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Abstract On June 26, 2016 an Mw 6.5 earthquake occurred in the Trans-Alai Range, about 37 km west of a 2008 Mw 6.7 earthquake, both near the town of Nura, China. We use Interferometric Synthetic Aperture Radar (InSAR) images from Sentinel-1A to image coseismic surface displacements from the most recent Nura earthquake. We invert the interferograms for both fault geometry and slip distribution. We find that the InSAR data can be described by either a steeply north-dipping fault or shallowly south-dipping fault, although only when slip on the south-dipping fault is constrained to be blind. The InSAR misfit for the north-dipping fault plane is slightly lower, and the north-dipping fault better describes the gradient of deformation in the near-field. We prefer the north-dipping fault geometry (~67° dip and 266° strike) as the source for the 2016 Nura earthquake, not only due to the better fit to the InSAR, but the resulting coseismic slip model is more consistent with the focal mechanism solutions and regional seismogenic thickness; however, it is important to reiterate that the InSAR data cannot uniquely determine which model is the definitive representation of the Nura earthquake. The maximum inferred coseismic slip is about 0.75 m at ~14 km depth beneath the Trans Alai range, with the largest slip in the depth range of about 8–20 km. Slip does not reach the surface, and is dominated by thrust displacement with a right-lateral strike-slip component. The geodetic moment of our preferred slip model is 6.61 × 1018 N·m, equivalent to Mw 6.5. The surface projection of our preferred fault is to the south of the Pamir Frontal Thrust (PFT), and we argue it is a back-thrust of the listric PFT, suggesting that the Trans Alai range is a 'pop-up' structure. Highlights • 2016 Mw 6.5 Nura earthquake was imaged by the Sentinel-1A satellite. • Coseismic deformation was measured on descending and ascending InSAR. • ~67° north-dipping fault is the preferred slip model of the Nura earthquake. • Coseismic slip is concentrated 8–20 km beneath the surface and is blind. • Preferred slip model is interpreted as backthrust to the Main Pamir Thrust. [ABSTRACT FROM AUTHOR]

The 2013 Mw6.8 Lushan, China earthquake occurred in the southwestern end of the Longmenshan fault zone. We jointly invert local strong motion data and geodetic measurements of coseismic surface deformation, including GPS and InSAR, to obtain a robust model of the rupture process of the 2013 Lushan earthquake. Our joint inversion best model involves the rupture of two opposing faults during the Lushan earthquake, a main fault and a secondary fault. It is only when the secondary fault is included that both the GPS and InSAR measurements are fit along with the near-field strong motion. Over 75% of the computed moment was released in slip on the main fault segment, a northwest dipping, listric thrust fault, with buried thrust and dextral strike-slip at hypocenter depths, and with only minor slip closer to the surface. The secondary fault mainly involved oblique thrust slip or pure dextral strike-slip at shallower depths, and accounts for just under 24% of the moment released in the Lushan earthquake. Coulomb stress changes of about 0.5 MPa on the secondary fault segment at the time coseismic slip initiated on that fault indicate that slip was likely triggered by the coseismic slip on the main blind thrust fault. Our coseismic slip model is consistent with a sub-horizontal and east–west to southeast–northwest trending most compressive stress. Our inferred coseismic slip model is also consistent with previous GPS derived models of strain accumulation on the Longmenshan fault system. [ABSTRACT FROM AUTHOR]

Abstract: The 12 May 2008, Mw 7.9 Wenchuan earthquake occurred on the Beichuan fault in the southwest to northeast trending Longmenshan fault zone (LMSfz), marking the border of the Sichuan basin and eastern Tibet. During the Wenchuan earthquake, the Beichuan fault slipped primarily in thrust motion on moderately dipping fault segments in the southwest, and as largely right lateral strike slip motion on more steeply dipping segments in the northeast. Additionally, the shallowly dipping Pengguan fault, located to the east of the Beichuan fault, slipped as mostly thrust slip. In this study, we explore the connection between sense of slip and fault geometry, and investigate whether models of coseismic slip yield information about the pre-earthquake background stress in the region of the Wenchuan earthquake. Specifically, we test whether coseismic slip models are consistent with a homogeneous regional stress, or if a heterogeneous stress field is required to explain the change in slip rake along strike. We assume that coseismic slip is parallel to the direction of the maximum shear stress on the fault, and consider several published coseismic slip models with differing fault geometries. Using a Bayesian probabilistic estimation, we find that the coseismic slip of the Wenchuan earthquake is consistent with a constant orientation of principal stresses along the strike of the LMSfz. This suggests a homogeneous state of stress prior to the earthquake, although our analysis only provides weak constraints on the relative magnitudes of the principal stresses. The inferred most compressive stress direction is sub-horizontal and approximately east–west trending. The intermediate compressive stress is sub-horizontal and north–south trending, and is most likely about 30% the magnitude of the most compressive stress. The least compressive stress is near-vertical. [Copyright &y& Elsevier]

