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We analyze time series from continuously recording GPS stations in Nepal spanning the pre- and post-seismic period associated to the M_w7.8 Gorkha earthquake which ruptured the Main Himalayan Thrust (MHT) fault on April 25th, 2015. The records show strong seasonal variations due to surface hydrology. After corrections for these variations, the time series covering the pre- and post-seismic periods do not show any detectable transient pre-seismic displacement. By contrast, a transient post-seismic signal is clear. The observed signal shows southward displacements consistent with afterslip on the MHT. Using additional data from stations deployed after the mainshock, we invert the time series for the spatio-temporal evolution of slip on the MHT. This modelling indicates afterslip dominantly downdip of the mainshock rupture. Two other regions show significant afterslip: a more minor zone updip of the rupture, and a region between the mainshock and the largest aftershock ruptures. Afterslip in the first ~ 7 months after the mainshock released a moment of [12.8 ± 0.5] × 10^(19) Nm which represents 17.8 ± 0.8% of the co-seismic moment. The moment released by aftershocks over that period of time is estimated to 2.98 × 10^(19) Nm. Geodetically observed post-seismic deformation after co-seismic offset correction was thus 76.7 ± 1.0% aseismic. The logarithmic time evolution of afterslip is consistent with rate-strengthening frictional sliding. According to this theory, and assuming a long-term loading velocity modulated on the basis of the coupling map of the region and the long term slip rate of 20.2 ± 1.1 mm/yr, afterslip should release about 34.0 ± 1.4% of the co-seismic moment after full relaxation of post-seismic deformation. Afterslip contributed to loading the shallower portion of the MHT which did not rupture in 2015 and stayed locked afterwards. The risk for further large earthquakes in Nepal remains high both updip of the rupture area of the Gorkha earthquake and West of Kathmandu where the MHT has remained locked and where no earthquake larger than M_w7.5 has occurred since 1505.

Great Himalayan earthquakes are rare. Analysis of surface motions in the months after the 2015 Gorkha earthquake reveals negligible aseismic slip, implying that stress may be stored in the crust to be tapped during future great earthquakes. The magnitude 7.8 Gorkha earthquake in April 2015 ruptured a 150-km-long section of the Himalayan decollement terminating close to Kathmandu1,2,3,4. The earthquake failed to rupture the surface Himalayan frontal thrusts and raised concern that a future Mw ≤ 7.3 earthquake could break the unruptured region to the south and west of Kathmandu. Here we use GPS records of surface motions to show that no aseismic slip occurred on the ruptured fault plane in the six months immediately following the earthquake. We find that although 70 mm of afterslip occurred locally north of the rupture, fewer than 25 mm of afterslip occurred in a narrow zone to the south. Rapid initial afterslip north of the rupture was largely complete in six months, releasing aseismic-moment equivalent to a Mw 7.1 earthquake. Historical earthquakes in 1803, 1833, 1905 and 1947 also failed to rupture the Himalayan frontal faults, and were not followed by large earthquakes to their south. This implies that significant relict heterogeneous strain prevails throughout the Main Himalayan Thrust. The considerable slip during great Himalayan earthquakes may be due in part to great earthquakes tapping reservoirs of residual strain inherited from former partial ruptures of the Main Himalayan Thrust.

We sampled six coral microatolls that recorded the relative sea level changes over the last 230 years east of Martinique, on fringing reefs in protected bays. The microatolls are cup-shaped, which is characteristic of corals that have been experiencing submergence. X-ray analysis of coral slices and reconstructions of the highest level of survival (HLS) curves show that they have submerged at rates of a few millimeters per year. Their morphology reveals changes in submergence rate around 1829 ± 11, 1895, and 1950. Tide gauges available in the region indicate a regional sea level rise at a constant mean rate of 1.1 ± 0.8 mm/yr, which contrasts with our coral record, implying additional tectonic subsidence. Comparing our coral morphology with that of synthetic corals generated with Matlab by using the Key West tide gauge record (Florida), we show that their growth was controlled by tectonics and that a sudden relative sea level increase drowned them around 1950. Simple elastic models show that this sudden submergence probably occurred during the 21 May 1946 earthquake, which ruptured the plate interface in front of Martinique, in the mantle wedge, in an area of sustained seismic activity. The 1839 M8+ earthquake probably occurred in the same area. Long-term subsidence of microatolls indicates that this deep portion of the megathrust is probably locked down to 60 km depth during the interseismic period. Our oldest coral recorded a long-lasting period (50 years) of stable relative sea level after the 1839 earthquake, indicating that transient interseismic strain rate variations may occur in the Lesser Antilles.

