rake. Analysis with a different virtual observations system indicates that stations above the fault tend to show spurious stress concentration areas on the fault surface, in agreement with previous studies. Analysis with synthetic data appears as a promising strategy to validate a VSFM based on geodetic data.

Late Holocene Slip Rate of the Mojave Section of the San Andreas Fault near Palmdale, California

ABSTRACT The geologic slip rate on the Mojave section of the San Andreas fault is poorly constrained, despite its importance for understanding earthquake hazard, apparent discrepancies between geologic and geodetic slip rates along this fault section, and long-term fault interactions in southern California. Here, we use surficial geologic mapping, excavations, and radiocarbon and luminescence dating to quantify the displacements and ages of late Holocene landforms offset by the fault at three sites. At the Ranch Center site, the slip rate is determined using the base of a fan marking incision and deflection of an ephemeral channel. At the adjacent Key Slide site, the margin of a landslide deposited on indigenous fire hearths provides a minimum rate. At the X-12 site, the slip rate is determined from a channel that incised into a broad fan surface, and is deflected and beheaded by the fault. We use maximum–minimum bounds on both the displacement and age of each offset feature to calculate slip rate for each site independently. Overlap of the three independent rate ranges yields a rate of 33–39 mm/yr over the last 3 ka, under the assumption that the sites share a common history, given their proximity. Considered in sequence, site-level epistemic uncertainties in the data permit but do not require a rate increase since ∼1200 cal B.P. Modest rate changes can be explained by aleatory variability in earthquake timing and magnitude; larger changes could suggest a shared regional variation with the Garlock and other faults. The new late Holocene slip rates are consistent with geodetic model estimates that include a viscoelastic crust and earthquake cycle effects. The geologic slip rates also provide average slip over dozens of earthquake cycles—a key constraint for long-term earthquake rupture forecasts.

Development of a Trans‐Dimensional Fault Slip Inversion for Geodetic Data

AbstractGeodetic fault slip inversions have generally been performed by employing a least squares method with a spatially uniform smoothing constraint. However, this conventional method has various problems: difficulty in strictly estimating non‐negative solutions, assumption that unknowns follow the Gaussian distributions, unsuitability for expressing spatially non‐uniform slip distributions, and high calculation cost for optimizing many hyper‐parameters. Here, we have developed a trans‐dimensional geodetic slip inversion method using the reversible‐jump Markov chain Monte Carlo (rj‐MCMC) technique to overcome these problems. Because sub‐fault locations were parameterized by the Voronoi partition and were optimized in our approach, we can estimate a slip distribution without the need for spatially uniform smoothing constraints. Moreover, we introduced scaling factors for observational errors. We applied the method to the synthetic data and the actual geodetic observational data associated with the 2011 Tohoku‐oki earthquake and found that the method successfully reproduced the target slip distributions including a spatially non‐uniform slip distribution. The method provided posterior probability distributions with the unknowns, which can express a non‐Gaussian distribution such as large slip with low probability. The estimated scaling factors properly adjusted the initial observational errors and provided a reasonable slip distribution. Additionally, we found that checkerboard resolution tests were useful to consider sensitivity of the observational data for performing the rj‐MCMC method. It is concluded that the developed method is a powerful technique to solve the problems of the conventional inversion method and to flexibly express fault‐slip distributions considering the complicated uncertainties.

Geodetic slip model of the November 26, 2019 Albania earthquake estimated from Sentinel-1 TOPS interferometry

This study uses Sentinel-1 Interferometric Synthetic Aperture Radar (InSAR) data to investigate the source model of the 26 November 2019 Albania earthquake, the largest earthquake to hit the northwestern region of the country in the last century. The preferred InSAR-based coseismic slip model shows that the slip is distributed in a relatively narrow area and is located at a depth of ~11 km with a maximum amplitude of ~0.89 m. The estimated geodetic moment of this earthquake is 3.70 × 1018 Nm, with a shear modulus of 40 GPa, which is equivalent to Mw 6.31. The preferred slip model shows that this earthquake ruptured an east-dipping fault, namely the Shijak Fault wherein the primary slip mechanism is the dip-slip motion. Two-dimensional (2.5-D) surface displacement derived from three pairs of InSAR data is used to ensure the dip direction of the seismogenic fault responsible for the 2019 Albania earthquake. The result of 2.5-D displacement shows the dominance of the westward and uplift motion with a maximum amplitude of ~0.1 m. It indicates that the source fault dips to the east, confirming the result of inversion analysis.

Seismic hazard of multi-segment rupturing for the Anninghe–Zemuhe–Daliangshan fault region, southeastern Tibetan Plateau: constraints from geological and geodetic slip rates

The Anninghe–Zemuhe–Daliangshan region is deformed by two parallel faults, i.e., the Anninghe–Zemuhe fault and the Daliangshan fault. A large seismic gap that characterizes the AZD region in the past 30 years has attracted significant research attentions to study the seismic hazard of these faults. However, it still remains a matter of debate whether the southern part of the Anninghe fault was ruptured with the Zemuhe fault in the 1850 M7.5 earthquake. The multi-segment rupture probabilities should be considered in the seismic hazard analysis. In this study, we divide the Anninghe fault, the Zemuhe fault and the Daliangshan fault into segments which are then used to build different multi-segment rupture combination models. First, we invert the fault slip rate in an elastic block model. Using both the geological and inverted geodetic slip rates as input constraints, we model the seismicity rates on the fault segments. Then, we compare the modeled seismicity rates with the historical earthquake catalog to test the validity of the models. The model, which does not consist of the multi-segment span the Anninghe fault and the Zemuhe fault, and the multi-segment combinations among the segments of the south Daliangshan fault and the north Xiaojiang fault, has the seismicity rates generally consistent with the fault slip rates and historical records. Finally, we use the compatible fault model to calculate the 475-year PGA values. Our modeled PGA values, from both the geological and geodetic slip rates, are averagely higher than those from the National Zonation Map. This methodology is more reasonable as it considers the fault slip rate and multi-segment rupture.