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The clustering of earthquake magnitudes is poorly understood compared to spatial and temporal clustering. Better understanding of correlations between earthquake magnitudes could provide insight into the mechanisms of earthquake rupture and fault interactions, and improve earthquake forecasting models. In this study we present a novel method of examining how seismic magnitude clustering occurs beyond the next event in the catalog and evolves with time and space between earthquake events. We first evaluate the clustering signature over time and space using double-difference located catalogs from Southern and Northern California. The strength of magnitude clustering appears to decay linearly with distance between events and logarithmically with time. The signature persists for longer distances (more than 50km) and times (several days) than previously thought, indicating that magnitude clustering is not driven solely by repeated rupture of an identical fault patch or Omori aftershock processes. The decay patterns occur in all magnitude ranges of the catalog and are demonstrated across multiple methodologies of study. These patterns are also shown to be present in laboratory rock fracture catalogs but absent in ETAS synthetic catalogs. Incorporating magnitude clustering decay patterns into earthquake forecasting models such as ETAS could improve their accuracy. 

Recent studies have identified an incomplete student understanding of how elastic rebound causes earthquakes. We hypothesized that realistic imaging of spatial patterns in ground motions over the course of the earthquake cycle would improve student understanding. Incorporating spatial change information in the form of both motion vectors and before-during-after contrasts should require most students to change an existing mental model or develop a new model. Using a quasi-experimental design, we developed instructional interventions for presenting variations in ground motion, including map views of fence bending and GPS velocity vectors. We measured the impact on student performance based on assignment questions related to the ground motion at different points in the earthquake cycle following several interventions in four undergraduate courses from introductory to upper level over 4 years. The first round of study was a free-response format and then multiple-choice answers were created from the most common answers, including new “worked example” questions inquiring about the reasons answers were correct or incorrect. We identified two key misconceptions based on student answer choices: (a) difficulty in recognizing velocity vector patterns when presented in a new reference frame, and (b) difficulty in reasoning that the fault must be locked for the strain to accumulate and produce an earthquake. Our analysis indicates the largest performance increases occur with simple animations that demonstrate the bending, breaking, and rebending of a fence, along with associated GPS vectors, plotted successively in different reference frames. This suggests difficulties in understanding elastic rebounds can be mitigated when spatial patterns are presented in a context with repeated opportunities to make predictions combined with animations to support mental models that connect the spatial patterns with ground movement. 

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. 

Abstract Seismicity in the Eagle Ford play grew to 33 times the background rate in 2018. We identified how hydraulic fracturing (HF) contributed to seismicity since 2014 by comparing times and locations of HF with a catalog of seismicity extended with template matching. We found 94 M_L≥ 2.0 earthquakes spatiotemporally correlated to 211 HF well laterals. Injected volume and number of laterals on a pad influence the probability of seismicity, but effective injection rate has the strongest effect. Simultaneous stimulation of multiple laterals tripled the probability of seismicity relative to a single, isolated lateral. The 1 May 2018 M_W4.0 earthquake may have been the largest HF‐induced earthquake in the United States. It occurred ~10 km from a M_W4.8 earthquake in 2011 and was thought to be induced by fluid extraction. Thus, faults in this area are capable of producing felt and potentially damaging earthquakes due to operational activities.