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.
Backprojection (BP) of teleseismic P waves is a powerful tool to study the evolution of seismic radiation of large earthquakes. The common interpretations on the BP results are qualitative comparisons with earthquake kinematic observations, such as the evolution of slip on the fault and rupture velocity. However, the direct relation between the BP images and physical properties of the earthquake rupture process remains unclear and is needed for further application of this technique. In this study, we start from a theoretical formulation of the BP images, which is linear in the frequency domain, and carry on a synthetic exercise with kinematic source representations and virtual receivers embedded in a homogeneous full- space. We find that the fundamental linear formulation of the BP method is most correlated with the true kinematic source properties: in frequency domain the BP images are proportional to the images of slip motion through a scaling matrix F(ω) that accounts for radiation pattern and source–receiver geometry and that acts as a spatial smoothing operator. Overall, the synthetic BP images match relatively well the kinematic models and our exercise validates that the BP image can be directly used to track the spatiotemporal propagation of rupture front. However, because F(ω) is not strictly an identity matrix due to limited station coverage in space (azimuth and distance) and to the limited frequency bands of the seismograms, it remains difficult to recover the details in the rupture fronts from BP images. We define a resolvability parameter εI(ω) built from F(ω) that incorporates fault geometry, radiation pattern and wave propagation (source–array geometry) to quantify the ability of the BP method to resolve details of the rupture on the fault. εI(ω) successfully captures the similarity between BP images and kinematic source. We analyse the resolvability of most tectonically active regions and the most commonly used seismic arrays. Based on this global resolvability analysis, we propose an empirical relation between the seismic frequency, resolvable area and earthquake magnitude. It provides general guidelines to choose the lowest frequency in seismic waveform (e.g. about 0.3 Hz for Mw 8 and 1 Hz for Mw 7 earthquakes) and to interpret the BP image in terms of the source kinematics. In general, this work attempts to provide a clear interpretation of the BP images in light of the real earthquake rupture process and give a systematic evaluation of seismic data limitations.
Abstract We develop a methodology that combines compressive sensing backprojection (CS-BP) and source spectral analysis of teleseismic P waves to provide metrics relevant to earthquake dynamics of large events. We improve the CS-BP method by an autoadaptive source grid refinement as well as a reference source adjustment technique to gain better spatial and temporal resolution of the locations of the radiated bursts. We also use a two-step source spectral analysis based on (i) simple theoretical Green's functions that include depth phases and water reverberations and on (ii) empirical P wave Green's functions. Furthermore, we propose a source spectrogram methodology that provides the temporal evolution of dynamic parameters such as radiated energy and falloff rates. Bridging backprojection and spectrogram analysis provides a spatial and temporal evolution of these dynamic source parameters. We apply our technique to the recent 2015 Mw 8.3 megathrust Illapel earthquake (Chile). The results from both techniques are consistent and reveal a depth-varying seismic radiation that is also found in other megathrust earthquakes. The low-frequency content of the seismic radiation is located in the shallow part of the megathrust, propagating unilaterally from the hypocenter toward the trench while most of the high-frequency content comes from the downdip part of the fault. Interpretation of multiple rupture stages in the radiation is also supported by the temporal variations of radiated energy and falloff rates. Finally, we discuss the possible mechanisms, either from prestress, fault geometry, and/or frictional properties to explain our observables. Our methodology is an attempt to bridge kinematic observations with earthquake dynamics.
The 2015 Mw8.3 Coquimbo, Chile earthquake is a typical megathrust earthquake, whose size and coseismic slip distribution are consistent with the interseismic locking model that was derived from GPS measurements. Our preliminary back-projection results show that the rupture of the Mw8.3 Coquimbo earthquake propagates updip from the hypocenter (~25 km in depth). Furthermore, we find frequency-dependent behavior of the radiation power of the rupture, similar to those observed during the 2010 Mw8.8 Maule earthquake, which occurred ~50 km to the south of the 2015 earthquake rupture area. These observations indicate that there is a systematic downdip variation of properties on the megathrust in southern Chile. The rich observations of the coseismic slip distributions of the 2015 Mw8.3 earthquake provide an excellent opportunity to validate numerical simulations of rupture process based on interseismic locking distributions.