Abstract: Over the past six years, the U.S. Ocean Bottom Seismograph Instrument Pool has undertaken a wide range of experiments. These experiments have addressed diverse scientific objectives through the deployment of instruments at different scales, geometries, water depths, and seasons. Overall, the experiments have been characterized by excellent instrument return rates, generally high data return, and an evolving set of instrument capabilities. These recent experiments provide insight and motivation for developments in areas such as instrument emplacement, deployment duration, communications (continuous or periodic), and standardized design elements – all key capabilities for future large-scale and/or long-term geophysical projects. Thus, we examine the characteristics, performance, and results of OBS experiments that have been done over the past six years as a key to understanding and motivating future technical directions for this important capability.
Poster:
OBSIP_Poster_Insights
Abstract: We present a low-cost seismic recording system intended to fulfill multiple seismic experiments and monitoring. The instrument was designed and crafted at the Institute of Engineering of the National Autonomous University of Mexico with the main objective of expanding the strong ground motion network and the pool of instruments for site and crustal characterization. The system consists of: 1) a 3-channel digital recorder with 24-bit resolution, 2) selectable sample rate of 100 or 200 samples per second, and 3) time control based on a real-time clock corrected by a GPS receiver. Moreover, the amplification stage input can select between various types of sensors, such as geophones, force balance accelerometers (FBA), MEMS type accelerometers, etc. The instruments will be used for large arrays in multiple experiments for recording earthquakes, aftershocks, seismic noise and measurement of vibrations in structures. Due, its low cost and the fact they are assembled in-house, modifications of the instruments can be made quickly to adjust the specific experiment requirements. Currently, three versions have been developed. The first version uses 2G force balance servo-accelerometers with the ability to continuously send data in real time through the Internet to a central registration point. The second instrument uses 1Hz geophones for noise measurements in large autonomous arrays; the data is stored in a microSD flash memory. The last model uses MEMS type low noise capacitive accelerometers mainly for strong earthquakes and measurements in structures.
Poster:
Congreso SSA2108 MTN Eng Ver3
Abstract: Several studies have generated low-frequency dynamic ground deformations (strains and rotations) in the near-fault region of actual or hypothetical earthquakes using physics-based simulation techniques that incorporate modeling of the seismic source and wave propagation processes. However, no systematic attempt has been made to compare observed and synthetic near-fault dynamic ground strains and rotations in order to demonstrate the effectiveness of the simulation techniques in predicting dynamic ground deformations in the vicinity of the fault. Such a comparison is presented in this study for the 2004 Mw 6.0 Parkfield, California, earthquake, which occurred on the San Andreas Fault and was recorded by a dense network of instruments. Following the work of Spudich and Fletcher (2008), estimates of near-fault dynamic ground strains and rotations are obtained by applying time-dependent geodetic analysis to displacement time histories recorded by the UPSAR and Turkey Flat dense arrays. A forward ground-motion simulation of the 2004 Parkfield earthquake is performed using the fault and crustal models proposed by Liu et al. (2006), and low-frequency translational motions, strains, and rotations are generated at selected stations. The computations are carried out using the discrete wavenumber representation method and the generalized transmission and reflection coefficient technique. The synthetic motions are generally in good agreement with the recorded translational motions, the recorded borehole volumetric strains, the array-derived strains and rotations, and the finite-difference approximated displacement gradients in terms of both amplitude and waveform characteristics. The results indicate that numerical simulations based on kinematic source models can be an effective tool for characterizing and predicting low-frequency dynamic ground deformations in the near-fault region.
