Stratigraphic Constraints of Rupture Kinematics and Scenarios of
Paleoearthquakes on the Southern Yangsan Fault, SE Korea
Abstract
After the 2016 Gyeongju Earthquake (MW 5.5) in SE Korea,
the NNE‒SSW-striking Yangsan Fault (YF) seismicity has emerged as a
significant scientific issue and hazard. Despite previous studies of the
seismicity of the YF, the paleoseismic information of the fault remain
uncertain. We carried out paleoseismic investigations to characterize
surface rupturing and the paleoseismic history of the northern section
of the southern YF. Deflected streams and displaced river terraces
indicate a dextral sense of slip, and fault splays cutting
unconsolidated sedimentary strata in trench walls show an east-side-up
geometry. The dextral strike-slip faulting with a minor reverse
component is characterized by several noticeable stratigraphic features,
including fissure-filling deposits, sag ponds, and cut-and-fill streams
along the fault splay. Based on fault‒sediment cross-cutting
relationships and optically stimulated luminescence ages of
unconsolidated sediments, we propose two possible rupture scenarios for
the most recent earthquake event along the studied fault section during
the Late Pleistocene: (1) a single rupture along the entire section
(WS‒MH‒IBN‒IB) at ca. 30 ka and (2) individual partial ruptures along
two segments of the section (WS‒MH and IBN‒IB) during 33‒30 and 29‒17
ka, respectively. It is also noted that the timings of earlier ruptures
(penultimate earthquake events) of the two segments are 37‒35 and 70‒52
ka, respectively. Furthermore, the timing(s) of the most recent
earthquake for our studied section is much older than that of the
southern section of the northern YF.