Anatomy of Old Faithful from Subsurface Seismic Imaging of Yellowstone National Park, Upper Geyser Basin

Abstract:

The Upper Geyser Basin (UGB) in Yellowstone contains one of the highest concentrations of hydrothermal features on Earth including the iconic Old Faithful geyser. Although this area has been the focus of many geological, geochemical, and geophysical studies, the shallow (<200 m) subsurface structure remains poorly characterized due to limited instrument implementations in this delicate and sensitive environment. The recent availability of seismic dense arrays (large-N) permits an environmental-friendly approach to investigate the detailed crustal structure from the low-cost and easy-deployed geophones. To probe the detailed structure in relation to the hydrothermal plumbing of the UGB, we deployed large-N arrays of 3C 5-Hz geophones in both November of 2015 and 2016, composed of 133 stations with ~50 m spacing, and 519 station locations, with an ~20 m spacing, respectively. We constructed cross-correlation functions (CCFs) and extracted Rayleigh-wave signals between 1-10 Hz via seismic signals excited by nearby hydrothermal features. We observe a clear lateral velocity boundary at 3.3 Hz frequency that delineates a higher phase velocity of ~1.6 km/s in the NE and a lower phase velocity of ~1.0 km/s in the SW corresponding to the local geologic formation of rhyolitic and glacial deposits, respectively. We also image a relatively shallow (10-60 m deep) large reservoir with an estimated porosity 30% located ~100 m southwest of Old Faithful from the significant spatial-dependent waveform distortions and delays between 5-10 Hz. This reservoir is likely controlled by the local geology with a rhyolitic deposit in the NE acting as a relatively impermeable barrier to vertical fluid ascent. In addition to the static structure, we observe temporal variations in both phase and amplitude from the minutely CCFs with regard to the potential influences from instrument resonance, seismic source, and structure. The preliminary results of variations will be demonstrated and discussed.

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Cascadia Onshore-Offshore Site Response, Submarine Sediment Mobilization, and Earthquake Recurrence

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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Run-up Estimation Using Non-Uniform Stochastic Sources: The South American Subduction Zone

Abstract:

Throughout history, megathrust earthquakes have produced large tsunamis that have devastated coastal cities in the near and far field. South America hosts one of the largest subduction zones in the world and it is important to study tsunamigenic earthquakes here to forecast and mitigate future catastrophes. We estimate the maximum magnitude of possible earthquakes along the South American subduction interface using scaling laws, subducting seafloor features, seismic-geodetic coupling and seismic history. We use the Slab2 subduction zone geometry model from the United States Geological Survey (USGS) to constrain the geometry of the interface. Then, we estimate tsunami run-up using numerical modeling for 100 non-uniform stochastic k² sources in each targeted area. Our results show great variability in run-up distribution along the Nazca-South America subduction zone. The most vulnerable areas are: Valparaíso in Chile, with a most likely scenario of 20 m run-up and a maximum of 33 m, and Lima in Perú, with a most likely scenario exceeding 25 meters of run-up and a maximum of 40 meters. Similar results are obtained in Huasco, Chile, and Iquique, Chile, and other areas along the Pacific Coast of South America. We conclude that tsunami hazard remains high along South America, even in areas where megathrust earthquakes have recently occurred.

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Station, Data, and Instrument Analysis of the Cascades Volcano Observatory’s Seismic Network Using Xmax and Other Tools

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.

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Development of Integrated Accelerographs Using Mems Technology with Efficient Real-Time Data Transmission and Deployment of a Collaborative Seismic Network.

Abstract:

Having a dense network of high-resolution accelerometers based in “Feedback” systems in a reasonable amount of time, is crucial for national seismic networks such as the Spanish seismic network IGN. However, this requirement is in conflict with the tight budgets that would made this virtually unreachable. IGN has designed, developed and manufactured integrated and comprehensive accelerographs that allow near real-time transmission of seismic data, based on what is known as MEMS technology. This type of instrumentation enables compliance with the requirement to produce accurate ShakeMaps based on large amounts of observed data, and not deducted (deducted of very simple analytical attenuation laws, almost in its entirety). It would also be very important for the development of Technical Building Regulations in areas of seismic risk. All of this with prices at least ten times cheaper than high-resolution accelerographs. With the aim to demonstrate its reliability, it has been realize a testing process of these MEMS accelerometers in a vibrant table at CEDEX and its comparison with a high resolution commercial accelerometer Guralp CMG-5T, and even the recorded accelerograms in 2013 seismic crisis at Torreperogil village, in the province of Jaén. Currently, the IGN is deploying several devices on focused areas such as Alhama Fault at Murcia Region (southeast of the Iberian Peninsula) and Aran Valley in the Catalonian Pyrenees. This new network is a densification of the existing accelerograph network based on standard commercial accelerometers and through volunteer citizens finds the installation places.

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The mblg 3.9 September 13, 2017, Earthquake on the Virginia-West Virginia Border: A Significant Shock in the Giles County Seismic Zone

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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