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On 2020 January 9, an M w 6.4 earthquake struck the central Koryak Highlands of eastern Siberia, northeast of the diffuse triple junction between the North American, Pacific and Eurasian plates. The largest earthquake recorded in the central Koryak Highlands to date, it provides an excellent opportunity to study the little-known active tectonics of this remote, sparsely instrumented region. We mapped coherent, coseismic surface deformation with Sentinel 1 Interferometric Synthetic Aperture Radar (InSAR), making this one of the highest latitude earthquakes to be captured successfully with satellite radar, in spite of the rugged, snow-covered terrain. Elastic dislocation modelling, teleseismic backprojections, calibrated hypocentral relocations and teleseismic moment tensor solutions are used to resolve a left-lateral fault trending northwestwards, proximal but perpendicular to a regional geological suture zone, the Khatyrka–Vyvenka Thrust. The earthquake probably ruptured unilaterally northwestwards along a 20 km long segment that appears indistinct in the local topography, and likely generated no surface rupture. We interpret that these observations are indicative of a structurally immature fault zone and estimate a seismogenic zone thickness of 10–15 km. The Koryak Highlands earthquake illustrates how terrane boundaries within cordilleran belts may continue to accommodate tectonic strain long after accretion, resulting in significant earthquakes even along hidden faults. [ABSTRACT FROM AUTHOR]

Airborne lidar (light detection and ranging) topography, acquired before and after an earthquake, can provide an estimate of the coseismic surface displacement field by differencing the preevent and postevent lidar point clouds. However, estimated displacements can be contaminated by the presence of large systematic errors in either of the point clouds. We present three-dimensional displacements obtained by differencing airborne lidar point clouds collected before and after the El Mayor-Cucapah earthquake, a M w 7.2 earthquake that occurred in 2010. The original surface displacement estimates contained large, periodic artifacts caused by systematic errors in the preevent lidar data. Reprocessing the preevent data, detailed herein, removed a majority of these systematic errors that were largely due to misalignment between the scanning mirror and the outgoing laser beam. The methodology presented can be applied to other legacy airborne laser scanning data sets in order to improve change estimates from temporally spaced lidar acquisitions. © 2014. American Geophysical Union. All Rights Reserved.

Subduction forearcs are subject to seismic hazard from upper plate faults that are often invisible to instrumental monitoring networks. Identifying active faults in forearcs therefore requires integration of geomorphic, geologic, and paleoseismic data. We demonstrate the utility of a combined approach in a densely populated region of Vancouver Island, Canada, by combining remote sensing, historical imagery, field investigations, and shallow geophysical surveys to identify a previously unrecognized active fault, the XEOLXELEK‐Elk Lake fault, in the northern Cascadia forearc, ∼10 km north of the city of Victoria. Lidar‐derived digital terrain models and historical air photos show a ∼2.5‐m‐high scarp along the surface of a Quaternary drumlinoid ridge. Paleoseismic trenching and electrical resistivity tomography surveys across the scarp reveal a single reverse‐slip earthquake produced a fault‐propagation fold above a blind southwest‐dipping fault. Five geologically plausible chronological models of radiocarbon dated charcoal constrain the likely earthquake age to between 4.7 and 2.3 ka. Fault‐propagation fold modeling indicates ∼3.2 m of reverse slip on a blind, 50° southwest‐dipping fault can reproduce the observed deformation. Fault scaling relations suggest a M 6.1–7.6 earthquake with a 13 to 73‐km‐long surface rupture and 2.3–3.2 m of dip slip may be responsible for the deformation observed in the paleoseismic trench. An earthquake near this magnitude in Greater Victoria could result in major damage, and our results highlight the importance of augmenting instrumental monitoring networks with remote sensing and field studies to identify and characterize active faults in similarily challenging environments. Plain Language Summary: Faults occurring in the upper plate above a subduction zone are often located near densely populated coastal areas, but their hazard is often underappreciated due to their low deformation rates. In the northern Cascadia forearc on the west coast of North America, high‐resolution topography and geologic mapping show a ∼2.3‐m‐high scarp across a ∼14,000 year‐old land surface 10 km north of downtown Victoria, British Columbia, Canada. This newly identified fault, the XEOLXELEK‐Elk Lake fault (XELF), crosses Saanich Peninsula within Greater Victoria and poses a hazard to the region's ∼400,000 inhabitants. Therefore, determining whether it produced recent large earthquakes is important for updating regional earthquake hazard models and increasing earthquake preparedness. To study the earthquake history of the fault, we used shallow geophysical techniques and excavated a trench across the scarp to examine the sedimentary record of deformation. These combined methodologies determined a single large earthquake, of magnitude 6.1–7.6, likely occurred on the XELF between ∼4,700 and 2,300 years ago. A similar future earthquake on the XELF Lake fault could cause major damage to the Greater Victoria area. Thus, our results can improve future earthquake hazard assessments. Key Points: We document a single Holocene earthquake on a previously unidentified fault in the northern Cascadia forearc near Victoria, CanadaTrenching, shallow geophysics, and fault‐fold modeling show 2.3–3.2 m of dip slip during an M 6.1–7.6 earthquake between 4.7 and 2.3 kaA combination of methodologies can enhance the study of active faults in forearcs, urbanized areas, and previously glaciated terrain [ABSTRACT FROM AUTHOR]