We explore the potential geodetic signature of mechanical stress shadows surrounding inferred major seismic asperities along the Japan-Kurile subduction megathrust. Such stress shadows result from a decrease in creep rates late in the interseismic period. We simplify the rupture history along this megathrust as the repeated rupture of several asperities, each with its own fixed recurrence interval. In our models, megathrust creep throughout the interseismic period evolves according to velocity strengthening friction, as opposed to common kinematic back-slip models of locked or partially locked (i.e. coupled) regions of the megathrust. Such backslip models are usually constrained by onshore geodetic data and typically find spatially extensive and smooth estimates of plate coupling, a likely consequence of model regularization necessitated by poor model resolution. Of course, these large coupled regions could also correspond to seismogenic asperities, some of which have not experienced a significant earthquake historically. A subset of existing kinematic models of coupling along the Japan Trench, particularly those that use both horizontal and vertical geodetic data, have inferred a surprisingly deep (∼100 km) locked zone along the megathrust or have called upon complex, poorly constrained megathrust processes, such as subduction erosion, to explain the geodetic observations. Here, we posit two scenarios for distributions of asperities on a realistic 3-D megathrust interface along the Japan-Kurile Trench off NE Japan. These scenarios reflect common assumptions made before and after the 2011 Mw 9 Tohoku-oki earthquake. We find that models that include two shallow M9-class asperities (one corresponding to the 2011 Tohoku-Oki earthquake and one offshore of Hokkaido) and associated stress-shadows can explain geodetic observations of interseismic strain along the eastern halves of Honshu and Hokkaido. Specifically, models including localized fault creep can explain most of the observed long-term vertical subsidence in this region during the past century and thus appealing to processes such as deep locking or subduction erosion may not be required. [ABSTRACT FROM PUBLISHER]

Abstract: We present velocity models determined by inverting refracted and reflected arrivals along two active source lines in the Changbaishan volcanic region, NE China. We resolve a prominent low-velocity zone (LVZ) in the crust, with velocities as low as 5.4 km/s. Away from the LVZ, the velocity gradients in the crust are relatively smooth, with average P-wave velocities of about 6.0–6.5 km/s. The Moho is at about 35 km depth, thickening to about 40 km under the Tianchi volcano, and thinning to about 30 km under the LVZ. The LVZ is located about 30–60 km to the north of the summit of the Tianchi volcano (the most recently active volcano in the region), is about 30–75 km in north–south extent, is at most 35 km in east–west extent, and is in the depth range of about 10–25 km below the surface. We use these results to constrain receiver function inversions, and show that the receiver functions in the region are compatible with our findings. With these data alone, the significance of the LVZ in non-unique, although we do not see any evidence to support the presence of partial melt in the crust, and favor the interpretation that the LVZ indicates a residual crustal magma chamber. [Copyright &y& Elsevier]

The July 3rd, 2015 Pishan, China Mw 6.4 earthquake occurred within the Hetian fold belt, a frontal thrust region between the Northwestern Tibetan Plateau and the Tarim basin. We investigate the fault geometry and the rupture process of the Pishan earthquake based on joint inversion of teleseismic body waves and InSAR measurements. Our results show that the top of the fault that ruptured in the Pishan earthquake is buried 5 ± 2 km beneath the surface and that the earthquake ruptured only a segment of a deeply-seated detachment lying under the Hetian fold belt. Our inferred coseismic slip model shows two slip asperities, located in the depth range of about 7–9 km and 10–11 km. Additionally, we resolve a slip deficit region between the slip asperities, which might indicate friction heterogeneity on the fault plane. The earthquake ruptured for ∼15 s, releasing a total seismic moment of 4.7 × 10 18 N m. We also show that the latest Pishan earthquake caused positive Coulomb stress changes on back thrust faults, potentially decreasing the time to the next earthquake on those faults. Based on our Pishan coseismic slip model, along with both the trend of the anticline system and the pattern of coseismic deformation, we envision an ongoing east-west extensional growth of the anticlines in the Western Kunlun frontal thrust region in addition to the northern underthrusting of the Northwestern Tibetan Plateau below the Tarim basin. [ABSTRACT FROM AUTHOR]