Poster:
Cao_etal_SSA_2018 (1)
Abstract: The use of seismology as a means to study glacier dynamics has rapidly expanded in recent years as we seek to better understand the present and future behaviour of glaciers in a rapidly changing climate. While smaller in mass than their Antarctic or Greenlandic cousins, sub-arctic mountain glaciers may be particularly vulnerable to rapid changes in climate, making them a worthy target of study. During June 2017, we acquired a 1-km-long seismic reflection and refraction survey on Lemon Creek Glacier, a 5.7-km-long subarctic mountain glacier in the Juneau Icefield of Southeast Alaska. We used 51 5-Hz Z-Land Fairfield nodal seismometers recording at 1000 Hz and a Betsy Seismic Gun source with 400-grain, 8-gauge blanks. Fairfield nodal seismometers are small, highly-portable sensors that may rapidly be deployed in dense networks in challenging environments. The seismic survey was located in the upper portion of the glacier within the accumulation zone, oriented parallel to the flow direction, and approximately followed the glacier’s center line. In this survey, the receivers were initially spaced at 20-m, and shots were fired between them at 20-m spacing. We fired multiple shots near the ends of the line to allow stacking to enhance the signal. After we recorded the initial active-source survey, we re-deployed the nodes in a regional array across the upper part of the glacier at ~350-m spacing. In mid-July, we fired additional shots adjacent to ~25 of those nodes to provide further constraints on ice thickness. We apply seismic reflection and refraction methods that have been applied successfully in high-arctic environments to image the interior and subglacial properties of Lemon Creek Glacier. In this abstract, we present our findings from the active-source deployment described here. Future work may include data from an additional, contemporaneous nodal deployment which includes the nearer-terminus region of the glacier, nearby broadband stations, and a coincident GPS deployment. Our work serves to elucidate the firn and ice structure, the rheology of the glacier, as well as the basal properties of the glacier.
Slidecast:
https://vimeo.com/277186018
Abstract: Auto-correlation of teleseismic P-wave coda is a recently developed technique capable of imaging crustal-scale features based on reflection signals from seismic discontinuities. Traditional P-wave receiver function analysis maps velocity contrasts based on P-to-S conversions and has been well utilized in a great many locations, but encounters difficulties in seismically complex environments: for example, where high impedance contrasts result in large magnitude reverberations that overwhelm the primary phases, or where high velocity layers or sloping interfaces invalidate assumptions of near-normal incidence. Deconvolution and multiple constraint alogrithms have been developed to address these issues but often introduce additional complexities and computational overhead. Auto-correlation has the advantages of being fast (when computed via the Cross Correlation Theorem), is not reliant on tuning parameters (e.g., water levels), directly exploits reverbatory phases for locating reflection horizons, and can be combined with a transformation to PSH wavevector coordinates to fully partition P- and S-wave energy into separate channels for joint inversion matching. We present a benchmark comparison of receiver function and auto-correlation inversions utilizing the first year of data from a 39 station broadband array spanning the Mackenzie Mountains, Yukon, Canada. Initial surveys of this region indicate a variety of seismically complicated crustal-scale structures, including large-scale strike-slip faulting, sloping interfaces, high velocity intrusions, and low velocity cratonic sediments. We also present application of the autocorrelation methodology to floating and grounded ice stations in Antarctica, where receiver function methods encounter particular difficulties due to the icecap and ocean.
Poster:
MGB_SSA_2018.compressed
Abstract: Increasing vulnerability of metropolitan areas within stable continental regions (SCR), such as Memphis, TN and St. Louis, MO near the New Madrid Seismic Zone (NMSZ), to earthquakes and the very low probability level at which short term earthquake forecasting is possible make an earthquake early warning system (EEWS) a viable alternative for effective real-time risk reduction in these cities. In this study, we explore practical approaches to earthquake early warning (EEWS), and test the adaptability and potential of the real-time monitoring system in the NMSZ. We determine empirical relations based on amplitude and frequency magnitude proxies from the initial four seconds of the P-waveform records available from the Cooperative New Madrid Seismic Network (CNMSN) database for magnitude M > 2.5. The amplitude based proxies include low pass filtered peak displacement (Pd), peak velocity (Pv), and integral of the velocity squared (IV2), whereas the frequency based proxies include predominant period (tau-p), characteristic period (tau-c), and log average period (tau-log). Very few studies have considered areas with lower magnitude events. With an active EEW system in the NMSZ, damage resulting from the catastrophic events, as witnessed in 1811-1812, may be mitigated in real-time.
Poster:
Ogweno_L_Determination_of_Earthquake_early_warning_parameters_for_the_new_madrid_seismic_zone
