Abstract: We present a comparison of Probabilistic Seismic Hazard Analyses computed at reference sites in Mexico City using Ground Motion Prediction Equations (GMPE) and synthetic seismograms. The computation of the latter is achieved by using a hybrid approach employing the Strain Green’s Tensor (SGT) formalism (e.g., Lee et al., 2011) with a realistic 3D model of Central Mexico (Juarez-Zuñiga, 2016) and Empirical Green’s functions. The assessment considers the seismotectonic regionalization of Mexico and seismicity parameters determined by Zuñiga (2017). Our estimates, constrained to response spectral accelerations for periods T=0.5, 1, 1.5, 2, 3 and 5 s, reveal substantial differences between GMPE and synthetically based values. Additionally, we discuss the implications of our results regarding the design spectra in Mexico’s capital. This project was funded by the Secretaria de Ciencia, Tecnología e Innovación (SECITI) of Mexico City. Project SECITI/073/2016
Slidecast:
https://vimeo.com/277177352
Abstract: Moczo et al. (2002, 2014), Kristek & Moczo (2003) and Kristek et al. (2017) developed the discrete representation of a strong discontinuous and smooth material heterogeneity in the elastic and viscoelastic media suitable for the finite-difference modelling of seismic wave propagation and earthquake ground motion. The representation is capable of sub-cell resolution and “sensing” an arbitrary shape and position of the interface in the grid. Extensive tests in canonical and complex realistic models confirmed accuracy and computational efficiency of the representation. With the proper discrete representation of material heterogeneity the most advanced finite-difference schemes can be more efficient in case of local surface sedimentary structures than the spectral-element and discontinuous-Galerkin methods (e.g., Chaljub et al. 2010, 2015; Maufroy et al. 2015). Is such a discrete representation possible also in the considerably mathematically and physically more complex poroelastic medium with larger number and more complex constitutive and governing equations? Yes, it is possible. We have developed a new discrete representation of the discontinuous and continuous material heterogeneity of the poroelastic medium. In our representation the stiffness matrix of the averaged medium has the same structure as the stiffness matrix for a smoothly heterogeneous medium but sufficiently accurately approximates boundary conditions at the interface. The same structure means that the number of algebraic operations in calculating stress and pressure is the same as in the smoothly heterogeneous poroelastic medium. We demonstrate the accuracy and the sub-cell resolution using a set of canonical configurations. We compare our seismograms with those calculated by the exact method developed by Diaz & Ezziani (2008). We also compare our seismograms with those obtained with an independent numerical method for a structurally complex model.
Slidecast:
https://vimeo.com/277155616
Abstract: Mass wasting processes such as rock falls, debris flows and land slides are often the initial elements of sediment cascades that shape the Earth’s surface, from high mountain peaks to the ocean basins, and they are driven by intrinsic and extrinsic mechanisms. While these drivers can usually be measured straightforwardly by classic approaches, the actual mass wasting processes are difficult to constrain, because they show infrequent and rapid occurrence in locations that are hard to predict, span several orders of magnitude in size, and typically involve a series of successive and combined processes. Environmental seismology, the study of the seismic signals emitted by Earth surface processes, provides a valuable alternative to this shortcoming. It allows insights to the evolution of mass wasting processes, assigning precise timing and location information (including tracking of moving sources), estimating the mobilised volumes, and monitoring precursor signals of large mass wasting processes. This contribution explores the validity and limitations of seismic methods to detect and locate small mass wasting processes, shows how retrospective analysis reveals the relative contribution of different process drivers, and how prospective analysis helps identifying early warning and rapid response opportunities. These themes are based on case studies from natural laboratory settings, explored by the GFZ Geomorphology Section: the Askja caldera flank collapse on Iceland, the rock fall prone Lauterbrunnen Valley and debris flow dominated Illgraben in Switzerland, and a chalk cliff coast on the island of Rügen in Germany.
Slidecast:
https://vimeo.com/277181958
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
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: 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
