Slip Deficit Rate Research Articles

Magnitudes of future large earthquakes near Istanbul quantified from 1500 years of historical earthquakes, present-day microseismicity and GPS slip rates

In the past 1500 years, seven sequences of significant earthquakes have occurred on a 160-km-long segment of the North Anatolian Fault zone (NAFZ) centered on Istanbul. The most recent of these sequences will be complete with the future occurrence of one or more Mw ≥ 7.0 earthquakes near the city. From a summation of the seismic moment released by historical earthquakes between 500 and 2000 CE we calculate a mean slip deficit rate of 9.9 mm/year. Our moment summation procedure sums the seismic moment associated with known historical earthquakes in the past 1500 years based on rupture length and assumed seismogenic depth. We observed that in the 1999 Izmit earthquake, strongly coupled fault patches were associated with the highest co-seismic slip. In our calculated map of present-day slip-potential, we recognize several such patches from their low present-day rate of microseismicity, e.g. segments beneath the Kumburgaz and Eastern (Çınarcık) Marmara Basins. In contrast, intervening weakly coupled patches host high present-day background seismicity, e.g. the segment beneath the Western (Tekirdağ) Basin. This lends credibility to an interpretation of future earthquakes occurring in three seismogenic segments of the NAFZ corresponding to three deep basins in the Sea of Marmara. The present-day slip deficits reach up to 1.7 m beneath the Western (Tekirdağ Basin) segment, and 4.0 m and 5.4 m beneath the Central (Central High and Kumburgaz Basin) and Eastern (Çınarcık Basin) segments, respectively. These segments most recently ruptured in August 1766, May 1766 and October 1509 and currently have a potential to generate Mw 7.2, Mw 7.4 and Mw 7.5, earthquakes respectively. Although contiguous ruptures have not occurred historically, ruptures of contiguous segments could occur as a Mw 7.5 earthquake in the west, or a Mw 7.6 earthquake in the east or as a single through-going Mw 7.7 rupture.

Mechanism of subsidence of the Northeast Japan forearc during the late period of a gigantic earthquake cycle

The forearc in Northeast Japan subsided (3–4 mm/year) in the interseismic ~100 years before the 2011 Tohoku earthquake (MW9.1) just like it did during this event. This study attempts to understand the mechanism of the vertical displacement of the forearc during gigantic earthquake cycles via numerical modeling. The results suggest that the interseismic subsidence rate in the forearc increases with the duration of the locking of the asperity of the gigantic earthquake over several hundred years, due to the increasing slip deficit rate on the deeper parts of the plate interface. The increasing slip deficit rate is caused by both the decreasing the shear stress in the shear zone owing to the continuous locking of the asperity and the increasing the mobility of the continental lithosphere owing to the viscoelastic relaxation in the mantle wedge. The deep slip deficit rate extending to ~100 km depth of the plate interface is necessary to explain the observed interseismic forearc subsidence rate. The results also suggest hundreds of years of continuous locking of the asperities of a gigantic earthquake in the western Kuril subduction zone, where fast forearc subsidence has been observed as well.

Global Plate Motions and Earthquake Cycle Effects

AbstractThe rotations of tectonic plates provide a partial description of the total observed displacements at the Earth's surface. The estimated number of kinematically distinct plates has increased from 12 in 1990 to 56 in 2010 as a result of the increase in the number of kinematic observables. At length scales <1,000 km, rotation‐only plate models are inaccurate because geodetic signals of long‐term plate motions are complicated by earthquake cycle effects. Here we present results from a global block model that unifies large‐scale plate motions and local earthquake cycle effects at plate boundaries. Incorporating the rotations of 307 distinct plates, elastic strain accumulation from 16 subduction zones and 1.59 × 107 km2 of fault system area, this model explains 19,664 interseismic GPS velocities at a resolution of 2.2 mm/year. Geodetically constrained fault slip deficit rates yield a cumulative global moment accumulation rate of 1.09 × 1022 N·m/year, 12% larger than the average annual coseismic moment release rate from 1900 to 2013. The potential contribution to the total moment rate budget can be estimated from the frequency distribution of the modeled fault slip‐deficit rates, which follow an exponential distribution. Integrating this frequency distribution over all possible slip rates indicates that the geologic structures included in this reference global block model account for 98% of the global moment budget. Comparing our results with population distribution, we find that ∼50% of the world's population lives within 200 km of an active fault with a slip rate >2 mm/year.

