Although blind normal faults are common in subduction environments, their rheology, kinematics and interaction with the upper crust are poorly constrained. A month-long shallow normal faulting sequence in the Ibaraki-Fukushima prefectural border (IFPB), northeast Japan, which followed the Mw9.0 Tohoku-Oki earthquake (TOE) and culminated in the Mw6.7 Iwaki earthquake, provides a window into megathrust-to-normal fault interaction. Stress change calculations indicate that direct triggering by the TOE co- and post-seismic slip does not provide a plausible explanation for the IFPB earthquake sequence. In quest for an alternative triggering mechanism, we analyzed post-TOE GNSS data from eastern IFPB. A key step in this analysis is the removal of the large-scale post-TOE displacement field, after which a distinct highly-localized strain along the coastline becomes apparent. The accumulation of this strain was mostly aseismic, and migrated with time prior to the Iwaki earthquake in a manner that correlates well with post-TOE local seismicity. We attribute the pre-Iwaki earthquake strain accumulation to aseismic slip along low-angle seaward dipping blind normal fault, activated by the TOE. Stresses transferred by this slip episode accelerated the failure along the IFPB shallow normal faults. This indirect triggering of the Iwaki earthquake sequence by the TOE highlights the complexity of stress transfers in subduction environments.

Tracking the onset of inelastic (permanent) deformation is critical to quantifying the stress experienced by an aquifer system so that the effects of current groundwater extraction practices are put in the context of the sedimentary and geological histories of a region. However, the pre-consolidation stress is rarely known due to the lack of multi-decadal ground-based data. In this paper, we propose a new approach to track the onset and spatial evolution of inelastic deformation based on a 2003-2020 multi-sensor Interferometric Synthetic Aperture Radar time series analysis. Our study reveals that in central Iran, many locations that used to experience elastic (recoverable) deformation just a few years ago, are now deforming inelastically, leading to irreversible lowering of the ground surface and irreversible loss of aquifer storage. Lithologic data reveals that the total thickness of the drained clay layers controls the extent and timing of the observed inelastic deformation, while groundwater data confirms that the multi-decadal lowering of groundwater levels is driving the long-term compaction. These results highlight that we are now at or near a tipping point in time between sustainability and permanent damage to our underground water resources, emphasizing the fact that current decisions have the potential to change the natural resources landscape permanently.

We use Interferometric Synthetic Aperture Radar (InSAR) data collected by the Sentinel-1 mission to study the co- and postseismic deformation due to the 2017 Mw 7.3 Sarpol-e Zahab earthquake that occurred near the Iran-Iraq border in Northwest Zagros. We find that most of the coseismic moment release is between 15 and 21 km depth, well beneath the boundary between the sedimentary cover and underlying basement. Data from four satellite tracks reveal robust postseismic deformation during ~ 12 months after the mainshock (from November 2017 to December 2018). Kinematic inversions show that the observed postseismic InSAR LOS displacements are well explained by oblique (thrust + dextral) afterslip both updip and downdip of the coseismic peak slip area. The dip angle of the shallow afterslip fault plane is found to be significantly smaller than that of the coseismic rupture, corresponding to a shallowly dipping detachment located near the base of the sediments or within the basement, depending on the thickness of the sedimentary cover, which is not well constrained over the epicentral area. Aftershocks during the same time period exhibit a similar temporal evolution as the InSAR time series, with most of aftershocks being located within and around the area of maximum surface deformation. The postseismic deformation data are consistent with stress-driven afterslip models, assuming that the afterslip evolution is governed by rate-strengthening friction. The inferred frictional properties updip and downdip of the coseismic rupture are significantly different, which likely reflect differences in fault zone material at different depths along the Zagros.

Long-term fault growth involves the dynamic evolution of fault zone architecture, structural maturity, and physical properties. Accurate characterization of these features is essential for improved understanding of fault mechanics, rupture dynamics and earthquake hazard. Fault structural maturity has traditionally been quantified via analysis of geological features. Nonetheless, the manifestations of an incipient fault are still poorly known, partly due to a lack of fault outcrops and limited diagnostic characteristics of this type of fault. In this study, we integrate coseismic and postseismic geodetic (InSAR/GPS) observations, relocated aftershocks, optical satellite imagery, and field measurements to characterize the fault kinematics of the May 21 2001 Mw7.4 Maduo earthquake, which occurred on an immature fault. Using relocated aftershocks, we determine the fault damage zone thickness and damage density decay at a comparable resolution with field geological investigations. We analyze surface inelastic strain along the rupture using both InSAR and optical images. We construct a buried slip model and refine the coseismic slip distribution to determine a shallow slip deficit, which we attribute to off-fault deformation. We also examine the afterslip distribution and moment release following the earthquake to probe its relationship with coseismic rupture. All pieces of evidence point to the dominant role of immaturity of the fault hosting the Maduo earthquake. Our study demonstrates that the combined analysis of seismological data, geodetic observations and field measurements helps to comprehensively characterize fault structural maturity and to better understand the role of single earthquakes in the long-term fault zone evolution.

