José Badal

Estimation of the Expected Number of Casualties Caused by Strong Earthquakes

The human losses after strong earthquakes that occurred in the world during the twentieth century have been analyzed, and a quantitative model for a preliminary assessment of casualties is proposed. It consists of a correlation between the number of casualties and the earthquake magnitude as a function of population density. We tackle the distribution of the total number of casualties within areas of different macroseismic intensity. Prognostic estimations of the expected number of killed or injured people caused by a supposed strong earthquake in Andalucia (Spain), using the model based on worldwide data, are suggested. Prognostic estimations based on specific data about the Kanto–Tokai (Japan) region are likewise given and compared with the number of casualties due to the 1995 Kobe (Japan) earthquake. In relation to the expected number of victims in areas affected by strong seismic impacts, we compute the casualty rate as the number of people killed divided into the inhabitants of a region and show its variation for different population density groups in the case of two extreme earthquake magnitudes.

Tomographic images of the Iberian subcrustal lithosphere and asthenosphere

The S wave velocity structure of the lithopheric mantle and asthenosphere beneath Iberia is displayed by means of tomographic images obtained from dispersion data of Rayleigh waves propagating across the Iberian region. We have used long-period data recorded at the broadband stations of the Network of Autonomously Recording Seismographs (NARS) installed in the Iberian Peninsula on the occasion of the Iberian Lithosphere Heterogeneity and Anisotropy (ILIHA) project. A total of 64 teleseismic events provided by the ILIHA array and 143 seismic paths have been studied. Surface wave dispersion analysis is carried out using various methods: from methods for a correct acquisition of data and subsequent two-station surface wave velocity measurements to inversion methods for velocity structure and methods for verifying the reliability of the inversion results. The phase and group velocity dispersion curves of the fundamental mode Rayleigh waves are the basic information from which we have obtained several Earth models represented by shear wave velocity distributions with depth. Using these inversion results, we display the most conspicuous features of the velocity stnicture of the Iberian lithosphere-asthenosphere system and propose a new regional model, the Iberian Lithosphere-Asthenosphere (ILA) model, for the deep structure of the Iberian Peninsula down to a depth of 200 km. We use a representation technique based on an iterative Laplacian interpolation method for obtaining tomographic images of the subcrustal lithosphere and asthenosphere of Iberia. We find significant lateral velocity variations in the peninsula, though these differences vary with depth interval. A low-velocity channel in the lithosphere (41–51 km) appears with nonuniform velocity structure. In contrast, at the greatest lithospheric depths (51–81 km), almost the entire peninsula shows a rather uniform velocity structure. The asthenosphere (81–181 km) is clearly a nonhomogeneous gross layer both laterally and with depth. The relatively higher velocities span the shallower depths within the asthenosphere, whereas the lower ones span the deeper part.

Inversion of Rayleigh wave phase and group velocities by simulated annealing

Abstract A joint inversion of regionalized phase and group velocities of fundamental mode Rayleigh waves is performed by means of a simulated annealing scheme which makes use of some thermodynamic analogies based on the Gibbs’ distribution. The correlations and the uncertainties on shear-wave velocities defining the elastic model are estimated by using a procedure based on the same thermodynamic analogies. As a relevant feature, the simulated annealing algorithm permits reducing the usual dependence of the solution on the sometimes arbitrary starting model. A shortcoming linked to the use of this procedure for solving an inversion problem is the necessity of a previous empirical determination of some parameters controlling the annealing process. With this aim, a simulation of a joint inversion of synthetic surface wave dispersion data has been performed. After that, path-averaged velocity dispersion curves corresponding to a Hercynian area of the Iberian Peninsula have been inverted to derive two possible crust and upper mantle elastic structures. Whereas, the first structure has been deduced by considering as unknown variables the shear velocities of the stratified Earth model, the second has been obtained also including the respective layer thicknesses. Both structures are compared with shear velocity and Qβ models derived by the stochastic inversion procedure in previous studies and from the same Rayleigh waves dataset. Finally, advantages and shortcomings of the inversion of phase and group velocity data by means of simulated annealing are discussed.

