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.
Media has not been submitted for this Presentation
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.
Media has not been submitted for this Presentation
Abstract: With the introduction of performance-based design, the tools and techniques for seismic design of structures are becoming more advanced and sophisticated. The requirements to account for nonlinear behaviour in performance-based design, construction of more complex structures (e.g., super tall and slender buildings), more accurate structural modelling and analysis tools, more powerful computers, and large amount of data collected from instrumented structures are also contributing to this. Consequently, the analysis results are now more sensitive to the characteristics of input motions, and engineers are asking for a more precise description of ground shaking. Response spectra are becoming no longer sufficient for the performance-based seismic design. This paper outlines some of the ground motion parameters that are not accounted for in current design codes, but can make a big difference in the results of performance-based seismic analysis and design. They are related to long-period components of ground motions, duration of shaking, rotational effects due to surface waves, processing of earthquake records, sensitivity of nonlinear response to the build-up of shaking intensity, and the bi-directional modelling of shaking. Examples are presented to justify the need for additional ground motion information, and suggestions are made how to quantify and utilize this information in seismic design.
Media has not been submitted for this Presentation
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.
Media has not been submitted for this Presentation
Abstract: For the seismic community quality waveform data is the starting point to quality seismic locations and research. Therefore, it is imperative that seismic station metadata be correct and up to date, station functionality monitored, and instrument response files be as accurate as possible to ensure that the network data is reliable. Quality and reliability of waveform data is the basis for completing a Quality Control (QC) study of the Cascades Volcano Observatory (CVO) seismic network (network code CC). The CC seismic network consists of 30 seismic stations throughout the Washington and Oregon Cascades focused on real-time monitoring the volcanoes that are classified as high-threat. CVO works in conjunction with the Pacific Northwest Seismic Network (PNSN) to provide metadata and real-time waveform data from the CC network to the Incorporated Research Institutions for Seismology (IRIS). QC analysis of this extent has never been completed on the CC network. A network wide analysis was completed in order to test sensor and data quality using open source software XMAX (ASL, https://github.com/usgs/xmax) and Evalresp (IRIS, https://ds.iris.edu/ds/nodes/dmc/software/downloads/evalresp) to review both metadata and sensor functionality. The findings of this study show mostly minor metadata issues, a few problem sensors and a noisy vault. We are working with PNSN to rectify all metadata and sensor issues by mid 2018 and will publish a USGS Open File Report by the end of 2018. Moving forward these tools will be important for maintaining knowledge and awareness of station health and data quality and will comprise the routine quality check procedures for CVO. This work is the building block for the future of the waveform data quality and reliability of the CC network.
Media has not been submitted for this Presentation
Abstract: Aftershock forecasts often depend on the characteristics of the ongoing sequence up to the time of the forecast. The observed aftershocks may be used to fit sequence-specific parameters for clustering models and to seed simulations of future aftershocks. However, the catalog magnitude of completeness may be elevated immediately following a large mainshock, due to decreased detection of smaller events, complicating the use of the early aftershocks. Simple functions have been found to describe the time-dependent magnitude of completeness following global (Page et al., BSSA, 2016) and California (Helmstetter et al., BSSA, 2006) mainshocks. As a further complication, the real-time earthquake catalog typically has omissions and errors not present in the final catalog. This real-time catalog incompleteness must also be quantified to avoid underestimating the probability of future aftershocks in real-time forecasts. We study the real-time completeness of the USGS ComCat catalog using snapshots downloaded periodically following selected M≥6 global and M≥5 U.S. earthquakes. We find that real-time catalog snapshots during roughly the first month of an aftershock sequence have a higher magnitude of completeness than the final catalog, with a typical difference of 0.3-0.8 magnitude units. We also find that the time-dependent magnitude of completeness equations developed using the final catalogs (Page et al., BSSA, 2016; Helmstetter et al., BSSA, 2006) can describe the completeness of the real-time catalogs during the first month with adjusted parameter values. Accounting for the real-time catalog completeness brings the aftershock productivity estimated from the real-time catalogs closer to the productivity estimated from the final catalog, increasing the accuracy of aftershock forecasts based on the real-time catalogs.
Media has not been submitted for this Presentation
