16:05 〜 16:20
[SEM15-03] Magnetotelluric Imaging of a Seismic Gap on the LiTang Strike-Slip Faults on the Southeastern Margin of the Tibetan Plateau
キーワード：seismic gap, magnetotelluric, LiTang faults, southeastern Tibetan Plateau, Earthquake
The southeastern margin of the Tibetan Plateau is one of the most earthquake-prone regions within mainland China. In this region, the most devastating earthquakes have occurred on the rapid strike-slip Xianshuihe faults and the massive thrust Longmenshan faults in recent decades. However, the LiTang faults, which parallel the Xianshuihe faults, have also hosted at least three large earthquakes with magnitudes over 7.0. Notably, the migration of these three earthquakes began in the north and moved toward the central segment sequentially, generating significant surface fractures with an approximate interval of 50 years (Xu et al., 2005). The latest event in 1948 produced ~41 km of surface fractures in the central part of these faults. No fractures have been observed in the southern segment of the LiTang faults, characterizing it as a major seismic gap.
Previous studies suggest that a seismic gap on a fault is typically dominated by strong mechanical coupling that accommodates higher elastic strains and stress, or it may be a segment characterized by mechanically soft rocks in the crust (Li et al., 2018). To investigate the nature of the LiTang seismic gap, we conducted a 3D magnetotelluric (MT) survey, incorporating newly collected MT data from 60 sites along with previous MT data from 55 sites. A 3D resistivity model was established using ModEM (Kelbert et al., 2014). This model represents the first 3D deep geophysical model for the southern LiTang faults to date. The model reveals significant heterogeneous variations in the resistivity structures along the fault zone, with the middle of the seismic gap characterized by extensive high conductivity extending to the middle crust. This high conductivity is located at the intersection of multiple faults from the eastern region. We preliminarily infer that this significant conductivity is controlled by fluid migration or advection within these faults, coinciding with hot spots on the surface. This unique structure suggests that the slip rate (~4 mm/yr) on the LiTang faults is likely influenced by the Xianshuihe faults (~10-17 mm/yr) to the east. The LiTang faults likely serve to accommodate the differential motions of the rapid strike-slip Xianshuihe faults to the east and the slow strike-slip Honghe faults(~2 mm/yr) to the west.
Previous studies suggest that a seismic gap on a fault is typically dominated by strong mechanical coupling that accommodates higher elastic strains and stress, or it may be a segment characterized by mechanically soft rocks in the crust (Li et al., 2018). To investigate the nature of the LiTang seismic gap, we conducted a 3D magnetotelluric (MT) survey, incorporating newly collected MT data from 60 sites along with previous MT data from 55 sites. A 3D resistivity model was established using ModEM (Kelbert et al., 2014). This model represents the first 3D deep geophysical model for the southern LiTang faults to date. The model reveals significant heterogeneous variations in the resistivity structures along the fault zone, with the middle of the seismic gap characterized by extensive high conductivity extending to the middle crust. This high conductivity is located at the intersection of multiple faults from the eastern region. We preliminarily infer that this significant conductivity is controlled by fluid migration or advection within these faults, coinciding with hot spots on the surface. This unique structure suggests that the slip rate (~4 mm/yr) on the LiTang faults is likely influenced by the Xianshuihe faults (~10-17 mm/yr) to the east. The LiTang faults likely serve to accommodate the differential motions of the rapid strike-slip Xianshuihe faults to the east and the slow strike-slip Honghe faults(~2 mm/yr) to the west.