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1. Re-Estimation of Earthquake Magnitudes Using a Relative Magnitude Method and the Effects of Magnitude Uncertainty on Seismic Hazard Estimates

   Gable, Sydney L; Huang, Yihe; Shelly, David R (December 2023, AGU)

   Accurate estimates of earthquake magnitude are necessary to improve our understanding of seismic hazard. Unbiased magnitudes for small earthquakes are especially important because magnitude exceedance probabilities for large earthquakes are derived from the behavior of small earthquakes. Also, accurate characterization of small events is becoming increasingly important for ground motion models. However, catalog magnitudes may vary for the same event depending on network procedures and capabilities. In addition, different magnitude scales are often used for events of varying sizes. For example, moment magnitude (Mw) is the widely preferred estimate for earthquake size but it is often not available for small earthquakes (M < 3.5). As a result, statistical measures such as magnitude frequency distribution (MFD) and b-value can be biased depending on magnitude type and uncertainties that arise during the measurement process. In this research we demonstrate the capability of the relative magnitude method to provide a uniform and accurate estimate of earthquake magnitude in a variety of regions, while only requiring the use of waveform data. The study regions include the Permian Basin in Texas, central Oklahoma, and southern California. We present results in which only relative magnitudes are used to estimate MFD and b-value as well as relative magnitudes that are benchmarked to an absolute scale using a coda-envelope derived Mw calibration for small events. We also discuss potential sources of uncertainty in the relative magnitude method such as acceptable signal-to-noise ratios, cross-correlation thresholds, and choice of scaling constant, as well as our attempts to mitigate those uncertainties. 

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2. Fault size–dependent fracture energy explains multiscale seismicity and cascading earthquakes

   https://doi.org/10.1126/science.adj9587  

   Gabriel, Alice-Agnes; Garagash, Dmitry I; Palgunadi, Kadek H; Mai, P Martin (July 2024, Science)

   Earthquakes vary in size over many orders of magnitude, often rupturing in complex multifault and multievent sequences. Despite the large number of observed earthquakes, the scaling of the earthquake energy budget remains enigmatic. We propose that fundamentally different fracture processes govern small and large earthquakes. We combined seismological observations with physics-based earthquake models, finding that both dynamic weakening and restrengthening effects are non-negligible in the energy budget of small earthquakes. We established a linear scaling relationship between fracture energy and fault size and a break in scaling with slip. We applied this scaling using supercomputing and unveiled large dynamic rupture earthquake cascades involving >700 multiscale fractures within a fault damage zone. We provide a simple explanation for seismicity across all scales with implications for comprehending earthquake genesis and multifault rupture cascades. 

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3. Improved Observations of Deep Earthquake Ruptures Using Machine Learning

   https://doi.org/10.1029/2023JB027334  

   Shi, Qibin; Denolle, Marine A. (December 2023, Journal of Geophysical Research: Solid Earth)

   Abstract Elevated seismic noise for moderate‐size earthquakes recorded at teleseismic distances has limited our ability to see their complexity. We develop a machine‐learning‐based algorithm to separate noise and earthquake signals that overlap in frequency. The multi‐task encoder‐decoder model is built around a kernel pre‐trained on local (e.g., short distances) earthquake data (Yin et al., 2022,https://doi.org/10.1093/gji/ggac290) and is modified by continued learning with high‐quality teleseismic data. We denoise teleseismic P waves of deep Mw5.0+ earthquakes and use the clean P waves to estimate source characteristics with reduced uncertainties of these understudied earthquakes. We find a scaling of moment and duration to beM₀ ≃ τ⁴, and a resulting strong scaling of stress drop and radiated energy with magnitude ( and ). The median radiation efficiency is 5%, a low value compared to crustal earthquakes. Overall, we show that deep earthquakes have weak rupture directivity and few subevents, suggesting a simple model of a circular crack with radial rupture propagation is appropriate. When accounting for their respective scaling with earthquake size, we find no systematic depth variations of duration, stress drop, or radiated energy within the 100–700 km depth range. Our study supports the findings of Poli and Prieto (2016,https://doi.org/10.1002/2016jb013521) with a doubled amount of earthquakes investigated and with earthquakes of lower magnitudes. 

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4. Improving Stress Drop Estimation through Point-Wise Spectral Ratio Stacking

   Liu, Meichen; Huang, Yihe; Ritsema, Jeroen (December 2023, AGU)

   Stress drop, a crucial source parameter in earthquake studies, significantly influences ground motion prediction and seismic hazard assessment. Despite several existing methods to estimate stress drops, the resulting stress drop estimates often exhibit a wide variation of up to 3-4 orders of magnitude. In this study, we address the robustness of stress drop estimation by introducing a point-wise spectral ratio stacking approach based on empirical Green’s functions (eGfs). Conventional trace-wise stacking can lead to data exclusion due to high signal-to-noise ratio requirements across a wide range of frequency. By adopting point-wise stacking, we maximize the utilization of useful recording information, leading to more accurate stress drop estimates. We applied the point-wise spectral ratio stacking method to a comprehensive dataset comprising global earthquakes from 1990 to 2020 with magnitude larger than Mw5.5 and depth shallower than 50 km. We first verified the moment magnitudes of earthquakes estimated from the resulting seismic moment ratios. We found that the moment magnitude of master events best consistent with catalog magnitudes when the magnitude difference between master and their eGfs differs by about 0.5. Our analysis indicates that stress drop of shallow earthquakes exhibits no depth dependence, while showing a slight increase with magnitude. The results obtained through our optimized stacking process shed new light on stress drop estimate of shallow earthquakes and have the potential to enhance the understanding of earthquake mechanics. 

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5. Spatiotemporal Variations of Intermediate‐Depth Earthquakes Before and After 2011 Tohoku Earthquake Revealed by a Template Matching Catalog

   https://doi.org/10.1029/2023GL104068  

   Zhai, Qiushi; Peng, Zhigang; Matsubara, Makoto; Obara, Kazushige; Wang, Yanbin (November 2023, Geophysical Research Letters)

   Abstract We investigate spatiotemporal changes of intermediate‐depth earthquakes in the double seismic zone beneath Central and Northeastern Japan before and after the 2011 magnitude 9 Tohoku earthquake. We build a template‐matching catalog 1 year before and 1 year after the Tohoku earthquake using Hi‐net recordings. The new catalog has a six‐fold increase in earthquakes compared to the Japan Meteorological Agency catalog. Our results show no significant change in the intermediate‐depth earthquake rate prior to the Tohoku earthquake, but a clear increase in both planes following the Tohoku earthquake. The regions with increased intermediate‐depth earthquake activity and the post‐seismic slips following the Tohoku earthquake are spatially separate and complementary with each other. Aftershock productivity of intermediate‐depth earthquakes increased in both planes following the Tohoku earthquake. Overall, aftershock productivity of the upper plane is higher than the lower plane, likely indicating that stress environments and physical mechanisms of intermediate‐depth earthquakes in the two planes are distinct. 

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