Showing papers by "Domenico Giardini published in 2001"

A theoretical investigation of average H/V ratios

Abstract: SUMMARY The mode summation method and a finite difference technique are applied to investigate the spectral ratio between the horizontal and vertical components (H/V ratio) of ambient vibrations and to explore the variation of the resonance frequency and the amplitude and shape of polarization as a function of the structure and the source positions. Layered structural models are used by assuming a large number of sources distributed around a receiver, with shallow source depths that are randomly assigned. We identify stable parts of the H/V ratios that are independent of the source distance and are dominated by the ellipticity of the fundamental-mode Rayleigh wave in the frequency band between the fundamental frequency of resonance of the unconsolidated sediments and the first minimum of the average H/V ratio. The ellipticity in this frequency band is determined by the layering of the sediments. The numerical simulations are compared with observations at a site where the thickness and velocity structure of the unconsolidated sediments are known from S-wave and surface wave measurements. Two methods are applied to compute the H/V ratio, the classical method in the frequency domain and a method based on frequency–time analysis that allows us to locate P–SV wavelets in the time-series. The main problem in comparing synthetics with observations is the contribution of SH waves in the observed H/V ratios. We propose a method to minimize these effects and the effects of the superposition of different incoming P–SV waves. An inversion scheme is applied to the stable parts of the observed H/V ratio, based on a genetic algorithm, to retrieve the S-wave velocity structure from a single ambient vibration record.

Active normal faulting in the upper Rhine graben and paleoseismic identification of the 1356 Basel earthquake.

Abstract: We have identified an active normal fault in the epicentral area of the Basel (Switzerland) earthquake of 18 October 1356, the largest historical seismic event in central Europe. The event of 1356 and two prehistoric events have been characterized on the fault with geomorphological analysis, geophysical prospecting, and trenching. Carbon-14 dating indicates that the youngest event occurred in the interval 610 to 1475 A.D. and may correspond to the 1356 Basel earthquake. The occurrence of the three earthquakes induced a total of 1.8 meters of vertical displacement in the past 8500 years for a mean uplift rate of 0.21 millimeters per year. These successive ruptures on the normal fault indicate the potential for strong ground movements in the Basel region and should be taken into account to refine the seismic hazard estimates along the Rhine graben.

A systematic test of the hypothesis that the b value varies with depth in California

Abstract: We spatially map the shallow to deep b value ratio in the crust in California. Previous studies of the frequency magnitude distribution, as a function of depth, for selected crustal regions indicated that b decreases from b > 1.1 in the 0–5 km depth range to b < 0.8 in the depth range 7–15 km. Our detailed mapping confirms that this pattern can be established at the 99% significance level for about 32% of the entire seismically active crust. About 2% of the crust displays the opposite b-gradient. One such area is the San Francisco Bay Area. We speculate that differences in stress levels are the main factor controlling the depth dependency of b. These results confirm that the b value should not always be considered a constant in studies such as seismic hazard estimations.

Unified seismic hazard modelling throughout the Mediterranean region

Abstract: The generation of a uniform seismic hazard modelling for the whole Mediterranean basin has been carried out under the coordination of activities of the IUGS-UNESCO IGCP-382 project Seismotectonics and Seismic Hazard Assessment of the Mediterranean Basin (SESAME) and the European Seismological Commission Working Group on Seismic Hazard Assessment. While the final approach for the Mediterranean region in the global hazard map of the Global Seismic Hazard Assessment Program was to assemble independent regional and national hazard maps, main efforts within recent SESAME activities have focused on the development of a unified seismic source model for the whole Mediterranean for homogeneous hazard computations. This unified source model is based on the compilation of existing data on seismogenic models for the region. The individual regional and national zonations have been joined, new seismic sources at border areas between different regions have been redesigned to avoid ambiguities coming from different approaches, and draft source zones have been established in areas where these were not yet availble. For the first time, a unified seismic source model for the whole of the Mediterranean region is presented and a suite of probabilistic hazard maps computed in a homogeneous fashion have been generated.

Eurasia-Africa plate boundary region yields new seismographic data

Abstract: The tectonic plate boundary between Eurasia and Africa is complex, in that it cannot be characterized as a single discrete plate boundary Deformation near this plate boundary varies from trans-tensional in the Azores archipelago, through strike-slip in the eastern Atlantic basin, to overall compressional between the European and African continents, with extensional sub-domains in the Mediterranean Sea. This complex pattern of deformation, related plate motion, and underlying driving forces leads to strong variations in seismic hazard throughout the region. A better understanding of the plate boundary processes requires knowing crust and upper mantle structure in the region, which is best investigated with three-component, broadband seismic data. To investigate the region's three-dimensional crust and upper mantle structure, we are carrying out a multiinstitutional project (MIDSEA) involving seismologists from 10 countries on the northern, southern, and western sides of the plate boundary.

The Future of Permanent Seismic Networks

Abstract: For the past 15 to 20 years, seismologists have been progressively building a hierarchy of global and regional seismic networks to study Earth structure and for earthquake surveillance, respectively The networks will be standardized, broadband, and digital, with data telemetry to a small number of regional data collection/archival centers, accessible freely over the Internet