Abstract: The increase of station coverage since 2014 due to the presence of the EarthScope USArray (TA) in Alaska provides an opportunity to investigate crustal attenuation in Alaska using Lg-waves.Lg waveforms provide a good measure of apparent attenuation because they propagate as multiple reflected shear waves trapped within the crust. Lg is often the strongest phase on the seismogram at regional distances from 2° to 25°. Although there have been other studies to determine attenuation in Alaska, they focused on small areas where data were available at the time of the study. The goal of our study is to investigate crustal attenuation throughout Alaska and define the locations and characteristics of Q transitions or boundaries, if there are any. Our approach is similar to that of Benz et al. (1997) and McNamara et al. (1996). This approach allows quantifying the attenuation of the crust using Lg Fourier spectral amplitude in narrow frequency bands of earthquakes that occurred within a given region and the inversion includes source and station terms. Initial results using the ML 5.3 Tok, Alaska, earthquake of 2017/02/13 that occurred in eastern south-central Alaska show Q at 1 Hz (Qo) to generally be around 100-150 in south central, eastern, and southeastern Alaska, with the power on frequency (eta) ranging from 0.75 to 1.1. These results are somewhat lower than the more restricted results of McNamara (2000) in south central Alaska of Qo=220 and eta = 0.66. There may be an azimuthal dependence in Qo in southeastern central Alaska with somewhat higher values parallel to the structural grain compared to perpendicular to the structural grain. For ray paths crossing the western cordillera into Canada, the Qo appears to be about 50 and eta about 1.1. Differences in Qo parallel and perpendicular to the structural grain have been observed elsewhere.
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Abstract: The objective of the Fault to Seismic Hazard Assessment (Fault2SHA) Working Group is to build a community of active fault-related researchers to exchange data, tools and ideas on how to best model faults in seismic hazard assessment in specific tectonic contexts. After a few meetings (Paris 2014, Chieti 2015) and thematic sessions at international conferences in 2016 (https://sites.google.com/site/linkingfaultpsha/home) the WG was officially established inside the European Seismological Commission (ESC) in 2016. Being a not-funded entity the WG acts on voluntary basis. The community involved is made of data providers, data modellers and data users willing to share data and methodological approaches. The WG milestones achieved since 2016 are: a paper on aftershock probabilistic seismic hazard based on fault data gathered by many European teams in the wake of the Amatrice, 2016 M6.0 earthquake (Peruzza et al., 2016); the organisation of an international workshop in Barcelonette in 2017, France, that gathered 50 participants from around the world; the publication of 10 papers in a special issue of the NHESS journal; the organisation of a training course in Paris in 2017, where geologists learned how to use some Fault2SHA tools. The WG has initiated other collaborative initiatives such as the establishment of natural laboratories in Italy and Spain. Preliminary results will be presented at SSA. In these laboratories we want to address specific issues and questions such as: Methods to define sections/ruptures; Physics-based approaches; Needs for the collection of data (volcanic area?) to update scaling laws; How to constrain slip on faults using geodesy? How to propagate uncertainty in fault-PSHA? The Fault2SHA session to be held during the 2018 SSA conference in Miami is an additional opportunity to widen the discussion beyond the European context and to open to new potential members the opportunity to join us at the next ESC meeting that will be held in 2018 in Malta.
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Abstract: Low-yield explosion monitoring introduces new challenges to the current approach to estimating source location, magnitude, and discrimination. Small sources require nearby short period observations, which have an increased sensitivity to geologic heterogeneity, poor signal-to-noise ratios, and in many cases are sparse. Local and regional short periods observations from small shallow seismic sources can be dominated by a regional phase shear waves and Rayleigh waves. Using teleseismic surface wave observations, Cleveland & Ammon [2013] and Cleveland et al. [2015] show the value of using surface waves to estimate precise, relative locations in regions without a nearby seismic network. Using common-station, nearby-event cross correlation time-shift measurements, much of the complexity in wave propagation caused by regional geological heterogeneity is removed (or at least, greatly reduced). In this work, we extend surface wave relocation methods to estimate precise relative locations of small (local magnitudes from 1 to 3) mine blast events across Pennsylvania using local and near-regional distance observations (out to 300 km in distance). We also exploit the cross correlation amplitude to estimate more precise relative magnitudes (actually log-moments) and develop a more consistent relationship between explosion yield and relative magnitude for various mines throughout the Commonwealth. Our locations are precise enough to allow us to image a time-dependent migration of a mine wall in north-central Pennsylvania. In west-central Pennsylvania, application of the relative location approach collapses a diffuse distribution of small-magnitude industrial events into five discreet clusters associated with particular operations in the area. The work demonstrates that cross correlation methods have the potential for achieving high precision relative location and magnitude estimates from local and regional observations of low yield seismic sources.
