Study of fault zone and basin structure of 2019 Mw5.5 Ye-U earthquake sequence beneath Central Myanmar Basin

Abstract

Accurate and precise location of earthquake sequence is critical to better understand seismotectonics, such as better delineation fault geometry and understanding of the rupture of the earthquakes. However, nearfield seismic observations are usually rare for such study. Here we study a unique dense nodal array data acquired by the deployment after the 31/08/2019 Mw5.5 crustal earthquake that is located ~50km to the west of Sagaing fault near Mandalay beneath ShweBo Central Myanmar Basin (CMB). The network, composed of 20 nodal stations with station spacing of ~5km, was deployed ~ 2 weeks after the mainshock and continuously recording for ~ 40 days. High quality waveforms containing clear P and S phase arrivals, and an interesting P-to-S phase converted at the basement of CMB were recorded for aftershocks. We applied a machine learning based automatic phase detection software (Earthquake-Transformer) to the dataset and detected 1143 events that were recorded by at least 3 stations. Double difference relocation of these aftershocks reveals a near E-W trending fault with a dimension of ~10km along strike and located between 7 to 12 km in depth. The strike of aftershock lineation is highly consistent with the focal mechanism derived from regional waveform inversion, indicating a left lateral strike-slip fault beneath CMB. Mainshock epicenter refined by a path calibration technique is located to the western edge of the seismicity, suggesting an eastward rupture directivity of the mainshock. Taking advantage of the P-basin-S converted phase at the basement of CMB, we constrained the thickness of the basin to be 5 ± 0.7 km. Strong strength of the P-basin-S phase requires sharp velocity change between the basin and bedrock. It is possible that the earthquake sequence is a result of small block rotation that has been taking place beneath the CMB due to the convergence of India plate. Another possible explanation is a conjugate fault system associated with 2012 Mw 6.8 Thabeikkyin earthquake sequence which ruptured close to Sagaing fault.