Subducting slab ultra-slow velocity layer coincident with silent earthquakes in southern Mexico

Great earthquakes have repeatedly occurred on the plate interface in a few shallow-dipping subduction zones where the subducting and overriding plates are strongly locked. Silent earthquakes (or slow slip events) were recently discovered at the down-dip extension of the locked zone and interact with the earthquake cycle. Here, we show that locally observed converted SP arrivals and teleseismic underside reflections that sample the top of the subducting plate in southern Mexico reveal that the ultra-slow velocity layer (USL) varies spatially (3 to 5 kilometers, with an S-wave velocity of ~2.0 to 2.7 kilometers per second). Most slow slip patches coincide with the presence of the USL, and they are bounded by the absence of the USL. The extent of the USL delineates the zone of transitional frictional behavior

Hydraulic fracturing‐induced seismicity

Hydraulic fracturing (HF) is a technique that is used for extracting petroleum resources from impermeable host rocks. In this process, fluid injected under high pressure causes fractures to propagate. This technique has been transformative for the hydrocarbon industry, unlocking otherwise stranded resources; however, environmental concerns make HF controversial. One concern is HF‐induced seismicity, since fluids driven under high pressure also have the potential to reactivate faults. Controversy has inevitably followed these HF‐induced earthquakes, with economic and human losses from ground shaking at one extreme and moratoriums on resource development at the other. Here, we review the state of knowledge of this category of induced seismicity. We first cover essential background information on HF along with an overview of published induced earthquake cases to date. Expanding on this, we synthesize the common themes and interpret the origin of these commonalities, which include recurrent earthquake swarms, proximity to well bore, rapid response to stimulation, and a paucity of reported cases. Next, we discuss the unanswered questions that naturally arise from these commonalities, leading to potential research themes: consistent recognition of cases, proposed triggering mechanisms, geologically susceptible conditions, identification of operational controls, effective mitigation efforts, and science‐informed regulatory management. HF‐induced seismicity provides a unique opportunity to better understand and manage earthquake rupture processes; overall, understanding HF‐induced earthquakes is important in order to avoid extreme reactions in either direction

The Great Sumatra-Andaman Earthquake of 26 December 2004

The two largest earthquakes of the past 40 years ruptured a 1600-kilometer-long portion of the fault boundary between the Indo-Australian and southeastern Eurasian plates on 26 December 2004 [seismic moment magnitude (M_w) = 9.1 to 9.3] and 28 March 2005 (M_w = 8.6). The first event generated a tsunami that caused more than 283,000 deaths. Fault slip of up to 15 meters occurred near Banda Aceh, Sumatra, but to the north, along the Nicobar and Andaman Islands, rapid slip was much smaller. Tsunami and geodetic observations indicate that additional slow slip occurred in the north over a time scale of 50 minutes or longer

Changes in Plate Motions and the Shape of Pacific Fracture Zones

Geosat passes, the new 2-min global gravity grid [Smith and Sandwell, 1995], and shipboard bathymetry across central Pacific fracture zones were used to identify features common to fracture zone segments that formed during times of changes in plate motions. These features are not predicted by current “locked fault” fracture zone models. During a change in spreading direction that induces tension across the transform fault, large-offset (greater than ∼500 km) transforms develop multiple parallel faults, spaced 50 to 100 km apart. The gravity signature of small-offset transform faults under tension includes a broader and more symmetric trough than observed on segments that formed during periods of steady spreading. Parts of fracture zones that form subsequent to a spreading reorientation that causes compression across the transform fault generally exhibit a single fault scarp that fits the locked fault model. Seafloor formed during a period of change usually marks a transition between structural styles, for example, between multiple fracture zone strands and a narrower single-fault fracture zone. Widening of the transform fault zone under tension and narrowing under compression are consistent with the assumption that during a change in spreading direction the new spreading ridges propagate to, but not across, the old transform fault

Evolution and Strength of Pacific Fracture Zones

Previous studies have shown that Pacific fracture zones are strong in some locations, sustaining the stresses associated with differential subsidence across a locked fault, while at other locations they show signs of weakness, acting as preferential conduits for volcanism or supporting anomalously low shear stresses. We find that half of all Geosat crossings of central Pacific fracture zones are inconsistent with a flexurally maintained scarp. We test two hypotheses for the origin of such anomalous crossings: (1) anomalous structures formed at the transform fault during times of changes in plate motions, and preserved at “strong” fracture zones; and (2) anomalous structures representing a posttransform response of the fracture zone to subsequent tectonic activity. We find that three-quarters of the anomalous crossings occur over the parts of fracture zones that formed during or immediately subsequent to times of changes in spreading directions. With the exception of several locations overprinted by hot spot volcanism, these same crossings show no obvious correlation with regional tectonic processes. These observations suggest that most anomalous fracture zone topography is inherited from the transform fault and is not a product of subsequent activity, consistent with the hypothesis that fracture zones generally remain strong throughout their lifetimes

A community experiment to record the full seismic wavefield in Oklahoma

Observing the full seismic wavefield by deploying large numbers of seismometers (also known as large-N deployments) and analyzing the resultant large datasets is now more feasible than ever before as a result of advances in instrumentation, computational power, and data analysis techniques. In 2015, the Incorporated Research Institutions for Seismology (IRIS) proposed a field deployment to provide the research community with experience in new techniques for obtaining full wavefield observations using a range of instrumentation (threecomponent [3C] nodal-style sensors, broadbands, and infrasound) at multiple spatial and temporal scales. The goals of the experiment were to demonstrate the field use of the nodal sensors, contribute a compelling dataset that could be analyzed through innovative techniques, and evaluate the performance of new array designs and instruments (particularly the 3C nodes). The resulting IRIS Wavefields Demonstration Community Experiment, conducted in north-central Oklahoma during the summer of 2016, collected data that were immediately made open and available at the IRIS Data Management Center (network code YW) and provided a unique and scientifically rich dataset to advance our understanding of the full seismic wavefield. A key finding was that by burying the 3C nodal sensors used in the deployment, substantially lower horizontal noise levels were achieved across a wide range of periods spanning 0.01-100 s