Focal mechanism of a Late Triassic large magnitude earthquake along the Longmen Shan fault belt, eastern Tibetan Plateau

Stress states near active fault zones are key to understand their seismogenic setting, geodynamic evolution and future seismic activity. The Longmen Shan Fault Belt (LSFB) is a prominent fault zone that has had a long history of activity and major recent earthquakes in the eastern Tibetan Plateau. However, we have no records of major events prior to the 2008 Wenchuan earthquake, and this hinders progress in understanding both the past and future seismicity of the LSFB. A Late Triassic pseudotachylyte, recently discovered and dated, provides an opportunity to compare modern stress states with those acting ca. 230 Ma ago. Here we use a new paleoseismological approach, based on the magnetic fabric of the fault rocks, to determine the focal mechanism of Late Triassic seismic events, each recorded in pseudotachylyte veins at the Bajiaomiao outcrop. The anisotropy of magnetic susceptibility (AMS) of pseudotachylytes and cataclasites arises from the shape preferred orientation of coseismic neoformed magnetite and monoclinic pyrrhotite, whereas that of protocataclasite arises mainly from the crystallographic preferred orientation of inherited clastic magnetite. The AMS of fault rocks shows well-defined magnetic foliation and lineation. The pseudotachylyte generation vein formed along the plane where seismic slip took place, while the AMS obliquity with respect to this plane indicates seismic slip with-248 degrees trend and 38 degrees dip. This result is consistent with the attitude of macroscopically visible subhorizontal striations on the pseudotachylyte plane itself. The consistent asymmetry of multiple injection veins with respect to the pseudotachylyte generation vein, along with the asymmetry of the AMS fabric, shows that seismic deformation resulted from left lateral, strike-slip motion. This study provides the oldest example, to date, of focal mechanism determination using the AMS method. The kinematics of the Late Triassic event departs from that of the 2008 Wenchuan earthquake thrust motion. Yet, with a displacement estimated between-4.4 and 17.4 m, the magnitude of the Late Triassic event was likely Mw 7.5-7.9.