Abstract: We present a preliminary model to systematically quantify the long-term earthquake risk of over six hundred thousand bridges located throughout the conterminous United States. The model uses (1) the 2014 U.S. Geological Survey’s long-term earthquake shaking hazard model, (2) the 2017 National Bridge Inventory (NBI) data available through the Federal Highway Administration (FHWA), and (3) earthquake fragility/vulnerability relationships for bridges available through the Federal Emergency Management Agency’s (FEMA) Hazus program. Each year, the FHWA compiles bridge inventory data from US states, federal agencies, and tribal governments. These agencies compile comprehensive details as part of the bridge inspection process in accordance with the National Bridge Inspection Standards. The NBI dataset contains the most up-to-date stock of the nation’s bridges, and lists a number of attributes pertaining to each bridge structure, such as, location, year built, bridge type, number of spans, length, skew, and inspection date. We use these attributes to categorize each bridge into a model structure type category, for example, using the Hazus Bridge Classification Scheme for vulnerability and risk analyses. For each bridge site, we first obtain an earthquake shaking hazard curve defined in terms of spectral acceleration (Sa) at a spectral period of 1.0 sec, and then integrate it with the bridge-specific fragility curve to compute long-term earthquake risk. Attributes such as the skewness and number of spans are accounted for in the evaluation of damage potential through fragility relationships from FEMA’s Hazus methodology for damage/risk assessment. Earthquake risk herein refers to a probability of experiencing slight, moderate, extensive, or complete damage states during the useful life of each bridge structure. In this presentation, we summarize some key findings and discuss potential improvements to these preliminary assessments.
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Abstract: The USArray project represents a once-in-a-generation opportunity to fundamentally change geophysical monitoring in the US Arctic. The addition of more than 200 stations capable of recording seismic, infrasound, ground temperature and meteorological data has brought a diverse group of organizations to the table, fostering new connections and collaborations between scientists whose paths otherwise would not cross. With the array slated for removal beginning in 2019, there is a window of opportunity to advocate for permanently retaining a subset of the USArray stations. The Alaska Earthquake Center has drafted a plan to permanently adopt a subset of the USArray stations and maintain them as part of the seismic network in Alaska. The expanded seismic network would substantially improve on the Alaska Earthquake Center’s ongoing mission to advance Alaska’s resilience to earthquake hazards. The many challenges in adopting USArray stations include choosing which stations to retain, upgrading the power systems to have 24/7 data transmission through the long Alaskan winter months, and lowering the costs of continuous telemetry. The final station selection will also carefully consider the needs of partner organizations, since the USArray network currently fills important gaps in the weather, wildfire and climate research monitoring networks across Alaska.
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Abstract: Induced seismicity as a result of hydraulic fracturing and the subsequent wastewater disposal process has been a concern in recent years. Although typically small magnitude events, these induced earthquakes can reach magnitudes large enough to cause damage to nearby structures. The majority of induced seismic events occur within old basement faults below the target formation. While most induced seismicity has occurred in the central United States, there are several instances of induced events occurring in the Appalachian Basin such as the magnitude 2.3 Lawrence County, PA event in 2016 and the magnitude 4.0 Youngstown, Ohio event in 2011. Using data from several permanent and temporary broadband seismic networks, this study aims to determine crustal structure and depth to basement across the Appalachian Basin by jointly inverting teleseismic P-wave receiver functions and surface wave dispersion measurements. P-wave receiver functions are primarily sensitive to shear-wave velocity contrasts and vertical travel times whereas surface waves are sensitive to shear-wave velocities. The joint inversion of the two methods bridges resolution gaps associated with each data set, enabling the development of a higher resolution subsurface model. A better understanding of the crustal structure across the basin will allow identification of areas that may be of high risk for induced seismicity. Preliminary results indicate the use of high frequency receiver functions with dispersion measurements can image basement structure.
