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1. Fast rupture of the 2009 M w 6.9 Canal de Ballenas earthquake in the Gulf of California dynamically triggers seismicity in California

   https://doi.org/10.1093/gji/ggac059  

   Fan, Wenyuan; Okuwaki, Ryo; Barbour, Andrew J; Huang, Yihe; Lin, Guoqing; Cochran, Elizabeth S (March 2022, Geophysical Journal International)

   SUMMARY In the Gulf of California, Mexico, the relative motion across the North America–Pacific boundary is accommodated by a series of marine transform faults and spreading centres. About 40 M> 6 earthquakes have occurred in the region since 1960. On 2009 August 3, an Mw 6.9 earthquake occurred near Canal de Ballenas in the region. The earthquake was a strike-slip event with a shallow hypocentre that is likely close to the seafloor. In contrast to an adjacent M7 earthquake, this earthquake triggered a ground-motion-based earthquake early warning algorithm being tested in southern California (∼600 km away). This observation suggests that the abnormally large ground motions and dynamic strains observed for this earthquake relate to its rupture properties. To investigate this possibility, we image the rupture process and resolve the slip distribution of the event using a P-wave backprojection approach and a teleseismic, finite-fault inversion method. Results from these two independent analyses indicate a relatively simple, unilateral rupture propagation directed along-strike in the northward direction. However, the average rupture speed is estimated around 4 km s−1, suggesting a possible supershear rupture. The supershear speed is also supported by a Rayleigh wave Mach cone analysis, although uncertainties in local velocity structure preclude a definitive conclusion. The Canal de Ballenas earthquake dynamically triggered seismicity at multiple sites in California, with triggering response characteristics varying from location-to-location. For instance, some of the triggered earthquakes in California occurred up to 24 hr later, suggesting that nonlinear triggering mechanisms likely have modulated their occurrence. 

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2. Ground motion characteristics of subshear and supershear ruptures in the presence of sediment layers

   https://doi.org/10.1093/gji/ggae422  

   Abdelmeguid, Mohamed; Elbanna, Ahmed; Rosakis, Ares (December 2024, Geophysical Journal International)

   SUMMARY We investigate the impact of sediment layers on ground motion characteristics during subshear and supershear rupture growth. Our findings suggest that sediment layers may lead to local supershear propagation, affecting ground motion, especially in the fault parallel (FP) direction. In contrast to homogeneous material models, we find that in the presence of sediment layers, a larger fault normal (FN) compared to FP particle velocity jump, reflects shear propagation at depth but does not rule out shallow supershear propagation. Conversely, a large FP compared to FN particle velocity jump indicates supershear propagation at depth. In the presence of a shallow layer, we also uncover a non-monotonic behaviour in the sediment’s influence on supershear transition and ground motion characteristics. During supershear propagation at depth we observe that sediment layers contribute to enhancing FP velocity pulses while minimally affecting the FN component. Furthermore, in the limit of global supershear propagation we identify local supersonic propagation within the sediment layers that significantly alters the velocity field around the rupture tip as observed on the free surface, creating both dilatational and shear Mach cones. In all our models with sediments we also find a significant enhancement in the fault vertical component of ground velocity. This could have particular implications for hazard assessments, such as in applications related to linear infrastructure, or a higher propensity to tsunami wave generation. Our research unravels the importance of considering heterogeneous subsurface material distribution in our physical models as they can have drastic implications on earthquake source physics. 

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3. Anatomy of strike-slip fault tsunami genesis

   https://doi.org/10.1073/pnas.2025632118  

   Elbanna, Ahmed; Abdelmeguid, Mohamed; Ma, Xiao; Amlani, Faisal; Bhat, Harsha S; Synolakis, Costas; Rosakis, Ares J (May 2021, Proceedings of the National Academy of Sciences)

   Tsunami generation from earthquake-induced seafloor deformations has long been recognized as a major hazard to coastal areas. Strike-slip faulting has generally been considered insufficient for triggering large tsunamis, except through the generation of submarine landslides. Herein, we demonstrate that ground motions due to strike-slip earthquakes can contribute to the generation of large tsunamis (>1 m), under rather generic conditions. To this end, we developed a computational framework that integrates models for earthquake rupture dynamics with models of tsunami generation and propagation. The three-dimensional time-dependent vertical and horizontal ground motions from spontaneous dynamic rupture models are used to drive boundary motions in the tsunami model. Our results suggest that supershear ruptures propagating along strike-slip faults, traversing narrow and shallow bays, are prime candidates for tsunami generation. We show that dynamic focusing and the large horizontal displacements, characteristic of strike-slip earthquakes on long faults, are critical drivers for the tsunami hazard. These findings point to intrinsic mechanisms for sizable tsunami generation by strike-slip faulting, which do not require complex seismic sources, landslides, or complicated bathymetry. Furthermore, our model identifies three distinct phases in the tsunamic motion, an instantaneous dynamic phase, a lagging coseismic phase, and a postseismic phase, each of which may affect coastal areas differently. We conclude that near-source tsunami hazards and risk from strike-slip faulting need to be re-evaluated. 