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Abstract: Four co-located 3-component (3-C) Eentec R-1 rotational velocity sensors and Episensor FBA ES-T translational accelerometers were deployed at the Nevada National Security Site to record three Source Physics Experiment (SPE) chemical explosions with yields of 90kg (SPE1), 997kg (SPE2), and 905kg (SPE3) equivalent TNT. The 4 co-located sensors were deployed 1km from ground zero within a granite outcrop. Three earthquakes were also recorded by this seismic array, a Ml 3.3 at 28km, a Ml 2.6 at 58km, and Ml 3.5 at 123km distance from SPE. Igel et al. (2005) demonstrated using long period teleseismic surface waves that the vertical rotational velocity (Ωz) is in phase and scales in amplitude with the transverse (SH) translational acceleration (üT) by the horizontal phase velocity c (üT / Ωz = -2c). We expect this also holds true for higher frequency body-waves at local distances and the radial and transverse rotational velocities should scale with the vertical and radial accelerations (P-SV) by the phase velocity, e.g., üZ / ΩR ~ c (Li and Baan, 2017). Using all 3-C of the rotational and translational motions, we measured the horizontal phase velocity of 450 m/s and 1125 m/s for 2 separate directions. In contrast, the horizontal velocity measured for the Ml 3.3 earthquake is 6 km/s in the 0.1 to 10 Hz band. While the earthquakes showed high coherency between 3-C rotational and translational motions, the explosions exhibited more coherency with P-SV wave but less coherency for SH-wave radiation. This may be due to explosion SH-waves originating from scattering rather than the source. This difference could be exploited as a discriminant between explosions and earthquakes. We explore such a discriminant using array-derived rotational motions from historical nuclear explosions recorded at regional distance. The ratio of peak-acceleration to peak-rotational-rate shows promise as a discriminant statistic. Prepared by LLNL under Contract DE-AC52-07NA27344.
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Abstract: Ecuador is a zone of high volcanic seismicity, in such sense it is important to recognize the behavior of volcanoes before they enter into an eruption process. This requires the analysis and to identification of different types of seismic events from a volcano. Cotopaxi volcano, is one of the most active and it has high risk due to the close proximity of populated areas in its surroundings and therefore it is one of the most supervised volcanoes in Ecuador. In this paper, a review of the most used spectral techniques for the analysis and extraction of discriminant features of microseisms is presented, since microseisms are some of the most important sources of information for analyzing the behavior of different volcanoes. Hence, our aim is to extract spectral features, which may help to classify events correctly, this information may also allow authorities to give early alerts in the case of increasing volcanic activity in order to alert and safeguard human lives. The analysis is performed by using parametric and non-parametric spectral techniques, enabling a more detailed study of the spectral content and the confidence intervals of specific events by using bootstrap technique, the empirical bootstrap procedure was specifically used, this consists in resampling the data from the empirical distribution. A database from the Cotopaxi volcano corresponding to a single station, with a broadband seismometer, containing several seismic signals representing different types of volcanic events such as volcano-tectonic (VT), long period (LP), hybrid (HYB) and tremors (TRE) registered in 2012, was used for the analysis. The results obtained show that LP events have a thin spectrum and their frequency range goes from 0.5 Hz to 6 Hz, with a major spectral component around 3.2 Hz, VT events have a wide spectrum and their frequency range goes up to 20 Hz with a major spectral component around 6.8 Hz, and TRE events have a spectrum that goes below 3 Hz.
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Abstract: In October 2014, a 2-week long slow slip event (SSE) occurred near Gisborne at the northern Hikurangi Margin, New Zealand. It was recorded by offshore instruments, deployed by the Hikurangi Ocean Bottom Investigation of Tremor and Slow Slip (HOBITSS) project. This study uses data compiled from May 2014 to July 2015, recorded on 15 HOBITSS ocean bottom seismometers as well as 12 Geonet stations around the time of this uniquely recorded SSE. We use the S-wave splitting technique to detect stress and fluid changes associated with slow slip. The shear wave splitting fast polarization direction is often inferred to represent the maximum horizontal stress directions (SHmax) in crustal studies and has been shown to vary temporally at volcanoes and in association with large earthquakes. Using Multiple Filter Automatic Splitting Technique (MFAST) we analyse more than 3000 local earthquakes to look for temporal changes in S-wave fast polarization directions and delay times during the Gisborne 2014 SSE. Because S-wave splitting results are sensitive to variations in earthquake locations, we also analyse results from individual spatial earthquake clusters to test the robustness of temporal changes and to better indicate where the measured anisotropy originates. The mean fast directions in the area show a NE-SW trending fast polarization direction similar to local SHmax directions mapped by previous studies and to local fault trends. Preliminary SWS results at LOBS stations show approximately 5-10 degrees of change in fast polarization direction from before to after the SSE. We also observe increased delay times (~ 0.5 seconds) occurring around the time of the SSE with a slow decrease in delay time in the following months. We hypothesize an initial movement of fluids in cracks due to changes in stress during the SSE, followed by a relaxation period where cracks slowly return to their previous state.
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