Sediment Structures Constrained by Converted Waves From Local Earthquakes Recorded by a Dense Seismic Array in the Tangshan Earthquake Region

To investigate the sediment structures within the Tangshan earthquake region in North China, we developed an effective and inexpensive method for reliably detecting the thickness of Quaternary sediments by using converted waves from local, small earthquakes recorded by a dense seismic array. From January to March 2017, we deployed 145 three-component seismographs with inter-station distances ranging between 1 and 4 km throughout the Tangshan earthquake region. Tens of local events with magnitudes between M L 0.1 and M L 2.4 were adequately recorded by the Tangshan dense seismic array and were accurately relocated. A synthetic seismogram analysis clearly showed that the travel-time differences between direct S and Sp converted waves or between direct P and Ps converted waves were almost linearly correlated with the sediment thicknesses, providing tight constraints on the basement depth. With this method, we generated a sediment thickness map of the Tangshan earthquake region. Overall, the measurements from the local converted wave method are consistent with those of previous microtremor horizontal-to-vertical spectral ratio studies. The sediment thicknesses indicate that the subsurface sedimentary structure throughout the Tangshan earthquake region has experienced significant transformations over time, partially controlled by the NE–SW-trending Tangshan Fault. The obtained sediment thickness map provides beneficial information for ensuring the safe construction of infrastructure in Tangshan city and can serve as an important model for simulating earthquake strong ground motions in the Tangshan earthquake region. Our study suggests that the proposed simple and efficient method of using converted waves could be applied to other populous plains or basins with high seismic activity and could be used to determine the general characteristics of the sediment structures in a given study area.