The ongoing collision of India with Asia is partly accommodated by slip on the Main Himalayan Thrust (MHT). The 25 April 2015, M w 7.8 Gorkha earthquake is the most recent major event to rupture the MHT, which dips gently northward beneath central Nepal. Although the geology of the range has been studied for decades, fundamental aspects of its deep structure remain disputed. Here, we develop a structural cross section and a three-dimensional, geologically informed model of the MHT that are consistent with seismic observations from the Gorkha earthquake. A comparison of our model to a detailed slip inversion data set shows that the slip patch closely matches an ovalshaped, gently dipping fault surface bounded on all sides by steeper ramps. The Gorkha earthquake rupture seems to have been limited by the geometry of that fault segment. This is a significant step forward in understanding the deep geometry of the MHT and its effect on earthquake nucleation and propagation. Published models of fault locking do not correlate with the slip patch or our fault model in the vicinity of the earthquake, further suggesting that fault geometry was the primary control on this event. Our result emphasizes the importance of adequately constraining subsurface fault geometry in megathrusts in order to better assess the sizes and locations of future earthquakes.

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Myanmar is located at the eastern margin of the ongoing Indo-Eurasian collision system, has experienced a complex tectonic history and is threatened by a high level of seismic hazard. Here we develop a crustal scale 3-D seismic velocity model of Myanmar, which is not only critical for understanding the regional tectonic setting and its evolution but can also provide the foundation for a variety of seismological studies, including earthquake location determinations, earthquake focal mechanism inversions, and ground motion simulations. We use the newly deployed Earth Observatory of Singapore-Myanmar broadband seismic network and other seismic stations in and around Myanmar to study the station-based 1-D velocity structure through a joint inversion of receiver functions, H/V amplitude ratio of Rayleigh waves, and surface wave dispersion measurements. Our results reveal a highly variable crustal structure across Myanmar region, characterized by a series of N-S trending sedimentary basins, with thicknesses up to~15 km in central Myanmar and an~5-km step in the depth of the Moho across the Sagaing-Shan Scarp fault system. We interpolate our station-based 1-D velocity profiles to obtain an integrated 3-D velocity model from southern Bangladesh to Myanmar. Using three regional earthquakes located to the south, within, and north of the seismic network, we show that our proposed model performs systematically better than the CRUST 1.0 model for both Pnl waves and surface waves. Our study provides a preliminary community velocity model for the region, with further refinements and interpretations anticipated in the near future.

We calculate two-station phase measurements using single-station measurements made on USArray Transportable Array data for surface waves at periods from 25 to 100 s. The phase measurements are inverted for baseline Love and Rayleigh wave phase velocity maps on a 0.5 • × 0.5 • grid. We make estimates of the arrival angle for each event at each station using a mini array method similar to beamforming, and apply this information to correct the geometry of the two-station measurements. These corrected measurements are inverted for an additional set of phase velocity maps. Arrival angles range from 0 • to ±15 • , and the associated corrections result in local changes of up to 4 per cent in the final phase velocity maps. We select our preferred models on the basis of the internal consistency of the measurements, finding that the arrival-angle corrections improve the two-station phase measurements, but that Love wave arrival-angle estimates may be contaminated by overtone interference. Our preferred models compare favourably with recent studies of the phase velocity of the Western United States. The corrected Rayleigh wave models achieve greater variance reduction than the baseline Rayleigh wave models, and the baseline Love wave models, which are more difficult to obtain, are robust and could be used in conjunction with the Rayleigh wave models to constrain radially anisotropic earth structure.

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The deep magmatic processes in volcanic arcs are often poorly understood. We analyze the shear wave velocity (V S ) distribution in the crust and uppermost mantle below Mount Rainier, in the Cascades arc, resolving the main velocity contrasts based on converted phases within P coda via source normalization or receiver function (RF) analysis. To alleviate the trade-off between depth and velocity, we use long period phase velocities (25-100 s) obtained from earthquake surface waves, and at shorter period (7-21 s) we use seismic noise cross correlograms. We use a transdimensional Bayesian scheme to explore the model space (V S in each layer, number of interfaces and their respective depths, level of noise on data). We apply this tool to 15 broadband stations from permanent and Earthscope temporary stations. Most results fall into two groups with distinctive properties. Stations east of the arc (Group I) have comparatively slower middle-to-lower crust (V S 5 3.4-3.8 km/s at 25 km depth), a sharp Moho and faster uppermost mantle (V S 5 4.2-4.4 km/s). Stations in the arc (Group II) have a faster lower crust (V S 5 3.7-4 km/s) overlying a slower uppermost mantle (V S 5 4.0-4.3 km/s), yielding a weak Moho. Lower crustal velocities east of the arc (Group I) most likely represent ancient subduction melanges mapped nearby. The lower crust for Group II ranges from intermediate to felsic. We propose that intermediate-felsic to felsic rocks represent the prearc basement, while intermediate composition indicates the mushy andesitic crustal magmatic system plus solidified intrusion along the volcanic conduits. We interpret the slow upper mantle as partial melt.