InSAR-Constrained Interseismic Deformation and Potential Seismogenic Asperities on the Altyn Tagh Fault at 91.5–95°E, Northern Tibetan Plateau

The present-day kinematic features of the different segments of the Altyn Tagh Fault (ATF) have been well-studied using geodetic data. However, on the eastern segment of the ATF at 91.5–95°E, high spatial resolution deformation has not been previously reported. Here, we processed 185 interferometric synthetic aperture radar (InSAR) images from three descending tracks of the C band ERS-1/2 and Envisat satellites spanning 1995–2011 and obtained the average deformation velocity field. Results show a left-lateral motion of ~4 mm/year along the fault-parallel direction across the ATF at 91.5–95°E, which is consistent with Global Positioning System (GPS) observations. The slip deficit rate distribution at shallow depths was resolved through the InSAR deformation velocity using a discretized fault plane. The slip deficit is capable of an Mw 7.9 earthquake, considering the elapsed time of the latest M 7.0 event. Two potential asperities that could be nucleation sites or rupture areas of future earthquakes were delineated based on the coupling coefficient and seismicity distributions along the fault plane. The larger asperity extends more than 100 km along the ATF at depths of 8–12 km. Our InSAR observations support the undeformed blocks model of the Indo-Eurasian collisional mechanism at the northern margin of the Tibetan plateau.

Gradient field of undersea sound speed structure extracted from the GNSS-A oceanography

After the twenty-first century, the Global Navigation Satellite System-Acoustic ranging (GNSS-A) technique detected geodetic events such as co- and postseismic effects following the 2011 Tohoku-oki earthquake and slip-deficit rate distributions along the Nankai Trough subduction zone. Although these are extremely important discoveries in geodesy and seismology, more accurate observation that can capture temporal and spatial changes are required for future earthquake disaster prevention. In order to upgrade the accuracy of the GNSS-A technique, it is necessary to understand disturbances in undersea sound speed structures, which are major error sources. In particular, detailed temporal and spatial variations are difficult to observe accurately, and their effect was not sufficiently extracted in previous studies. In the present paper, we reconstruct an inversion scheme for extracting the effect from GNSS-A data and experimentally apply this scheme to the seafloor sites around the Kuroshio. The extracted gradient effects are believed to represent not only a broad sound speed structure but also a more detailed structure generated in the unsteady disturbance. The accuracy of the seafloor positioning was also improved by this new method. The obtained results demonstrate the feasibility of using the GNSS-A technique to detect a seafloor crustal deformation for oceanography research.

A b map implying the first eastern rupture of the Nankai Trough earthquakes

The Nankai Trough megathrust earthquakes inflicted catastrophic damage on Japanese society and more widely. Most research is aimed at identifying strongly coupled regions that are considered as a major source of future disastrous earthquakes. Here we present a b-value map for the entire Nankai Trough zone. The b value, which represents the rate of occurrence of small earthquakes relative to larger ones, is inversely dependent on differential stresses, and has been used to detect highly stressed areas on fault planes in various tectonic situations. A remarkable finding is that the b value is inversely correlated with the slip-deficit rate (SDR). Moreover, the b value for the areas of high SDR in the eastern part is lower than that in the western part, indicating that differential stress on asperities in the eastern part is higher than that in the western part. This may explain the history of the Nankai Trough earthquakes, in which the eastern part tends to rupture first.

