Tiechanshan-Tunghsiao anticline earthquake analysis: Implications for northwestern Taiwan potential carbon dioxide storage site seismic hazard

We analyze the seismicity and earthquake focal mechanisms beneath the Tiechanshan-Tunghsiao (TCS-TH) anticline over the last two decades for seismic hazard evaluation of a potential carbon dioxide storage site in northwestern Taiwan. Seismicity in the TCS-TH anticline indicates both spatial and temporal clustering at a depth range of 7-12 km. Thirteen 3.0 ≤ M_L ≤ 5.2 earthquake focal mechanisms show a combination of thrust, strike-slip and normal faulting mechanisms under the TCS-TH anticline. A 1992 M_L 5.2 earthquake with a focal depth of ∼10 km, the largest event ever recorded beneath the TCS-TH anticline during the last two decades, has a normal fault mechanism with the T-axis trending NNE-SSW and nodal planes oriented NNW-SSE, dipping either gently to the NNE or steeply to the SSW. Thrust fault mechanisms that occurred with mostly E-W or NWW-SEE striking P-axes and strike-slip faulting events indicate NWW-SEE striking P-axes and NNE-SSW trending T-axes, which are consistent with the regional plate convergence direction. For the strike-slip faulting events, if we take the N-S or NNW-SSE striking nodal planes as the fault planes, the strike-slip faults are sinistral motions and correspond to the Tapingting fault, which is a strike-slip fault reactivated from the inherited normal fault and intersects the Tiechanshan and Tunghsiao anticlines.