Abstract: The Teton normal fault runs along the eastern base of the prominent Teton range for ~70 km and defines the northeastern margin of the Basin and Range extensional province (Wyoming, USA). The fault has a latest Pleistocene vertical slip rate of ~1–2 mm/yr as indicated by faulted Pinedale glacial surfaces. However, a paleoseismic record of two surface-faulting earthquakes on the fault at ~8 and ~7–5 ka (Granite Canyon site) suggests a Holocene slip rate of only ≤0.5 mm/yr. Uncertainty remains in which slip-rate values are most appropriate for hazard modeling. Additional questions stem from the fault’s sparse paleoseismic data, including the completeness of the paleoseismic record and the rupture extent and recurrence of past large earthquakes. To address these questions, we excavated a trench across a ~9-m-high scarp at the Buffalo Bowl site on the southernmost third of the fault, ~4 km southwest of Granite Canyon. The trench exposed ~20º-east-dipping, coarse-grained alluvial-fan deposits vertically offset a total of ~5 m. The Teton fault is expressed as a 50º–70º-east-dipping plane that juxtaposes weathered footwall bedrock and overlying fan gravel with scarp-derived colluvium. A 6-m-wide graben has trapped colluvial sediments near the base of the scarp. We interpret three surface-faulting earthquakes at the site using colluvial-wedge texture and fabric, cross-cutting fault relations, and evidence of weak pedogenic horizons within the wedges. Likely correlative footwall and hanging-wall fan sediments are possibly latest Pleistocene to early Holocene in age and preserve displacement from a previously unrecorded Teton-fault earthquake. Pending radiocarbon ages for charcoal and luminescence ages for fine sand and silt will help resolve the timing of these earthquakes. Comparison of our paleoseismic results with the Granite Canyon record and other ongoing investigations will help resolve the fault’s late Quaternary earthquake history and slip rate.
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Abstract: In collaboration with the VA, the U.S. Geological Survey has developed structural health monitoring (SHM) software that utilizes vibration inputs to continually analyze and archive the response characteristics of a building in near real-time. The SHM software is built on the Earthworm (EW) system (Johnson et al., 1995), which is an open data processing platform that allows any continuous waveform data to be collected into ring buffers from a digitizer for further analyses (http://www.isti2.com/ew). The SHM software initially determines baselines for a suite of structural response parameters, and then continuously examines the response for changes in these parameters. The structural parameters monitored currently are inter-story drift ratios, shear-wave travel times throughout the building, and base-shear capacity-demand ratio. The SHM software is integrated with a web-enabled SHM data management framework to support aggregation, storage, and reporting of SHM data obtained and analyzed from instrumented hospital buildings to record strong shaking from earthquakes. By analyzing and characterizing the threshold values for building-specific engineering demand parameters, the SHM software can determine inspection priority to be low, moderate, high or very high and thus assist efforts in evaluating the safety and integrity of buildings in the aftermath of an earthquake. The SHM software is scalable—to support an arbitrary number of sensors, and it is extensible—to accommodate new data streams without the need to rewrite storage and display logic. The SHM software works on site or remotely. The software was validated using both ambient and low- and high-intensity shaking data inputs to a full-scale seven-story reinforced concrete building section tested on the UC San Diego shake table.
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Abstract: On September 13, 2017 the USGS NEIC reported a duration magnitude MD 3.2 earthquake at 37.473N 80.703W, depth 18 km near Lindside, West Virginia, close to the Virginia-West Virginia border. The earthquake was felt primarily in Monroe, Mercer and Summers counties, West Virginia and in Giles, Montgomery, Pulaski and Bland counties, Virginia. The maximum intensity reported to the USGS Did You Feel It? program was IV MM. The earthquake occurred in an area of moderate seismicity known as the Giles County Seismic Zone (GCSZ). The largest shock in the GCSZ occurred in 1897 near Pearisburg, VA, with mblg magnitude estimated from the felt area at 5.8. We relocated the hypocenter of the September, 2017 earthquake using a locally specific velocity model, at 37.4775N, 80.7035W, depth 21 km. We estimated the mblg magnitude at 3.90 +/- 0.26 using 26 stations at regional distances, and determined a duration magnitude MD of 3.71 +/- 0.17, using 33 stations. The duration magnitude is based on a correlation between the log of short-period signal duration and mblg. We determined a focal mechanism using 27 P polarities, 12 SH polarities and 16 SH/P amplitude ratios. The nodal planes with least rms amplitude ratio error are: strike N91E, dip 69 deg., rake -22 deg.; auxiliary plane strike N189E, dip 69 deg., rake -158 deg. This event is notable because it is the largest shock in the GCSZ since May, 1974 (mblg 3.7). This recent shock, like many others in the GCSZ, shares characteristics with those in the Eastern Tennessee Seismic Zone (ETSZ), which is also in the Appalachian Valley and Ridge province. The 2017 GCSZ focal mechanism is mostly strike-slip with a small normal component, on steeply dipping nodal planes trending approximately N-S and E-W. This type of mechanism is dominant in the ETSZ. Also, in both areas, focal depths tend to be greater than 12 km, unlike shocks to the east in the Blue Ridge, Piedmont and Coastal Plain provinces which tend to occur at shallower depths.