Tsunami Scenarios Based on Interseismic Models Along the Nankai Trough, Japan, From Seafloor and Onshore Geodesy

AbstractThe recent availability of Global Positioning System‐Acoustic seafloor geodetic observations enables us to resolve the spatial distribution of the slip deficit rate near the Nankai trough, southwestern Japan. Considering a tectonic block model and the transient deformation due to the major earthquakes in this area, the slip deficit rate between the two relevant blocks can be estimated. In this study, we remove the time‐dependent postseismic deformation of the 2004 southeastern off the Kii Peninsula earthquakes (MJMA 7.1, 7.4), which had led to the underestimation of the slip deficit rate in earlier studies. We model the postearthquake viscoelastic relaxation using the 3D finite element model with bi‐viscous Burgers rheology, as well as the afterslip on the finite faults. The corrected Global Positioning System‐Acoustic and land‐based Global Navigation Satellite Systems data are aligned to the existing tectonic model and used to estimate the slip deficit rate on the plate boundary. We then calculate the coseismic displacements and tsunami wave propagation with the simple assumption that a hundred years of constant slip deficit accumulation was released instantaneously. To evaluate the influence of uncertainties in the plate interface geometry on a tsunami model for the Nankai trough, we investigated two different geometries and performed checkerboard inversion simulations. Although the two models indicate roughly similar results, the peak height of the tsunami wave and its arrival time at several points are significantly different in terms of the expected hazard.

Slow slip rate and excitation efficiency of deep low-frequency tremors beneath southwest Japan

We estimated the long-term average slip rate on the plate interface across the Nankai subduction zone during 2002–2013 using deep low-frequency tremors as a proxy for short-term slow slip events based on empirical relations between the seismic moment of short-term slow slip events and tremor activities. The slip rate in each region is likely to compensate for differences between the convergence rate and the slip deficit rate of the subducting Philippine Sea plate estimated geodetically, although the uncertainty is large. This implies that the strain because of the subduction of the plate is partially stored as the slip deficit and partially released by slow slip events during the interseismic period. The excitation efficiency of the tremors for the slow slip events differs among regions: it is high in the northern Kii region. Some events in the western Shikoku region show a somewhat large value. Antigorite serpentinite of two types exists in the mantle wedge beneath southwest Japan. Slips with more effective excitation of tremors presumably occur in high-temperature conditions in the antigorite+olivine stability field. Other slip events with low excitation efficiency are distributed in the antigorite+brucite stability field. Considering the formation reactions of these minerals and their characteristic structures, events with high excitation efficiency can be correlated with a high pore fluid pressure condition. This result suggests that variation in pore fluid pressure on the plate interface affects the magnitude of tremors excited by slow slip events.

Elastic block and strain modeling of GPS data around the Haiyuan-Liupanshan fault, northeastern Tibetan Plateau

Based on the dense GPS velocity field in the northeastern margin of the Tibetan Plateau from 1999 to 2016, we have produced the deformation and strain characteristics of the Haiyuan fault and the Liupanshan fault. Estimated long-term slip rate along the Haiyuan-Liupanshan fault zones show a gradual decrease from 6.4 ± 1.6 mm/yr at the Tuolaishan fault to 2.9 ± 1.2 mm/yr at the Southern Liupanshan fault. Left-lateral thrusting movement was inverted for the Xiangshan-Tianjingshan fault (XS-TJS), which has an average slip rate of 2.1 ± 3.4 mm/yr during the study period. We also calculated the heterogeneous distribution of interseismic coupling along the fault zones. Our result also shows the locking depth of the Tianzhu seismic gap is ∼22 km. The slip rate deficit, the seismic moment accumulation rate, and the Coulomb stress accumulation rate are high on the fault planes, whereas the second invariant of the strain rate is low at the surface. The Liupanshan fault is locked to a depth of ∼23 km, and the corresponding seismic moment accumulation rate on the fault plane is high, while the strain rate at the surface is low. The accumulated strain along the Tianzhu seismic gap and the Liupanshan fault could be balanced by earthquakes with magnitudes of Mw7.9 and Mw7.4, considering the absence of large earthquakes over the last 1000 years and 1400 years respectively. The Haiyuan segments had ruptured during 1920 Haiyuan earthquake, and the estimated locking depth for period 1999–2016 is 5–10 km. Its seismic moment accumulation rate at depth is low and the strain rate at the surface is high. Our result indicates that 70% of the strike-slip along the Haiyuan segments transforms into thrusting along the Liupanshan fault, while the remaining 30% is related to the orogeny of the Liupanshan. For slip between the Haiyuan fault and the XS-TJS, about 27–34% of the slip is partitioned on the XS-TJS.