Abstract: The 2011 M w 9.1 Tohoku-Oki earthquake had unusually large slip (over 50m) concentrated in a relatively small region, with local stress drop inferred to be 5–10 times larger than that found for typical megathrust earthquakes. Here we conduct a detailed analysis of foreshocks and aftershocks (M w 5.5–7.5) sampling this megathrust zone for possible clues regarding such differences in seismic excitation. We find that events occurring in the region that experienced large slip during the M w 9.1 event had steeper dip angles (by 5–10°) than the surrounding plate interface. This discrepancy cannot be explained by a single smooth plate interface. We provide three possible explanations. In Model I, the oceanic plate undergoes two sharp breaks in slope, which were not imaged well in previous seismic surveys. These break-points may have acted as strong seismic barriers in previous seismic ruptures, but may have failed in and contributed to the complex rupture pattern of the Tohoku-Oki earthquake. In Model II, the discrepancy of dip angles is caused by a rough plate interface, which in turn may be the underlying cause for the overall strong coupling and concentrated energy-release. In Model III, the earthquakes with steeper dip angles did not occur on the plate interface, but on nearby steeper subfaults. Since the differences in dip angle are only 5–10°, this last explanation would imply that the main fault has about the same strength as the nearby subfaults, rather than much weaker. A relatively uniform fault zone with both the main fault and the subfaults inside is consistent with Model III. Higher resolution source locations and improved models of the velocity structure of the megathrust fault zone are necessary to resolve these issues. [Copyright &y& Elsevier]

Geophysical observations from the 2011 moment magnitude (Mw) 9.0 Tohoku-Oki, Japan earthquake allow exploration of a rare large event along a subduction megathrust. Models for this event indicate that the distribution of coseismic fault slip exceeded 50 meters in places. Sources of high-frequency seismic waves delineate the edges of the deepest portions of coseismic slip and do not simply correlate with the locations of peak slip. Relative to the Mw 8.8 2010 Made, Chile earthquake, the Tohoku-Oki earthquake was deficient in high-frequency seismic radiation--a difference that we attribute to its relatively shallow depth. Estimates of total fault slip and surface secular strain accumulation on millennial time scales suggest the need to consider the potential for a future large earthquake just south of this event. [ABSTRACT FROM AUTHOR]

We used three-component surface velocities in southwest Japan to estimate plate coupling on the subducting plate interface at the Nankai trough. We analyzed continuous GPS data from the Japanese GEONET network from 1996 to 2006 using a consistent analysis strategy that generates bias-fixed solutions for the entire network. We applied systematic time-series analysis methods to estimate common mode error, which improved position solutions for the entire network. To allow for differences in regional deformation sources, we modelled the plate coupling on the plate interface beneath Shikoku island to Kii Peninsula and the Tokai-Suruga trough separately. The results show strong coupling at a depth of ∼10–30 km off Shikoku and Kii Peninsula. The spatial variation in plate coupling coincides well with the coseismic rupture zones of the past large earthquakes. Maximum slip deficit rates of ∼2–3 cm yr−1 at the depth of ∼5–25 km are found beneath the Tokai area, consistent with results from other studies. The downdip limits of the highly coupled areas and transition zones beneath Shikoku and the Kii Peninsula correspond approximately to estimates of the 450 °C isotherms. Good correlation is observed between the lateral variations of the slip deficit distribution, low frequency earthquakes, and coseismic slip. This correlation suggests that temperature, and possibly fluid variations, contribute to such correlation in space. The interplate slip deficit derived from the GPS velocities over the 10 yr of observations is generally compatible with the results over shorter time spans, suggesting that plate coupling in SW Japan does not change significantly over the period of these GPS measurements. [ABSTRACT FROM AUTHOR]

We model the crustal deformation caused by two long-term subduction slip transients in southwest Japan, which we refer to as the 2000–2004 Tokai and the 2002–2004 Bungo Channel slow slip events (SSEs). We use re-analysed GEONET position time-series, and a Kalman filter based network inversion method to image the spatiotemporal slip variation of the two events on the plate interface during the period of 1998–2004.67 and 2000–2005. Both events are found to have complex slip histories with multiple subevents. In addition to a newly identified slip subevent in 2002–2003, we find that the major event in the Bungo Channel SSE initiated in early 2003 beneath the northeastern corner of the region and expanded southwestward, in contrast to the slip characteristics suggested by other studies. The re-analysed GPS data in the Tokai region shows a renewed slip activity for the Tokai SSE in early 2003–2004 at a similar location as in the period of 2001–2002. The equivalent Mw for both the Tokai and Bungo Channel SSEs are about 7.0. Our results show that the Tokai SSE appears to start before the Miyaki-Kozu seismovolcanic event. Integrating plate coupling and SSEs shows that the transient slip zones are located in a region between the locked zones and the epicentres of the low frequency earthquakes (LFEs). At least part of the interseismic slip deficit is released by episodic SSEs beneath the Bungo Channel region. We find excellent temporal correspondence between transient slip and adjacent LFEs for both SSE, suggesting that they are closely related and possibly reflect that long-term slow slip may modulate the occurrence of LFEs. [ABSTRACT FROM AUTHOR]