Attenuation and Basin Amplification Revealed by the Dense Ground Motions of the 12 July 2020 Ms 5.1 Tangshan, China, Earthquake

Abstract On 12 July 2020, an Ms 5.1 moderate earthquake occurred on the north segment of the Tangshan fault in North China, which was the seismogenic fault of the 1976 Ms 7.8 Tangshan earthquake and numerous small-to-moderate earthquakes in recent decades in the Tangshan seismic zone. The Ms 5.1 event was well-recorded by dense ground-motion observation stations, including the national strong-motion stations and seismic intensity stations. This many ground-motion recordings, obtained for such a moderate event in North China for the first time, provided a rare opportunity to investigate the attenuation and site effects on ground motion. The distance decay in the Tangshan seismic zone was first explored using the spectral amplitudes from the vertical component. The strong anelastic attenuation and weak geometrical spreading effects were clearly found. The hinged trilinear form may be more effective at describing the geometrical spreading. No geometrical spreading decay was visible at medium distances (60–100 km). Anomalous areas with extraordinary high amplitudes occurred in the spatial distribution of peak ground accelerations and peak ground velocities that we attribute to significant basin amplification effects, which was confirmed by the wideband and high amplifications on the standard spectral ratio and the later-arriving, long-period surface waves observed in waveforms in the Ninghe–Baodi area and south of Beijing. The basin-induced surface waves in the 2–5 s period were most prominent in the Ninghe–Baodi area. We further inferred that basin effects may be responsible for the high-intensity anomaly areas observed in the 1976 Ms 7.8 Tangshan earthquake.