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Abstract: Corinth Gulf is one the most seismically active rifts worldwide, with several low in magnitude earthquakes as well as a few stronger ones (M>6), especially in its western part. This study focuses on the spatiotemporal properties of the seismic activity occurred between 2008-2014, when the national seismological network was denser. In this respect, a highly accurate earthquake catalog consisting of ~22,000 events was compiled using double difference technique and differential times from both phase picked data and cross correlation measurements. The locations showed the existence of a very shallow north dipping structure in the western part of Corinth Gulf, which is void of spatiotemporal clusters, whereas seismic excitations are placed above that zone in shallower depths. The waveform database was searched for repeating events (i.e., events with identical waveforms) and the repeaters were classified into multiplets of repeating sequences. These repeaters revealed two patterns of activity namely continuous type and burst-like repeaters. Continuous type repeaters last the entire study period, have low slip rates and are located on the shallow north dipping zone, which was found void of spatiotemporal clusters. On the other hand, burst-like repeaters are located above the shallow north dipping zone, in areas where seismic excitations occur and evidence was found that their occurrence can be related to fluid intrusion. The major finding of this study is that the spatial distribution of the relocated seismicity revealed two patterns of activity in the western subarea, namely, strongly clustered seismicity in both space and time in depths shallower than 10 km and below that activity a very narrow shallow north dipping zone which consists of continuous type repeaters. Based on the properties of the continuous type repeaters, the aseismic slip along the shallow dipping zone, was calculated.
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Abstract: Aftershock forecast modeling is an important tool for investigating the locations and magnitudes of historical earthquakes as well as for short-term earthquake forecasting. Although the 16 July 1936 M6 Milton-Freewater earthquake is the largest historical earthquake in eastern Oregon, having been widely felt in eastern Washington, northeastern Oregon, and northern Idaho, its location is uncertain. Various studies have reported its epicenter as lying within 30 km of the intersection of the Hite and Wallula faults. In the absence of reported coseismic surface rupture, we estimate the mainshock location and magnitude from aftershock forecast modeling that considers the numbers of reported aftershocks at six different locations in the day following the quake. The estimated epicenter and magnitude derived from the aftershock modeling compare favorably to the ISC-GEM solution, an S-P time recorded at Spokane, the directions of observed horizontal motions, and the distribution of ground failure and liquefaction. The aftershock modeling constrains the epicenter of the 1936 earthquake with an accuracy of about 5 km and is most consistent with an epicenter midway between Umapine and Milton-Freewater, above an estimated 10 km long subsurface rupture between them. This suggested epicenter and rupture plane are consistent with the elongation of ground failure along the Wallula fault and the fault’s strike along one of the focal planes indicating that the earthquake may have primarily ruptured the eastern end of NW-trending Wallula fault. Aftershock forecast modeling indicates a mainshock magnitude between 6.2 to 6.4, in agreement with reported instrumental magnitudes but higher than the range derived from seismic intensity data (5.1 to 5.5). This discrepancy may result from a systematic underestimation of the seismic intensities in the epicentral region: felt reports justify a maximum Modified Mercalli intensity of VIII instead of the VII previously assigned.
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Abstract: Local geologic structure and topography may modify arriving seismic waves. The consequent variation in shaking, or ‘site-response’, may affect the distribution of slope-failures and redistribution of submarine sediments. I used seafloor seismic data from the 2011-2015 Cascadia Initiative and permanent onshore seismic networks to derive estimates of site-response, denoted Sn, in low- and high-frequency (0.02-1 and 1-10 Hz) passbands. Three shaking metrics (peak velocity, peak acceleration, and energy density) Sn vary similarly throughout the study region (onshore and offshore) and change primarily in the convergence direction, roughly east-west. In the two passbands, Sn patterns offshore are nearly opposite one another and range over an order of magnitude or more across Cascadia. Sn patterns may be attributed broadly to sediment resonance and attenuation. These findings, and an abrupt step in the east-west trend of Sn suggest that changes in topography and structure at the edge of the continental shelf significantly impact shaking. The variations in Sn also correlate with the edges of gravity lows diagnostic of marginal basins and with methane plumes channeled within shelf-bounding faults. The offshore Sn exceeds the onshore Sn in both passbands. The relatively greatest and smallest Sn estimates at low- and high-frequencies, respectively, coincide with the steepest slopes and the shelf. These results should be considered in submarine shaking-triggered slope-stability failure studies. Significant north-south Sn variations are not apparent from the sparse sampling, but do not permit rejection of the hypothesis that the southerly decrease in intervals between shaking-triggered turbidites and inferred great earthquakes inferred by Goldfinger et al. [2012; 2013; 2016] and Priest et al. [2017] may be due to inherently stronger shaking southward.
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