On the complexity of seismic waves trapped in non-flat geologic features

Most earthquake engineering and seismological models make the sweeping assumption that the world is flat. The ground surface topography, however, has been repeatedly shown to strongly affect the amplitude, frequency, duration and damage induced by earthquake shaking, effects mostly ignored in earthquake simulations and engineering design. In this talk, I will show a collection of examples that highlight the effects of topography on seismic ground shaking, and I will point out what these results suggest in the context of the current state-of-earthquake engineering practice. Examples will range from semi-analytical solutions of wave propagation in infinite wedge to three-dimensional numerical simulations of topography effects using digital elevation map-generated models and layered geologic features. I will conclude by demonstrating that ‘topography’ effects vary strongly with the stratigraphy and inelastic behavior of the underlying geologic materials, and thus cannot be accurately predicted by studying the effects of ground surface geometry alone

On the complexity of seismic waves trapped in non-flat geologic features

Most earthquake engineering and seismological models make the sweeping assumption that the world is flat. The ground surface topography, however, has been repeatedly shown to strongly affect the amplitude, frequency, duration and damage induced by earthquake shaking, effects mostly ignored in earthquake simulations and engineering design. In this talk, I will show a collection of examples that highlight the effects of topography on seismic ground shaking, and I will point out what these results suggest in the context of the current state-of-earthquake engineering practice. Examples will range from semi-analytical solutions of wave propagation in infinite wedge to three-dimensional numerical simulations of topography effects using digital elevation map-generated models and layered geologic features. I will conclude by demonstrating that ‘topography’ effects vary strongly with the stratigraphy and inelastic behavior of the underlying geologic materials, and thus cannot be accurately predicted by studying the effects of ground surface geometry alone

Basin Effects in Strong Ground Motion: A Case Study from the 2015 Gorkha, Nepal, Earthquake

The term "basin effects" refers to entrapment and reverberation of earthquake waves in soft sedimentary deposits underlain by concave basement rock structures. Basin effects can significantly affect the amplitude, frequency, and duration of strong ground motion, while the cone-like geometry of the basin edges gives rise to large amplitude surface waves through seismic wave diffraction and energy focusing, a well-known characteristic of basin effects. In this research, we study the role of basin effects in the mainshock ground motion data recorded at the Kathmandu Basin, Nepal, during the 2015 M_w7.8 Gorkha earthquake sequence. We specifically try to understand the source of the unusual low frequency reverberating pulse that appeared systematically across the basin, and the unexpected depletion of the ground surface motions from high frequency components, especially away from the basin edges. In order to do that we study the response of a 2D cross section of Kathmandu Basin subjected to vertically propagating plane SV waves. Despite the scarcity of geotechnical information and of strong ground motion recordings, we show that an idealized plane-strain elastic model with a simplified layered velocity structure can capture surprisingly well the low frequency components of the basin ground response. We finally couple the 2D elastic simulation with a 1D nonlinear analysis of the shallow basin sediments. The 1D nonlinear approximation shows improved performance over a larger frequency range relative to the first order approximation of a 2D elastic layered basin response

Observations and Simulations of Basin Effects in the Kathmandu Valley During the 2015 Gorkha, Nepal, Earthquake Sequence

The M7.8 Gorkha, Nepal main shock ruptured a segment of the Main Himalayan Thrust (MHT) directly below Kathmandu Valley, causing strong shaking levels across the valley. Strong-motion data reveal an initial 6 s source pulse that was amplified and reverberated within the basin. One of the striking features of the observed ground motions in the valley was the exceptionally low energy of periods less than 2 s, which likely limited the extent and severity of structural damage in Kathmandu compared with alternative rupture scenarios of the same magnitude in the region. Isolated cases of liquefaction and lateral spreading of unconsolidated sediments were also observed, but have not yet revealed a systematic damage pattern. Initial analysis of available data suggests that several different factors, including source and path as well as site effects, were responsible for the unusual ground motions characteristics. In this paper, we provide a short description of the Kathmandu Valley geology and analyze available strong-motion records from the main shock and three strong aftershocks, with the intent to shed light on earthquake reconnaissance observations from this earthquake

Observations and Simulations of Basin Effects in the Kathmandu Valley During the 2015 Gorkha, Nepal, Earthquake Sequence

The M7.8 Gorkha, Nepal main shock ruptured a segment of the Main Himalayan Thrust (MHT) directly below Kathmandu Valley, causing strong shaking levels across the valley. Strong-motion data reveal an initial 6 s source pulse that was amplified and reverberated within the basin. One of the striking features of the observed ground motions in the valley was the exceptionally low energy of periods less than 2 s, which likely limited the extent and severity of structural damage in Kathmandu compared with alternative rupture scenarios of the same magnitude in the region. Isolated cases of liquefaction and lateral spreading of unconsolidated sediments were also observed, but have not yet revealed a systematic damage pattern. Initial analysis of available data suggests that several different factors, including source and path as well as site effects, were responsible for the unusual ground motions characteristics. In this paper, we provide a short description of the Kathmandu Valley geology and analyze available strong-motion records from the main shock and three strong aftershocks, with the intent to shed light on earthquake reconnaissance observations from this earthquake

