ALBERT

Description: 〈span〉〈div〉Abstract〈/div〉We adapt the relative polarity method from 〈a href="https://pubs.geoscienceworld.org/bssa#rf29"〉Shelly 〈span〉et al.〈/span〉 (2016)〈/a〉 to compute focal mechanisms for microearthquakes associated with the 2014 hydroshearing stimulation at the Newberry volcano geothermal site. We focus the analysis on events relocated by 〈a href="https://pubs.geoscienceworld.org/bssa#rf2"〉Aguiar and Myers (2018)〈/a〉, who report that six event clusters predominantly comprise the 2014 sequence. Data quality allows focal mechanism analysis for four of the six event clusters. We use 〈a href="https://pubs.geoscienceworld.org/bssa#rf13"〉Hardebeck and Shearer (2002〈/a〉, 〈a href="https://pubs.geoscienceworld.org/bssa#rf14"〉2003〈/a〉; hereafter HASH) to compute focal mechanisms based on first‐motion polarities and 〈span〉S〈/span〉/〈span〉P〈/span〉 amplitude ratios. We manually determine 〈span〉P〈/span〉‐ and 〈span〉S〈/span〉‐wave polarities for a well‐recorded reference event in each cluster, then use waveform cross correlation to determine whether recordings of other events in the cluster are the same or reversed polarity at each network station. Most waveform polarities are consistent with the affiliated reference event, indicating similar focal mechanisms within each cluster. The deeper clusters are east–west‐striking normal faults, whereas the shallower clusters, close to the top of the open‐hole section of the borehole, are strike slip with east–west motion. Regional studies and prestimulation borehole breakouts find the maximum stress direction is vertical and maximum horizontal stress is approximately north–south. Fault geometry and focal mechanisms of microseismicity during the stimulation suggest that increased pressure from fluid injection predominantly caused changes in horizontal stress, consistent with predictions from numerical studies of stress change caused by fluid injection. At shallow depths, where previous studies suggest the difference between vertical and horizontal stress is lowest, injection appears to have rotated the direction of maximum stress from vertical to horizontal, resulting in strike‐slip motion. At greater depth, vertical stress continued to be the dominant direction during the stimulation, but fault orientation indicates either reactivation of pre‐existing fractures or rotation of the direction of maximum horizontal stress from approximately north–south to east–west.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉On 3 September 2017, the Democratic People’s Republic of Korea (DPRK) conducted its sixth and largest declared nuclear test at the Punggye‐ri test site. Recently, we have been using regional waveform envelopes to estimate the explosive yield and overburden of chemical and nuclear explosions by coupling explosion source models to propagation parameters. Similar to most yield determination methods, there can be trade‐offs between yield and depth, leading to uncertainties in both parameters. The relative locations are well constrained by small timing differences in seismic phase arrivals at stations that recorded multiple events, but there are potential uncertainties on the absolute locations. Depths are poorly constrained by the relative arrival times. In this study, we performed a coupled location and yield analysis of the DPRK nuclear tests. We obtain highly accurate travel times using correlation methods and relative locations using a Bayesian location method. Then, while keeping the relative locations of the six tests constant, we consider the consequences of shifts to the absolute locations on the resulting overburden for each event. Given that overburden, we determine the yield that minimizes the waveform misfit. We also test and compare a number of explosion source models. By considering the coupled location/depth/yield problem, we reduce uncertainties in the absolute locations, and determine yield and depth estimates of the events. Based on statistical analysis, we estimate that the 2017 test has a yield of 125 kt (equivalent trinitrotoluene [TNT]) with a 1 sigma uncertainty range of 103–150 kt at about 600 m of overburden.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉Absolute location of the announced Democratic People’s Republic of Korea (DPRK) nuclear test on 6 January 2016 is constrained by fitting associated, Interferometric Synthetic Aperture Radar (InSAR)‐based ground displacement with elastic finite‐element modeling of an underground explosion source, including the effects of topography. The other five announced nuclear tests are located using arrival times and differential arrival times of regional and teleseismic body waves and constraints on the 6 January 2016 event location. Nuclear tests on 6 January 2016, 9 September 2016, and 3 September 2017 are under the summit ridge of Mt. Mantap, and tests on 25 May 2009 and 12 February 2013 are south of the topographic crest under the steep southern face of the mountain. The first test on 9 October 2006 was located near the crest of a separate topographic ridge approximately 2.87 km east of the 6 January 2016 event. Several unannounced events have occurred in the vicinity of the test site. The location uncertainty ellipse of an event approximately 8 min 30 s after the 2017 announced test covers the DPRK test site and is likely to have occurred there. Additional events on 27 September 2017 and 12 October 2017 are 4–8 km northwest of the test site, and location probability regions do not overlap the test site.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉We adapt the PageRank method to define signal‐correlation topology for microearthquakes at the Newberry Volcano enhanced geothermal systems (EGS) demonstration in central Oregon. The Newberry EGS was stimulated in 2012 and 2014, producing hundreds of microearthquakes. Event locations based on 〈span〉P〈/span〉‐wave and 〈span〉S〈/span〉‐wave picks and a double‐difference method resulted in diffuse clouds of seismicity for both stimulations (〈a href="https://pubs.geoscienceworld.org/bssa#rf16"〉Foulger and Julian, 2013b〈/a〉; 〈a href="https://pubs.geoscienceworld.org/bssa#rf9"〉Cladouhos 〈span〉et al.〈/span〉, 2016〈/a〉). We expand on the seismological application of PageRank (〈a href="https://pubs.geoscienceworld.org/bssa#rf1"〉Aguiar and Beroza, 2014〈/a〉) to define event families based on signal‐correlation topology and analyze each event family by measuring differential arrival times of 〈span〉P〈/span〉 and 〈span〉S〈/span〉 waves. We relocate each family using the Bayesloc method (〈a href="https://pubs.geoscienceworld.org/bssa#rf25"〉Myers 〈span〉et al.