3‐D Magnetotelluric Imaging of the Easternmost Kunlun Fault: Insights Into Strain Partitioning and the Seismotectonics of the Jiuzhaigou Ms7.0 Earthquake

Abstract

We present a three‐dimensional electrical resistivity model of the crust and upper mantle beneath the easternmost Kunlun fault (EKLf), obtained by three‐dimensional inversion of magnetotelluric (MT) data. The crust of the Songpan‐Ganzi block is characterized by high resistivity from the surface to a depth of around 20 km, and by low resistivity in the mid‐lower crust in the depth range 20–40 km. The eastern edge of the high conductivity layer is coincident with the EKLf and the Huya fault. The electrical resistivity structure provides new insights into both (1) the generation of recent M > 6 earthquakes and (2) strain partitioning on this segment of the EKLf. Our model reveals that the Huya fault is the main branch of the EKLf in the region. Together with the EKLf, the Huya fault defines the boundary between the Songpan‐Ganzi and Bikou blocks. The mid‐lower crust of the Songpan‐Ganzi block in this region has a low resistivity that likely represents a mechanically weak layer. The 2017 Jiuzhaigou Ms7.0 earthquake and other recent M > 6 earthquakes may have been controlled by the change in viscosity in the mid‐lower crust that occurs across this boundary. The high conductivity may be acting as either (1) a channel of lower crustal flow, or (2) as a weak layer that decouples the upper and lower crust. The fact that the high conductivity layer does not extend along the north side of the Sichuan Basin questions the idea that crustal flow occurs in this area.

Crustal Density Structure of the Jiuzhaigou Ms7.0 Earthquake Area Revealed by the Barkam–Jiuzhaigou–Wuqi Gravity Profile

“…In general, compared with Crust1.0 [ 30 , 31 ], obtained from active source seismic studies as well as from receiver function studies ( Figure 4 ), spatial variation of the crustal density structures and Moho boundary obtained from our gravity study is in good agreement with previous studies [ 8 , 9 , 10 , 11 , 12 , 23 , 24 , 25 , 26 ]. The similarity of interface’s depth between our model and Crust1.0 indicates the robustness of our result.…”

“…There are several published crustal models in our study region. These models are constructed either from wide-angle deep reflection, P-wave receiver functions earthquake, surface wave tomography or ambient noise tomography [ 8 , 9 , 10 , 11 , 12 , 23 , 24 , 25 ]. The study of the P-wave receiver functions of tele-seismic data shows that multiple low-speed, low-density belts exist in the upper and middle crust below the plateau from the Bayan Har block to the central-southern Longmenshan region at depths of approximately 10~15 km and 24–45 km [ 23 , 24 , 25 ], which is consistent with the results of Aba-Wuqi deep seismic reflection profile (from project report of the geophysical exploration center, China earthquake administration).…”

“…There is little difference in the Moho interface compared with the average thickness of the Bayan Har block, approximately 60–62 km. The initial model is constructed based on the seismic velocity results in this area [ 8 , 9 , 10 , 11 , 12 , 23 , 24 , 25 , 26 ]. Based on the above geophysical observations and velocity/density conversion [ 27 ], an initial model of the crustal structure is obtained and shown in Table 2 .…”

“…There are many geophysical studies in this area [ 8 , 9 , 10 , 11 , 12 ], for instance, surface waves, deep reflection seismic profile, magnetotelluric (MT). However, geodetic field observations are very sparse because of the harsh natural environment, high altitude, and poor traffic.…”

Lithospheric Flexural Isostasy Background of the 2017 Ms7.0 Jiuzhaigou Earthquake and Its Implications on Material Extrusion in the Northeastern Bayan Har Block

“…values are recovered in that region, and there are extremely low T e (<10km) in the seismogenic zone of the Jiuzhaigou earthquake, which means mechanically weak lithosphere consistent with seismic and magnetotelluric tomography results (Du et al, 2019;Riaz et al, 2019;Sun et al, 2020). The T e structure also suggested that there is a banded low T e (<40km) region stretched from the northeastern Bayan Har block to the southwestern Ordos block.…”