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1. Teleseismic Converted Waves Image of the Indo-Burma Subduction System Across the Bangladesh-India-Myanmar (BIMA) Array

   Ajala, R. ; Persaud, P. ; Steckler, M. ; Sandvol, E. ; Islam, M. ; Gaherty, J. ; Than, O. ; Oo, K. ; Akhter, S. ; Ni, J. ( January 2020 , American Geophysical Union, Fall Meeting 2020)

   null (Ed.)

   Recent GPS studies show that the Indo-Burma subduction system is locked with the implication of a potential large-magnitude earthquake. To inform better seismic hazard models in the region, we need an improved understanding of the crustal structure and the dynamics of the Indo-Burma subduction system. The Bangladesh-India-Myanmar (BIMA) tripartite project deployed 60 broadband seismometers across the subduction system and have been continuously recording data for ~2 years. In this study, we computed receiver functions from 30 high-quality earthquakes (M≥5.9) with epicentral distances between 30º and 90º recorded by the array. The algorithm utilized ensures the uniqueness of the seismic model and provides an uncertainty estimate of every converted wave amplitude. We stacked all the receiver functions produced at each station along the entire transect to generate a cross-sectional model of the average crustal structure. The level of detail in the image is improved by computing higher frequency receiver functions up to 4 Hz. The results represent some of the strongest constraints on crustal structure across the subduction system. Beneath the Neogene accretionary prism's outer belt, we observe a primary conversion associated with the Ganges Brahmaputra Delta that ranges in depth from ~10 km near the deformation front up to ~12 km at the eastern boundary. From the eastern end of the Neogene accretionary prism to the Sagaing Fault, we image the Indian subducting slab and the Central Myanmar basin. The depth-extent of seismicity associated with the Wadati-Benioff zone is consistent with the locations of primary conversions from the subducting plate. We further verify the converted phases of the slab by analyzing azimuthal moveout variations. The Central Myanmar basin is roughly bowl-shaped in cross-section with a maximum thickness of ~15 km about halfway between the Kabaw and Sagaing faults. The average crustal thickness beneath the Ganges-Brahmaputra delta is ~20 km, most likely representing a transitional crust formed from thinning of the continental crust intruded and underplated by igneous rocks. In contrast, the average thickness of the continental crust beneath the Central Myanmar basin is ~40 km. Our results provide a baseline model for future geophysical investigations of the Indo-Burma subduction zone. 

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2. Geodetic Extension Across the Southern Basin and Range and Colorado Plateau

   https://doi.org/10.1029/2020JB021355  

   Broermann, James ; Bennett, Richard A. ; Kreemer, Corné ; Blewitt, Geoffrey ; Pearthree, Philip A. ( June 2021 , Journal of Geophysical Research: Solid Earth)

   Rates of crustal deformation in the southern Basin and Range (SBR) and Colorado Plateau (CP) provinces are relatively low in the context of the Pacific‐North America plate boundary (PA–NA); however, the accumulation of small amounts of strain over long periods of time can lead to large earthquakes such as theM_w7.5 1887 Sonoran earthquake in northern Mexico. SBR and CP rates of deformation are difficult to quantify due to a dearth of young faulting and seismicity. Moreover, strain accumulation and release related to the adjacent, more active San Andreas and Gulf of California fault systems to the west and southwest can mask the background strain rates associated with SBR and CP tectonics. With data from an enhanced continuous GPS network, we estimate crustal surface velocities of the SBR and CP, after removing coseismic and postseismic displacements, and elastic loading effects arising from major fault zones to the (south)west. We use cluster analysis and geologic data to separate the GPS velocity field into regions and calculate distinct block rotation and uniform strain rates for each region. We find the highest strain rate region includes southwestern Arizona; an area with sparse Quaternary faults, relatively low seismicity, and a relatively large discrepancy between geodetic and geologic rates of deformation. This anomalous strain rate may reflect residual, unmodeled PA‐NA strain seeping into the Arizona study area from the west. Alternatively, it may represent the potential for one or more rare, future, large‐magnitude earthquakes or indicate strain is being released through other process(es).

    

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3. New GNSS and Geological Data From the Indo‐Burman Subduction Zone Indicate Active Convergence on Both a Locked Megathrust and the Kabaw Fault

   https://doi.org/10.1029/2022JB025550  

   Oryan, Bar ; Betka, Paul M. ; Steckler, Michael S. ; Nooner, Scott L. ; Lindsey, Eric O. ; Mondal, Dhiman ; Mathews, Austin M. ; Akhter, Syed Humayun ; Singha, Sanju ; Than, Oo ( April 2023 , Journal of Geophysical Research: Solid Earth)