〈/span〉, 2007〈/a〉, 〈a href="https://pubs.geoscienceworld.org/bssa#rf26"〉2009〈/a〉) and find that, after relocation, event families are tightly clustered spatially, indicating that the rock volume affected by stimulation was more than an order of magnitude smaller than would be inferred from double‐difference analyses. Relocation also reveals that most events occurred at the contacts between lithologic units, suggesting preferential fluid flow and hydroshearing along pre‐existing weaknesses, as well as microseismic activity progressing away from the well with time.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Kremasta seismic sequence in western Greece is one of the most commonly cited examples of reservoir‐induced seismicity (RIS). Here, we show that this sequence is a result of normal tectonic activity and that only some small, unrelated microseismic events are reservoir induced. Shortly after the beginning of the impoundment of the Kremasta Dam in 1965, the then newly established seismic monitoring network in Greece recorded two Ms≥6.0 events and numerous small shocks spread over a 120‐km‐wide region. These were interpreted as a single seismic sequence (namely the Kremasta seismic sequence) and assumed to be reservoir induced. We revisit the epicenter locations of these events and interpret them in the framework of the regional tectonic context and the local hydrogeology. Placing these events into the local context shows that they represent an amalgamation of separate, ordinary (tectonic) seismic sequences. Further, the regional rocks are highly fragmented by small faults and the spatial distribution of seismic events is not consistent with a model of stress transfer from reservoir loading. In addition, it is not likely that events at such long (〉20–30  km) distances from the reservoir could be induced by an initial reservoir load head of 30 m. Although the larger magnitude events are tectonic, after impoundment local residents reported an unusual frequency of small microseismic events felt only within 10 km of the dam. We provide evidence that these are a result of the collapse of numerous shallow karstic cavities adjacent and beneath the reservoir due to increased water load (locally 100–150 m depth). This study has significant implications for interpretation of seismic triggering mechanisms in other regions: earthquake occurrence within the proximity of reservoirs during and after impoundment time cannot be assumed to be RIS unless supported by seismological, geological, and hydrogeological evidence.〈/span〉

Description: 〈span〉〈div〉ABSTRACT〈/div〉Ocean‐bottom seismometers (OBSs) allow us to extend seismological research to the oceans to constrain offshore seismicity but also image the marine subsurface. A challenge is the high noise level on OBS records, which is created not only by bottom currents but also by the specific seismometer models used. We present a quantitative noise model for the LOBSTER OBS, which is the main instrument of DEPAS, currently the largest European OBS pool, stationed at Alfred‐Wegener‐Institut (AWI) Bremerhaven. Studying sensor noise in vault conditions and current sensitivity at an oceanographic measurement mast, we can show that the previously reported high noise level of the instrument is caused by the original sensor (Güralp CMG‐40T‐OBS). We also show that a strong signal that has been reported between 1 and 5 Hz can be attributed to head‐buoy cable strumming. We provide a current‐dependent quantitative noise model that can be used for experiment design in future deployments and show that the performance of the pool OBS can be improved at moderate cost by replacing the CMG‐40T‐OBS with a sensor of a proven noise floor below 10−8  nm/s2, for example, a Trillium compact.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉We adapt the relative polarity method from 〈a href="https://pubs.geoscienceworld.org/bssa#rf29"〉Shelly 〈span〉et al.〈/span〉 (2016)〈/a〉 to compute focal mechanisms for microearthquakes associated with the 2014 hydroshearing stimulation at the Newberry volcano geothermal site. We focus the analysis on events relocated by 〈a href="https://pubs.geoscienceworld.org/bssa#rf2"〉Aguiar and Myers (2018)〈/a〉, who report that six event clusters predominantly comprise the 2014 sequence. Data quality allows focal mechanism analysis for four of the six event clusters. We use 〈a href="https://pubs.geoscienceworld.org/bssa#rf13"〉Hardebeck and Shearer (2002〈/a〉, 〈a href="https://pubs.geoscienceworld.org/bssa#rf14"〉2003〈/a〉; hereafter HASH) to compute focal mechanisms based on first‐motion polarities and 〈span〉S〈/span〉/〈span〉P〈/span〉 amplitude ratios. We manually determine 〈span〉P〈/span〉‐ and 〈span〉S〈/span〉‐wave polarities for a well‐recorded reference event in each cluster, then use waveform cross correlation to determine whether recordings of other events in the cluster are the same or reversed polarity at each network station. Most waveform polarities are consistent with the affiliated reference event, indicating similar focal mechanisms within each cluster. The deeper clusters are east–west‐striking normal faults, whereas the shallower clusters, close to the top of the open‐hole section of the borehole, are strike slip with east–west motion. Regional studies and prestimulation borehole breakouts find the maximum stress direction is vertical and maximum horizontal stress is approximately north–south. Fault geometry and focal mechanisms of microseismicity during the stimulation suggest that increased pressure from fluid injection predominantly caused changes in horizontal stress, consistent with predictions from numerical studies of stress change caused by fluid injection. At shallow depths, where previous studies suggest the difference between vertical and horizontal stress is lowest, injection appears to have rotated the direction of maximum stress from vertical to horizontal, resulting in strike‐slip motion. At greater depth, vertical stress continued to be the dominant direction during the stimulation, but fault orientation indicates either reactivation of pre‐existing fractures or rotation of the direction of maximum horizontal stress from approximately north–south to east–west.〈/span〉