Abstract: Seismic source discrimination at local distances (< 200 km) is becoming increasingly important within the nuclear monitoring community. A study conducted by Koper et al. (2016) found that the difference between local magnitude (ML) and coda/duration magnitude (MC) could distinguish between mining-induced seismicity and natural seismicity in Utah. They found that the shallower mining-induced earthquakes (depths < 2–3 km) had more negative ML-MC values than the generally deeper (> 5 km) tectonic events. Similar results showing that ML-MC decreases as source depth approaches the surface have recently been found in Yellowstone, Oklahoma, and Italy. Here we investigate how well direct measurements of peak amplitude (A) and duration (t) made at individual stations can be used as proxies for the network averaged ML–MC values. In particular, we investigate how log10(A/t) varies as a function of distance, how quickly the variance decreases as more stations are averaged together, and whether individual station corrections are warranted. We also examine how changes to the procedures used to measure A and t affect the performance of log10(A/t) as a depth discriminant. We aim to replicate and explain our ML-MC observations using a three-dimensional, fourth-order, finite-difference code (SW4) to synthesize high-frequency waveforms in realistic Earth models that contain topographic and volumetric scattering. The ultimate goal of this study is to introduce a new depth discriminant to nuclear monitoring practices that can be applied to all networks and help differentiate mid- and lower- crustal earthquakes from potential explosions.
Poster:
Scales_SSA2018
Abstract: In the last years, the Monitoring and Early Warning Center for Earthquakes and Tsunamis at INETER, Nicaragua, has developed rapidly due to new responsibilities INETER acquired at national and international level. In 2015, it became home of the Central American Tsunami Advisory Center CATAC). In 2016, INETER started a cooperation with the Swiss Seismological Service on Earthquake Early Warning (EEW). To permit tsunami services and EEW for Nicaragua and Central America we extended our seismic network and intensified data exchange with the networks in the region. Computing hardware was greatly enhanced. We abandoned SEISAN (in place since 1991) for routine processing and rely now completely on the SeisComP3 package. In 2016, we started to use experimentally the EEW modules included in SeisComP3. In January of 2018, we installed the tsunami modules of the package. Seismic and tsunami processing is first done automatically but after a few minutes the seismologist on duty revise the results. We receive and process real time data from about 90 stations in Nicaragua, and around 250 stations from the other Central America countries, Guatemala, El Salvador, Honduras, Costa Rica and Panama. Additionally, we process around 200 global seismic station for our global locator. The monthly number of processed seismic events increased from around 120 to around 1000. The personnel working 24×7 was capacitated in the use of the software and the scientific exploitation of the data. The earthquake detection and location quality increased drastically for the events below the Pacific Ocean of Nicaragua, in the border regions to the neighboring countries and in the Caribbean Sea. Information and alarm messages are sent out automatically and manually to seismological and civil protection agencies in Central America. We thank JICA-Nicaragua, JMA/Tokio and University of Hokkaido for cooperation on CATAC, and of Swiss Technical Cooperation DEZA and SED/ETHZ on the development of EEW.
Poster:
Strauch Poster Warning Center
Abstract: The geothermal field of Humeros Puebla, Mexico, is one of the most important in the country due to its production of 85 Mwh. Previous studies indicate that most of the seismicity is concentrated in two large areas mostly located in the northern part of the field, near the areas of injection and re-injection of fluids (Urban et al, 2013, Lermo et al, 2001, 2002, 2005, 2008). In the framework of the Working Package 5.2 of the GEMex bilateral project between Mexico and the European Union, a seismic network has been installed consisting of 26 short-period and 17 broad band stations. This seismic network represents the largest monitoring network installed in a geothermal field in Mexico. The seismic noise of the stations has been analyzed to determine i) the quality of the seismograms and ii) the level of noise in the high frequency range ( >1 Hz), which could be attributed to the extraction plants, steam pipelines or the effect of human activity. For this purpose, we analyzed the power spectral density function, through the implementation of routines in free software such as Seismic Analysis Code (SAC) and Jupyter. Likewise, the spectral ratios have been calculated for each station of the network in order to characterize and determine the site effects. We also present the location of the seismic events registered in the field since September 2017, calculated using a 1D velocity model presented by Urban and Lermo (2013). SEISAN software is used to locate the seismicity (Havskov and Ottemoller, 2003) and HypoDD double difference method (Waldhauser, 2001) to refine the event location. Our preliminary results indicate that the noise levels are acceptable compared with the model of Peterson (1993), which allows us to characterize local seismicity and low magnitude within the geothermal field.
