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We explore a novel acquisition geometry that can be used to estimate the linear component of site amplification using a dense nodal seismic network installed in Yangon, Myanmar’s largest city. The city is surrounded by several seismically active faults, including the Sagaing Fault, which is capable of generating M_w > 7.0 earthquakes. As part of the Irrawaddy delta system, this densely populated city sits on young water-saturated alluvium that is likely to amplify earthquake ground motions. Assessing site response is crucial for understanding the seismic hazard potential to minimize the loss of property and lives. Using a dense seismic array comprised of 110 three-component nodes, we estimated the frequency-dependent site amplification pattern of Yangon from regional (Lg) and local (Sg) seismic phases. Since this acquisition geometry is not sensitive to Q or geometric spreading, this approach provides a fast and cost-effective way to estimate the linear component of site response as a function of frequency. Our Lg and Sg site response results identify regions with high site amplification that have significantly greater seismic hazard risks for regional and local distance earthquakes. We observed consistent site response characteristics between both Lg and Sg phases. Site amplification patterns correlate well with the surficial geology and subsurface structure beneath the city. De-amplification is observed across all frequencies at stations located above an anticlinal structure composed of older Pliocene rocks (i.e. the Irrawaddy Formation). Conversely, highly amplified areas correspond to younger Pleistocene to recent alluvial plains consisting of loose, unconsolidated alluvium. We found a dominant horizontal-to-vertical spectral ratio (HVSR) peak at ∼1.0 Hz from ambient noise, likely corresponding to the thickness of unconsolidated sediments. We suggest that the growing number of nodal networks worldwide can be used to estimate frequency-dependent site amplification, addressing key data gaps in seismic hazard assessment. 

Myanmar is surrounded by complex seismotectonic elements and threatened by a high seismic risk. The Central yanmar Basin (CMB) hosts the largest and fastest growing cities of Myanmar. The CMB is bounded by the Indo- Myanmar subduction zone to the west and the Sagaing fault to the east and is a seismically active tectonic block that has experienced large earthquakes (up to magnitude 8.0). A large earthquake in this region would affect Yangon and its surrounding population of around 8 million. Sedimentary basins have a significant contribution to seismic wave propagation, amplification and duration of ground shaking. Thus, to more accurately estimate the seismic hazard, a clear understanding of the detailed basin structures is required. The goal of our study is to map crustal structures, i.e. crustal thickness, crustal blocks, basin shape, size and depth, fault geometry, dipping layers and intra-crustal layers beneath the Yangon region. We will present receiver functions from a dense array of 168 nodal seismometers with the goal of revealing high-resolution seismic images of the basin. Our dense array will improve basin imaging by reducing uncertainties in receiver function interpretations. Developing a better understanding of basin structures will help our understanding of seismic amplification in the basin and thus will help to more accurately estimate the seismic hazard of this region. 

Abstract This study represents the first campaign‐style teleseismic shear wave splitting (SWS) investigation of central Myanmar, an area that is tectonically controlled by the oblique subduction of the Indian Plate underneath the Eurasian Plate. The resulting 678 well‐defined and 247 null SWS measurements obtained from recently deployed 71 broadband seismic stations show that the Indo‐Burma Ranges (IBR) possess mostly N‐S fast orientations that are parallel to the trend of the depth contours of the subducted slab. Relative to the global average of 1.0 s, extremely large splitting times with station‐averaged values ranging from 1.28 to 2.79 s and an area‐averaged value of 2.09 ± 0.55 s are observed in the IBR. In contrast, the Central Basin (CB) and the Shan Plateau (SP) are characterized by slightly larger than normal splitting times. The fast orientations observed in the CB are mostly NE‐SW in the northern part of the study area, N‐S in the central part, and NW‐SE in the southern part. The fast orientations change from nearly N‐S along the N‐S oriented Sagaing Fault, to NW‐SE in the central and eastern portions of the SP. These observations, together with SWS measurements using local S events, crustal anisotropy measurements using P‐to‐S receiver functions, and the estimated depth of the source of anisotropy using the spatial coherency of the splitting parameters, suggest the presence of a trench‐parallel sub‐slab flow system driven by slab rollback, a trench‐perpendicular corner flow, and a trench‐parallel flow possibly entering the mantle wedge through a slab window or gap.