Yong Zhang

Key Points• Joint centroid moment tensor inversion, back-projection imaging, multi-point-source inversion, and finite-fault inversion reveal the multi-scale rupture process of the 2026 Mw 7.5 Venezuela earthquake.• The rupture initiated on a northeast-striking secondary fault and propagated eastward for ~180 km with an average rupture velocity of ~2.9 km/s.• The results provide new evidence for slow cascaded rupture controlled by fault geometry and structural complexity.

SUMMARY: In geodetic slip inversions, resolution decreases rapidly with depth because deformation data are measured at the ground surface. Traditionally, this decrease in resolution has been attributed to the weak deformation signals caused by slips at greater depths. However, this study demonstrated that the primary cause is the stronger smoothing applied to deeper slips compared to shallower ones. This work proposes a method that scales the Green's functions to equalize smoothing effects across depths. The method's effectiveness was validated through both synthetic tests and real earthquake applications. In synthetic tests, it improved recovery of deep slips in both location and amplitude. When applied to the 2008 MW 7.9 Wenchuan earthquake, the method produced smaller slips near the ground surface and larger slips at depth. For the land-based deformation inversion of the 2011 MW 9.0 Tohoku earthquake, the method resulted in larger shallow slips near the trench and greater sea-floor uplift compared to the conventional inversion, which is valuable for accurate prediction of tsunami wave height. Additionally, this method may also be applicable to other inversions where smoothing is used and observation amplitudes vary with distance.

Rapidly determining seismic source characteristics, particularly the moment tensor and finite-fault inversion, is critical for providing timely and detailed information for rapid responses to large earthquakes. We proposed an automatic method to improve the efficiency of these inversions, which was limited previously by using far-field data in moment tensor inversions and manual operation in finite-fault inversions. Using near-field data, we simultaneously determined the moment tensor solution and the horizontal moment distribution. It can recover the source mechanism and identify moment-concentrated regions based solely on preliminary location and magnitude results. In addition, by solving the horizontal moment distribution, this approach can handle ruptures on complex fault systems, including curved, branched, parallel, and conjugated faults. The effectiveness of this method was validated through numerical tests and applications to the 2008 Wenchuan and 2016 Kaikōura earthquakes. By utilizing real-time near-field data, this method can identify meizoseismal areas within minutes after an earthquake, providing valuable insights for intensity distribution and disaster assessment.

The kinematic characteristics of an earthquake source offer insights into its physical properties, with macroscopic parameters such as seismic moment and radiated energy serving as key indicators for describing seismic hazards. In this study, we explored the characteristics of earthquake sources in the frequency domain. Specifically, we introduced a hybrid method using teleseismic P-wave data to derive source spectra for over 200 large (MW > 7) shallow (depth < 70 km) earthquakes from 2000 to 2024. Our analysis reveals that in a simple ω-n model, the power exponent n is consistently less than 2 when the corner frequency aligns with the earthquake's total duration. To further characterize earthquake source properties, we defined the centroid frequency (fe) as the centroid of the energy spectral density which can represent the dimension of asperities on the fault plane. We also proposed the ratio between observed radiated energy and that predicted by the ω-² decay model as a metric for quantifying rupture heterogeneity (RH). Relationships of fe and RH with focal mechanism, magnitude, and centroid depth were analyzed within the conventional asperity model framework. Our source spectra dataset can also provide a robust foundation for future investigations into earthquake sources.

The 18 December 2023 Mw 6.0 Jishishan earthquake struck the Dahejia basin in the northeastern Tibetan Plateau. Three isolated sites of concentrated fractures with a total length of ∼8 km were observed in field surveys. It remains unclear that such surface faulting was the extension of mapped active thrusts or just ground cracking. Moreover, inaccurate and incomplete routine earthquake catalog based on a sparse permanent seismic network makes it difficult to determine the seismogenic fault within a multifault zone. In this study, we develop a high precision aftershock catalog based on a self-deployed dense seismic array, illuminating an unrecognized blind northeast-dipping seismogenic fault and an activated southwest-dipping conjugate fault. We simulate the dynamic rupture process to investigate slip behaviors and delineate the previously suspected ruptures are ground cracking. Our results present a unilateral rupture event with a relatively slow rupture velocity and low radiation efficiency, suggesting that this earthquake may have ruptured an immature basin thrust.

A Simple Method for Improving the Resolution of Geodetic Slip InversionYong Zhang School of Earth and Space Sciences, Peking University, Beijing 100871, China zhang-yong@pku.edu.cn

SUMMARY: Centroid moment tensor inversion involves the determinations of the moment tensor solution, which is usually done with low-frequency signals to meet the requirement of the point-source approximation, and the moment centroid, which requires high-frequency signals to improve the resolution. Traditional centroid moment tensor inversion techniques, such as the W-phase inversion, mainly use low-frequency data to estimate the magnitude and fault parameters, which limits the resolution of the moment centroid. In this study, we combine the P wave and W phase to constrain both the moment tensor and moment centroid. The rupture directivity is considered in the P-wave inversion, and the rupture velocity is resolved by inverting the P wave solely. The moment centroid is estimated by the rupture velocity from the P-wave inversion and the grid search in the W-phase inversion. The final moment tensor solution is determined based on the moment centroid by jointly inverting both P-wave and W-phase data. The resulting centroid moment tensor solution can fit a broad frequency band (0.001–0.1 Hz) of waveforms. Through synthetic inversion tests and applications to several large earthquakes, we demonstrate that the magnitudes and fault parameters from our joint inversion are more stable than the P-wave inversion, and the moment centroids, especially the centroid depths, seem more reasonable than those from the W-phase inversion.

A devasting Mw7.2 earthquake struck southern Haiti on 14 August 2021, leading to over 2000 casualties and severe structural failures. This earthquake, which ruptured ~70 km west of the 2010 Mw7.0 event, offers a rare opportunity to probe the mechanical properties of southern Haiti. This study investigates the kinematic multi-fault coseismic rupture process by jointly analyzing teleseismic and interferometric synthetic aperture radar (InSAR) datasets. We determined the optimal dip of different segment faults through finite-fault inversion, and the results show that the dips of the first, second and third faults are 62°, 76° and 76°, respectively, coinciding with the relocated aftershock distribution. The results estimated from our joint inversion revealed that the slip was dominated by reverse motion in the first segment and strike-slip motion in the second and third segments. Three slip patches were detected along the strike, with a peak slip of 3.0 m, and the rupture reached the surface at the second segment. The kinematic rupture process shows a unilateral rupture with a high centroid rupture velocity (5.5 km/s), and the rupture broke through the stepover and caused a cascade rupture. The rupture front experiences a directivity pulse of high ground motions with high amplitude and short duration, which may be an additional factor explaining the many landslides concentrated on the western end of the fault. The Coulomb failure stress change result indicates the increases in the probability of future events to the east and west of the 2021 main shock.