Pre-computed tsunami inundation database and forecast simulation in Pelabuhan Ratu, Indonesia

We built a pre-computed tsunami inundation database in Pelabuhan Ratu, one of tsunami-prone areas on the southern coast of Java, Indonesia, which can be employed for a rapid estimation of tsunami inundation during an event. The pre-computed tsunami waveforms and inundations are from a total of 340 scenarios ranging from 7.5 to 9.2 in moment magnitude scale (Mw), including simple fault models of 208 thrust faults and 44 tsunami earthquakes on the plate interface, as well as 44 normal faults and 44 reverse faults in the outer-rise region. Using our tsunami inundation forecasting algorithm (NearTIF), we could rapidly estimate the tsunami inundation in Pelabuhan Ratu for three different hypothetical earthquakes. The first hypothetical earthquake is a megathrust earthquake type (Mw 9.0) offshore Sumatra which is about 600 km from Pelabuhan Ratu to represent a worst-case event in the far-field. The second hypothetical earthquake (Mw 8.5) is based on a slip deficit rate estimation from geodetic measurements and represents a most likely large event. The third hypothetical earthquake is a tsunami earthquake type (Mw 8.1) which often occurs south of Java. We compared the tsunami inundation maps produced by the NearTIF algorithm with results of direct forward inundation modeling for the hypothetical earthquakes. The tsunami inundation maps produced from both methods are similar for the three cases. However, the tsunami inundation map from the inundation database can be obtained in much shorter time (1 min) than the one from a forward inundation modeling (40 min). These indicate that the NearTIF algorithm based on pre-computed inundation database is reliable and useful for tsunami warning purposes. This study also demonstrates that the NearTIF algorithm can work well, though the earthquake source is located outside the area of fault model database because it uses a time shifting procedure for the best-fit scenario searching.

Continuous GPS measurements of crustal deformation in Garhwal-Kumaun Himalaya

We report GPS measurements of crustal deformation from five continuous sites in the Garhwal Kumaun Himalaya. The motion at these sites confirms strain accumulation on the Main Himalayan Thrust (MHT) under the Outer and Lesser Himalaya with a slip deficit rate of 18 mm/year over a width of ∼100 km. Apart from secular plate motion, these sites also exhibit seasonal variations. We model these seasonal variations using the global models of atmospheric and hydrological loads. The influence of these loads is maximum in the vertical component which decreases in the north and then in the east component. All the four sites in the Himalayan region and a site at Delhi show consistency between the observed seasonal variations with that predicted by the model. However, the annual variations in the horizontal component of displacement time series at BDRI (Badrinath) GPS site are not consistent with the predictions which could be due to local site conditions.

Subducting an old subduction zone sideways provides insights into what controls plate coupling

The Hikurangi Plateau has had two episodes of subduction beneath New Zealand – firstly at ca. 100 Ma during north–south convergence with Gondwana, and currently during east–west convergence between the Pacific and Australian plates. As a result of this ninety-degree change in convergence direction, an old subduction zone is now being subducted sideways, and the tectonic history of the subducted plate varies dramatically along the strike of the Hikurangi Margin. Here we identify the location of the underplated Hikurangi Plateau along the shallow part of the Hikurangi Margin, using results from both relocated seismicity and seismic tomography. Next we decipher the tectonic history of the plateau along strike, particularly in terms of the hydration state of the plateau, and the nature of any sedimentary rock units capping the plateau. We then use this information to understand plate coupling at two scales: on the large scale, the southward transition from typical subduction in the North Island to continental collision in the South Island; and at a smaller scale, the strong lateral change from a high deficit in slip rate at the plate interface in the southern North Island to a low deficit in slip rate in the northeastern North Island. We find that the southward transition from subduction to continental collision is controlled by the plateau being more dehydrated to the south, as a result of being more deeply subducted at the Gondwana margin. The southward transition from localized slip at the plate interface to distributed upper plate deformation with no active plate interface occurs in Cook Strait and is relatively sharp. The high deficit in slip rate at the plate interface in the southern North Island is likely due to a relatively smooth plate interface from sedimentary rocks capping the Hikurangi Plateau, an impermeable terrane in the overlying plate, and the hydrated plateau acting in concert to produce an interseismically sealed plate interface. Further northeast where the slip rate deficit at the plate interface is low, the plate interface has a rough topography (including seamounts), there is no impermeable terrane in the overlying plate, and fluid from the dehydrating plateau migrates across the plate interface.

