Abstract: At least nine small seismic events were detected and located around the North Korean nuclear test site following the 3 September 2017 underground nuclear test. The magnitude of these shocks range from 2.3 to 3.5. Based on their proximity to the 3 September 2017 UNT, these shocks may be considered as aftershocks of the UNT. We assess the best method to classify these small events based on spectral amplitude ratios of regional P and S wave from the shocks. None of these shocks are classified as explosion-like based on P/S spectral amplitude ratios. We examine additional possible small seismic events around the North Korean test site by using seismic data from stations in South Korea and northeastern China including IMS seismic arrays, GSN stations, and regional network stations. A linear discriminant function based on Mahalanobis methods applied to P/S spectral ratios does a better job of screening events, than does a simple average of such ratios.
Slidecast:
https://vimeo.com/278019415
Abstract: Following the discovery of the Shoreline Fault along the coast of central California by Hardebeck (2010) and the occurrence of the tsunami disaster at the Fukushima nuclear power plant in 2011, there has been increased attention on the potential seismic hazards of the central California region in relation to the Diablo Canyon nuclear power plant. The Pacific Gas & Electric Company has supported related research both offshore and onshore, and the Southern California Earthquake Center (SCEC) has established a Central California Seismic Project (CCSP) to develop a Community Velocity Model (CVM) and Community Fault Model for the region and use them for strong motion simulations and hazard estimation. The existing SCEC Central California CVM was developed from a USGS velocity model, velocity measurements from wells and seismic surveys, and refinement using waveform tomography. We are working to further refine the Central California CVM by (1) incorporating surface-wave dispersion data from an expanded set of stations and extended to shorter periods (3 to 4 seconds) and (2) incorporating body-wave arrival-time data from earthquakes and P-wave travel times from explosions. The current SCEC Central California CVM will be used as the starting model for new surface-wave inversions, new body-wave inversions, and ultimately joint body wave-surface wave inversions. Inversions using the new data will result in a finer resolution CVM for central California. Research supported by SCEC Award #17188 and USGS Award G16AP00111.
Slidecast:
https://vimeo.com/278026457
Abstract: Active plate tectonics in Colombia is dominated by the convergence of the oceanic Nazca and Caribbean plates along the northwestern margins of continental South American plate. As a consequence, two subduction zones and a complex fault system are present inside the northern Andean region. As the result of these interactions, large earthquakes occur in Colombia as well as shallow to intermediate-deep seismic events with a wide range of magnitudes. In this study we compile 3D active fault plane geometry models for 54 seismogenic sources for probabilistic seismic hazard applications. Three types of seismogenic sources are defined: 39 crustal fault systems (21 thrust, 9 sinistral, 9 dextral), 12 subducting slab segments (6 inslab, 6 interface) and 3 localized seismic clusters. This model considers a wide variety of geometric constraints, based on relevant references for the region. The earthquake distribution in depth illuminates the general extent of slab segments as planar clusters of events, while the main activity for localized seismic clusters is identified as punctual at different depth ranges. A compilation of active mapped fault corridors, tectonic block configuration and crustal thickness estimates is used to approximate planes for crustal rupture, for which associated seismicity do not exceed 50 km depth. Different scaling relationships for fault rates and geometries are used to estimate values for the maximum magnitude in terms of moment magnitude. Estimates range from Mw 7.2-8.4 for crustal sources, Mw 8.4-8.8 for subduction segments and Mw 5.9-7.5 for clusters. The seismic activity seems concentrated accross subduction zones rather than along the major fault systems, but with a clear contribution of crustal sources and localized clusters. Improving the characterization of the Colombian seismogenic systems, including their seismic activity, is essential to improve the seismic hazard models for Colombia.
Slidecast:
https://vimeo.com/278022989
Abstract: The Cordillera Blanca batholith is an intrusive emplacement of Miocene age (5 Ma) that lies over the northern-central Peruvian Andes. Its western flank is bounded by a normal fault that extends for about 220 km in a NW-SE direction and dips west at a low angle. There is no historical evidence of large earthquakes associated with this fault, but geological studies indicate Quaternary or Holocene activity with as much as 2.5 m of vertical slip. Other related events in the neighborhood of the Cordillera Blanca Fault are the 1942 Quiches (M 6) earthquake and the 1970 Chimbote earthquake (Mw 8). The Chimbote event caused the highest death toll of all times in Peru (70,000 casualties) as it triggered a massive mudslide that swept all along its path and buried the town of Yungay. The ice-and-snow covered, elevated peaks (up to 18,000 ft.) in the Cordillera Banca mountains, as well as the existance of the active bounding normal fault system, represent a big hazard to this region. This study presents the first geodetic estimates of the modern kinematics of the Cordillera Blanca Fault and uses GPS measurements carried out between 2014 and 2017, as well as baseline changes observed between two cGPS stations across its southern end.
Slidecast:
https://vimeo.com/278051014
Abstract: In October 2016, we acquired high-resolution P- and S-wave seismic data along a 120-m-long, SW-NE-trending profile in Napa, California. Our seismic survey was designed to image a strand of the West Napa Fault Zone (WNFZ), which ruptured during the 24 August 2014 Mw 6.0 South Napa Earthquake. We separately acquired P- and S-wave data at every station using multiple hammer hits, which were edited and stacked into individual shot gathers in the lab. Each shot was co-located with and recorded by 118 P-wave (40-Hz) geophones, spaced at 1 m, and by 118 S-wave (4.5-Hz) geophones, spaced at 1 m. We developed both P- and S-wave tomographic velocity models, as well as Poisson’s ratio and a Vp/Vs ratio models. We observed a well-defined zone of elevated Vp/Vs ratios below about 10 m depth, centered beneath the observed surface rupture. P-wave reflection images show that the fault forms a flower-structure in the upper few tens of meters. This method has been shown to delineate fault structures even in areas of rough terrain.
Slidecast:
https://vimeo.com/278053943
Abstract: Interpretations of differing types of data, analyses, and modeling yield different rates of recurrence for Cascadia’s great (M8-9) megathrust earthquakes. Learning the source of these differences will improve seismic hazard assessments and forecasts. Understanding these differences will require asking fundamental questions about cycles of earthquake strain accumulation and release on various temporal and spatial scales, how earthquake ruptures nucleate and evolve, and how seismic energy is radiated. To address these questions the Cascadia Recurrence Project Team is undertaking an effort to produce a detailed and holistic knowledge of the history of Cascadia’s past megathrust events. We are focusing on improving our understanding Cascadia earthquake history and processes by integrating the results of studies of earthquake-triggered 1) ground failures (particularly landslides), 2) coastal land-level changes, 3) tsunami inundation and deposition, and 4) offshore slope failures and turbidites. Other contextual studies examine 5) structural controls on rupture segmentation, 6) rupture modeling, 7) seismic ground motion simulation and onshore and offshore site response, 8) modes of plate-boundary locking and slip, and 9) interactions among upper- and lower-plate faults.
Slidecast:
https://vimeo.com/278062302
