Abstract: Aquifers are vital groundwater reservoirs for domestic, agricultural, and industrial activities worldwide. Tracking their state with high temporal and spatial resolution is critical for water resource management, yet rarely achieved from a single data set. Here, we show that groundwater level varitions can be mapped at basin-scale using perturbations in seismic velocity, dv/v. We recover daily changes in dv/v in the San Gabriel Valley, California, from cross-correlation of the ambient seismic field for the years 2000 – 2017. dv/v recovers the multi-year depletions and rapid recharge that mark the drought-wet cycles in southern California. dv/v correlates spatially with vertical surface displacements and deformation measured from local GPS stations. These results suggest that velocity change measurements have the potential to improve established monitoring efforts of aquifers.
Slidecast:
https://vimeo.com/277185258
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: Large historical mega-thrust subduction earthquakes, such as the 2004 Aceh-Andaman, 2010 Maule, and 2011 Tohoku earthquakes, have triggered numerous aftershocks in subduction plate interfaces and continental crusts. The crustal seismicity occurs much closer to the population and buildings than the subduction earthquake which is likely to occur with a larger magnitude and at a greater distance. Therefore, the crustal earthquake can have a greater potential impact on seismic damage and loss than the subduction earthquake. Generally, times between major events may be too short to inspect and repair damaged buildings; in such situations, damage accumulation of buildings can be major issues. A new method for assessing spatiotemporal seismic hazard and risk due to a mega-thrust subduction earthquake that triggers both subduction and crustal aftershocks is developed. The Epidemic Type Aftershock Sequences (ETAS) model is used to generate synthetic earthquake catalogs and capture spatiotemporal earthquake clustering. The conventional isotropic ETAS simulation is extended to account for spatial anisotropic distribution of aftershocks by applying the scaling law of the rupture model, and implementing a 2D uniform distribution and a power law decay inside and outside of the rupture area, respectively. Moreover, to evaluate seismic hazard and risk, the ETAS model is convolved with ground motion prediction equations (GMPEs) and seismic fragility curves. A case study is set up for the 2011 Mw9 Tohoku event in Japan. By incorporating more realistic spatial anisotropy of aftershocks, we quantify how the spatiotemporal seismic hazard rate is changed by the triggered crustal and subduction aftershocks in comparison with long-term time-independent hazard rate. Furthermore, we propose to evaluate the impact of increased crustal and subduction aftershocks to seismic hazard and risk assessments for making various risk management decisions more effectively in the post-mainshock period.
Slidecast:
https://vimeo.com/277156673
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: Earthquake Intensity Measures (IM) are commonly employed in seismic risk assessment to link seismological parameters (M, R, ε) with Engineering Demand Parameters (EDP). Consequently, the selection of appropriate IMs consistent with the structural type and location has become increasingly important using criteria such as efficiency (standard deviation in EDP given an IM) and sufficiency (conditional independence of EDP from seismological parameters given an IM). As sufficiency of IMs has been only assessed qualitatively using p-values–which provide no information beyond pass/fail–, we first describe a recently proposed quantitative method to gauge sufficiency using a Total Information Gain (TIG) metric. We apply the TIG to a four-story steel moment frame building for a large number of combinations of IMs, EDPs, and record-sets to realize that the choice of ground motion set can play a huge role in determining an IM’s sufficiency. Next, we investigate the relation between the efficiency and sufficiency metrics and find that these quantities are Bi-variate Normal distributed with a weak correlation. We further utilize this finding to derive a unified metric for sufficiency and efficiency of an IM using a Mahalanobis transformation and a Euclidean norm.
Slidecast:
Abstract: Ground motions are in first order a function of magnitude and distance of a given earthquake; this observation is fundamental in the study of earthquakes, and is widely used for the different estimation of earthquake magnitudes. However, in the second order and above, there is a wide range of factors affecting ground motions, and in many cases obscuring the contributions of magnitude and distance, and leading to large uncertainties related to the observed ground motions. Such observations are the basic motivation for developing Ground Motion Prediction Equations (GMPEs), trying to make some sense out of all those second and third order factors at the site, path or source of the analyzed earthquakes. We present a method in which GMPEs are used not only for obtaining attenuation curves for the practical needs of seismic hazard analysis, but also as a powerful research tool for exploring the factors controlling ground motions. In this method we set several stages of regression analysis in order to isolate the different factors controlling ground motions at the San Jacinto Fault Zone (SJFZ), such as rupture directivity and fault zone amplification. The regression scheme includes a built-in time domain tool for the analysis of rupture directivity, and is showing: a) reasonable solutions in reference to the local tectonics at the SJFZ, and b) consistent solutions from a statistical point of view. There are several levels of confidence when using this tool, and in the process of obtaining the GMPEs we have used the basic level of confidence. Since then we added extra levels of confidence, in which the results are more robust, and could be used also as an alternative to focal mechanism techniques. Recent earthquake rupture simulations we have been doing lately are adding even more confidence, aiming to achieve not only statistical consistency of the time-domain directivity analysis, but also robustness on an event by event basis.
Slidecast:
https://vimeo.com/277154849
