Abstract: A software tool has been developed to utilize time-frequency representations (TFR) of time domain signals based on the continuous wavelet transform (CWT) where the seismogram is converted into a 2D map of wavelet scale (analogous to Fourier period) as a function of time. The 2D map is an intuitive description of the signal since major signal arrivals in the CWT can often be seen correlating with inferred signals in the seismogram and separated from bands of noise. The tool allows the choice of simple block thresholding schemes analogous to bandpass filtering to remove obvious noise bands. The remainder can further be analyzed by interactively choosing irregular blocks that represent different parts of the signal for removal or for separate analysis. Inverse transforming the processed CWT yields signals with substantial Signal-to-Noise (SNR) improvement. The tool is used to process explosion data recorded by the IRIS Community Wavefields experiment in early July 2016. Four explosions (909kg, 455kg, 2 at 228kg) at 3 different shot points (ranges of ~16km, 35km, and 65km) from the experiment were detonated by the Air Force Research Laboratory to examine local wave propagation and signal characteristics. Interactive block thresholding incorporating the Morlet wavelet significantly improves the SNR for 909kg and 455kg shots. Three dominant wavetrains are evident in the data including the low scale (high frequency) P wave and 2 dispersed higher and fundamental mode surface wave trains. Each wave train can be extracted from the CWT map separately for further modeling, such as for group and phase velocity dispersion. The smaller shots are usually lost in the ‘noise’ that sometimes include swarms of local induced earthquakes, easily seen in the CWT map. Preprocessing the data to improve the SNR also improves array beamforming at high frequency for dense arrays constructed from subsets of nodal stations of the IRIS experiment.
Slidecast:
https://vimeo.com/278567479
Abstract: A Century Later is an historical novel based on the life of a Catholic nun who died in the San Antonio Hospital in Mayagüez, Puerto Rico, as a consequence of the October 11, 1918, M 7.3 earthquake. During 100 years her story and tragedy were unknown. The author, Fernando Bayrón Toro, a professor, historian, writer, political scientist and lawyer, researched archives in Puerto Rico and Europe for this work. The novel includes many facts and photographs from the earthquake and tsunami and helps clarify the earthquake and tsunami impact. In Mayagüez, for example, one of the oft asked questions is where did the tsunami reach when it flooded the city. Gleaming over maps and plans and in discussion with scientists he helps answer this question. While the novel makes for interesting reading, it also serves the purpose of sensitizing and educating the reader on earthquakes and tsunamis. People reading this book hopefully will be more likely to take proactive measures to prepare for such an event. Earthquake and tsunami awareness also is increased thru the public presentations on the book.
Slidecast:
https://vimeo.com/278532092
Abstract: In September 2017 the Northeastern Caribbean was battered by two Category 5 hurricanes, Irma and Maria. While Irma’s destructive forces were most devastating to Antigua & Barbuda, Anguilla, the British Virgin Islands, the Northern US Virgin Islands, Maria completed the destruction of the region with its fury focusing on Dominica, St. Croix (USVI) and Puerto Rico. There are many parallels between these hurricanes and earthquakes and tsunamis which are examined in the hopes of improving resilience to the more infrequent geophysical events. Issues like onset, duration, impact and damage, response and recovery will be addressed. Unlike Earthquakes and Tsunamis which are no notice/extremely rapid onset events, the hurricanes provided advance warning giving some time to take safety measures. In the case of the hurricanes, the winds blew for hours limiting any type of emergency response, in this case there is closer tie in with tsunamis rather than earthquakes. The images of the damage along the coastlines due to storm surge and river flooding recall scenes from recent tsunamis and are a powerful reminder of the destructive forces of water. One of the most significant impacts from the hurricanes was the sudden complete loss of power, communication and road connectivity. In the response and recovery phase personal, community and institutional empowerment has been recognized. Although the Puerto Rico Seismic Network was left with limited earthquake data due to the loss of all but 4 stations after Maria, it used the resources it had to serve as a focal point for emergency communications. Once again the elderly and those with less economic resources have been the most vulnerable. Debris has been a major issue, but probably small in comparison with what we would be facing from an earthquake and tsunami. The presentation will focus on how the Irma and Maria experience can be used to educate also on earthquakes and tsunamis and build back better, more resilient.
