Empirical evidence of local seismic effects at sites with pronounced topography: a systematic approach

The recent growth of seismic monitoring networks allows for systematic studies of local seismic effects at sites with pronounced topography.We applied a terrain classification method to identify such sites within Swiss and Japanese networks and compiled a data set of high-quality earthquake recordings. As a number of recent studies have found local effects to be directional at sites with strong topographic features, polarization analysis of particle motion was performed and azimuthally dependent resonant frequencies were estimated. The same procedure was also applied for available ambient vibration recordings. Moreover, average residuals with respect to ground motion prediction models for a reference bedrock were calculated to estimate the average amplification or deamplification for each station. On one hand, observed amplifications are found to be tightly linked with ground motion directionality as estimated by polarization analysis for both earthquake and ambient vibration recordings. On the other hand, we found no clear relation between local topographic features and observed amplification, so the local subsurface properties (i.e. shear wave velocity structure) seem to play the key role and not the geometry itself

High-Frequency Directivity in Strong Ground Motion Modeling Methods

We are investigating two distinct strong ground motion simulation techniques as regards their high-frequency directivity: i) the composite model with a fractal subevent size dis- tribution, based on the method of summation of empirical Green’s functions, and ii) the integral model with the k-squared slip model with k-dependent rise time, based on the representation theorem. We test the simulations in a 1D layered crustal model against em- pirical PGA attenuation relations, particularly with regard to their uncertainty, described by the standard deviation ( ). We assume that any synthetic model for a particular earth- quake should not provide a PGA scatter larger than the observed scatter for a large set of earthquakes. The 1999 Athens earthquake (Mw=5.9) is studied as a test example. In the composite method, the synthetic data display a scatter of less than ±2 around the empirical mean. The k-squared method displays a larger scatter, demonstrating strong high-frequency directivity. It is shown that the latter can be reduced by introducing a formal spectral modification. 1 Introduction Low-frequency directivity effects are well known. For example, there is a number of seismic recordings of recent earthquakes (e.g., 1992 Landers, 1994 Northridge, 1995 Kobe, 1999 Chi-Chi), which show long-period velocity pulses caused by rupture propagation towards a station. This effect can be successfully explained by the apparent source time function varying with azimuth (Haskell, 1964). 2Unpublishedope

High-Frequency Directivity in Strong Ground Motion Modeling Methods

We are investigating two distinct strong ground motion simulation techniques as regards their high-frequency directivity: i) the composite model with a fractal subevent size dis- tribution, based on the method of summation of empirical Green’s functions, and ii) the integral model with the k-squared slip model with k-dependent rise time, based on the representation theorem. We test the simulations in a 1D layered crustal model against em- pirical PGA attenuation relations, particularly with regard to their uncertainty, described by the standard deviation ( ). We assume that any synthetic model for a particular earth- quake should not provide a PGA scatter larger than the observed scatter for a large set of earthquakes. The 1999 Athens earthquake (Mw=5.9) is studied as a test example. In the composite method, the synthetic data display a scatter of less than ±2 around the empirical mean. The k-squared method displays a larger scatter, demonstrating strong high-frequency directivity. It is shown that the latter can be reduced by introducing a formal spectral modification. 1 Introduction Low-frequency directivity effects are well known. For example, there is a number of seismic recordings of recent earthquakes (e.g., 1992 Landers, 1994 Northridge, 1995 Kobe, 1999 Chi-Chi), which show long-period velocity pulses caused by rupture propagation towards a station. This effect can be successfully explained by the apparent source time function varying with azimuth (Haskell, 1964).

Fundamental and higher two-dimensional resonance modes of an Alpine valley

ISSN:0956-540XISSN:1365-246

Using ellipticity information for site characterization

Network of Research Infrastructures for European Seismology, Deliverable JRA4-D

Coupled seismogenic geohazards in Alpine regions

COupled seismogenic GEohazards in Alpine Regions (COGEAR) is an interdisciplinary natural hazard project investigating the hazard chain induced by earthquakes. It addresses tectonic processes and the related variability of seismicity in space and time, earthquake forecasting and short-term precursors, and strong ground motion as a result of source and complex path effects. We study non-linear wave propagation phenomena, liquefaction and triggering of landslides in soil and rock, as well as earthquake-induced snow avalanches. The Valais, and in particular parts of the Rhone, Visper, and Matter valleys have been selected as study areas. Tasks include detailed field investigations, development and application of numerical modeling techniques, assessment of the susceptibility to seismically induced effects, and installation of different monitoring systems to test and validate our models. These systems are for long-term operation and include a continuous GPS and seismic networks, a test installation for observing earthquake precursors, and a system to study site-effects and non-linear phenomena in two test areas (Visp, St. Niklaus / Randa). Risk-related aspects relevant for buildings and lifelines are also considered. © 2012-OGS