High-Frequency Rupture Processes of the 2014 Mw 8.2 Iquique and 2015 Mw 8.3 Illapel, Chile, Earthquakes Determined from Strong-Motion Recordings

ABSTRACT Strong-motion recordings of the 2014 Mw 8.2 Iquique and 2015 Mw 8.3 Illapel, Chile, earthquakes were analyzed to determine rupture propagation and the location, timing, and strength of subevents that produce most of the high-frequency (≥1 Hz) ground motions. A moving window,cross-correlation analysis of recordings from a local dense array, band-pass filtered at 1 Hz, directly shows that the Iquique earthquake ruptured to the southeast over a distance of about 60 km. Array analysis of lower frequency energy (0.03–0.1 Hz) indicates that it occurred updip of the high-frequency rupture. A methodology was developed for inverting the envelopes of acceleration records (1–5 Hz) to map high-frequency source factors on the rupture zone and was applied to the two earthquakes. Waveforms of Mw 6 earthquakes were used as empirical Green’s functions in the inversions. High-frequency subevents within the two Mw 8 earthquakes were located at depths ranging from 25 to 55 km and mostly occurred downdip of the peak slip of these earthquakes. Fourier spectral ratios of the Iquique mainshock with respect to Mw 5–6 aftershocks were fit to determine their stress drops. The stress drops were roughly constant from Mw 5 to 8 at 10–20 MPa. A compound rupture model is described in which subevents occur in areas of spatially heterogeneous strength and stress on the rupture, and produce the high-frequency radiated energy of the overall earthquake, but are not located in the areas of peak slip. The stress drop of the overall earthquake is shown to equal the root mean square stress drop of subevents averaged over the rupture area.