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.

Earthquake source time functions carry information about the complexity of seismic rupture.We explore databases of earthquake source time functions and find that they are composed of distinct peaks that we call subevents. We observe that earthquake complexity, as represented by the number of subevents, grows with earthquake magnitude. Patterns in rupture complexity arise from a scaling between subevent moment and main event moment. These results can be explained by simple 2-D dynamic rupture simulations with self-affine heterogeneity in fault prestress. Applying this to early magnitude estimates, we show that the main event magnitude can be estimated after observing only the first few subevents.Plain Language Summary Seismograms are measurements of waves from earthquakes.They give us information about what happened on the fault at the place where the earthquake occurred. Seismograms can be difficult to interpret because they are often very complicated. Why? One reason is that the waves change when they travel long distances between the fault and a seismometer. Seismologists correct for this effect, however, by constructing something called a source time function. Source time functions are much easier to understand than raw seismograms. We examine a catalog of source time functions from around the world. We find that large earthquakes are composed of many smaller events that we call subevents. The size of a subevent is related to the size of the main earthquake. One important outcome is that we can predict the final size of an earthquake after observing only the first few subevents.

Earthquake source time functions carry information about the complexity of seismic rupture. We explore databases of source time functions of earthquakes and find that source time functions are composed of distinct peaks that we call subevents. We observe that earthquake complexity, as represented by the number of subevents, grows with earthquake magnitude. We find that subevent magnitudes are nearly proportional to their corresponding main event magnitude. We show that the main event magnitude can be estimated after observing only the first few subevents.