Overcoming the limitations of empirical Green's functions: High-frequency source imaging of the 2013 Mw 6.6 Lushan, China, earthquake with an extended envelope inversion

Abstract

This study develops an improved envelope inversion method that overcomes a key limitation of traditional empirical Green’s function based envelope inversion, the requirement that every station must record both a large earthquake and suitable small earthquakes. Instead, we establish an envelope parameter attenuation model using regional historical small thrust-type earthquakes along the Longmen Shan fault zone, thereby accounting for source mechanism, propagation path, and, to some extent, local site conditions in a statistical sense. The attenuation model represents the average trend of envelope parameters and, despite its inherent smoothing of fine waveform details, enables the use of all available near-field stations without requiring collocated small earthquake records. The method is applied to the 2013 Lushan M_w6.6 earthquake using horizontal-component records from 24 near-field stations. A non-normalized objective function is adopted to give greater weight to large-amplitude near-field records, which are most sensitive to high-frequency radiation and provide stronger constraints on its distribution. The results indicate that: (1) The optimal high-frequency radiation model for the Lushan earthquake requires not only a primary fault plane but also a secondary fault on the hanging wall. High-frequency radiation from this secondary fault significantly improves the synthetic envelope fit at stations BXD, BXY, BXZ, and BXM, which are located immediately adjacent to the fault. (2) On the main fault, high-frequency radiation is primarily concentrated in three regions. One region is located around the initial rupture point. This may be attributable to both rupture initiation and the failure of high-stress drop area. The second region is located near the surface at the southwest end of the fault. The third region is located along the fault margins, where rupture ceased to excite high-frequency radiation. High-frequency radiation propagates along the secondary fault plane from the rupture initiation point to the near-surface region on the northeast side. (3) On the main fault, the strongest high-frequency radiation at the rupture initiation point occurs in the shallow zone characterized by large slip. Variations in vertical energy radiation may be caused by the distinctive blind-thrust source structure of the Lushan earthquake. The peak in high-frequency radiation occurs 6–12 s after rupture initiation, following the peak in low-frequency energy radiation.