Slidecast:
https://vimeo.com/278530081
Abstract: The Pacific Northwest Seismic Network is responsible for monitoring seismic activity in Cascadia and is also part of the USGS ShakeAlert earthquake early warning system. Part of our mission is operating and maintaining our network of over 400 stations. Fulfilling our role effectively requires monitoring station state of health metrics, those related to waveform quality such as power spectral density estimates, and those specific to ShakeAlert. The later include frequency of potential triggers that exceed certain amplitude and signal-to-noise ratio criteria. Multiple tools are available for seismologists to assess state of health and waveform quality across many stations such as MUSTANG (IRIS), SeisNetWatch (ISTI), and Data Quality Analyzer (ASL) though none meet all of our needs. Some operate in near real-time and focus on state of health while others include metrics that require long data segments and cannot scale to near real-time for our network. To bridge the wide range of needs by the PNSN, we are developing the Seismic Quality Assessment Console (SQUAC). The interface will be a web-service based API to generate a page with table and map views of the network metrics with variable levels of network-station-location-channel granularity. The web interface will also include one-click on the fly generated charts of time histories of selected metrics as well as station-level plots of latest hour metrics and waveforms useful for technicians in the field. SQUAC will be able to digest any properly formed JSON message sent to the API and thus can ingest metrics from databases, AQMS messages, other APIs, upload via wget, etc. Currently at the PNSN we are seeding this with over 30 state of health and waveform quality metrics calculated hourly in near real-time using a light-weight ObsPy based package. These metrics are stored in a postgres database and monitored so that when thresholds are exceeded, SQUAC can “squawk” and quickly alert interested parties.
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Abstract: Lately, there have been newly identified, potentially seismogenic crustal faults (PSCFs) in Chile, many of them are now published by the South America Risk Assessment Project of the Global Earthquake Model. These faults pose a great risk to the community and infrastructure; therefore, it is necessary to estimate ground motion that can be produced by these seismic sources. Unfortunately, there are very few recorded ground motion data from these active faults, not sufficient to build ground motion prediction equations—a necessary element to estimate earthquake hazard. Because of the lack of recorded data, it has become necessary to estimate ground motion with physics-based models. With recorded ground motion, we have estimated path and site attenuation parameters, namely kappa, for different sites in Central Chile. Preliminary results show that the regional value of kappa is approximately 40 ms. The PSCFs database contain variable information depending on the different sources used to compile the fault datum. Nevertheless, if absent, the necessary parameters to simulate seismic scenarios such as the dimension, sense of slip and dip angle and direction can be extracted from the fault traces themselves and inferred from the seismotectonic setting of each individual fault. We used the 2010 Leonard scaling relationships to determine a magnitude based on the length of each active fault. For each fault, we computed several kinematic earthquake rupture scenarios, and propagated seismic waves using Green’s functions with the UCSB method (Crempien and Archuleta, 2015). The Green’s functions incorporate the attenuation measured previously and are calculated up to 25 Hz. For all earthquake scenarios, we estimate peak ground acceleration intensities of over 1g within 10km of the closest distance to the fault. These preliminary results show that these active crustal faults need to be studied in depth in order to better characterize the earthquake hazard in Central Chile.
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Abstract: Probabilistic seismic hazard analysis (PSHA) is typically performed by combining an earthquake rupture forecast (ERF) with a set of empirical ground motion prediction equations (GMPEs). ERFs have typically relied on observed fault slip rates, scaling relationships, and regional magnitude-frequency distributions to estimate the rate of large earthquakes on pre-defined fault segments. GMPEs, which regress against recorded ground motions, often lack data at short site-rupture distances and for large, complex ruptures. The CyberShake platform (Graves et al., 2011) replaces GMPEs with deterministic three-dimensional ground motion simulations, characterizing the effects of basin response and other path effects which are parameterized or treated as aleatory variability in GMPEs. We replace traditional ERFs with a multi-cycle physics-based earthquake simulator, the Rate-State Earthquake Simulator (RSQSim), developed by Dieterich & Richards-Dinger (2010). RSQSim simulations on the Uniform California Earthquake Rupture Forecast, Version 3 (UCERF3) fault system produce seismicity catalogs that match long term rates on major faults and yield remarkable agreement with UCERF3 when carried through to GMPE-based PSHA calculations. Averaged over a representative set of sites, the RSQSim-UCERF3 hazard-curve differences are comparable to, or even less than, the differences between UCERF3 and its predecessor, UCERF2, used in all CyberShake studies to date. Unlike traditional ERFs, RSQSim produces full slip-time histories for all simulated ruptures which can be used directly as input to deterministic wave propagation simulations. We couple the RSQSim model with CyberShake and the SCEC Broadband Platform to create the first fully physics-based PSHA model. Resultant ground motions match GMPE estimates of mean and variability of shaking well over magnitudes and distances for which GMPEs are well constrained. We will present these comparisons and preliminary CyberShake results using RSQSim.
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