Fault locking and slip rate deficit of the Haiyuan-Liupanshan fault zone in the northeastern margin of the Tibetan Plateau

GPS-derived horizontal velocities, geologic fault slip rates and earthquake-derived fault slip vector azimuths are inverted simultaneously for fault coupling and slip rate deficit on the Haiyuan-Liupanshan fault in the northeastern margin of the Tibetan Plateau. Along the Haiyuan fault, the results show 3.2–6.2mm/yr of left-lateral strike-slip, and the strike-slip transformed into thrusting deformation (2.8–3.5mm/yr) along the Liupanshan fault. The results suggest full coupling down to ∼10km along the Haiyuan fault. Significant portions from the Jinqianghe fault to the Maomaoshan fault, however, are locked to ∼23km depth. The abrupt change in fault coupling coincides with the Tianzhu seismic gap. High slip rate deficit (3.0–4.5mm/yr) and seismic moment accumulation rate are also interpreted along the seismic gap. The Liupanshan fault is locked to 15–20km depth. A high seismic moment accumulation rate, low slip rate deficit (2.0–3.2mm/yr) and scarcity of large seismic event over the last 1400 years may imply high strain accumulation on the fault. The results demonstrate that crustal deformation in the northeastern Tibetan Plateau is dominated by elastic block rotation. We also conclude that deformation due to fault coupling is limited along the near-field (less than 50km with velocities less than 2mm/yr) of the Haiyuan-Liupanshan fault.

Seafloor geodetic constraints on interplate coupling of the Nankai Trough megathrust zone

Interplate megathrust earthquakes have inflicted catastrophic damage on human society. Such an earthquake is predicted to occur in the near future along the Nankai Trough off southwestern Japan--an economically active and densely populated area in which megathrust earthquakes have already occurred. Megathrust earthquakes are the result of a plate-subduction mechanism and occur at slip-deficit regions (also known as 'coupling' regions), where friction prevents plates from slipping against each other and the accumulated energy is eventually released forcefully. Many studies have attempted to capture distributions of slip-deficit rates (SDRs) in order to predict earthquakes. However, these studies could not obtain a complete view of the earthquake source region, because they had no seafloor geodetic data. The Hydrographic and Oceanographic Department of the Japan Coast Guard (JHOD) has been developing a precise and sustainable seafloor geodetic observation network in this subduction zone to obtain information related to offshore SDRs. Here, we present seafloor geodetic observation data and an offshore interplate SDR-distribution model. Our data suggest that most offshore regions in this subduction zone have positive SDRs. Specifically, our observations indicate previously unknown regions of high SDR that will be important for tsunami disaster mitigation, and regions of low SDR that are consistent with distributions of shallow slow earthquakes and subducting seamounts. This is the first direct evidence that coupling conditions might be related to these seismological and geological phenomena. Our findings provide information for inferring megathrust earthquake scenarios and interpreting research on the Nankai Trough subduction zone.

Extreme multi-millennial slip rate variations on the Garlock fault, California: Strain super-cycles, potentially time-variable fault strength, and implications for system-level earthquake occurrence

