High-resolution teleseismic tomographic crustal imaging for potential seismogenic segment of the central Tan-Lu Fault Zone, East China

Abstract

The Tan-Lu Fault Zone (TLFZ) is the largest strike-slip system in East Asia. The central TLFZ has four main branch faults across a width of 20– 40 km, and both the great 1668 Tancheng Earthquake (M8.5) and the Anqiu Earthquake (M7.0) occurred on its two eastern branch faults. Due to the lack of recent earthquakes and stations around a seismic-quiescent segment of the TLFZ centered near the 70 BCE Anqiu Earthquake, previous tomographic images have resolutions coarser than 20 km in the middle and lower crust, unable to show how the crust is faulted along the TLFZ. To improve the resolution, we deployed a 70-km-long W-E-trending seismic array with 38 portable seismographs across the TLFZ at 36.1°N. Using one month of teleseismic records and a newly developed multiscale teleseismic tomography method, our P-wave velocity profile has achieved a resolution of 5 km × 5 km in shallow crust, and 10 km × 10 km in deep crust, as shown by the resolution tests. The profile's velocity variation correlates well with surface geology and crustal electrical resistivity profiles. The new profile provides clues for the depth distribution of major TLFZ branches, and indicates the two eastern TLFZ branch faults cut through the crust along a column-shaped low-velocity anomaly. This low-velocity column is associated with low-resistivity anomalies in the upper and lower crust, indicating a likely presence of fluids in the faulted rocks. This crustal faulting interpretation is consistent with the Moho geometry of previous receiver function studies, with an alignment between a receiver function Moho dome and a large low-velocity anomaly. Though seismicity is sparse near the profile, our interpreted crust-cutting location is in line with a linear cluster of recent TLFZ earthquakes about 20 km south of the profile, indicating that this quiescent segment could be a 150 km seismic gap along the central TLFZ.