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1. Earthquake focal mechanisms with distributed acoustic sensing

   https://doi.org/10.1038/s41467-023-39639-3  

   Li, Jiaxuan; Zhu, Weiqiang; Biondi, Ettore; Zhan, Zhongwen (July 2023, Nature Communications)

   Abstract Earthquake focal mechanisms provide critical in-situ insights about the subsurface faulting geometry and stress state. For frequent small earthquakes (magnitude< 3.5), their focal mechanisms are routinely determined using first-arrival polarities picked on the vertical component of seismometers. Nevertheless, their quality is usually limited by the azimuthal coverage of the local seismic network. The emerging distributed acoustic sensing (DAS) technology, which can convert pre-existing telecommunication cables into arrays of strain/strain-rate meters, can potentially fill the azimuthal gap and enhance constraints on the nodal plane orientation through its long sensing range and dense spatial sampling. However, determining first-arrival polarities on DAS is challenging due to its single-component sensing and low signal-to-noise ratio for direct body waves. Here, we present a data-driven method that measures P-wave polarities on a DAS array based on cross-correlations between earthquake pairs. We validate the inferred polarities using the regional network catalog on two DAS arrays, deployed in California and each comprising ~ 5000 channels. We demonstrate that a joint focal mechanism inversion combining conventional and DAS polarity picks improves the accuracy and reduces the uncertainty in the focal plane orientation. Our results highlight the significant potential of integrating DAS with conventional networks for investigating high-resolution earthquake source mechanisms. 

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2. 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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3. Ubiquitous Earthquake Dynamic Triggering in Southern California

   https://doi.org/10.1029/2023JB026487  

   DeSalvio, Nicolas D.; Fan, Wenyuan (June 2023, Journal of Geophysical Research: Solid Earth)

   Abstract Earthquakes can be dynamically triggered by the passing waves of other distant events. The frequent occurrence of dynamic triggering offers tangible hope in revealing earthquake nucleation processes. However, the physical mechanisms behind earthquake dynamic triggering have remained unclear, and contributions of competing hypotheses are challenging to isolate with individual case studies. To gain a systematic understanding of the spatiotemporal patterns of dynamic triggering, we investigate the phenomenon in southern California from 2008 to 2017. We use the Quake Template Matching catalog and an approach that does not assume an earthquake occurrence distribution. We develop a new set of statistics to examine the significance of seismicity‐rate changes as well as moment‐release changes. Our results show that up to 70% of 1,388 globalM ≥ 6 events may have triggered earthquakes in southern California. The triggered seismicity often occurred several hours after the passing seismic waves. The Salton Sea Geothermal Field, San Jacinto fault, and Coso Geothermal Field are particularly prone to triggering. Although adjacent fault segments can be triggered by the same earthquakes, the majority of triggered earthquakes seem to be uncorrelated, suggesting that the process is primarily governed by local conditions. Further, the occurrence of dynamic triggering does not seem to correlate with ground motion (e.g., peak ground velocity) at the triggered sites. These observations indicate that nonlinear processes may have primarily regulated the dynamic triggering cases. 

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4. Examining the Role of Elevated and Sustained Strain in Dynamically Triggering Earthquakes on the Anza Section of the San Jacinto Fault

   https://doi.org/10.1785/0120240079  

   DeSalvio, Nicolas D; Barbour, Andrew J; Fan, Wenyuan (February 2025, Bulletin of the Seismological Society of America)

   ABSTRACT Microearthquakes can be dynamically triggered in southern California by remote earthquakes. However, directly connecting dynamic triggering mechanisms with observational data remains challenging. One proposed failure mechanism suggests that both the amplitude and duration of cyclic fatigue caused by the passing seismic wave contribute to triggering occurrence. Here, we measure dynamic strains recorded by borehole strainmeters in the Anza section of the San Jacinto fault zone from 710 earthquakes that occurred over 300 km away between 2008 and 2017 to systematically investigate the role of elevated and sustained strain in controlling dynamic triggering. We design a suite of tests to evaluate whether specific amplitude thresholds and durations of strain can predict dynamic triggering cases. We further test whether the peak dynamic strain (PDS) can predict triggering occurrence in combination with the strain amplitude and duration. Based on these tests, there is no strain amplitude–duration threshold that can distinguish triggering occurrence in Anza. Dynamic triggering is more likely to occur if a remote earthquake causes a PDS above 100 nanostrain, though many cases were triggered at smaller PDSs. The lack of clear correlation between triggering and characteristics of the dynamic strain field suggests that the tested features of the incoming waves do not determine triggering occurrence and local fault conditions and slip processes are more important in controlling dynamic triggering in Anza. 

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5. Testing Earthquake Nucleation Length Scale with Pawnee Aftershocks

   https://doi.org/10.1785/0220210184  

   Wu, Bill S.; McLaskey, Gregory C. (April 2022, Seismological Research Letters)

   Abstract The interpretation of precursory seismicity can depend on a critical nucleation length scale h*, yet h* is largely unconstrained in the seismogenic crust. To estimate h* and associated earthquake nucleation processes at 2–7 km depths in Oklahoma, we studied seismic activity occurring prior to nine M 2.5–3.0 earthquakes that are aftershocks of the 3 September 2016 M 5.8 Pawnee, Oklahoma, earthquake. Four of the nine M 2.5–3.0 aftershocks studied did not have detectable seismicity within a 2 km radius of their hypocenters in the preceding 16 hr time windows. For the other five events, which did exhibit foreshock sequences, we estimated the static stress changes associated with each event of each sequence based on precise earthquake relocations and magnitude estimates. By carefully examining the spatiotemporal characteristics, we found all five of these M 2.5–3.0 aftershocks, and 70% of our studied events were plausibly triggered via static stress transfer from nearby earthquakes occurring hours to seconds earlier, consistent with the cascade nucleation model and a small h* in this region. The smallest earthquakes we could quantitatively study were M −1.5 events, which likely have 1–2 m rupture dimensions. The existence of these small events also supports a small nucleation length scale h*≤1  m, consistent with laboratory estimates. However, our observations cannot rule out more complicated earthquake initiation processes involving interactions between foreshocks and slow slip. Questions also remain as to whether aftershocks initiate differently from more isolated earthquakes. 

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