Paleoseismic investigation of the seismic gap between the seismogenic structures of the 2008 Wenchuan and 2013 Lushan earthquakes along the Longmen Shan fault zone at the eastern margin of the Tibetan Plateau

Abstract

The 2008 Mw 7.9 Wenchuan earthquake occurred along the middle and northern segments of the Longmen Shan fault zone at the eastern margin of the Tibetan Plateau. Five years later, the 2013 Mw 6.6 Lushan earthquake ruptured a section of the southern segment of the Longmen Shan fault zone, leaving a 50-km-long seismic gap between the seismogenic structures of the two earthquakes. In our study, we use trenching and calibrated radiocarbon age models to assess the rupture behavior of the gap over multiple earthquakes. At least two paleoseismic events were identified with age constraints between A.D. 1350–1830 and 525–760 B.C., respectively. Trench stratigraphy suggests the presence of another possible event with an age constraint of A.D. 590–1210. Using cumulative vertical displacement of ∼1.5 m for the lowest unit exposed in the trench (U1) and its age of ca. 2500 yr B.P., we estimate the vertical slip rate of the Dachuan-Shuangshi fault, the primary fault along the southern segment, to be ∼0.6 mm/yr. The lack of correlation of events between multiple paleoseismic sites along the Dachuan-Shuangshi fault suggests that the seismic gap has a low possibility of rupturing completely during paleoearthquakes. A comparison of the rupture behavior of the southern segment with the middle segment of the Longmen Shan fault zone indicates that the likelihood of cascading ruptures between the two segments is low.