Detailed geodetic imaging of earthquake ruptures enhances our understanding of earthquake physics and associated ground shaking. The 25 April 2015 moment magnitude 7.8 earthquake in Gorkha, Nepal was the first large continental megathrust rupture to have occurred beneath a high-rate (5-hertz) Global Positioning System (GPS) network. We used GPS and interferometric synthetic aperture radar data to model the earthquake rupture as a slip pulse ~20 kilometers in width, ~6 seconds in duration, and with a peak sliding velocity of 1.1 meters per second, which propagated toward the Kathmandu basin at ~3.3 kilometers per second over ~140 kilometers. The smooth slip onset, indicating a large (~5-meter) slip-weakening distance, caused moderate ground shaking at high frequencies (>1 hertz; peak ground acceleration, ~16% of Earth’s gravity) and minimized damage to vernacular dwellings. Whole-basin resonance at a period of 4 to 5 seconds caused the collapse of tall structures, including cultural artifacts.

The Mw 7.7 2007 November 14 earthquake had an epicentre located close to the city of Tocopilla, at the southern end of a known seismic gap in North Chile. Through modelling of Global Positioning System (GPS) and radar interferometry (InSAR) data, we show that this event ruptured the deeper part of the seismogenic interface (30–50 km) and did not reach the surface. The earthquake initiated at the hypocentre and was arrested ∼150 km south, beneath the Mejillones Peninsula, an area already identified as an important structural barrier between two segments of the Peru–Chile subduction zone. Our preferred models for the Tocopilla main shock show slip concentrated in two main asperities, consistent with previous inversions of seismological data. Slip appears to have propagated towards relatively shallow depths at its southern extremity, under the Mejillones Peninsula. Our analysis of post-seismic deformation suggests that small but still significant post-seismic slip occurred within the first 10 d after the main shock, and that it was mostly concentrated at the southern end of the rupture. The post-seismic deformation occurring in this period represents ∼12–19 per cent of the coseismic deformation, of which ∼30–55 per cent has been released aseismically. Postseismic slip appears to concentrate within regions that exhibit low coseismic slip, suggesting that the afterslip distribution during the first month of the post-seismic interval complements the coseismic slip. The 2007 Tocopilla earthquake released only ∼2.5 per cent of the moment deficit accumulated on the interface during the past 130 yr and may be regarded as a possible precursor of a larger subduction earthquake rupturing partially or completely the 500-km-long North Chile seismic gap.

The 2012 Brawley earthquake swarm occurred in the Brawley Seismic Zone (BSZ) within the Imperial Valley of southern California (Fig. 1). The BSZ is the northernmost extensional segment of the Pacific–North America plate boundary system. Johnson and Hill (1982) used the distribution of seismicity since the 1930s to outline the geographical extent of the BSZ, defining boundaries of the BSZ as shown in Figure 1. Its north–south extent ranges from the northern section of the Imperial fault, starting approximately 10 km north of the United States–Mexico international border and connecting to the southern end of the San Andreas fault, where it terminates in the Salton Sea. Larsen and Reilinger (1991), who defined a similar geographical extent of the BSZ, argued that the BSZ was migrating to the northwest, which they associated with the propagation of the Gulf of California rift system into the North American continent. During the seismically active period of the 1970s, the BSZ produced close to half of the earthquakes recorded in California (Johnson and Hill, 1982; Hutton et al., 2010). However, for two decades following the 1979 Imperial Valley mainshock M_w 6.4 and its aftershock sequence, the BSZ was much less active. In general, the BSZ seismicity is indicative of right-lateral strike-slip plate motion accompanied by crustal thinning as well as possible associated fluid movements in the crust (Chen and Shearer, 2011). The 2012 Brawley swarm produced more than 600 events recorded by the United States Geological Survey (USGS)–California Institute of Technology (Caltech) Southern California Seismic Network (SCSN). Other monitoring instruments in the region, such as the Global Positioning System (GPS) network, creepmeters, and the Wildlife Liquefaction Array (WLA) also recorded signals from the largest events. In addition, Interferometric Synthetic Aperture Radar (InSAR) satellites collected images from space.