Finishing the euchromatic sequence of the human genome

The sequence of the human genome encodes the genetic instructions for human physiology, as well as rich information about human evolution. In 2001, the International Human Genome Sequencing Consortium reported a draft sequence of the euchromatic portion of the human genome. Since then, the international collaboration has worked to convert this draft into a genome sequence with high accuracy and nearly complete coverage. Here, we report the result of this finishing process. The current genome sequence (Build 35) contains 2.85 billion nucleotides interrupted by only 341 gaps. It covers ∼99% of the euchromatic genome and is accurate to an error rate of ∼1 event per 100,000 bases. Many of the remaining euchromatic gaps are associated with segmental duplications and will require focused work with new methods. The near-complete sequence, the first for a vertebrate, greatly improves the precision of biological analyses of the human genome including studies of gene number, birth and death. Notably, the human enome seems to encode only 20,000-25,000 protein-coding genes. The genome sequence reported here should serve as a firm foundation for biomedical research in the decades ahead

Geometry and stratigraphy parameterization of topography effects: From the infinite wedge to 3D convex features

Although the problem of seismic wave scattering by topographic irregularities has been studied for several decades, only recently it has attracted the attention of geotechnical earthquake engineering researchers. Macroseismic observations and recorded evidence from large earthquakes have highlighted that structural damage intensity is frequently higher on the surface of irregular topographies than on adjacent flat ground sites. Numerical and semi-analytical published studies have qualitatively corroborated these observations, but when compared to field recordings, have been shown to systematically underestimate the absolute level of topographic amplification up to an order of magnitude or more in some cases. This discrepancy between theory and observations has been attributed, at least in part, to idealizations of the above studies such as the assumptions of 2D geometry, homogeneous medium, linear elastic response, and monochromatic or narrowband ground shaking. In this research, we bridge the quantitative gap between previous theoretical studies and observations by systematically studying the role of geometry, stratigraphy, and ground motion characteristics through a series of elaborate numerical analyses. We specifically start from the topographic amplification caused by a 2D infinite wedge on the surface of a homogeneous elastic halfspace, and extend the state-of-the-art understanding of wave focusing and scattering by this fundamental block of irregular ground surface geometries. From there, we gradually increase the geometric and stratigraphic complexity up to a 3D convex layered topographic feature, identifying in each level the controlling factors of topographic amplification. Our results provide new insights into the effects of surface topography and its nonlinear coupling with subsurface soil layering, and suggest that in real conditions, topographic amplification can only be quantitatively captured when geometry and stratigraphy of the site are simultaneously accounted for in theoretical predictive models.Ph.D

Simulation and Validation of Topographic Effectds on Mt Pleasant, Christchurch, New Zealand

Damage distribution maps from strong earthquakes and recorded data from field experiments have repeatedly shown that the ground surface topography and subsurface stratigraphy play a decisive role in shaping the ground motion characteristics at a site. Published theoretical studies qualitatively agree with observations from past seismic events and experiments; quantitatively, however, they systematically underestimate the absolute level of topographic amplification up to an order of magnitude or more in some cases. We have hypothesized in previous work that this discrepancy stems from idealizations of the geometry, material properties, and incident motion characteristics that most theoretical studies make. In this study, we perform numerical simulations of seismic wave propagation in heterogeneous media with arbitrary ground surface geometry, and compare results with high quality field recordings from a site with strong surface topography. Our goal is to explore whether high-fidelity simulations and realistic numerical models can – contrary to theoretical models – capture quantitatively the frequency and amplitude characteristics of topographic effects. For validation, we use field data from a linear array of nine portable seismometers that we deployed on Mount Pleasant and Heathcote Valley, Christchurch, New Zealand, and we compute empirical standard spectral ratios (SSR) and single-station horizontal-to-vertical spectral ratios (HVSR). The instruments recorded ambient vibrations and remote earthquakes for a period of two months (March-April 2017). We next perform two-dimensional wave propagation simulations using the explicit finite difference code FLAC. We construct our numerical model using a high-resolution (8m) Digital Elevation Map (DEM) available for the site, an estimated subsurface stratigraphy consistent with the geomorphology of the site, and soil properties estimated from in-situ and non-destructive tests. We subject the model to in-plane and out-of-plane incident motions that span a broadband frequency range (0.1-20Hz). Numerical and empirical spectral ratios from our blind prediction are found in very good quantitative agreement for stations on the slope of Mount Pleasant and on the surface of Heathcote Valley, across a wide range of frequencies that reveal the role of topography, soil amplification and basin edge focusing on the distribution of ground surface motion