Focal depth of the Yunnan Jinggu <italic>M</italic><sub>w</sub>6.1 earthquake: Discussion on depth of the brittle-ductile transition zone of a young fault

Abstract

Since most continental earthquakes occur in the brittle layer instead of ductile layers, the focal depth of an earthquake provides an important constraint on the rheology structure of fault zone. According to the results of previous studies, the typical depth of brittle-ductile transition zone on mature faults is about 10 km. However, several moderate earthquakes occurred on young faults in stable cratons, and focal depths of those events are very shallow (e.g. the 1993 M w6.1 India Killari earthquake and a series of shallow earthquakes in Australia). Those observations suggest that depth of the brittle- ductile zone on young faults in cratons is shallower than that on mature faults. In this study, we report a case on a young fault in a tectonically active region that has not been well studied and discuss implications for the rheology structure. On October 7, 2014, an M w6.1 earthquake occurred in Jinggu, Yunnan (China), the southeastern part of the tectonically active Yunnan-Myanmar block, followed by two aftershocks of M >5 in December. According to historical seismicity and geologic studies in this region, it is suggested that the seimogenic fault of the Jinggu earthquake is a young fault. We use data from global and regional seismic networks, as well as aftershock data from the Lozhadu reservoir network and temporary stations, to study focal depths of the mainshock and aftershocks. The hypocenter of a reference event ( M w4.3) that was recorded with two close temporary stations is determined, and then the hypocenters of the mainshock and two moderate aftershocks are relocated with a relative method based on Pn/Pg arrival times. The relocated hypocenter of the mainshock is 9.5 km deep, whereas the hypocenters of the two aftershocks are about 10 km deep. The centroid depth is investigated with the CAP method based on waveform modeling. Both teleseismic and regional waveforms are used to invert for the centroid depth of the mainshock. The optimal waveform fit is obtained with a depth of 5 km. The two aftershocks are studied with regional waveforms only and the results are close to the hypocenter depths. Since the hypocenter indicates the initial point of rupture whereas the centroid location is close to the center of the main rupture patch, we propose the mainshock initiated at 10 km depth and expanded to the brittle layer at shallower depth. For two aftershocks, the consistency of hypocenter depth and centroid depth suggests circular rupture patterns. Therefore, we propose the bottom of the seismogenic layer and brittle-ductile transition zone is about 10 km deep. This hypothesis is also supported by other observations. Magnetotelluric study in this region reveals a low conductivity layer above 10 km that is regarded as brittle rock, whereas the high conductivity layer beneath 15 km is regarded as ductile rock. Furthermore, a regional rheology model, which is obtained with surface temperature, seismic velocity, and GPS data, also suggests the brittle-ductile transition zone is at about 9 km depth. In summary, the depth of the brittle-ductile transition zone on the young fault in the tectonic active region is similar to that of mature faults, which is useful information for lithosphere geodynamics studies.