Observations of fold growth in fold-thrust belt settings show that brittle deformation can be localized or distributed. Localized shear is associated with frictional slip on primary faults, while distributed brittle deformation is recognized in the folding of the bulk medium. The interplay of these processes is clearly seen in fault-bend folds, which are folds cored by a fault with an abrupt change in dip (e.g., a ramp-décollement system). While the kinematics of fault-bend folding were described decades ago, the dynamics of these structures remain poorly understood, especially the evolution of fault slip and off-fault deformation over different periods of the earthquake cycle. In order to investigate the dynamics of fault-bend folding, we develop a numerical modeling framework that combines a long-term elasto-plastic model of folding in a layered medium with a rate-state frictional model of fault strength evolution in order to simulate geologically and mechanically consistent earthquake sequences. In our simulations, slip on the ramp-décollement fault and inelastic fold deformation are mechanically coupled processes that build geologic structure. As a result, we observe that folding of the crust does not occur steadily in time but is modulated by earthquake cycle stresses. We suggest combining seismological and geodetic observations with geological fault models to uncover how elastic and inelastic crustal deformation generate fault-bend folds. We find that distinguishing between the elastic and inelastic response of the crust to fault slip is possible only in the postseismic period following large earthquakes, indicating that for most fault systems this information currently remains inaccessible.

Groundwater extraction at rates exceeding recharge is occurring throughout Iran for agricultural and industrial activities, resulting in land subsidence in many areas, particularly the Yazd-Ardakan Plain (YAP) in the dry and desert regions of central Iran. In this study, Interferometric Synthetic Aperture Radar (InSAR) time series analysis and statistical models are used to characterize the controls on land subsidence in the YAP, from 2003 to 2020. Our results reveal the existence of a northwest-southeast elongated area of 363 experiencing subsidence at rates up to 15 cm/yr. In the YAP, the international Airport, railway, transit road, and several industrial and historical sites are threatened by the differential subsidence. Well data confirm that groundwater levels have decreased by 18 meters between 1974 and 2018, driving the compaction of sediments within the underlying aquifer system. Our statistical analysis shows that the thickness of a shallow, clay-rich aquitard layer controls the extent of the observed subsidence and an Independent Component Analysis of the InSAR time series shows that inelastic compaction dominates. This work reveals that in central Iran, current groundwater extraction practices are not sustainable and result in permanent subsidence, ground fractures with impact on infrastructures, and a permanent decrease in water storage capacity.

Recent studies of anthropogenically induced seismicity have improved our understanding of the causal relationships between earthquakes and industrial activity. Whether larger-magnitude earthquakes can be triggered and how human injection and production of fluids interact with active faults remain poorly understood. The 2017 Mw 6.5 Leyte earthquake nucleated at a depth near the production zone and within 1 km of an actively producing geothermal field in the Philippines. We use satellite radar data to constrain the pre-earthquake ground deformation across the field and the Leyte fault and to determine the coseismic source parameters. From consideration of regional historical seismicity and fluid extraction model constrained by fluid injection and extraction rates, we find evidence suggesting that the mainshock is directly associated with the geothermal production efforts. Our findings demonstrate that the extraction of geothermal power close to active fault zones is capable of triggering damaging earthquakes, a hazard that was previously underappreciated.

We explore seasonal mass oscillations by continental water storage in Southeast Asia and Himalayan arc region using continuous Global Positioning System (cGPS) measurements and satellite data from the Gravity Recovery and Climate Experiment (GRACE). While the interaction between seasonally induced non-tectonic and tectonic deformation along the Himalayan plate boundary is still debated, we propose that tectonic deformation along this plate boundary can be significantly influenced by the deformation induced by the non-tectonic hydrological loading cycles. We suggest that the substantially higher transient displacements above the base of the seismogenic zone indicate a role of changes in aseismic slip rate on the deep megathrust that may be controlled by seasonal hydrological loading. We invoke modulation of aseismic slip on the megathrust down-dip of the seismogenic zone due to a fault resonance process induced by the seasonal stress changes. This process modulates mid-crustal ramp associated micro-seismicity and influences the timing of Central Himalayan earthquakes.