   The Indo‐Burma subduction zone is a highly oblique subduction system where the Indian plate is converging with the Eurasian plate. How strain is partitioned between the Indo‐Burma interface and upper plate Kabaw Fault, and whether the megathrust is a locked and active zone of convergence that can generate great earthquakes are ongoing debates. Here, we use data from a total of 68 Global Navigation Satellite System (GNSS) stations, including newly installed stations across the Kabaw Fault and compute an updated horizontal and vertical GNSS velocity field. We correct vertical rates for fluctuating seasonal signals by accounting for the elastic response of monsoon water on the crust. We model the geodetic data by inverting for 11,000 planar and non‐planar megathrust fault geometries and two geologically viable structural interpretations of the Kabaw Fault that we construct from field geological data, considering a basin‐scale wedge‐fault and a crustal‐scale reverse fault. We demonstrate that the Indo‐Burma megathrust is locked, converging at a rate ofmm/yr, and capable of hosting >8.2M_wmegathrust events. We also show that the Kabaw Fault is locked and accommodating strike‐slip motion at a rate ofmm/yr and converging at a rate ofmm/yr. Our interpretation of the geological, geophysical, and geodetic datasets indicates the Kabaw Fault is a crustal‐scale structure that actively absorbs a portion of the convergence previously ascribed to the Indo‐Burma megathrust. This reveals a previously unrecognized seismic hazard associated with the Kabaw Fault and slightly reduces the estimated hazard posed by megathrust earthquakes in the region.

    

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4. Rupture Directivity of the 2021 M W 6.0 Yangbi, Yunnan Earthquake

   https://doi.org/10.1029/2022JB024321  

   Gong, Wenzheng ; Ye, Lingling ; Qiu, Yuxin ; Lay, Thorne ; Kanamori, Hiroo ( September 2022 , Journal of Geophysical Research: Solid Earth)

   The 2021M_W6.0 Yangbi, Yunnan strike‐slip earthquake occurred on an unmapped crustal fault near the Weixi‐Qiaoho‐Weishan Fault along the southeast margin of the Tibetan Plateau. Using near‐source broadband seismic data from ChinArray, we investigate the spatial and temporal rupture evolution of the mainshock using apparent moment‐rate functions (AMRFs) determined by the empirical Green's function (EGF) method. Assuming a 1D line source on the fault plane, the rupture propagated unilaterally southeastward (∼144°) over a rupture length of ∼8.0 km with an estimated rupture speed of 2.1 km/s to 2.4 km/s. A 2D coseismic slip distribution for an assumed maximum rupture propagation speed of 2.2 km/s indicates that the rupture propagated to the southeast ∼8.0 km along strike and ∼5.0 km downdip with a peak slip of ∼2.1 m before stopping near the largest foreshock, where three bifurcating subfaults intersect. Using the AMRFs, the radiated energy of the mainshock is estimated as ∼. The relatively low moment scaled radiated energyof 1.5 × 10⁻⁵and intense foreshock and aftershock activity might indicate reactivation of an immature fault. The earthquake sequence is mainly distributed along a northwest‐southeast trend, and aftershocks and foreshocks are distributed near the periphery of the mainshock large‐slip area, suggesting that the stress in the mainshock slip zone is significantly reduced to below the level for more than a few overlapping aftershock to occur.

    

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5. Heterogenous late Miocene extension in the northern Walker Lane (California-Nevada, USA) demonstrates vertically decoupled crustal extension

   https://doi.org/10.1130/GES02409.1  

   Say, Michael C. ; Zuza, Andrew V. ( September 2021 , Geosphere)

   Abstract The spatial distribution and kinematics of intracontinental deformation provide insight into the dominant mode of continental tectonics: rigid-body motion versus continuum flow. The discrete San Andreas fault defines the western North America plate boundary, but transtensional deformation is distributed hundreds of kilometers eastward across the Walker Lane–Basin and Range provinces. In particular, distributed Basin and Range extension has been encroaching westward onto the relatively stable Sierra Nevada block since the Miocene, but the timing and style of distributed deformation overprinting the stable Sierra Nevada crust remains poorly resolved. Here we bracket the timing, magnitude, and kinematics of overprinting Walker Lane and Basin and Range deformation in the Pine Nut Mountains, Nevada (USA), which are the westernmost structural and topographic expression of the Basin and Range, with new geologic mapping and 40Ar/39Ar geochronology. Structural mapping suggests that north-striking normal faults developed during the initiation of Basin and Range extension and were later reactivated as northeast-striking oblique-slip faults following the onset of Walker Lane transtensional deformation. Conformable volcanic and sedimentary rocks, with new ages spanning ca. 14.2 Ma to 6.8 Ma, were tilted 30°–36° northwest by east-dipping normal faults. This relationship demonstrates that dip-slip deformation initiated after ca. 6.8 Ma. A retrodeformed cross section across the range suggests that the range experienced 14% extension. Subsequently, Walker Lane transtension initiated, and clockwise rotation of the Carson domain may have been accommodated by northeast-striking left-slip faults. Our work better defines strain patterns at the western extent of the Basin and Range province across an approximately 150-km-long east-west transect that reveals domains of low strain (∼15%) in the Carson Range–Pine Nut Mountains and Gillis Range surrounding high-magnitude extension (∼150%–180%) in the Singatse and Wassuk Ranges. There is no evidence for irregular crustal thickness variations across this same transect—either in the Mesozoic, prior to extension, or today—which suggests that strain must be accommodated differently at decoupled crustal levels to result in smooth, homogenous crustal thickness values despite the significantly heterogeneous extensional evolution. This example across an ∼150 km transect demonstrates that the use of upper-crust extension estimates to constrain pre-extension crustal thickness, assuming pure shear as commonly done for the Mesozoic Nevadaplano orogenic plateau, may not be reliable. 

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