We document geodetic strain across the Nepal Himalaya using GPS times series from 30 stations in Nepal and southern Tibet, in addition to previously published campaign GPS points and leveling data and determine the pattern of interseismic coupling on the Main Himalayan Thrust fault (MHT). The noise on the daily GPS positions is modeled as a combination of white and colored noise, in order to infer secular velocities at the stations with consistent uncertainties. We then locate the pole of rotation of the Indian plate in the ITRF 2005 reference frame at longitude = − 1.34° ± 3.31°, latitude = 51.4° ± 0.3° with an angular velocity of Ω = 0.5029 ± 0.0072°/Myr. The pattern of coupling on the MHT is computed on a fault dipping 10° to the north and whose strike roughly follows the arcuate shape of the Himalaya. The model indicates that the MHT is locked from the surface to a distance of approximately 100 km down dip, corresponding to a depth of 15 to 20 km. In map view, the transition zone between the locked portion of the MHT and the portion which is creeping at the long term slip rate seems to be at the most a few tens of kilometers wide and coincides with the belt of midcrustal microseismicity underneath the Himalaya. According to a previous study based on thermokinematic modeling of thermochronological and thermobarometric data, this transition seems to happen in a zone where the temperature reaches 350°C. The convergence between India and South Tibet proceeds at a rate of 17.8 ± 0.5 mm/yr in central and eastern Nepal and 20.5 ± 1 mm/yr in western Nepal. The moment deficit due to locking of the MHT in the interseismic period accrues at a rate of 6.6 ± 0.4 × 10^(19) Nm/yr on the MHT underneath Nepal. For comparison, the moment released by the seismicity over the past 500 years, including 14 M_W ≥ 7 earthquakes with moment magnitudes up to 8.5, amounts to only 0.9 × 10^(19) Nm/yr, indicating a large deficit of seismic slip over that period or very infrequent large slow slip events. No large slow slip event has been observed however over the 20 years covered by geodetic measurements in the Nepal Himalaya. We discuss the magnitude and return period of M > 8 earthquakes required to balance the long term slip budget on the MHT.

Coral records of relative sea level change provide a history of vertical interseismic and coseismic deformation along the coast of northern Simeulue Island, Sumatra, and reveal details about earthquakes in the 10th and 14th–15th centuries A.D. along the southern end of the December 2004 Mw 9.2 Sunda megathrust rupture. Over a 56 year period between A.D. 1390 and 1455, northern Simeulue experienced a cluster of megathrust ruptures, associated with total uplift that was considerably more than in 2004. Uplifted corals at two sites constrain the first event of the cluster to A.D. 1393 ± 3 and 1394 ± 2 (2σ). A smaller but well-substantiated uplift occurred in northern Simeulue in 1430 ± 3. An inferred third uplift, in A.D. 1450 ± 3, killed all corals on the reef flats of northern Simeulue. The amount of uplift during this third event, though confirmed only to have exceeded 28 and 41 cm at two sites, probably surpassed the 100 and 44 cm that occurred, respectively, at those sites in 2004, and it was likely more than in 2004 over all of northern Simeulue. The evidence for past earthquake clustering combined with the inference of considerably greater uplift in A.D. 1390–1455 than in 2004 suggests that strain may still be stored along the southernmost part of the 2004 rupture. Interseismic subsidence rates recorded by northern Simeulue coral microatolls have varied by up to a factor of 4 at some sites from one earthquake cycle to another. Published version