The left-lateral Xianshuihe Fault is located at the eastern boundary of the Tibetan Plateau and is one of the most active faults in China. It is associated with substantial seismic potential, with more than 20 Mw>6 earthquakes since 1700. The fault has been documented to be creeping at the surface for decades; however, the spatial and temporal distributions of shallow creep along the fault are not well resolved. In this study, we obtain high-resolution interseismic velocity maps along the 350-km-long central Xianshuihe Fault and Interferometric Synthetic Aperture Radar (InSAR) timeseries along the Kangding segment using ascending and descending Sentinel-1 data. The InSAR data reveal multiple creeping sections and the estimated surface creep rates show high along-strike variability. A coupling model characterizes the distribution of creep with depth. The seismic potential of apparent rupture asperities along the Xianshuihe Fault is further refined by considering fault-crossing baseline data and the distribution of historical ruptures and microseismicity. Moreover, a stress-driven afterslip model of the observed accelerated creep around the Mw 5.9 2014 Kangding rupture indicates substantial shallow afterslip, which provides further constraints on the distribution of locked and creeping patches along this section of the Xianshuihe Fault.

The Calaveras Fault (CF) branches from the San Andreas Fault (SAF) near San Benito, extending sub-parallel to the SAF for about 50 km with only 2-6 km separation and diverging northeastward. Both the SAF and CF are partially coupled, exhibit spatially variable aseismic creep and have hosted moderate to large earthquakes in recent decades. Understanding how slip partitions among the main fault strands of the SAF system and establishing their degree of coupling is crucial for seismic hazard evaluation. We perform a timeseries analysis using more than 5 years of Sentinel-1 data covering the Bay Area (May 2015-October 2020), specifically targeting the spatiotemporal variations of creep rates around the SAF-CF junction. We derive the surface creep rates from cross-fault InSAR timeseries differences along the SAF and CF including adjacent Sargent and Quien Sabe Faults. We show that the variable creep rates (0-20 mm/yr) at the SAF-CF junction are to first order controlled by the angle between the fault strike and the background stress orientation. We further examine the spatiotemporal variation of creep rates along the SAF and CF and find a multi-annual coupling increase during 2016-2018 the subparallel sections of both faults, with the CF coupling change lagging behind the SAF by 3 to 6 months. Similar temporal variations are also observed in both b-values inferred from declustered seismicity and aseismic slip rates inferred from characteristic repeating earthquakes. The high correlation of b-value and slip-rate changes may indicate that the SAF is extremely sensitive to small stress perturbations.

The Ridgecrest sequence (Mw6.4 and Mw7.1, July 2019, California) is a cross-fault earthquake that has been observed using a wide range of geophysical and geological methods. The sequence ruptured consecutively two orthogonal cross-fault systems within 34 hours (northeast- and northwest-trending). It raised the question of the relation between the two systems of faults both at depth and at the surface, and its impact on the surface displacement pattern. Here we use high-resolution (50 cm) satellite optical image correlation to measure the 3D surface displacement field at 0.5 meters ground resolution for the two earthquakes. Because our images bracket the whole sequence, our displacement and deformation maps include both earthquakes. Our data allow for measuring series of slip profiles in the components parallel and perpendicular to the rupture, and in the vertical direction, to look at the correlation between slip distribution and rupture complexity at the surface. We point out significant differences with previous geodetic and geological-based measurements and show the essential role of distributed faulting and diffuse deformation in the comprehension of surface displacement patterns. We discuss the segmentation of the rupture regarding the fault geometry and along-strike slip variations. We image several surface deformation features with similar orientation to the deeply embedded fabric identified in seismic studies. This northeast-trending fabric influenced the surface deformation both during the foreshock and the mainshock earthquakes. We also derive strain fields from the horizontal displacement maps and show the predominant role of rotational and shear strains in the rupture process. We finally compare our results to kinematic inversions and show that the foreshock did influence the mainshock by clamping the fault and encouraging off-fault diffuse deformation rather than fault slip in some areas.

The 2011 Mw 9.0 Tohoku-oki earthquake is one of the world’s best-recorded ruptures. In the aftermath of this devastating event, it is important to learn from the complete record. We describe the state of knowledge of the megathrust earthquake generation process before the earthquake, and what has been learned in the decade since the historic event. Prior to 2011, there were a number of studies suggesting the potential of a great megathrust earthquake in NE Japan from geodesy, geology, seismology, geomorphology, and paleoseismology, but results from each field were not enough to enable a consensus assessment of the hazard. A transient unfastening of interplate coupling and foreshock activity were recognized before the earthquake, but did not lead to alerts. Since the mainshock, follow-up studies have (1) documented that the rupture occurred in an area with a large interplate slip deficit, (2) established large near-trench coseismic slip, (3) examined structural anomalies and fault-zone materials correlated with the coseismic slip, (4) clarified the historical and paleoseismic recurrence of M~9 earthquakes, and (5) identified various kinds of possible precursors. The studies have also illuminated the heterogeneous distribution of coseismic rupture, aftershocks, slow earthquakes and aseismic afterslip, and the enduring viscoelastic response, which together make up the complex megathrust earthquake cycle. Given these scientific advances, the enhanced seismic hazard of an impending great earthquake can now be more accurately established, although we do not believe such an event could be predicted with confidence.