Abstract: Jiuzhaigou occurred Ms7.0 earthquakes which caused serious human and economic losses. We simulated the strong ground motion characteristic using the Empirical Green Function Method. We also estimated the ground motion intensity of Zhangzha town which lost earthquake record data. All the stations are within the scope of around 100 km and all the stations’ PGA are larger than 10gal. Because of lacking appropriate aftershock record data, we try to use the aftershocks of Wenchuan earthquake and Dingxi earthquake as green function to simulate Jiuzhaigou earthquake in the first time. The simulated results as a whole can indicate the characteristics of the ground motion intensity especially the high frequency component. PGA, Duration and Response Spectrum are also fitted well between the simulated values and observed values. This attempt also indicated that using other earthquake’ s aftershock to simulate this event is feasible when lacking aftershock records. The PGA of Zhangzha town is estimated about 180gal-300gal. We also discussed the criterion about how to choose aftershock as green function.
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Abstract: The Merida Andes (MA) is an orogeny delimiting the Maracaibo block to the west, in response to the subduction of the Caribbean Plate beneath South American continent. 2-D seismic and gravity modeling of the crustal structure of MA was carried out along the profile Central Andes with a length of 380 km from the coast of Falcon, crossing perpendicular to the mountain range of the MA until its southeastern end in the Barinas-Apure basin. In the seismic data obtained from 11 shot points with 0.2 to 1 tons of explosive charges and recorded by 480 Texan recorders, we observed critical PmP reflections at distances of about 60 km for the northern part of the profile (Falcón Basin), and 120 to 90 km for the central and southern part of the profile, corresponding to the axis of the orogen and the Barinas Basin, respectively. Derived values for the depth of the Moho discontinuity range from 29 km depth for the Falcon basin, to 40-53 km close to the core of the chain, and 35 km at the southern end of the profile in Barinas basin. The crustal root is defined with a maximum depth of 53 km, which is displaced with respect to the highest part of the chain at this segment, approximately 10 km towards the northwest. 325 gravity stations were acquired along the profile and modeled together with satellite gravity data in a high resolution 2D gravity forward model, which confirms the strong variations of the thickness of the crustal root and its asymmetry. Cenozoic sediments with a P-wave velocity (Vp) ranging from 2 to 4 km/s are underlain by Cretaceous sediments, (Vp = 4 to 5 km/s) with a maximum depth of 10 and 6 km in the Maracaibo and Barinas-Apure basins, respectively. The crystalline basement shows Vp between 5.5 and 6.3 km/s and a density of 2.78 g/cm3 down to about 15 km beneath the basins and 25 km beneath the orogen. The lower crust is modeled with Vp of 6.5 to 7 km/s and a density of 2.84 g/cm3, underlain by the upper lithospheric mantle, with Vp greater than 7.7 km/sand a density of 3.22 g/cm3. The density model suggests the existence of an incipient A-subduction of continental South America towards the Maracaibo block.
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Abstract: We develop a method for quasi-automated estimation of directivity, rupture area, duration, and centroid velocity of earthquakes with second seismic moments. The method is applied to small to moderate earthquakes in southern California. P and S phase picks are given by a 1-D ray tracing algorithm and cataloged event locations. These are refined for deconvolution by using a grid search on zero-crossings within a short time window around the automated P/S picks. Source Time Functions (STFs) of target events are derived using deconvolution with a stacked empirical Green’s function (seGf). The use of seGf suppresses non-generic source effects such as directivity in individual eGf’s. The seGf for each target event is based on stacking individual eGfs (normalized by seismic potencies) selected by spatial and magnitude criteria as well as performances in the projected Landweber deconvolution. A weighted stack of eGfs, with weight coefficients grid searched and determined by waveform fits, helps further to correct inaccuracies of focal mechanisms. Compared with a single eGf, analysis with a weighted stack can significantly improve waveform fit and typically allows getting STFs at 5-10 more stations. The method is suitable for analysis of large seismic datasets and it works for target events in southern California with magnitudes as small as 3.5. Most events analyzed so far have significant directivities.
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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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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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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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