Characteristics of the 2017 Pan’an M L 4.0 Earthquake Sequence

Abstract

Abstract An earthquake sequence occurred between February and September 2017 in Pan’an, Zhejiang, China. In this study, the Brune source model and spectral analysis were used to obtain the source parameters (stress drop and apparent stress) from the Zhejiang seismic monitoring network data. Five low-frequency spectral-amplitudes from four nearfield stations were used to calculate the spectral-amplitude correlations. The events were then clustered, and a time series of their moving average correlation was plotted. Focal source mechanisms of the ML 4.0 mainshock were obtained using Cut-and-Paste inversions to analyze focal mechanism similarity for the small and medium earthquakes. The results showed that the Pan’an earthquake sequence had low spectral-amplitude correlations. The moving average of the correlations varied from 0.86 to 0.95, indicating a low similarity in focal mechanism across the sequence as they became increasingly divergent over time. The overall level of tectonic stress in the seismic source region was low, as the stress drops ranged from 0.00 MPa to 0.80 MPa. As stress accumulation did not show a dominant direction, the Pan’an earthquake sequence could be attributed to low-friction faulting. The spectral correlation analysis thus shows that the Pan’an earthquake sequence was an ordinary swarm of small earthquakes. Our approach should be utilized in further analyses, as the resulting data could facilitate rapid post-earthquake sequence assessments.