Poster:
SSA_FINAL
Abstract: We develop an empirical ground-motion model (GMM) for Fourier amplitude spectra (FAS) using the PEER NGA-W2 database of ground motions from crustal earthquakes (Ancheta et al., 2014). GMMs have traditionally been developed for 5%-damped response spectra (PSA), however, there are some available models for FAS (e.g. Bora et al., 2015; Stafford et al., 2006; Yenier and Atkinson, 2015). There are two key advantages of developing FAS model over PSA models: the FAS is more closely related to the physical behavior so that it is easier to apply seismological constraints to the GMM; and the Fourier spectrum is linear which allows ground motion from small magnitude earthquake to be used to constrain the linear path and linear site effects without the spectral shape dependence of PSA. The GMM features a magnitude scaling formulation guided by Chiou and Youngs (2014). Large-magnitude near-fault saturation to capture the effects of extended ruptures is included and guided by broadband simulations up to M8.0. The GMM features stronger near-fault magnitude scaling (less saturation) than the CY14 model for PSA. The model includes terms for anelastic attenuation, site effects (Vs and kappa), style of faulting, hanging wall, and depth to top of rupture. The model coefficients are smoothed in a series of steps to assure smooth spectra and to constrain the extrapolation. An essential feature of forward-application for GMMs is their ability to reliably extrapolate to the key large magnitude and short distance ranges that often control the hazard. This allows for the GMMs to be applied beyond the ranges well constrained by empirical data. Our model is applicable to crustal earthquakes in the range M3-8, with rupture distances 0-300 km, for sites with Vs30 between 150-2,000 m/s, and over the frequency range 0.1-100 Hz. Models for the between-event, within-event, and total standard deviation are provided.
Poster:
SSA_2018_Poster
Abstract: The concept of picking earthquake phase arrivals and associating them to a common event is simple, but hard to implement in a robust and reliable manner when there are multiple overlapping events and false arrival time picks. Related issues include determining precisely if and when an event has occurred when noisy and false arrivals are included in the data set, and determining how many different unique events have occurred when move-out times are possibly overlapping. Interestingly when these challenges are posed as a precise mathematical problem, simple and accurate solutions arise through the use of basic multilayer perceptron (MLP) supervised classification and regression frameworks. For a pre-determined velocity model, we obtain distinct (shallow) MLP networks to classify an arbitrary set of arrival times on $n$ stations between a true P or S phase recorded event and noise (i.e. a false set of arrivals), determine the distinct number of unique events in a time window, determine which set of arrivals associate to a common event, and also, using a denoising autoencoder framework, predict arrival times on stations missing arrival time detections during an event. Discrete (i.e. vectors of arrival time data) and continuous (i.e. encoding of arrival time data as time-dependent Gaussian pulses) implementations are used. Synthetic tests of the method and applications to field data from the IPOC network of Northern Chile are presented.
Poster:
IWM_Automatic_Association_Earthquake_Arrivals
Abstract: Using the earthquake catalogues from China and ISC (International Seismological Centre), the uniform catalogue of Himalaya and adjacent areas has been established for the seismic hazard analysis and seismic hazard map of Himalaya areas according to according to the following principles. 1) The earthquake parameters should be taken from China catalog if the events occurred in China. 2) The earthquake parameters should be taken from ISC catalog if the events occurred out of China. 3) The earthquake parameters should be checked if the events occurred on the China border areas based on the data of earthquake stations. There are 17886 events with event magnitude 4.0 and greater in the catalogue, among them, there are 65 ones with magnitude 7 and greater. Based on the catalogue, the seismicity of the Himalaya and adjacent areas has been analyzed, and the following conclusions are made. 1) The epicenters generally follow a NW-trending band distribution along the Himalayan arc. At the eastern and western ends, that is, the structural knots of the Himalayas, the epicenter strips are respectively turned to the NE-trending. 2) Intermediate-depth earthquake are distributed on both east and west ends of the Himalayan arc structure belt, that is, the Pamir at the west end and the Assam at the east end. In the Qinghai-Tibet Plateau and the front of the Himalaya arc, events are basically shallow earthquakes. 3) Earthquakes with magnitude greater than 4.5 are basically complete since 1960 A.D.. 4) The b-values of magnitude – frequency relationship for the study areas are between 0.9 and 1.0.
Poster:
SSA2018poster-XU-116