The 15 May 2020 Mw 6.5 Monte Cristo Range earthquake (MCRE) in Nevada, United States, is the largest instrumental event in the Mina deflection--a zone of east-trending left-lateral faults accommodating a right step between northwest-trending right-lateral faults of the Walker Lane. The MCRE ruptured a highly distributed faulting area with muted geomorphic expressions, motivating us to characterize the behavior of an earthquake on a structurally immature fault system. Inverse modeling of Interferometric Synthetic Aperture Radar (InSAR) and Global Navigation Satellite System (GNSS) displacements reveals left-lateral slip on an east-striking, eastern fault and left-lateral-normal slip on an east-northeast-striking, western fault. Unusually, the two faults cross one another and ruptured together in the mainshock. The maximum slip of 1 m occurs at 8-10 km depth, but less than 0.1 m of slip reaches the surficial model fault patches, yielding a pronounced shallow slip deficit (SSD) of 91%. Relocated hypocenters indicate that the mainshock initiated at 9 km depth and that aftershocks span depths of 1-11 km, constraining the local seismogenic thickness. Our new field observations of fracturing and pebble-clearing in the western MCRE characterize a third, shorter, northern fault that is at the resolution limit of the InSAR-GNSS modeling. The segmented and intersecting fault geometry, off-fault aftershocks with variable mechanisms, distributed surface fractures, limited long-term geomorphic offsets, and a 600-700 m (cumulative) bedrock offset are all characteristic of a structurally immature fault system. However, the large SSD is not unusual for an earthquake of this magnitude, and a larger compilation of InSAR models (28 Mw=6.4 strike-slip events) shows that SSDs correlate with magnitude rather than structural maturity. This study demonstrates the importance of integrating geodesy, seismology, and field observations to capture the full complexity of large earthquakes, and further suggests that seismic hazard assessments in shattered crustal regions consider the potential for multi- and cross-fault rupture. [ABSTRACT FROM AUTHOR]

The north-striking sinistral reverse Papatea Fault ruptured with a very large (up to 12 m) oblique slip as part of the 2016 MW 7.8 Kaikōura earthquake in the northeastern South Island. Paleoseismic studies were undertaken at three sites along the Papatea Fault, named Murray's roadcut, Jacqui's Gully (both on the main strand), and Wharekiri trench (western strand). These sites provide evidence for up to three Late Holocene paleoearthquakes prior to 2016 (=E0) on this previously unmapped active fault, with preferred OxCal-modelled timings of 98–149 (E1), 546–645 cal yr BP (E2), and >738 cal yr BP (E3). Event correlations between the sites are generally consistent across these past events, implying that the two strands of the Papatea Fault link at depth and rupture together co-seismically as in 2016. Comparisons of its paleoseismic record with the Kekerengu Fault and uplift data from Waipapa Bay and Kaikōura, suggest that the Papatea Fault may have three distinct rupture modes: (i) Kaikōura-type multi-fault ruptures with multi-metre, anelastic block displacements and associated major landscape change; (ii) multi-fault earthquake ruptures with other regional fault combinations; and (iii) single-fault Papatea ruptures with metre-scale displacement. OxCal models offer the possibility that the E1 fault rupture occurred in 1855 CE. [ABSTRACT FROM AUTHOR]

