Preliminary seismic hazard assessment for the proposed Bohai Strait subsea tunnel based on scenario earthquake studies

In this paper we present a preliminary seismic hazard assessment for the proposed Bohai Strait subsea tunnel (BSST) based on scenario earthquakes in a strike-slip stress regime. We studied two scenarios: 1) a right lateral, strike-slip faulting earthquake event with surface wave magnitude (Ms) 7.4 occurred on the north to northeast (NNE) oriented Tan-Lu fault zone (TLFZ) with a location about 100 km away from BSST (the far field); and 2) a left-lateral strike-slip event with magnitude Ms4.1 occurred in the direct vicinity to BSST (the near field) on the northwest to west (NWW) Zhangjiakou-Penglai fault zone (Zhang-Peng Fault Zone, or ZPFZ). Selections of these two scenario events are based on the characterization of the actual seismotectonic settings and representation of earthquake history in the Bohai region. We studied the impacts of these two scenario events on BSST in terms of longitudinal compression, bending, and tunnel cross-section's ovaling deformation. As a preliminary study, we excluded the analysis of formation-structure interaction and focused on the ground motion parameters expressed as the maximum displacement, the peak ground velocity (PGV) and peak ground acceleration (PGA) of the free seismic wave field surrounding the tunnel walls. This simplification is acceptable for the fact that BSST is planned to be constructed in the granite bedrock formation in which the inertial contrast between the formation and the structure is minimal. Seismic hazard assessment for tunnels is completely different from the assessment for above-ground buildings. It is also substantially different from assessment for tunnels built in soil formations, for which the formation (soil) structure interaction evaluation is critical. We use semi-analytical approach for assessing the seismic impact of the far field, large event to BSST; whereas use the two-dimensional (2D) finite difference time domain (FDTD) numerical simulation technique for assessing the response of BSST to the seismic load of the near field, smaller event. The preliminary results suggest that 1) the impact of the low frequency, far field event has very subtle impact to BSST in terms of longitudinal compression, bending, and cross-section ovaling; and 2) the impact of the high-frequency, near field event has little impact to BSST in terms of longitudinal compression and bending; while has a measurable impact to cross-section ovaling. However, this kind of deformation is within the limit of tolerance under earthquake-resistant design. Over all, the seismic risk posed by regional large earthquakes and local small events to BSST is within allowable risk threshold and manageable with the enforcement of earthquake resistant code. (C) 2019 Published by Elsevier B.V.