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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. 

A recent study found ∼90% of earthquakes in the Eagle Ford shale in Texas from 2014 to 2018 were spatiotemporally related to hydraulic fracturing (HF) wells. Since then, earthquakes have remained prevalent, including a new region of seismicity in Live Oak county. We sought to perform a deeper exploration of how HF has contributed to recent seismicity using template matching and repeating signal detection (RSD), which employs machine learning to search for repeating earthquakes with the potential to find something different than the template catalog. RSD identified new bursts of seismicity with a shorterS‐Ptime (∼2 s) than the previous catalog (>4 s), which temporally correlated with HF near station N4 735B. The shortS‐Pevents have smaller magnitudes (M_L < 2.0), consistent with the idea that HF‐induced seismicity in the Eagle Ford is likely more pervasive than previously reported, but detection is limited by the density of stations. RSD and template matching identified 1,600 earthquakes correlated with HF from 2019 to February 2020. We confirmed newly detected HF‐induced seismicity in Live Oak county did not occur until January 2019. Despite similar cumulative volume prior to and after 2019, the onset of detectable seismicity did not occur until HF injection exceeded 2 million barrels per month over this area, supporting the notion that injection flux is a stronger influence on the seismicity occurrence than cumulative volume. When considering the full catalog, the likelihood of seismicity also correlated with proximity to mapped faults, with 13% of wells <2 km from faults having seismicity compared to only 3% of wells >2 km.

 

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. 

The Mexican subduction zone is an ideal location for studying subduction processes due to the short trench-to-coast distances that bring broad portions of the seismogenic and transition zones of the plate interface inland. Using a recently generated seismicity catalog from a local network in Oaxaca, we identified 20 swarms of earthquakes (M < 5) from 2006 to 2012. Swarms outline what appears to be a steeply dipping structure in the overriding plate, indicative of an origin other than the plate interface. This steeply dipping structure corresponds to the northern boundary of the Xolapa terrane. In addition, we observed an interesting characteristic of slow slip events (SSEs) where they showed a shift from trenchward motion toward an along-strike direction at coastal GPS sites. A majority of the swarms were found to correspond in time to the along-strike shift. We propose that swarms and SSEs are occurring on a sliver fault that allows the oblique convergence to be partitioned into trench-perpendicular motion on the subduction interface and trench-parallel motion on the sliver fault. The resistivity structure surrounding the sliver fault suggests that SSEs and swarms of earthquakes occur due to high fluid content in the fault zone. We propose that the sliver fault provides a natural pathway for buoyant fluids attempting to migrate upward after being released from the downgoing plate. Thus, sliver faults could be responsible for the downdip end of the seismogenic zone by creating drier conditions on the subduction interface trenchward of the sliver fault, promoting fast-slip seismogenic rupture behavior.

 

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.

 

Wastewater disposal is generally accepted to be the primary cause of the increased seismicity rate in Oklahoma within the past decade, but no statewide analysis has investigated the contribution of hydraulic fracturing (HF) to the observed seismicity or the seismic hazard. Utilizing an enhanced seismicity catalog generated with multistation template matching from 2010 to 2016 and all available hydraulic fracturing information, we identified 274 HF wells that are spatiotemporally correlated with bursts of seismicity. The majority of HF‐induced seismicity cases occurred in the SCOOP/STACK plays, but we also identified prominent cases in the Arkoma Basin and some more complex potential cases along the edge of the Anadarko Platform. For HF treatments where we have access to injection parameters, modeling suggests that poroelastic stresses are likely responsible for seismicity, but we cannot rule out direct pore pressure effects as a contributing factor. In all of the 16 regions we identified, ≥75% of the seismicity correlated with reported HF wells. In some regions, >95% of seismicity correlated with HF wells and >50% of the HF wells correlated with seismicity. Overall, we found ~700 HF‐induced earthquakes withM ≥ 2.0, including 12 events withM3.0–3.5. These findings suggest state regulations implemented in 2018 that require operators in the SCOOP/STACK plays to take action if aM > 2 earthquake could have a significant impact on future operations.