The Kepingtag (Kalpin) fold-and-thrust belt of the southern Chinese Tian Shan is characterized by active shortening and intense seismic activity. Geological cross-sections and seismic reflection profiles suggest thin-skinned, northward-dipping thrust sheets detached in an Upper Cambrian décollement. The 2020 January 19 M w 6.0 Jiashi earthquake provides an opportunity to investigate how coseismic deformation is accommodated in this structural setting. Coseismic surface deformation resolved with Sentinel-1 Interferometric Synthetic Aperture Radar (InSAR) is centred on the back limb of the frontal Kepingtag anticline. Elastic dislocation modelling suggests that the causative fault is located at ∼7 km depth and dips ∼7° northward, consistent with the inferred position of the décollement. Our calibrated relocation of the main shock hypocentre is consistent with eastward, unilateral rupture of this fault. The narrow slip pattern (length ∼37 km but width only ∼9 km) implies that there is a strong structural or lithological control on the rupture extent, with updip slip propagation possibly halted by an abrupt change in dip angle where the Kepingtag thrust is inferred to branch off the décollement. A depth discrepancy between main shock slip constrained by InSAR and teleseismic waveform modelling (∼7 km) and well-relocated aftershocks (∼10–20 km) may suggest that faults within sediments above the décollement exhibit velocity-strengthening friction. [ABSTRACT FROM AUTHOR]

We investigate active tectonics in southwestern Turkey along the trend between Fethiye, near the eastern end of the Hellenic subduction zone, and Burdur, on the Anatolian plateau. Previously, regional GNSS velocities have been used to propose either (1) a NE-trending zone of strike-slip faulting coined the Fethiye–Burdur Fault Zone, or (2) a mix of uniaxial and radial extension accommodated by normal faults with diverse orientations. We test these models against the available earthquake data, updated in light of recent earthquakes at Arıcılar (24 November 2017, M w 5.3), Acıpayam (20 March 2019, M w 5.6) and Bozkurt (8 August 2019, M w 5.9), the largest in this region in the last two decades. Using Sentinel-1 InSAR and seismic waveforms and arrival times, we show that the Arıcılar, Acıpayam and Bozkurt earthquakes were partially or fully buried ruptures on pure normal faults with subtle or indistinct topographic expressions. By exploiting ray paths shared with these well-recorded modern events, we relocate earlier instrumental seismicity throughout southwestern Turkey and incorporate these improved hypocentres in an updated focal mechanism compilation. The southwestern Fethiye–Burdur trend is dominated by ESE–WNW trending normal faulting, even though most faults evident in the topography strike NE–SW. This hints at a recent change in regional strain, perhaps related to eastward propagation of the Gökova graben into the area or to rapid subsidence of the Rhodes basin. The northeastern Fethiye–Burdur trend is characterized by orthogonal normal faulting, consistent with radial extension and likely responsible for the distinct physiography of Turkey's Lake District. We find that the 1971 M w 6.0 Burdur earthquake likely ruptured a NW-dipping normal fault in an area of indistinct geomorphology near Salda Lake, contradicting earlier studies that place it on well-expressed faults bounding the Burdur basin, and further highlighting how damaging earthquakes are possible on faults that would prove difficult to identify beforehand. Overall, our results support GNSS-derived kinematic models that depict a mix of uniaxial and radial extension throughout southwestern Turkey, with no evidence from focal mechanisms for major, active strike-slip faults anywhere along the Fethiye–Burdur trend. Normal faulting orientations are consistent with a stress field driven primarily by contrasts in gravitational potential energy between the elevated Anatolian plateau and the low-lying Rhodes and Antalya basins. [ABSTRACT FROM AUTHOR]

