Abstract: Like earthquakes, acoustic signals can consist of a “direct arrival” followed by a coda of scattered waves. In the acoustic case, the coda is generated from wave interactions with topography. The shape of the coda carries information about both the scatterer geometry and the atmospheric state between secondary sources and the receiver. This may permit inversion for the acoustic velocity structure of the lower atmosphere, particularly in regions with repeating infrasound sources in the same area (e. g. quarries, bombing ranges). This study presents results from an experiment using gradient flow Independent Component Analysis (ICA) on signals recorded from two 800 kg TNT equivalent explosions near Socorro, NM, USA. We show that gradient flow ICA is able to track signals scattered by the surrounding mountain ranges within the acoustic coda out to 30 seconds following the first arrival. Results are then compared with results from the Progressive Multi-Channel Cross Correlation (PMCC) method. The ICA algorithm strongly outperforms PMCC in this scenario. Suggestions for future research are given, including details on how this method can be used to invert for the vector wind field. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc. for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.
Poster:
WED.Brickell.1630.Albert
Abstract: The search for key elements characterizing the earthquake rupture process is challenged by the specificities of each individual event. This results in a large diversity when looking at earthquakes as a whole. This diversity is well documented by the moment rate functions (or Source Time Functions – STF), one of the most robust seismological observables of the rupture process. Teleseismic STFs also have the potential to be automately extracted for each earthquake with magnitude above 5.7-6, since the development of the digital global broadband seismic networks. This potential access to thousands of STFs, in all earthquake contexts and depths, motivated the development of the SCARDEC method, which simultaneously retrieves the static source parameters (depth, focal mechanism and magnitude) together with the STF. More precisely, the SCARDEC method retrieves apparent STFs (for each location and P/S phase), which also offers the possibility to track first-order features of the space-time rupture process. The present study uses these STFs to further document how, and at which velocity, rupture develops. In a first step, we will review the information provided by a systematic search of the average rupture velocities. Second, we will focus on the most energetic phase of the STFs. Even if the times at which rupture strongly accelerates appears unpredictable (resulting in very different STF shapes), the characteristics of this acceleration provide constraints on the dynamics of the rupture. As a matter of fact, this acceleration is on average faster than what is predicted by a classical self-similar rupture growth (where the STF grows quadratically with time). This shows that during the main phase, rupture velocity and/or slip rate increase.
Poster:
WED.Brickell.1430.Vallée
Abstract: Source scaling is a fundamental issue to understand earthquakes. A magnitude and log area (M-logA) relation is a key scaling to link modeling of seismic source and ground motion. Currently, scenario seismic hazard maps in Japan adopt a 3-stage M-logA scaling for crustal earthquakes, although many national seismic hazard maps use a linear or bilinear scaling. The 3-stage M-logA scaling is originally proposed by Scholz (2002). Recent development of slip inversions enabled us to improve quantitative estimates of the scaling. Based on the source characterization of slip inversions, the M-logA scaling for crustal earthquakes are: The first circular-crack model stage of A (km2) = 2.23 x 10-15 (Mo (Nm) x 107)2/3 by Somerville et al. (1999) for Mw<6.5, the second L-model stage of A (km2) = 4.24 x 10-11 (Mo (Nm) x 107)1/2 by Irikura and Miyake (2001, 2011) for Mw = 6.5~7.4 after fault width saturation, and the third W-model stage of A (km2) = 1.0 x 10-17 Mo (Nm) by Murotani et al. (2015) for Mw>7.4 after fault displacement saturation. The 3-stage M-logA scaling shows the first bending at L~Wmax without significant gaps that pointed out by past 2-D numerical simulations. The second L-model stage is similar to Hanks and Bakun (2002) that is well constraint by megafault systems. We also confirmed that dynamic rupture simulations for strike-slip faulting using 3-D FDM of Dalguer et al. (2008) naturally reproduce the 3-stage M-logA scaling. To fit the scaling between slip inversions and dynamic rupture simulations, slight increase of stress drop from 2.3 to over 3.0 MPa is required in the second L-model stage. Those are compatible with the static models by Fujii and Matsu’ura (2000) and Shaw and Scholz (2001). Finally, we validate the 3-stage source scaling and other published scaling (e.g., Leonard, 2010) for recent crustal earthquakes with slip inversions, and compare their performance.