We quantify fore-arc deformation using fossil reefs to test the assumption commonly made in seismic cycle models that anelastic deformation of the fore arc is negligible. Elevated coral microatolls, paleoreef flats, and chenier plains show that the Sumatran outer arc island of Nias has experienced a complex pattern of relatively slow long-term uplift and subsidence during the Holocene epoch. This same island rose up to 2.9 m during the Mw 8.7 Sunda megathrust rupture in 2005. The mismatch between the 2005 and Holocene uplift patterns, along with the overall low rates of Holocene deformation, reflects the dominance of elastic strain accumulation and release along this section of the Sunda outer arc high and the relatively subordinate role of upper plate deformation in accommodating long-term plate convergence. The fraction of 2005 uplift that will be retained permanently is generally

Records of relative sea-level change extracted from corals of the Mentawai islands, Sumatra, imply that this 700-kilometer-long section of the Sunda megathrust has generated broadly similar sequences of great earthquakes about every two centuries for at least the past 700 years. The moment magnitude 8.4 earthquake of September 2007 represents the first in a series of large partial failures of the Mentawai section that will probably be completed within the next several decades.

International audience; Geodetic and paleogeodetic measurements of interseismic strain above the Sumatran portion of the Sunda subduction zone reveal a heterogeneous pattern of coupling. Annual banding in corals provides vertical rates of deformation spanning the last half of the 20th century, and repeated GPS surveys between 1991 and 2001 and continuous measurements at GPS stations operated since 2002 provide horizontal velocities. Near the equator, the megathrust is locked over a narrow width of only a few tens of kilometers. In contrast, the locked fault zone is up to about 175 km wide in areas where great interplate earthquakes have occurred in the past. Formal inversion of the data reveals that these strongly coupled patches are roughly coincident with asperities that ruptured during these events. The correlation is most spectacular for rupture of the Mw 8.7 Nias-Simeulue earthquake of 2005, which released half of the moment deficit that had accumulated since its previous rupture in 1861, suggesting that this earthquake was overdue. Beneath the Mentawai islands, strong coupling is observed within the overlapping rupture areas of the great earthquakes of 1797 and 1833. The accumulated slip deficit since these events is slowly reaching the amount of slip that occurred during the 1833 earthquake but already exceeds the slip that occurred during the 1797 earthquake. Thus, rerupture of part of the Mentawai patch in September 2007 was not a surprise. In contrast, coupling is low below the Batu islands near the equator and around Enggano island at about 5°S, where only moderate earthquakes (Mw < 8.0) have occurred in the past two centuries. The correlation of large seismic asperities with patches that are locked during the interseismic period suggests that they are persistent features. This interpretation is reinforced by the fact that the large locked patches and great ruptures occur beneath persistent geomorphologic features, the largest outer arc islands. Depth- and convergence-rate-dependent temperature might influence the pattern of coupling, through its effect on the rheology of the plate interface, but other influences are required to account for the observed along-strike heterogeneity of coupling. In particular, subduction of the Investigator Fracture Zone could be the cause for the low coupling near the equator.

The great Sumatra - Andaman earthquake and tsunami of 2004 was a dramatic reminder of the importance of understanding the seismic and tsunami hazards of subduction zones(1-4). In March 2005, the Sunda megathrust ruptured again, producing an event(5) of moment magnitude (M-w) 8.6 south of the 2004 rupture area, which was the site of a similar event in 1861 (ref. 6). Concern was then focused on the Mentawai area, where large earthquakes had occurred in 1797 ( M-w = 8.8) and 1833 (M-w = 9.0)(6,7). Two earthquakes, one of M-w = 8.4 and, twelve hours later, one of M-w = 7.9, indeed occurred there on 12 September 2007. Here we show that these earthquakes ruptured only a fraction of the area ruptured in 1833 and consist of distinct asperities within a patch of the megathrust that had remained locked in the interseismic period. This indicates that the same portion of a megathrust can rupture in different patterns depending on whether asperities break as isolated seismic events or cooperate to produce a larger rupture. This variability probably arises from the influence of non- permanent barriers, zones with locally lower pre- stress due to the past earthquakes. The stress state of the portion of the Sunda megathrust that had ruptured in 1833 and 1797 was probably not adequate for the development of a single large rupture in 2007. Themoment released in 2007 amounts to only a fraction both of that released in 1833 and of the deficit of moment that had accumulated as a result of interseismic strain since 1833. The potential for a large megathrust event in the Mentawai area thus remains large.

