RESUMEN
The state of Oklahoma has experienced an unprecedented increase in earthquake activity since 2009, likely driven by large-scale wastewater injection operations. Statewide injection rates peaked in early 2015 and steadily decreased thereafter, approximately coinciding with collapsing oil prices and regulatory action. If seismic activity is primarily driven by fluid injection, a noticeable seismogenic response to the decrease in injection rates is expected. Langenbruch and Zoback suggest that "the probability of potentially damaging larger events, should significantly decrease by the end of 2016 and approach historic levels within a few years." We agree that the rate of small earthquakes has decreased toward the second half of 2016. However, their specific predictions about seismic hazard require reexamination. We test the influence of the model parameters of Langenbruch and Zoback based on fits to observed seismicity distributions. The results suggest that a range of realistic aftershock decay rates and b values can lead to an increase in moderate earthquake probabilities from 37 to 80% in 2017 without any further alteration to the model. In addition, the observation that all four M ≥ 5 earthquakes to date occurred when injection rates were below the triggering threshold of Langenbruch and Zoback challenges the applicability of the model for the most societally significant events.
RESUMEN
The Pawnee M5.8 earthquake is the largest event in Oklahoma instrument recorded history. It occurred near the edge of active seismic zones, similar to other M5+ earthquakes since 2011. It ruptured a previously unmapped fault and triggered aftershocks along a complex conjugate fault system. With a high-resolution earthquake catalog, we observe propagating foreshocks leading to the mainshock within 0.5 km distance, suggesting existence of precursory aseismic slip. At approximately 100 days before the mainshock, two M ≥ 3.5 earthquakes occurred along a mapped fault that is conjugate to the mainshock fault. At about 40 days before, two earthquakes clusters started, with one M3 earthquake occurred two days before the mainshock. The three M ≥ 3 foreshocks all produced positive Coulomb stress at the mainshock hypocenter. These foreshock activities within the conjugate fault system are near-instantaneously responding to variations in injection rates at 95% confidence. The short time delay between injection and seismicity differs from both the hypothetical expected time scale of diffusion process and the long time delay observed in this region prior to 2016, suggesting a possible role of elastic stress transfer and critical stress state of the fault. Our results suggest that the Pawnee earthquake is a result of interplay among injection, tectonic faults, and foreshocks.
RESUMEN
Calving of glacial ice into the ocean from the Greenland Ice Sheet is an important component of global sea level rise. The calving process itself is relatively poorly observed, understood, and modeled; as such, it represents a bottleneck in improving future global sea level estimates in climate models. We organized a pilot project to observe the calving process at Helheim Glacier in East Greenland in an effort to better understand it. During an intensive one-week survey, we deployed a suite of instrumentation including a terrestrial radar interferometer, GPS receivers, seismometers, tsunameters, and an automated weather station. This effort captured a calving process and measured various glaciological, oceanographic, and atmospheric parameters before, during, and after the event. One outcome of our observations is evidence that the calving process actually consists of a number of discrete events, spread out over time, in this instance over at least two days. This time span has implications for models of the process. Realistic projections of future global sea level will depend on accurate parametrization of calving, which will require more sustained observations.
RESUMEN
Subglacial discharge influences glacier basal motion and erodes and redeposits sediment. At tidewater glacier termini, discharge drives submarine terminus melting, affects fjord circulation, and is a central component of proglacial marine ecosystems. However, our present inability to track subglacial discharge and its variability significantly hinders our understanding of these processes. Here we report observations of hourly to seasonal variations in 1.5-10 Hz seismic tremor that strongly correlate with subglacial discharge but not with basal motion, weather, or discrete icequakes. Our data demonstrate that vigorous discharge occurs from tidewater glaciers during summer, in spite of fast basal motion that could limit the formation of subglacial conduits, and then abates during winter. Furthermore, tremor observations and a melt model demonstrate that drainage efficiency of tidewater glaciers evolves seasonally. Glaciohydraulic tremor provides a means by which to quantify subglacial discharge variations and offers a promising window into otherwise obscured glacierized environments.