Shallow structure of part of northwestern Iberia from short-period Rayleigh-wave observations

Propagation of 0.2–2.5 s short-period Rayleigh-waves across a part of northwestern Iberia is investigated. Analysis of a seismic profile line with recorded seismograms up to distances of 70 km from the shot-point, provides group-velocities and attenuation coefficients of the fundamental mode. First higher-mode group-velocity has been obtained for only one of the studied cases. The studied region has been divided into two subrogions, the northwestern and the southeastern side from the shot-point. Inversion procedures applied to the velocity and attenuation data yield the shear velocity and the Q−1β models up to a depth of about 1.5 km for the NW, and 2.4 for the SE side. From the shear-wave obtained models corresponding to the NW and to the SE sides, it is possible to infer the existence of some degree of lateral variation of S velocity. The NW side presents velocities varying between 1.8 km · s−1 (at the surface) and 3.3 km · s−1 (at 1.2 km depth). The velocities in the SE model, for the same depths, are 2.2 and 3.4 km · s−1, respectively. Using only the fist six traces of the SE side, it is possible to infer a low-velocity region located between 0.4 km and 0.7 km depth. It seems that the velocities are higher in the complex geological region of the SE side than in the NW one, which is in agreement with P velocity models found by others. From the tentative anelastic attenuation coefficients, Q−1β models have been inferred for both sides of the seismic refraction profile. Although the obtained Q−1β models are somehow different in shape, the maximum Q−1β value is about 0.05 in both cases.

Moment Magnitudes for Early (1923–1961) Instrumental Iberian Earthquakes

A collection of 18 Iberian earthquakes, felt with epicentral intensity I 0 ≥ VI (MSK) in the Iberian Peninsula during the period 1923–1961, just before the introduction of the WWSSN stations in 1962, have been investigated through digital reconstruction of old seismograms recorded in four Spanish seismological observatories: Alicante, Almeria, Ebro, and Toledo. Spectral analysis of the seismic waves recorded at the different stations has been performed and the seismic moment of the early instrumentally recorded earthquakes considered has been calculated according to the model of Brune (1970) and applying corrections by instrumental response, anelastic attenuation, free surface, and radiation pattern. A seismic moment-magnitude relation has been obtained, and the result has been compared with other regressions obtained for different seismo-active regions in Spain and in the world. The moment magnitudes of the earthquakes considered have been calculated according to the scale of Kanamori (1977) and Hanks and Kanamori (1979). Finally, a magnitude correction for the conversion of surface wave magnitude to moment magnitude has been proposed: M = 0.96 M (A/T) – 0.36. This conversion should be considered valid for the 4.0–6.0 surface-wave magnitude interval, which spans well the characteristic range for the moderate seismic activity in the Iberian Peninsula.

Spatial prediction procedures for regionalization and 3-D imaging of Earth structures

Abstract The imaging of three-dimensional (3-D) Earth structures from tomographic results is an especially delicate subject due to various problems. When the data provided by the seismic surveys are averaged values that do not describe locally the medium, surface wave tomography is a relevant example of this, the regularization constraints to be imposed in the inverse process are fairly subjective. In fact, the methods for regionalization of the seismic information (dispersion data, attenuation parameters, etc.) involve an inverse problem that usually must be solved by following a mathematical approach. Opposite to this, we propose various non-inverse procedures with a common target: to reconstruct 3-D Earth structures from irregularly sampled seismic data consisting of path-averaged values depending on the wavelength or varying with depth. For this purpose we review different imaging techniques aimed at volumetric modeling and visualization of data. We refer to special rendering methods and the key operations concerning the methodology to follow: gridding and interpolation. In order to get a faster and simpler volume visualization way, we use a regular voxel grid that we achieve by means of a selected gridding algorithm under specific controls. Exact-type methods (inverse distance weighting, kriging, splines, finite differences, gridding triangulation, wavelets and gradient-wavelets) and approximate methods (least-square fitting with splines) are very briefly described. In particular, both a modified 2-D slicing Laplacian method, based on interpolation by finite differences, and the 3-D direct wavelets and gradient-wavelets methods are original. To compare the accuracy and computational efficiency of all these methods, we have them applied to synthetic data trying the reconstruction of specific volumes whose (geometrical and physical) characteristics are known. We also have used real data and hereby show some tomographic solutions related to the research of domains at different scales and depths.

Anelastic structure of the Iberian Peninsula obtained from an automated regionalization algorithm and stochastic inversion

