We cluster a global data base of 3529 M>5.5 earthquakes in 1995-2018 based on a dynamic time warping dissimilarity of their source time functions (STFs). The clustering exhibits different degrees of STF shape complexity and suggests an association between STF complexity and earthquake source parameters. Thrust events are in large proportion with simple STF shapes and at all depths. In contrast, earthquakes with complex STF shapes tend to be located at shallow depth in complicated tectonic regions with preferentially strike slip mechanism and relatively longer duration. With 2D dynamic modeling of earthquake ruptures on heterogeneous pre-stress and linear slip-weakening friction, we find a systematic variation of the simulated STF complexity with frictional properties. Comparison between the observed and synthetic clustering distributions provides useful constraints on elements of the frictional properties. In particular, the characteristic slip-weakening distance could be constrained to be generally short (< 0.1 m) and depth dependent.

Megathrust earthquakes exhibit a ubiquitous seismic radiation style: low-frequency (LF) seismic energy is efficiently emitted from the shallowest portion of the fault, whereas high-frequency (HF) seismic energy is efficiently emitted from the deepest part of the fault. Although this is observed in many case-specific studies, we show that it is ubiquitous in global megathrust earthquakes between 1995 and 2021. Previous studies have interpreted this as an effect of systematic depth variation in either the plate interface frictional properties (Lay et al., 2012) or the P wavespeeds (Sallarès & Ranero, 2019). This work suggests an alternative hypothesis: the interaction between waves and ruptures due to the Earth’s free surface is the leading mechanism that generates this behavior. Two-dimensional dynamic rupture simulations of subduction zone earthquakes support this hypothesis. Our simulations show that the interaction between the seismic waves reflected at the Earth’s free surface and the updip propagating rupture results in LF radiation at the source. In contrast, the downdip propagation of rupture is less affected by the free surface and is thus dominated by HF radiation typical of buried faults. To a second degree, the presence of a realistic Earth structure derived from P-wave velocity (VP) tomographic images and realistic VP/VS ratio estimated in boreholes further enhances the contrast in source radiation. We conclude that the Earth’s free surface is necessary to explain the observed megathrust earthquake radiation style, and the realistic structure of subduction zone is necessary to better predict earthquake ground motion and tsunami potential.

Backprojection (BP) of teleseismic P waves is a widely-used method to study the evolution of earthquake radiation and is particularly effective for large earthquakes. We can harness key information on the spatiotemporal evolution during the rupture process from waveform similarity or coherency. Understanding the relation between earthquake physics and the spatiotemporal evolution from BP imaging, which are usually obtained from high frequency seismic waveforms, is of great importance. Theoretical studies indicate that the high-frequency bursts can be related to abrupt changes in rupture velocity (e.g. stopping of rupture or kinks on the fault). Moreover, the BP images are thought to be equivalent to either slip or slip rate on the fault, provided that the Green’s functions from the sources to the receivers are incoherent delta functions. Furthermore, recent studies propose that the frequency dependent features of BP results can reflect the stress status, frictional and/or geometrical heterogeneity on the fault surface. It is promising that we can obtain more observational constraints and information about the earthquake dynamic source from the backprojection results combined with other independent techniques. In this study, we attempt to figure out the relation between the BP results and earthquake source process by testing both kinematic and dynamic source models. With these source models, we can synthesise the seismic waveforms and trace them back to the fault surface using the BP method. Therefore, we can directly compare the BP results with the already-known earthquake sources and further explore the possible relation to the source properties by varying our source models such as the friction laws, fault geometries. To simplify our problem and exclude the potential effects from complex earth structure, our tests are carried out in a purely elastic medium, whole space, allowing us to solve analytically for the far-field body waves. From these systematical tests and comparisons, we aim at building a comprehensive relation between the BP images and various source properties. Moreover, our results can provide significant help to better understand the physics of earthquake source process from seismic observations.

Earthquake signals in seismic data are inevitably contaminated with signals from unwanted sources. Separating noise from earthquake signals can greatly improve the analysis of the seismic data, such as earthquake characterization and ambient noise analysis. In this work, we develop a new auto-encoder to extract transient signals from ambient signals directly in the time domain for 3-component seismograms. We benchmark our architecture development and performance against a time-frequency counterpart (similar to the DeepDenoisier). We explore the generalization of our time-domain denoiser by training on various scales of seismic data. First, we train purely on observed seismograms of local ( < 350 km) events using the STandford EArthquake Dataset (STEAD) data set. Second, we generate a data set of observed seismograms from regional earthquakes (350 km-2000 km), which we complement with seismograms generated by hybrid low-frequency deterministic, high-frequency stochastic synthetic waveforms. We explore the robustness of the denoiser on various noise structures. Finally, we explore the quality of the extracted signals, for earthquake characterization and for ambient noise seismology.