Journal of Geophysical Research, v. 112, p. B02404, 2007. http://dx.doi.org/10.1029/2006JB004450; International audience; The geomorphology and internal stratigraphy of modern coral microatolls show that all the outer arc Mentawai islands of West Sumatra have been subsiding over the past several decades. These same islands rose as much as 3 m during the giant megathrust earthquakes of 1797 and 1833, and the current subsidence probably reflects strain accumulation that will lead to future large earthquakes. Average subsidence rates over the past half century vary from 2 to 14 mm yr−1 and increase southwestward, toward the subduction trench. The pattern is consistent with rates of subsidence measured by a sparse network of continuously recording Global Positioning System (cGPS) stations and with locking of a 400-km-long section of the underlying subduction megathrust, between about 1°S and 4°S. This record of subsidence and tilting, extending nearly a century into the past, implies that the region is advancing toward the occurrence of another giant earthquake. However, evidence of episodic rather than steady subsidence reflects a behavior that is more complex than simple elastic strain accumulation and relief. Most prominent of these episodes is an extensive emergence/subsidence couplet in about 1962, which may be the result of rapid, aseismic slip on the megathrust, between the islands and the trench. Lower subsidence rates recorded by the corals since about 1985 may reflect failure on many small patches within the locked section of the megathrust.

We determine coseismic and the first-month postseismic deformation associated with the Sumatra–Andaman earthquake of 26 December 2004 from near- field Global Positioning System (gps) surveys in northwestern Sumatra and along the Nicobar-Andaman islands, continuous and campaign gps measurements from Thailand and Malaysia, and in situ and remotely sensed observations of the vertical motion of coral reefs. The coseismic model shows that the Sunda subduction megathrust ruptured over a distance of about 1500 km and a width of less than 150 km, releasing a total moment of 6.7–7.0 × 1022 N m, equivalent to a magnitude Mw ∼9.15. The latitudinal distribution of released moment in our model has three distinct peaks at about 4° N, 7° N, and 9° N, which compares well to the latitudinal variations seen in the seismic inversion and of the analysis of radiated T waves. Our coseismic model is also consistent with interpretation of normal modes and with the amplitude of very-long-period surface waves. The tsunami predicted from this model fits relatively well the altimetric measurements made by the jason and topex satellites. Neither slow nor delayed slip is needed to explain the normal modes and the tsunami wave. The near-field geodetic data that encompass both coseismic deformation and up to 40 days of postseismic deformation require that slip must have continued on the plate interface after the 500-sec-long seismic rupture. The postseismic geodetic moment of about 2.4 × 1022 N m (Mw ∼8.8) is equal to about 30 ± 5% of the coseismic moment release. Evolution of postseismic deformation is consistent with rate-strengthening frictional afterslip. Online material: Summary of geodetic data used in this study.

Continuously recording Global Positioning System stations near the 28 March 2005 rupture of the Sunda megathrust [moment magnitude ( M w ) 8.7] show that the earthquake triggered aseismic frictional afterslip on the subduction megathrust, with a major fraction of this slip in the up-dip direction from the main rupture. Eleven months after the main shock, afterslip continues at rates several times the average interseismic rate, resulting in deformation equivalent to at least a M w 8.2 earthquake. In general, along-strike variations in frictional behavior appear to persist over multiple earthquake cycles. Aftershocks cluster along the boundary between the region of coseismic slip and the up-dip creeping zone. We observe that the cumulative number of aftershocks increases linearly with postseismic displacements; this finding suggests that the temporal evolution of aftershocks is governed by afterslip.