Abstract An automated regionalization of a reduced domain of the Earths surface, without a-priori seismotectonic information, and the corresponding anelastic structures are achieved by means of a sequential application of a principal component analysis, a clustering procedure and the stochastic inversion algorithm to anelastic attenuation coefficients of Rayleigh waves. Our database is formed by path-averaged attenuation coefficients derived from fundamental mode Rayleigh waves crossing the Iberian Peninsula. The wavetrains were recorded at the broad-band stations installed some years ago in the Iberian Peninsula for the ILIHA project. Before the sequential application of the algorithms, the area covered by the seismic paths was characterized by 22 maps of local attenuation coefficients corresponding to the 10–120 s period range. All these maps were obtained by applying Yanovskayas formulation for laterally heterogeneous media to the set of path-averaged attenuation coefficients mentioned above. After that, we divided the Iberian Peninsula into six homogeneous regions in terms of the local attenuation coefficients for 22 different periods and by means of the principal component analysis and the clustering algorithm. We then obtained, by stochastic inversion, the respective anelastic structure down to a depth of 225 km for the six homogeneous regions. The coefficients vary from 1.0 to 2.0×10−3 km−1 and the Q−1β structures, ranging from 110×10−3 to 40×10−3, suggest for five of the regions an asthenosphere with its upper and lower boundaries close to 80 and 180 km depth, respectively. A shallow beginning of the asthenosphere is detected for the sixth homogeneous region. This range is very similar to the depths deduced from previous tomographic studies of the Iberian Peninsula based on Rayleigh wave phase and group velocities. Finally, correlations obtained between the regions and some seismotectonic characteristics of the Peninsula are discussed. It is noteworthy that we cannot associate each of the six homogeneous regions with a single Hercynian, Alpine or Neogene domain.

Shear velocity structure beneath the Iberian Massif from broadband Rayleigh wave data

Abstract Up to a few years ago, dispersion analyses of surface waves across the Iberian Peninsula and adjacent zones were based on analog data recorded at the long-period Iberian stations. The installation of the NARS array on Iberian territory for a period of one year, has provided a greater station density than was previously available with the very few permanent long-period seismological stations installed on the peninsula. The NARS array also provided quality digital records, and increased the path coverage for two-station surface wave velocity measurements. Fundamental mode Rayleigh waves recorded at broadband stations belonging to this array have been analyzed to produce phase and group velocity dispersion curves for the period range 10–90 s. With the dataset now available, the elastic structure beneath the Iberian Massif has been investigated in terms of the shear velocity distribution as a function of depth. Time-variable filtering is employed to remove higher mode interference efficiently and to improve isolation of the fundamental mode Rayleigh wave from the seismograms. Multiple filtering is then used to compute group velocities at each station. The interstation Rayleigh wave group velocity can thus be easily calculated. Frequency-domain Wiener deconvolution is used to determine the interstation phase velocity. We perform inversion of velocity dispersion data containing both Rayleigh wave phase velocities and group velocities according to the generalized inversion theory by means of the stochastic inverse operator. The theoretical models for the lower crust and uppermost mantle beneath the Iberian Massif obtained by joint inversion, show a continental lithosphere with a thickness of 81 km. The crustal and subcrustal velocities are greater than in other areas of the Iberian Peninsula. The asthenosphere appears as a layer 100 km thick defined by very low velocities when compared with the rest of the peninsular area. Both the lithosphere and the asthenosphere exhibit a low velocity channel. In the lithosphere a subcrustal low velocity channel has velocity constrained between 4.33 and 4.62 km s−1. In the asthenosphere the low velocity channel is constrained between 4.13 and 4.36 km s−1.

Imaging of the Near-Surface Shear-Wave Velocity Structure of the Granada Basin (Southern Spain)

The seismic velocity structure of the Granada Basin (southern Spain) at depths of 1-4 km is investigated by dispersion analysis of 0.5- to 5.9-sec Rayleigh waves. We have used records from 12 quarry blasts detonated between 1990 and 2000 and 16 local earthquakes with magnitudes between 2.6 and 3.8 that occurred between 1990 and 1997. All events were recorded on the Regional Seismic Network of Andalucia, Spain, and on the National Seismic Network. P -, S - and Rg -phases were recorded at 13 stations at distances between 10 and 72 km from the source. The analysis and inversion of Rg waveforms is performed using multiple filtering techniques and a generalized inversion approach. We measured ray-path group velocities from 0.83 to 2.15 km/sec in the 0.6- to 5.8-sec period interval, and obtained shear-wave velocity profiles for 29 source-station paths. The shear velocities lie between 0.8 and 3.34 km/sec within a standard deviation band of 0.05-0.14 km/sec. From these models, it is possible to infer the most conspicuous features in the upper 4 km of the region and the existence of some degree of lateral variation in velocity. After applying an imaging method aimed at volumetric modeling and data visualization, we present the first velocity images obtained for the Granada Basin. Our results characterize the basin and are correlated with the surface geology. Manuscript received 7 September 2001.

Magnitude Formulae and Intensity–Magnitude Relations for Early Instrumental Earthquakes in the Iberian Region

The magnitudes of early instrumentally recorded earthquakes in the Iberian region (1912–1962) have been studied through processing of digitized seismograms of Wiechert seismograph and analysis of macroseismic information. A magnitude system based on instrumental registrations and macroseismic observations has been proposed. It consists of two compatible magnitude formulae depending on the total duration of seismic oscillations and on the maximum ground amplitude/period ratio of surface waves and includes correspondent intensity–magnitude relationships.