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Clustering of earthquake magnitudes is still actively debated, compared to well-established spatial and temporal clustering. Magnitude clustering is not currently implemented in earthquake forecasting but would be important if larger magnitude events are more likely to be followed by similar sized events. Here we show statistically significant magnitude clustering present in many different field and laboratory catalogs at a wide range of spatial scales (mm to 1000 km). It is universal in field catalogs across fault types and tectonic/induced settings, while laboratory results are unaffected by loading protocol or rock types and show temporal stability. The absence of clustering can be imposed by a global tensile stress, although clustering still occurs when isolating to triggered event pairs or spatial patches where shear stress dominates. Magnitude clustering is most prominent at short time and distance scales and modeling indicates >20% repeating magnitudes in some cases, implying it can help to narrow physical mechanisms for seismogenesis.

 

Abstract The recent Indios, Puerto Rico earthquake sequence has drawn attention, as the increased seismicity rate in this area was unprecedented. The sequence began on 28 December 2019, caused a 6.4 magnitude earthquake on 7 January 2020, and remained active over a year later. This sequence fits the nominal definition of an earthquake swarm in that it had an abrupt onset, a sustained high rate of seismicity without a clear triggering mainshock or evidence for Omori decay, and a lack of adherence to Bath’s law. However, the sequence also had several prominent mainshock–aftershock (MS–AS) sequences embedded within it. We applied three-station waveform cross correlation to the early part of this sequence using the Puerto Rico Seismic Network (PRSN) catalog as templates, which confirmed the mixture of swarm and MS–AS patterns. In an effort to place this intriguing sequence in the context of the previous seismicity in Puerto Rico, we investigated the existence of swarms and MS–AS sequences recorded by the PRSN since 1987 by identifying sequences with increased seismicity rate when compared to the background rate. About 59 sequences were manually verified and characterized into swarms or MS–AS. We found that 58% of the sequences follow traditional swarm patterns and 14% adhere to traditional MS–AS behavior, whereas 29% of the sequences have a mixture of both swarm and MS–AS behaviors. These findings suggest that it is not unusual for the Indios sequence to have a mixture of both the characteristics. In addition, the detection of many swarms distributed over a broad area of the subduction interface indicates stress heterogeneity and low-coupling consistent with prior studies indicating that the potential for a magnitude ∼8 megathrust earthquake along the Puerto Rico trench is unlikely. 

Abstract The COVID-19 pandemic led to the suspension of many summer research opportunities for science, technology, engineering and mathematics students. In response, the Incorporated Research Institutions for Seismology Education and Outreach program, in collaboration with Miami University, offered a free online Seismology Skill Building Workshop to increase undergraduates’ knowledge, skills, self-efficacy, and interest in observational seismology and scientific computing. Registrations were received from 760 undergraduates representing 60 different countries. U.S. participants consisted of 59% women and 29% from populations traditionally underrepresented in the geosciences. The workshop design consisted of a tailored Linux virtual machine, regular webinars, a Slack workspace, tutorial-style active e-learning assignments, and an optional final project. Every other week for 12 weeks, a module with ∼6 assignments was released to build skills with Linux, Generic Mapping Tools, Seismic Analysis Code, webservices, seismic network processing, Python, ObsPy, and Jupyter notebooks. A final module focused on competitiveness for graduate school, summer internships, and professional jobs. Evaluation of the workshop relied on registration data, pre- and post-workshop surveys, and performance data from the learning management system. 440 participants completed at least one assignment, 224 completed at least 80% of the assignments, and 191 completed all 35 assignments, significantly higher than most comparable large-scale, open-access courses. Participants invested ∼6 hrs per week and averaged a score of 88% on assignments. We identified >60% normalized gain in scientific computing skills. There is evidence that the inclusive design of the workshop was able to attract and retain a diverse population. However, some additional investigation is needed to ensure that benefits were evenly experienced. Regardless of the degree of completion, participants perceived the workshop quite positively: on average 96% described it as high to very high quality, 83% satisfied to very satisfied with their experience, and 70% very likely to recommend to peers. We identify future directions for running a second iteration of the workshop, including strategies to continue broadening participation and improving retention. 

Abstract The La Crucecita earthquake ruptured on the megathrust, generating strong shaking and a modest but long-lived tsunami. This is a significant earthquake that illuminates important aspects of the behavior of the megathrust as well as the potential related hazards. The rupture is contained within 15–30 km depth, ground motions are elevated, and the energy to moment ratio is high. We argue that it represents a deep megathrust earthquake, the 30 km depth is the down-dip edge of slip. The inversion is well constrained, ruling out any shallow slip. It is the narrow seismogenic width and the configuration of the coastline that allow for deformation to occur offshore. The minor tsunamigenesis can be accounted for by the deep slip patch. There is a significant uplift at the coast above it, which leads to negative maximum tsunami amplitudes. Finally, tide-gauge recordings show that edge-wave modes were excited and produce larger amplitudes and durations in the Gulf of Tehuantepec.