Abstract Pronounced variations in fault slip rate revealed by new measurements along the Garlock fault have basic implications for understanding how faults store and release strain energy in large earthquakes. Specifically, dating of a series of 26.0 + 3.5 / − 2.5 m fault offsets with a newly developed infrared-stimulated luminescence method shows that the fault was slipping at > 14.0 + 2.2 / − 1.8 mm / yr , approximately twice as fast as the long-term average rate, during a previously documented cluster of four earthquakes 0.5–2.0 ka. This elevated late Holocene rate must be balanced by periods of slow or no slip such as that during the ca. 3300-yr-long seismic lull preceding the cluster. Moreover, whereas a comparison of paleoseismic data and stress modeling results suggests that individual Garlock earthquakes may be triggered by periods of rapid San Andreas fault slip or very large-slip events, the “on–off” behavior of the Garlock suggests a longer-term mechanism that may involve changes in the rate of elastic strain accumulation on the fault over millennial time scales. This inference is consistent with most models of the geodetic velocity field, which yield slip-deficit rates that are much slower than the average latest Pleistocene-early Holocene (post-8–13 ka) Garlock slip rate of 6.5 ± 1.5 mm / yr . These observations indicate the occurrence of millennia-long strain “super-cycles” on the Garlock fault that may be associated with temporal changes in elastic strain accumulation rate, which may in turn be controlled by variations in relative strength of the various faults in the Garlock-San Andreas-Eastern California Shear Zone fault system and/or changes in relative plate motion rates.

The source model and recurrence interval of Genroku-type Kanto earthquakes estimated from paleo-shoreline data

In the southern Kanto region of Japan, where the Philippine Sea plate is descending at the Sagami trough, two different types of large interplate earthquakes have occurred repeatedly. The 1923 (Taisho) and 1703 (Genroku) Kanto earthquakes characterize the first and second types, respectively. A reliable source model has been obtained for the 1923 event from seismological and geodetical data, but not for the 1703 event because we have only historical records and paleo-shoreline data about it. We developed an inversion method to estimate fault slip distribution of interplate repeating earthquakes from paleo-shoreline data on the idea of crustal deformation cycles associated with subduction-zone earthquakes. By applying the inversion method to the present heights of the Genroku and Holocene marine terraces developed along the coasts of the southern Boso and Miura peninsulas, we estimated the fault slip distribution of the 1703 Genroku earthquake as follows. The source region extends along the Sagami trough from the Miura peninsula to the offing of the southern Boso peninsula, which covers the southern two thirds of the source region of the 1923 Kanto earthquake. The coseismic slip takes the maximum of 20 m at the southern tip of the Boso peninsula, and the moment magnitude (Mw) is calculated as 8.2. From the interseismic slip-deficit rates at the plate interface obtained by GPS data inversion, assuming that the total slip deficit is compensated by coseismic slip, we can roughly estimate the average recurrence interval as 350 years for large interplate events of any type and 1400 years for the Genroku-type events.

Slow slip events and time‐dependent variations in locking beneath Lower Cook Inlet of the Alaska‐Aleutian subduction zone

AbstractWe identify a series of abrupt changes in GPS site velocities in Lower Cook Inlet, Alaska, in late 2004, early 2010, and late 2011. The site motions during each time period are nearly linear. The surface deformations inferred from GPS for pre‐2004 and 2010–2011 are similar to each other, as are 2004–2010 and post‐2011. We estimate the slip distribution on the Alaska‐Aleutian subduction plate interface accounting for upper plate block rotations and interpret this toggling between two deformation patterns as caused by transient slip. We find that by allowing negative slip deficit rates (i.e., creep rates in excess of relative plate motion), the data in Lower Cook Inlet are fit significantly better during pre‐2004 and 2010–2011, suggesting the occurrence of slow slip events (SSEs) there during those time periods. The earlier SSE lasted at least 9 years (observations in that area began in 1995) with Mw ~7.8. The latter SSE had almost the same area as the earlier one and a duration of ~2 years with Mw ~7.2. During 2004–2010 and post‐2011, the inversions result in only positive slip deficit rates (i.e., locking) in Lower Cook Inlet. Slip rates are nearly constant during the Lower Cook Inlet SSEs, and the events start and stop abruptly. Both of these properties contrast with observations of SSEs in Upper Cook Inlet and elsewhere. The Lower Cook Inlet SSEs are consistent with previously proposed duration‐magnitude scaling laws and demonstrate that slow slip events can last as long as a decade.

