Rapid deformation rates due to development of diapiric anticline in southwestern Taiwan from geodetic observations

We adopted 106 campaign-mode GPS observations and 310 precise leveling measurements between 2002 and 2010 to understand the present-day crustal deformation in mudstone area and to estimate the earthquake potential of the Hsiaokangshan (HKSF) and the Chishan faults (CHNF) in southwest Taiwan. Horizontal velocities east of the CHNF are ~ 66 mm/yr, 270° and gradually decrease westward to ~ 15 mm/yr, N259°. A horizontal velocity gradient of ~ 15 mm/yr is shown between the HKSF and CHNF. Subsidence rates west of the HKSF and east of the CHNF are ~ 5-10 mm/yr, while the uplift is observed between these two faults in the highest elevation with the maximum rate of ~ 18 mm/yr. The observed deformation patterns are difficult to be fully modeled by 2D kinematic fault model. Field relationships within the vertical shear zones of the mudstone therefore indicate that the deformation pattern may be also controlled by a relic onshore mud diapir that is still experiencing vertical uplift. Consistency between the geological and geodetic vertical velocities, weak rock strength, and no destructive earthquakes over the last 100 years imply that faults (HKSF) within the mudstone area are creeping. However, the CHNF or the associated décollement may still have earthquake potential.