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Abstract We propose that the mantle lithospheric density and crustal thickness are correlated in such a way as to produce a flat Tibetan Plateau. We observe that the mantle lithosphere is relatively uniform beneath the Himalaya and southern and central Tibet, despite a near doubling of crustal thickness relative to India. Farther north, cratonic mantle lithosphere disappears over large regions of north-central Tibet, giving rise to large lateral variations in uppermost mantle Vs anomalies (>12%) that are uncorrelated with changes in surface elevation but are closely related to changes in crustal thickness. This decoupling of surface topography from spatial variations in upper mantle seismic velocity, and assumed buoyancy, implies that Tibetan topography is controlled by a crust-mantle interaction that is able to maintain its near constant elevation. This crust-mantle interaction is likely driven by gravitational potential energy with a very weak crust. Magmatism, with ages of ca. 20 Ma to Present, spatially correlated with this region with no sub-Moho mantle lithosphere implies destabilization of mantle lithosphere in northern Tibet. Cratonic Indian underthrusting for the past 25 m.y. has also not led to significant topography in the plateau through time. The magmatism may have helped weaken the crust, allowing it to respond to changes in uppermost mantle buoyancy, resulting in a flat plateau. 

Abstract The devastating 6 February 2023 Kahramanmaraş earthquake sequence in southeastern Türkiye started with a moment magnitude (Mw) 7.8 earthquake, for which the initial rupture broke the Sakçagöz segment near Nurdağı and then jumped into a bilateral rupture along multiple segments of the Eastern Anatolian fault zone (EAFZ). This complicated rupture was followed nine hours later by an Mw 7.6 event near Ekinözü. To better understand the spatiotemporal evolution of aftershocks, site amplification, and the structural and tectonic framework of the EAFZ in this diffuse triple junction, we deployed a dense seismometer array covering both aftershock zones for nearly four months. The main Eastern Anatolian Seismic Temporary (EAST) array includes 125 nodal, 10 broadband, and 6 strong-motion seismic stations distributed around the rupture zone. An additional linear array of 73 nodal stations was also installed across the Pazarcık segment of the EAFZ and the Sakçagöz segment near the Mw 7.8 epicenter to record fault-zone waves for ∼30 days. This article shows example recordings and the EAST array geometry, preliminary research results, and the metadata related to all of the stations in this array. A deep-learning-based phase picking for one month of continuous recording yielded millions of seismic phase readings and tens of thousands of aftershock locations after phase associations. We also give examples of both local and teleseismic waveforms recorded by the nodal arrays, which can be used for subsequent high-resolution earthquake relocation, imaging of crustal structures, and fault-zone imaging. 

We utilized shear wave splitting analysis of teleseismic SKS, SKKS, and PKS phases to infer upper mantle deformational fabrics across a substantial area of Southeast Asia, where splitting measurements were previously limited. We used newly available permanent and temporary broadband seismic networks deployed across the Indo-Burma subduction zone and the eastern Indochina peninsula. The resulting 492 well-constrained splitting and 654 null measurements from 185 stations reveal clear large-scale patterns in the mantle deformational fabrics in response to the highly oblique active subduction and a large transform plate boundary. We identified two distinct domains of mantle deformation fabrics in the western Burma microplate and the eastern Indochina peninsula. In the former, trench parallel N-S fast polarization directions with an average lag time (δt) of 1.9 s are observed beneath the Indo-Burman Ranges. We suggest the observed splitting is partly due to anisotropy in the sub-slab region and relates to shear induced by the north moving Indian plate. The lithospheric fabric within the Indo-Burman Ranges and underlying subducting slab fabric contribute to produce the observed average δt of 1.9 s. The δt value decreases to an average of 1.0 s towards the back-arc until we reach the dextral Sagaing fault. In the second domain, starting approximately 100 km east of the Sagaing fault, we observe a consistent E-W fast direction with an average δt of 1.10 s in the eastern Shan-Thai and Indochina blocks. We interpret the E-W fabric as due to the deformation associated with the westward spreading of the Hainan mantle plume, possibly driven by overriding plate motion. Low velocities in the shallow mantle and late Cenozoic intraplate volcanism in this region support the plume-driven asthenospheric flow model in the Indochina peninsula. The sudden transition of the fast polarization direction from N-S to E-W along the eastern edge of the Burma microplate indicates the Sagaing fault acts as a mantle flow boundary between the subduction dominated trench parallel flow to the west and plume induced asthenospheric flow to the east. We also observed no net splitting beneath the Bengal basin which is most likely due to the presence of frozen vertical fabric resulting from the Kerguelen plume activity during Early Cretaceous. 

Abstract The Thai Meteorological Department (TMD) seismic network began development in 2008. There are a total of 71 seismic stations consisting of 26 borehole stations and 45 surface stations currently installed. The three-component data from the TMD seismic network have been widely used in previous seismological studies. In a recent analysis, we have found that sensor orientation as reported in the site metadata is sometimes significantly incorrect, especially for borehole stations. In this study, we analyze P-wave polarization data from regional and teleseismic earthquakes recorded in the network to estimate the true instrument orientation relative to geographic north and compare that to station metadata. Of the 45 surface stations, we found that at present, ~ 82% are well oriented (i.e., aligned within 0–15° of true north). However, 8 sites have sensors misoriented by more than 15°, and some stations had a temporal change in sensor orientation during an upgrade to the seismic system with replacement of the sensor. We also evaluated sensor orientations for 26 TMD borehole seismic stations, from 2018 to the 2022. For many of the borehole stations, the actual sensor orientation differs significantly from the TMD metadata, especially at short-period stations. Many of those stations have sensor misorientations approaching 180°, due to errors in the ambient noise analysis calibration techniques used during installation. We have also investigated how this sensor misorientation affects previous seismic studies, such as regional moment tensor inversion of earthquakes sources and receiver function stacking. We have found that the large deviations in sensor orientation can result in erroneous results and/or large measurement errors. A cause of the orientation error for borehole sites could be a combination of strong background surface ambient seismic noise coupled with an incorrect reference instrument response. 

The seismology component of this experiment will consist of a 2.5 dimensional transect that will cross from Bangladesh into Myanmar. We will install as many stations as possible on hard rock sites to minimize noise, although this will not be possible in low-lying deltaic areas. The array will consist of three lines. The middle line will be closely spaced in order to image shallow crustal features. It will have a station spacing of 5-10 km in Bangladesh expanding to 15 km in eastern Myanmar. To image the detachment megathrust at 10-20 km depth in the accretionary prism, a 100-km-long section spanning the Bangladesh and India border will have station spacing of 5 km or less. Two flanking lines located ~40 km on either side will have ~40 km spacing. This 80 km wide swath is critical for earthquake locations and body- and surface-wave tomography. The stations will operate for ~2 years, providing ample recordings from a wide backazimuth distribution of local, regional, and teleseismic events, and ambient noise for analysis