Despite the importance of viscoelasticity in the evolution of crustal stress/strain being widely recognized, the interpretation of interseismic geodetic measurements for assessing earthquake potential is still based overwhelmingly on elastic models. The reasons for this disparity include conflating deformation rates with deformation itself and the lack of a succinct representation of the seismic readiness of a locked fault in a viscoelastic Earth. Using a classical viscoelastic model for strike-slip faults, we reiterate the commonly overlooked message that, if the recurrence interval is long, most of the strain energy for the next earthquake accrues early in the cycle, and low strain rates later in the cycle by no means indicate diminished rupture potential. Fault stress stays near failure for much of the late interseismic period which may explain why slow slip-rate faults have more variable recurrence intervals than fast slip-rate faults. We propose to use displacement deficit instead of slip deficit to represent seismic readiness. Plain Language Summary Modern satellite measurements can reveal how quickly faults are being loaded by tectonic plate motions, and seismic hazard models use these loading rates as proxy for the likelihood of a pending earthquake. However, because of the partially fluid-like behavior of Earth's interior, these loading rates have actually evolved with time since the last rupture. For faults with long intervals between successive earthquakes, these rates slow down substantially as the next event draws near. We, therefore, caution that slow rates of loading should not be assumed to reflect limited earthquake potential. [ABSTRACT FROM AUTHOR]

The 2020 Mw 6.8 Elazığ earthquake was the largest along the East Anatolian Fault (EAF) in over a century, providing valuable insights into its rupture behavior. We use satellite geodesy and seismology to detail the mainshock rupture, postseismic deformation, and aftershocks. The mainshock propagated mostly westward at ∼2 km/s from a nucleation point on an abrupt ∼10° fault bend. Only one end of the rupture corresponds to an established EAF segment boundary, and the earthquake may have propagated into the slip zone of the 1874 M ∼ 7.1 Gölcuk Gölu earthquake. It exhibits a pronounced (∼80%) shallow slip deficit, only a small proportion of which is recovered by early aseismic afterslip. The slow rupture velocity, shallow slip deficit, and minimal afterslip are characteristic of earthquakes hosted by faults of low‐to‐intermediate structural maturity, indicating that faults continue to evolve in important ways even as they accrue cumulative offsets of tens of kilometers. Plain Language Summary: We investigate the 2020 Mw 6.8 Elazığ (Turkey) earthquake, the largest along the East Anatolian Fault (EAF) in over a century. Anatolian faults are emblematic within the earthquake science community, but most attention has focused on the North Anatolian fault which ruptured repeatedly during the twentieth century, and relatively little is known about the EAF. We use satellite geodesy and seismology to map fault motions during the earthquake, after the earthquake, and in its aftershock sequence. Documenting relations between this earthquake, previous earthquakes, and early postseismic deformation is pivotal to gain a better understanding in what drives rupture behavior. Our results show that previous structural models of the EAF were only partially successful in predicting the end points of the 2020 rupture and that many aspects of this earthquake are characteristic of structurally immature faults. These results are important for seismic hazard assessment in this region. Key Points: The mainshock propagated mostly westward from a nucleation point on an abrupt ∼10° fault bendOnly one rupture termination corresponds to an established EAF segment boundary, and the rupture may partially overlap with an 1874 earthquakeThe mainshock exhibits a pronounced shallow slip deficit that is not fully recovered through early shallow afterslip [ABSTRACT FROM AUTHOR]

Back projection methods are increasingly used for mapping large earthquake ruptures in space and time. Unlike other seismological source imaging methods, back projections require no prior knowledge of earthquake source parameters or fault geometries except for the epicentre and average depth, making them a convenient tool for studying complex, multisegmented ruptures. We developed a new 3-D Phase-Weighted Relative Back Projection method (3-D PWBP) that yields improved spatial resolutions by enacting two advances. First, we exploit both the phase and amplitude of the seismogram signal to enhance the distinction of correlated signals. Second, we implement a 3-D velocity model to provide more accurate traveltimes. We vindicate these refinements with several synthetic tests and an analysis of the 1997 M w 7.2 Zirkuh (Iran) earthquake, which we show ruptured mostly unilaterally southwards at ∼3.0 km s−1 along its ∼125 km-long, mostly single-stranded surface rupture. Then, we apply the new method to the more complex case of the 2016 M w 7.8 Kaikōura (New Zealand) earthquake, which we demonstrate is divided into two major stages separated by a gap of ∼8 s and ∼30–40 km. The overall rupture speed is ∼1.7 km s−1 and the overall duration is ∼84 s, considerably shorter than some earlier estimates. We see no clear evidence for a continuous failure of the subduction interface that underlies the known, surface-rupturing crustal faults, though we cannot rule out its involvement in the second major stage in the northern part of the rupture area. The late (∼80 s) peak in relative energy is likely a high-frequency stopping phase, and the rupture appears to terminate southwest of the offshore Needles fault. [ABSTRACT FROM AUTHOR]