Is the Lishan fault of Taiwan active?

•Microearthquakes were recorded by a dense seismic array near the Lishan fault of Taiwan.•The Lishan fault is a high-angle left-lateral structure based on seismicity and focal mechanisms.•The Lishan fault is an active structure from the surface to about 35 km.

The Lishan fault has been characterized alternately as a major discontinuity in stratigraphy, structures and metamorphism, a ductile shear zone, a tectonic suture or non-existent. In addition to being a geological boundary, it also marks transitions in subsurface structures. Thus, the seismicity to the west of the fault permeates through the upper and mid-crust while beneath the Central Range it is noticeably less and largely concentrated in the upper 12 km. A prominent west-dipping conductive zone extends upward to meet the Lishan fault. Also, the eastward increase of crust thickness from ~ 30 km in the Taiwan Strait quickens under the Lishan fault to form a root of over 50 km under the Central Range. In the past, the small magnitude seismicity along the Lishan fault has been noticed but is too diffuse for definitive association with the fault. Recent processing of aftershock records of the 1999 Mw 7.6 Chi-Chi earthquake using Central Weather Bureau data and, especially, data from three post-Chi-Chi deployments of seismic stations across central Taiwan yielded hypocenters that appear to link directly to the Lishan structure. The presence of a near 4-km-long vertical seismic zone directly under the surface trace of the Lishan fault indicates that it is an active structure from the surface down to about 35 km, and the variety of focal mechanisms indicates that the fault motion can be complex and depth-dependent.