Location of the mainshock and aftershock sequences of the <italic>M</italic>6.9 Mainling earthquake, Tibet

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We measured the location of the mainshock of the November 18, 2017, earthquake ( M 6.9) that occurred in Mainling County, Nyingchi City, Tibet, by using arrival-time data recorded by temporary seismic stations deployed on the Namche Barwa syntaxis and surrounding regions. The seismic stations offered good coverage of the hypocenter region and provided a good opportunity to precisely locate the mainshock and aftershock sequences. The Mainling earthquake was located at 29.87° N, 95.05° E, and the focal depth is 12 km. We investigated the focal mechanism of the mainshock by applying the W phase source inversion algorithm from the broad-band records of global seismic networks accessed from the IRIS data center. First, we resampled the original waveform to 1 SPS and then removed the mean, trend, and instrument response of the waveform, filtering it with a bandwidth of 150–70 s. After inversion, we obtained a solution with M w = 6.45 and a best double couple given by strike/dip/rake=319.9°/61.7°/101.0°. This result suggested that the seismogenic fault of the Mainling earthquake was characterized by a high-angle and NE-dipping thrust. We determined the location of the aftershock sequences by applying Geiger and double difference (DD) algorithms. Due to the lack of the local-scale tomographic model and reliable Poisson’s ratio data in the hypocentral region, we used only the P-wave arrival times to locate the aftershock sequences. We determined absolute locations for 148 aftershocks with high quality P-wave arrivals recorded by 14 temporary seismic stations and estimated the locations using the 1-D velocity model, in which the P-wave velocity was 6 km/s in the upper crust, 6.6 km/s in the lower crust, and 8.1 km/s in the uppermost mantle. The location errors were 0.6 km in the horizontal direction with an average residual of 0.07 s. In addition, we used the DD location algorithm in an attempt to minimize uncertainties in the aftershock locations. It was confirmed that the DD location algorithm greatly improved the relative location accuracy of events that are close to one another compared to the traditional Geiger location method. After DD location, the location errors of the aftershock sequences are 0.23, 0.26, and 1.43 km in the north-south, east-west, and vertical directions, respectively. The results indicated that the aftershocks were distributed along both sides (NW and SE directions) of the mainshock, which spread approximately 40 km in length and 10 km in width. Results of the focal depth profiles suggested that the aftershocks were located at depths above 15 and 10 km in the NW and SE directions of the mainshock, respectively. The aftershocks were mainly located at two sub-parallel faults. In the first two hours, the M 6.9 Mainling earthquake and aftershocks were mainly distributed at fault F1, and after that the aftershocks were distributed at fault F2. This suggested that the Xixingla fault zone is composed of a series of high-angle and NE-dipping faults, and the M 6.9 Mainling earthquake occurred at the south most one. The M 6.9 Mainling earthquake triggered seismic activities of a sub-parallel fault located to the northeast. The occurrence of the M 6.9 Mainling earthquake indicates that the Namche Barwa syntaxis is still in a high tectonic compression state at present. The Namche Barwa terrane continues moving towards the NE direction, leading the shortening deformation in the interior of the terrane. It is possible that a large earthquake may occur at Motuo and Mainling faults in the future along both sides of the Namche Barwa syntaxis.