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Abstract: We examine locations, magnitudes and faulting types of post-2000 earthquakes in the trifurcation area of San Jacinto fault zone to clarify basic aspects of failure processes in the area. Most M > 3.5 events have strike-slip mechanisms, occur within 1 km of the main faults (Clark, Buck Ridge, and Coyote creek) and have hypocenter depths of 10-13 km. In contrast, many smaller events have normal source mechanisms and hypocenters in intra-fault areas deeper than 13 km. Additional small events with hypocenter depth < 13 km occur in off-fault regions and have complex geometries including lineations normal to the main faults. Five moderate earthquakes with M 4.7-5.4 have high aftershock rates (~150 M > 1.5 events within 1 day from the mainshock). To obtain more details on aftershock sequences of these earthquakes, we detect and locate additional events with the matched filter method. There are almost no aftershocks within 1 km from the mainshocks, consistent with large mainshock stress drops and low residual stress. The five aftershock sequences have little spatial overlap. While the mainshocks are on the main faults, most aftershocks are located in intra-fault and off-fault regions. Their locations and spatial distribution reflect the mainshock rupture directions and many also follow structures normal to the main faults. The significant diversity of observed features highlights the essential volumetric character of failure patterns in the area. The increasing rate of moderate events, highly productive aftershock sequences and large inferred stress drops may be indicative of approaching major event.
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Abstract: Seismicity in Pennsylvania results from natural earthquakes, mining blasts, and other induced events. Earthquakes occur primarily in the northwestern and southeastern portions of the state. Seismic events caused by mine and quarry blasts occur throughout the state, mainly in the coal mining regions, and although rare, there have been seismic events associated with hydraulic fracturing. However, to date there are no known seismic events linked to wastewater injection. In 2006, The Pennsylvania State University, in collaboration with the Bureau of Topographic and Geologic Survey within the Pennsylvania Department of Conservation and Natural Resources (DCNR), began constructing a network of seismic stations to detect and locate seismicity in the state. Between 2006 and 2013, the network grew to a total of 10 seismic stations providing near real time, open access seismic data. In late 2015, an expansion of the network to 30 broadband seismic stations began with funding from the DCNR and the Pennsylvania Department of Environmental Protection (DEP). Construction of the 30-station network was completed in August 2016. The 30 stations in the network plus another 13 broadband stations operated by other organizations provide fairly even data coverage across the Commonwealth. Data from these stations, in addition to data from 28 stations in neighboring states, are used to monitor seismicity within Pennsylvania. In addition to the broadband seismic stations, in 2017 7 short-period stations were installed at two wastewater injection sites and have been included in the current PASEIS network. Data from the 71 broadband and 7 short-period stations are fed into an EarthWorm system at Penn State for automatic event detection and location. To improve the automatic locations, the arrival times of P and S waves are repicked by hand and then used with the HYPOINVERSE code to obtain a refined location.
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Abstract: We will discuss and show the results obtained from an integrated SeismoGeodetic System, model SG160-09, installed in the Chile (Chilean National Network), Italy (University of Naples Network), and California. The SG160-09 provides the user high rate GNSS and accelerometer data, full epoch-by-epoch measurement integrity and, using the Trimble RTPD Client, the ability to create combined GNSS and accelerometer high-rate (200Hz) displacement time series in real-time for Earthquake Early Warning application. The SG160-09 combines seismic recording with GNSS geodetic measurement in a single compact, ruggedized package. The system includes a low-power, 220-channel GNSS receiver powered by the latest Trimble-precise Maxwell™6 technology and supports tracking GPS, GLONASS and Galileo signals. The receiver incorporates on-board GNSS point positioning using Real-Time Precise Point Positioning (PPP) technology with satellite clock and orbit corrections delivered over IP networks. The seismic recording element includes an ANSS Class A, force balance triaxial accelerometer with the latest, low power, 24-bit A/D converter, which produces high-resolution seismic data. The SG160-09 processor acquires and packetizes both seismic and geodetic data and transmits it to the central station using an advanced, error-correction protocol with back fill capability providing data integrity between the field and the processing center. The SG160-09 has been installed in three seismic stations in different geographic locations with different Trimble global reference stations coverage The hardware includes the SG160-09 system, external Zephyr Geodetic-3 GNSS antenna, and both radio and high-speed Internet communication media. Both acceleration and displacement data was transmitted in real-time to the centralized Data Acquisition and Processing Centers for real-time data processing. Command/Control of the field station and real-time GNSS position correction are provided via the Pivot software suite. Data from the SG160-09 system was used for seismic event characterization along with data from traditional stand-alone broadband seismic and geodetic stations installed in the network. Our presentation will focus on the key improvements of the network installation with the SG160-09 system, RTX correction accuracy obtained from Trimble Global RTX tracking network, rapid data transmission, and real-time data processing for strong seismic events and aftershock characterization.
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