Abstract: Treasure Island is located in the central San Francisco Bay, immediately north of Yerba Buena Island, between the active San Andreas and Hayward faults. Treasure Island was constructed by placing hydraulic sand fill over natural shoal deposits within perimeter rock dikes. The natural shoal deposit consists of layers of clean sand, silty sand, and lenses of highly plastic clay. Full-scale and high-energy in-situ dynamic ground improvement test results indicated that, unlike the fill material, no appreciable ground improvement (i.e. densification) was observed within the shoal deposits. From a thorough geologic characterization of the shoal deposit and the results of laboratory cyclic direct simple shear tests on high-quality samples, it was concluded that the dynamic behavior of the natural shoal deposit could not be adequately captured by simplified conventional analytical methods, as the shoal deposit was found to be more resistant to seismically induced lateral deformation than could be predicted by simplified methods. Therefore, this study was undertaken to evaluate the seismic deformation of the existing shoreline at Treasure Island through a nonlinear dynamic deformation analysis. The scope of the study included seismic site response analyses, lateral deformation analyses using two-dimensional finite-element models in PLAXIS, pseudo-static hybrid deformation analyses, and comparisons with observed seismic performance of similar sites during past earthquakes. The shoal deposit was modeled using the UBC Sand model, with input parameters carefully selected to capture material behavior obtained through cyclic simple shear tests. Examination of PLAXIS analysis results indicates that the magnitude of lateral deformations at the location of the proposed development was negligible. A simplified method was also developed to be used as a screening tool for estimating the potential for lateral movement at other sites along the Treasure Island shoreline.
Slidecast:
https://vimeo.com/277159601
Abstract: Costa Rica is characterized by a high seismicity due to its location in a subduction zone and near to the triple point of the plates Coco, Caribbean and Nazca. This study evaluates the stability of the mean rate earthquake occurrence and the spatial and temporal distribution of seismicity in Costa Rica, in order to calculate the completeness magnitude for the RSN seismic catalog and the Gutenberg-Richter relationship for 1975-2017. Results show that the completeness magnitude for the catalog is 5.0, but it could be as low as 3.0 Mw for certain periods of time. The preliminary “b” value and maximum likelihood magnitude obtained for this catalog are 0.96, and 7.8, respectively. The geographic distribution of seismicity highlights the main active tectonic structures. In particular, clusters of seismicity revels highly deformed crustal areas, which are located along the inland projected path of seamount chains and the Panama Fracture Zone. These clusters do not correlate exactly with the largest energy released zones. These results are vital for understanding the tectonic setting of the region and the correct assessment of the seismic hazard and risk mitigation.
Slidecast:
https://vimeo.com/277181876
Abstract: In order to deal more effectively with the future earthquake risk, two important issues need to be solved: 1. How can the government and the public imagine the disaster scenario caused by the earthquake risk in these areas? 2. Can officials and the general public systematically see the damage caused by historical earthquakes in the area or other areas, and they understand the characteristics and distribution of earthquakes? Both of these issues are related to earthquake damage photos and survey data. In order to improve the utilization of survey data, this paper proposes a method to classify and attribute the seismic damage photos based on the need of future seismic hazard assessment. 1. The Mark of the seismic intensity or ground motion parameters in pictures. Also includes the name and number of the earthquake damage pictures, shooting location. According to the contents of the earthquake damage pictures, the following two categories can be used for the division and annotation. 2. The surface features, include faults, landslides, dammed lakes, sand liquefaction etc. 3. Engineering structure categories: building structure, water conservancy, electricity, petrochemicals, transportation, infrastructure and other important projects. Based on the above, seismic damage images are more refined attribute classification, 4. The details of the damage attribute, according to the different characteristics of the image object were marked. According to the above method, this paper presents some examples of earthquake damage photographs that have been classified and defined by attributes, and will try hard to set up an earthquake disaster database in the future. The authors suggest that international organizations should establish a global database of earthquakes and respond more appropriately to future earthquake risks.
Slidecast:
https://vimeo.com/277176971
Abstract: Over the past six years, the U.S. Ocean Bottom Seismograph Instrument Pool has undertaken a wide range of experiments. These experiments have addressed diverse scientific objectives through the deployment of instruments at different scales, geometries, water depths, and seasons. Overall, the experiments have been characterized by excellent instrument return rates, generally high data return, and an evolving set of instrument capabilities. These recent experiments provide insight and motivation for developments in areas such as instrument emplacement, deployment duration, communications (continuous or periodic), and standardized design elements – all key capabilities for future large-scale and/or long-term geophysical projects. Thus, we examine the characteristics, performance, and results of OBS experiments that have been done over the past six years as a key to understanding and motivating future technical directions for this important capability.
Poster:
OBSIP_Poster_Insights
Abstract: Crustal structure is an important parameter in global and regional seismic studies. A widely used method to obtain important features of the crust and upper mantle is receiver function, that uses teleseismic data (in distances raging from 30 to 95 degrees). In Brazil it began to be utilized since 1993, however there is still a lack of information in some areas, where the results obtained show a poor lateral resolution and great uncertainties due local complications and density of stations, as in the area of Pantanal and Chaco basin. In order to obtain more detailed crustal information about theses areas and update the map of Brazilian crust thickness, we used data of 1452 teleseismic events occurred from 2010 to 2016, registered at 166 RSBR (Brazilian Seismographic Network) and XC (FAPESP 3-basins project) stations. Automated selection of traces is performed based on azimuth recovering, radial SNR values and fit percentage of the original trace recovering. Deconvolution of the signal were done in time domain using low frequencies, a move-out correction was made for each phase and the estimation of crustal thickness was performed on receiver function traces by the Zhu and Kanamori modified method. Results shows a thin crust of 32 km +/- 1.2 km in Pantanal basin and a thicker crust in Chaco basin (37 km +/- 4 km), we also found unreliable results in the Amazon area. These new results corroborate the pre-existent models of crustal thickness in the most studied area of the eastern part of South America.
Poster:
Carolina_Rivadeneyra
Abstract: On 30 November 2016 at 18:25 (1st December at 00:25, UTC time) a Mw 5.5 earthquake occurred at 2.7 km depth near the town of Capellades de Alvarado, Costa Rica. It was the main shock of an earthquake sequence including foreshocks and aftershocks, located between the active volcanoes Irazú and Turrialba. This is the most recent of a series of damaging earthquakes originated in the faults crossing the Central Volcanic Range, which constitutes the northern boundary of the most populated area of the country. Using mainly the seismic records from the National Seismological Network (RSN), we present in this study a seismological analysis of the earthquake sequence and the location and characteristics of the fault that originated this seismicity. Additionally, we describe the geotectonic context of the fault and the Capellades earthquake effects. The earthquake sequence shows a clear 8-km long alignment striking north-northwest between Irazú and Turrialba volcanoes. The joint interpretation of the earthquake relocation, the main-shock moment tensor solution, and the focal mechanisms of 17 events allows for determining the source in a nearly vertical strike-slip fault, in agreement with regional active fault systems. This structure had not been previously recognized and has been named Liebres Fault in this study. The main shock was felt in most of the country, with a maximum intensity of VI+. This earthquake has been the largest in the eastern part of the Central Volcanic Range since the 1952 Patillos earthquake (Ms 5.9) and the first Mw > 5.0 earthquake recorded by the RSN in the Turrialba volcano edifice. Despite the proximity to this active volcano, which has been erupting periodically since 2010, there were no immediate eruptive effects.
Poster:
Linkimer_etal_Capellades_SSA_2018
