While classic double-couple earthquake models explain seismic wavefields accurately at low frequencies, at higher frequencies, seismic radiation exhibits significantly more complex and stochastic characteristics. Various on-fault and off-fault mechanisms have been proposed to explain high-frequency radiation, yet their relative contributions and trade-offs remain debated. 

In this study, we analyze near-fault high-frequency characteristics of the 2023 Mw 7.8 Kahramanmaraş earthquake with 19 strong-motion stations within approximately 10 km of its southern rupture. Above ~0.4 Hz, we observe a loss of horizontal polarity and reduced coherence between the two horizontal components, which cannot be explained by heterogeneous rupture on a planar fault. Additionally, the ~0.4 Hz transition frequency is lower than the commonly accepted rule-of-thumb value of 1 Hz. The near-fault high-frequency energy arrives concurrently with low-frequency signals, suggesting that high-frequency radiation originates near the fault rather than from medium scattering. Comparison with regional stations and back-projection analysis suggests that high-frequency signatures from the source persist even at greater distances. 

These findings indicate that the small-scale radiation processes near the rupture front are more complex than those described in conventional earthquake source representations. Our results highlight the need for improved earthquake source parameterization to assess high-frequency ground motion hazards and may provide valuable constraints for theoretical studies on high-frequency radiation mechanisms.

Learn more in our publication Wu et al., 2025, JGR

Last updated Nov-28-2025

With the deployment of continental scale seismic arrays, seismologists can quickly locate the high-frequency seismic radiation sources and track the earthquake rupture propagation using a technique called back-projection. It is a signal beamforming technique application in seismology, and similar applications can be found in fields such as radar, wireless communication, and radio astronomy. Recent studies have proposed multiple advancements in improving the back-projection location. However, the physical interpretation of the amplitude of stacked high-frequency source radiations, which is commonly referred to as beam power, is still challenging since the analysis is not based on a forward model. 

In this project, we conduct synthetic experiments to investigate the physical significance of back-projection beam power. We find that beam power is mainly controlled by the spatial heterogeneity wavelength near the rupture front, rupture directivity, and the seismogram frequency. In addition, back-projection alone may be unable to distinguish which type or types of source heterogeneity are responsible for the signal. It is in contrast with some previous studies that link the beam power to the maximum slip rate (acceleration) amplitude near the rupture front. Based on the results, we develop a novel theoretical framework that can quantitatively interpret the frequency- and array-dependent back-projection results not only in our synthetic experiments, but also the 2019 bilateral rupture M7.6 New Ireland earthquake.

Learn more in our publication Li et al., 2022, and my Feb-2023 presentation slides @USC. 

Last updated Aug-28-2023