Slidecast:
https://vimeo.com/277547944
Abstract: The Collaboratory for the Study of Earthquake Predictability (CSEP) is global platform for conducting prospective earthquake forecasting and prediction experiments. Since its inception in 2006 in California, CSEP has grown to encompass four testing centers around the world, and is now prospectively evaluating over four hundred models in testing regions in California, Italy, New Zealand, Japan, the western Pacific and at global scale. Here, we review a decade of CSEP achievements, and state our priorities for future activities. Achievements encompass new scientific insights into earthquakes and their predictability, progress in evaluation methodology, and broader impacts in seismic hazard assessments and risk reduction strategies. Scientific highlights include: (1) the numerous small earthquakes provide information about future moderate-to-large earthquakes; (2) geodetic strain rate models capture earthquake potential, and, when coupled with smoothed seismicity models, provide the most informative forecasts; (3) a new generation of physics-based Coulomb/rate-state models are now able to compete with statistical models in forecasting the space-time evolution of earthquake sequences;. More broadly, CSEP has highlighted the benefits and scientific importance of prospective and independent testing to establish credible benchmarks of the forecast skill of competing hypotheses and models. These and other CSEP results have effected changes to several earthquake source models of official seismic hazard models, including in California, New Zealand and Italy. In this way, CSEP has contributed to safer and better-informed societies. Future activities will be guided by three main objectives: (1) Improve the discrimination capability of forecast testing by expanding the spatial and temporal distribution of earthquake data. (2) Develop procedures and requisite cyberinfrastructure for testing earthquake forecasts worldwide, focusing on new types of earthquake forecasts. (3) Test key hypotheses that underlie earthquake forecasting models.
Slidecast:
https://vimeo.com/277546924
Abstract: Spatial and temporal distributions of aftershocks were studied for recent moderate earthquakes that occurred at shallow depth onshore of Japan and were well recorded by the regional networks. These events include the 2000 Western Tottori (Mw 6.7), 2004 Niigata Chuetsu (Mw 6.6), 2005 Fukuoka (Mw 6.6), 2007 Noto Peninsula (Mw 6.7), 2007 Niigata Chuetsu-oki (Mw 6.8), 2008 Iwate-Miyagi-ken (Mw 6.8), 2016 Kumamoto (Mw6.2) and 2017 Tottori (Mw6.2) earthquakes. All of these earthquakes are approximately of similar size, however, the rates of aftershock activity are quite different. The 2004 Niigata and 2008 Iwate-Miyagi earthquakes have significantly more aftershocks than the other 7 events. In the spatial locations of the aftershocks, these two earthquakes have more complex spatial distributions with more aftershocks occurring away from the mainshock fault plane. There appears to be a correlation between the rate of aftershock activity and the spatial complexity of the locations. The sequences with higher rates of aftershock occurrence may be associated with aftershocks triggered in a volume around the mainshock. In contrast, for the other sequences, aftershocks occur mainly in a planar pattern close to the mainshock fault plane. The early time sequences of the aftershocks for these events were also examined. Using continuously recorded seismograms from nearby borehole stations of Hi-net, aftershocks were identified and counted. From about one minute following the mainshock origin time, we estimate that we can identify aftershocks with magnitudes down to Mj 3.5. For the first few minutes the rate of aftershocks is quite similar for all of the mainshocks. The higher rate of aftershocks for the 2004 Niigata and 2008 Iwate-Miyagi earthquakes appears to begin about 10 minutes after the mainshock. This suggests that the enhanced triggering of aftershock for these two earthquakes may be caused by some changes in the aftershock region several minutes after the mainshock.
Slidecast:
https://vimeo.com/277548143
Abstract: The current deployment of the EarthScope Transportable Array (TA) in Alaska affords an unprecedented opportunity to study explosive volcanic eruptions using a relatively dense regional seismo-acoustic network. Active volcanism in the Aleutian Arc poses a risk to both regional and international air traffic. Infrasound monitoring has demonstrated utility for the detection and characterization of explosive volcanism, but previous studies have utilized relatively sparse networks of infrasound arrays in comparison to the TA in Alaska (which uses single-sensor stations). Here we present capabilities for the detection, location, and characterization of remote explosive volcanic eruptions using seismic, infrasonic, and ground-coupled airwave phases. We combine data from the TA and additional regional networks, including data from the Alaska Volcano Observatory (AVO) and Alaska Earthquake Center (AEC). We implement a Reverse Time Migration (RTM) technique to locate explosive eruptions in Alaska, with a focus on the recent explosive activity at locally-unmonitored Bogoslof volcano (December 2016 – August 2017). More than 60 eruptive events from Bogoslof provide a unique validation dataset, allowing experimentation and optimization of different RTM strategies. We also apply RTM to eruptions from other Alaskan volcanoes (Cleveland, Pavlof) and Kamchatkan volcanoes (Bezymianny, Shiveluch). We are experimenting with different strategies and parameter choices for the RTM; challenges include varying signal durations and amplitudes, the source-receiver geometries, and most volcanic eruptions occurring outside the network. We employ Receiver Operating Characteristic (ROC) curves to characterize parameter choices, and investigate coherence weighting of infrasound for signal cleaning and selection. Our methods are useful for both (1) event detection using real-time data and (2) scanning data archives to identify and discriminate volcanic and non-volcanic events.
Poster:
WED.Brickell.1700.Sanderson