Large uplifts and tilts occurred on the Sumatran outer arc islands between 0.5° and 3.3°S during great historical earthquakes in 1797 and 1833, as judged from relative sea level changes recorded by annually banded coral heads. Coral data for these two earthquakes are most complete along a 160-km length of the Mentawai islands between 3.2° and 2°S. Uplift there was as great as 0.8 m in 1797 and 2.8 m in 1833. Uplift in 1797 extended 370 km, between 3.2° and 0.5°S. The pattern and magnitude of uplift imply megathrust ruptures corresponding to moment magnitudes (M_w) in the range 8.5 to 8.7. The region of uplift in 1833 ranges from 2° to at least 3.2°S and, judging from historical reports of shaking and tsunamis, perhaps as far as 5°S. The patterns and magnitude of uplift and tilt in 1833 are similar to those experienced farther north, between 0.5° and 3°N, during the giant Nias-Simeulue megathrust earthquake of 2005; the outer arc islands rose as much as 3 m and tilted toward the mainland. Elastic dislocation forward modeling of the coral data yields megathrust ruptures with moment magnitudes ranging from 8.6 to 8.9. Sparse accounts at Padang, along the mainland west coast at latitude 1°S, imply tsunami runups of at least 5 m in 1797 and 3–4 m in 1833. Tsunamis simulated from the pattern of coral uplift are roughly consistent with these reports. The tsunami modeling further indicates that the Indian Ocean tsunamis of both 1797 and 1833, unlike that of 2004, were directed mainly south of the Indian subcontinent. Between about 0.7° and 2.1°S, the lack of vintage 1797 and 1833 coral heads in the intertidal zone demonstrates that interseismic submergence has now nearly equals coseismic emergence that accompanied those earthquakes. The interseismic strains accumulated along this reach of the megathrust have thus approached or exceeded the levels relieved in 1797 and 1833.

Seismic rupture produced spectacular tectonic deformation above a 400-kilometer strip of the Sunda megathrust, offshore northern Sumatra, in March 2005. Measurements from coral microatolls and Global Positioning System stations reveal trench-parallel belts of uplift up to 3 meters high on the outer-arc islands above the rupture and a 1-meter-deep subsidence trough farther from the trench. Surface deformation reflects more than 11 meters of fault slip under the islands and a pronounced lessening of slip trenchward. A saddle in megathrust slip separates the northwestern edge of the 2005 rupture from the great 2004 Sumatra-Andaman rupture. The southeastern edge abuts a predominantly aseismic section of the megathrust near the equator.

We utilize coral microatolls in western Sumatra to document vertical deformation associated with subduction. Microatolls are very sensitive to fluctuations in sea level and thus act as natural tide gauges. They record not only the magnitude of vertical deformation associated with earthquakes (paleoseismic data), but also continuously track the long-term aseismic deformation that occurs during the intervals between earthquakes (paleogeodetic data). This paper focuses on the twentieth century paleogeodetic history of the equatorial region. Our coral paleogeodetic record of the 1935 event reveals a classical example of deformations produced by seismic rupture of a shallow subduction interface. The site closest to the trench rose 90 cm, whereas sites further east sank by as much as 35 cm. Our model reproduces these paleogeodetic data with a 2.3 m slip event on the interface 88 to 125 km from the trench axis. Our coral paleogeodetic data reveal slow submergence during the decades before and after the event in the areas of coseismic emergence. Likewise, interseismic emergence occurred before and after the 1935 event in areas of coseismic submergence. Among the interesting phenomenon we have discovered in the coral record is evidence of a large aseismic slip or “silent event” in 1962, 27 years after the 1935 event. Paleogeodetic deformation rates in the decades before, after, and between the 1935 and 1962 events have varied both temporally and spatially. During the 25 years following the 1935 event, submergence rates were dramatically greater than in prior decades. During the past four decades, however, rates have been lower than in the preceding decades, but are still higher than they were prior to 1935. These paleogeodetic records enable us to model the kinematics of the subduction interface throughout the twentieth century.