Coseismic radiation and stress drop during the 2015 Mw 8.3 Illapel, Chile megathrust earthquake

On 16 September 2015, an M-w 8.3 earthquake struck middle Chile due to the subduction of the Nazca plate beneath the South America plate. This earthquake is the consequence of 72 years of strain accumulation in the region since the 1943 M 8.3 event. In this study, we apply the compressive sensing method (CS) to invert for the spatiotemporal distribution of the coseismic radiation at different frequencies of this event. The results show clear frequency-dependent feature of earthquake rupture with low-frequency (LF) radiation located in the updip region while high-frequency (HF) radiation concentrated in the downdip region of the megathrust. We also compare the CS results with three coseismic slip models as well as the stress drop distributions inferred from these slip models. The comparison confirms our understanding of coseismic radiation that energy sources are mostly located in the margin of large coseismic slip regions. Furthermore, we find that the LF radiation sources are mainly within the stress-decreasing (releasing) regions while the HF radiation sources are mainly located in the stress-increasing (loading) regions due to rupturing of relatively large asperities nearby (stress decreasing and releasing). These results help to better understand the physics of the rupture process during megathrust earthquakes. Moreover, our results do not show radiation sources south of the epicenter, suggesting that the subducting Juan Fernandez Ridge probably stopped the rupture of this earthquake toward the south.