Locking degree and slip rate deficit distribution on MHT fault before 2015 Nepal Mw 7.9 earthquake

The spatial pattern and rate of strain accumulation on a fault during the pre- and inter-seismic phases are very important for interpreting the mechanism of earthquakes and evaluating seismic potentials. Here we use global positioning system (GPS) data and the block-dislocation model to invert for the locking degree and slip rate deficit of the Main Himalayan Thrust (MHT) fault in the southern margin of Tibet before the 2015 Mw 7.9 Nepal earthquake. Results show that the locking depth and slip rate deficit increase from the west to the east. Along the western segment of the MHT fault (80°E–84°E), the locking depth is estimated to be 12–17km with a slip rate deficit of 0–5mm/a; along the central Nepal segment (84°E–87°E), the locking depth is 16–21km with a slip rate deficit of 6–10mm/a, whilst along the eastern segment (87°E–90°E), the locking depth increases to 23–26km with a slip rate deficit of 8–13mm/a. The 2015 Nepal earthquake initiated at the boundary between the western and central segments, an area with as where the slip rate deficit varies dramatically from 0 to 9mm/a within 50km resulting in high energy gradients. High strain concentration along the central and eastern segments leads to unilateral propagation of the rupture to the east. Given the paucity of large seismic events over the previous decades and the current high slip rate deficit, seismic hazard on the eastern Nepal segment remains high.

Transition from collision to subduction and its relation to slab seismicity and plate coupling

The Philippine Sea plate exhibits an along-trench variation in structure, from an eastern volcanic arc to a typical oceanic crust in the west. The regional difference of intra-slab seismicity implies that this transition occurs around the Itoigawa-Shizuoka Tectonic Line (ISTL). However, the nature of the subducting slab in this region has not been studied in detail. Here, we investigate the structure of the Philippine Sea plate subducting beneath the ISTL. Using active source data, we found reflective portions at depths of 14–18 km. An amplitude evaluation for the reflection phases showed that the reflective zone has a P wave velocity as low as 3.0 km/s. Together with its slightly northwestward-dipping geometry, we interpreted that the low-velocity zone is at the plate boundary and can be attributed to high water content supplied by the slab. This feature is in great contrast to the collision zone further east, where the slab top is less reflective and the dehydration process is inactive. This structural difference also correlates well with the regional distribution of slab seismicity. The reflective zones we found are likely located at the down-dip end of the locked zone, where high slip deficit rates are currently observed. This may suggest that changing fluid pressures and the resulting frictional properties in the down-dip direction control the transition from a coseismic rupture zone to a deeper aseismic zone.

Geodetic imaging of potential seismogenic asperities on the Xianshuihe‐Anninghe‐Zemuhe fault system, southwest China, with a new 3‐D viscoelastic interseismic coupling model

AbstractWe use GPS and interferometric synthetic aperture radar (InSAR) measurements to image the spatial variation of interseismic coupling on the Xianshuihe‐Anninghe‐Zemuhe (XAZ) fault system. A new 3‐D viscoelastic interseismic deformation model is developed to infer the rotation and strain rates of blocks, postseismic viscoelastic relaxation, and interseismic slip deficit on the fault surface discretized with triangular dislocation patches. The inversions of synthetic data show that the optimal weight ratio and smoothing factor are both 1. The successive joint inversions of geodetic data with different viscosities reveal six potential fully coupled asperities on the XAZ fault system. Among them, the potential asperity between Shimian and Mianning, which does not exist in the case of 1019 Pa s, is confirmed by the published microearthquake depth profile. Besides, there is another potential partially coupled asperity between Daofu and Kangding with a length scale up to 140 km. All these asperity sizes are larger than the minimum resolvable wavelength. The minimum and maximum slip deficit rates near the Moxi town are 7.0 and 12.7 mm/yr, respectively. Different viscosities have little influence on the roughness of the slip deficit rate distribution and the fitting residuals, which probably suggests that our observations cannot provide a good constraint on the viscosity of the middle lower crust. The calculation of seismic moment accumulation on each segment indicates that the Songlinkou‐Selaha (S4), Shimian‐Mianning (S7), and Mianning‐Xichang (S8) segments are very close to the rupture of characteristic earthquakes. However, the confidence level is confined by sparse near‐fault observations.