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

Abstract

On 16 September 2015, an Mw 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.

Subduction zone megathrust earthquakes

“…The deeper parts of the megathrust produce significantly more high-frequency energy than the shallower megathrust for most of the largest earthquakes in recent decades, such as the 2010 Maule Chile M w 8.8 (e.g., Kiser and Ishii, 2011;Koper et al, 2012), 2011 Tohoku M w 9.1 (e.g., Koper et al, 2011;Ide et al, 2011;Ishii, 2011), and the 2015 M w 8.3 Illapel, Chile, event (e.g., Melgar et al, 2016;Yin et al, 2016). Ye et al (2016c) documented modest depth dependence in highfrequency radiation for the 100+ large events, with lower spectral decay rates for deeper events on subduction zone megathrusts.…”

Modeling tsunami observations to evaluate a proposed late tsunami earthquake stage for the 16 September 2015 Illapel, Chile, Mw 8.3 earthquake

“…The 16 September 2015 Illapel, Chile, M w 8.3 megathrust earthquake has been extensively studied using seismic, geodetic, and tsunami data sets, and the rupture has been consistently characterized to involve a largeslip patch offshore extending from about 30°S to 31.6°S, with a secondary slip patch downdip below the coast [e.g., Heidarzadeh et al, 2016;Li et al, 2016;Ye et al, 2016;Yin et al, 2016]. The rupture has at least a 95 s duration and a seismic moment of about 3.…”

Foreshocks, b Value Map, and Aftershock Triggering for the 2011 Mw 5.7 Virginia Earthquake

“…Moreover, the seismicity at the southwestern end is located in areas with little slip during the mainshock (Hartzell et al, ) and marked the shallow limit of mainshock's slip (Q. Wu et al, ), which is opposite to the suggestion that large slip areas tend to have large b values (e.g., Tormann et al, ). Indeed, it has been pointed out that a large slip area may not be necessarily associated with high stress (Yin et al, ). Finally, the b values are also strongly affected by material and geometrical heterogeneity (Mogi, ).…”