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4. Shallow Fault Zone Structure Affects Rupture Dynamics and Ground Motions of the 2019 Ridgecrest Sequence to Regional Distances

   https://doi.org/10.1029/2025JB031194  

   Schliwa, Nico; Gabriel, Alice‐Agnes; Ben‐Zion, Yehuda (June 2025, Journal of Geophysical Research: Solid Earth)

   Abstract Seismic faults are surrounded by damaged rocks with reduced rigidity and enhanced attenuation. These damaged fault zone structures can amplify seismic waves and affect earthquake dynamics, yet they are typically omitted in physics‐based regional ground motion simulations. We report on the significant effects of a shallow, flower‐shaped fault zone in foreshock‐mainshock 3D dynamic rupture models of the 2019 Ridgecrest earthquake sequence. We find that the fault zone structure both amplifies and reduces ground motions not only locally but at distances exceeding 100 km. This impact on ground motions is frequency‐ and magnitude‐dependent, particularly affecting higher frequency ground motions from the foreshock because its corner frequency is closer to the fault zone's fundamental eigenfrequency. Within the fault zone, the shallow transition to a velocity‐strengthening frictional regime leads to a depth‐dependent peak slip rate increase of up to 70% and confines fault zone‐induced supershear transitions mostly to the fault zone's velocity‐weakening roots. However, the interplay of fault zone waves, free surface reflections, and rupture directivity can generate localized supershear rupture, even in narrow velocity‐strengthening regions, which are typically thought to inhibit supershear rupture. This study demonstrates that shallow fault zone structures may significantly affect intermediate‐ and far‐field ground motions and cause localized supershear rupture penetrating into velocity‐strengthening regions, with important implications for seismic hazard assessment. 

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5. NH22B-04: Field survey of the 28 September 2018 Sulawesi tsunami (Invited)

   Fritz, M (April 2019, American Geophysical Union https://agu.confex.com/agu/fm18/meetingapp.cgi/Paper/476809)

   On September 28, 2018, a magnitude Mw 7.5 earthquake occurred in the neck of Sulawesi’s Minahasa peninsula. The combined effects of the earthquake and tsunami caused catastrophic damage and more than 2000 deaths. An international tsunami survey team (ITST) was deployed 3 weeks after the event to document flow depths, runup heights, inundation distances, sediment deposition, damage patterns at various scales, performance of the man-made infrastructure and impact on the natural environment. The 23 to 29 October ITST covered a 100 km stretch of coastline circling the entire Palu Bay and adjacent coastlines along the Makassar Strait. The ground-based field survey was complemented by a 200 km long reconnaissance flight with an Indonesian Army Mil Mi-17 helicopter. The collected survey data includes more than 100 tsunami runup and flow depth measurements. The tsunami impact peaked inside Palu Bay with flow depths above ground reaching 6 m and maximum runup heights of 9 m. Inundation and tsunami damage was mostly limited to within 0.5 km of the shoreline except along river estuaries. A significant variation in tsunami impact was observed inside Palu Bay with rapid decreases of the tsunami heights towards the bay entrance and outside of the Palu Bay along the coastlines on the Makassar Strait. Selected tsunami eyewitness video recording locations inside Palu Bay were surveyed for subsequent video image calibration, tsunami hydrograph and flow velocity analysis. We acquired precise topographic data using terrestrial laser scanning (TLS) at selected video sites with multiple scans acquired from different instrument positions at each site. These ground-based LiDAR measurements produce 3-dimensional “point cloud” datasets. Digital photography from 3 scanner-mounted cameras yields full dome panoramic images overlaid on highly accurate point clouds. Integrated GPS measurements allow accurate georeferencing of the TLS data. We deployed a Leica BLK360 scanner from the NHERI RAPID facility. Field observations, drone videos, helicopter and satellite imagery are presented. The team interviewed numerous eyewitnesses based on established protocol and educated residents about tsunami hazards since community-based education and awareness programs are essential to save lives in locales at risk from locally generated tsunamis. 

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