J. Delgado

Microtremors as a Geophysical Exploration Tool: Applications and Limitations

Abstract—The usefulness of microtremors as a geophysical exploration tool has been analyzed. This application is possible due to the relationship between the main resonance frequency of a given soil, obtained from the H/V spectral ratios of microtremors, its thickness and average shear velocity. We first measured the ambient noise at 33 sites and determined their main resonance frequency. Detailed geotechnical information was available for 23 of the sites, thereby allowing us to establish a quantitative relationship between the resonance frequency and the thickness of the soil, and indirectly between either of them and the shear velocity of the soil. The practical application of this relationship has revealed its usefulness in determining the surface structure of a valley with excellent accuracy, with an error of only 15% in the depths calculated. These errors are due to the simplification of the problem that this method implies: it requires that the shear velocity vary constantly with depth throughout the study region, which is evidently not always so, and that the input data themselves intrinsically have a certain degree of uncertainty. This method is therefore not valid when there is no mechanical contrast between the study soil and the underlying layer or when the shear velocity varies irregularly with depth in the study area.

Mapping soft soils in the Segura river valley (SE Spain): a case study of microtremors as an exploration tool

This study analyses the spatial distribution of soft soils in a valley with a significant amount of recently accumulated .Late Pleistocene-Holocene sediments, but where the available geotechnical information on the subsurface is inadequate to study the entire valley. To analyse this distribution, we have used a new geophysical prospecting technique, which consists of estimating the thickness of soft soils based on their resonance frequency. This frequency has been determined from the HrV ratios of microtremors measured at 180 sites in the valley. The results indicate the generalised presence of soft soils along almost the entire valley, with thicknesses of up to 67 m. Based on the interpolation of the results, we have drawn up a map showing the spatial distribution and thickness of the soft soils in the valley. This map is of great interest both for future plans regarding the use of the soil in the region as well as for seismic zoning and soil-response studies. q 2000 Elsevier Science B.V. All rights reserved.

Shallow seismicity and active faults in the Betic Cordillera. A preliminary approach to seismic sources associated with specific faults

Abstract Seismicity in the Betic Cordillera is quite active but in general moderate. The maps of I ≥ VI or M1 ≥ 3.5 show some significant groupings. In turn, the network of Neogene and Quaternary faults in the Cordillera is very dense, with three main sets striking N60–70° to east-west, northwest-southeast and northeast-southwest. The fault segments in this network which are active or potentially active from a neotectonic perspective have been marked. We have also compared the fracture network with the epicentral positions of the major seisms, as there is often considerable uncertainty about the location of these positions, and we have indicated the clearest correlations with various fault segments. By using both neotectonic and seismic data, we obtain a series of fault segments considered to be active or potentially active in the Betic Cordillera. These results, while open to future improvements, represent a preliminary step towards establishing the seismic sources associated with segments of specific faults.

The b parameter in the Betic Cordillera, Rif and nearby sectors. Relations with the tectonics of the region

Abstract The a and b parameters of the Gutenberg-Richter ratio for intensities and magnitudes are evaluated according to the methods of calculation, treatment and quality of the data used. When these variability factors have been controlled by the corresponding confidence indices, the mean values of these parameters are used to regionalise the Iberian-Maghreb area as regards shallow and intermediate seismicity. Analysis of the various sectors under consideration (seismic groupings, differentiated seismic sources in the Betic Cordillera, crustal domains and lithospheric domains) provides values of the afore-mentioned parameters coherent with the known tectonic evolution of this area. We also show that the b parameter is not reliable for small-size seismic sources because of the poor statistical representativity of the sample (very few years for small regions). The b parameter values obtained for both crustal and lithospheric domains are remarkably similar and can be linked with what is to be expected in highly fractured sectors. This supports the hypotheses that explain the existence of intermediate seismicity by movement of faults cutting almost the entire lithosphere, or even by the possible presence of a zone of incipient subduction.

Attenuation of Intensity with Epicentral Distance in the Iberian Peninsula

We have classified the attenuation of the Medveded, Sponheuer, and Karnik (MSK) intensity into five types (each as a function of the epicentral intensity I o) based on the mean radii of 254 isoseismal maps, mainly historical earthquakes in the Iberian Peninsula. Geographically representing each earthquake with its corresponding attenuation tendency, it can be seen that those with low attenuation lie west of the Peninsula and those with high attenuation in the south and east. This regionalization seems to be due as much to the seismotectonic characteristics (different crustal types and size of the earthquakes) as to the different construction types in each region. These attenuation values are similar to those of southern Europe, but much higher than those found in the United States. From the point of view of seismic-hazard evaluation, these laws represent an improvement with regard to those used so far. We have extended previous attenuation studies to the whole of the Iberian Peninsula, and, in some points, differences of attenuation assignment of almost two degrees of intensity have been corrected.

Stress fields in the Iberian-Maghrebi region

This study concerns the present stressfield between the Eurasian and Africanplates in the Iberian-Maghrebi region(Portugal, Spain, Morocco, Algeria andTunisia). In addition to an up-to-datecatalogue of earthquakes in this area, acatalogue of the focal mechanisms composedof 486 solutions of fault planes,standardized in terms of notation andinformation type, was used. These data wereused applying the right-dihedron method ofAngelier and Mechler (1977), to obtaindifferent zones with homogeneous stress.The results obtained for shallowearthquakes (h < 30 km) coincide, in themajority of cases, with the general stressfields proposed by numerous authors forthis region, according to which there isNW-SE compression. However, the stressorientation appears to vary in certainareas, perhaps perturbed by the opening ofthe Atlantic Ocean, the approach of Iberiaand Africa, or the extension of the AlboranSea.For the intermediate earthquakes (30 < h< 150 km) no general pattern was found,and the P and T axes seem to be randomlyoriented for the depth intervalsconsidered. For the very deep earthquakes(h > 600 km), however, the P axis lies ina NNW-SSE direction, dipping towards theSSE, while the T axis is subhorizontal in aNE-SW direction.The determinations from the focalmechanisms highlight the existence of aregional stress field with a subhorizontalcompression axis trending NW-SE. Superimposed are others that specificallyaffect particular sectors; these arerelated to the opening of the AtlanticOcean, the extension of the BeticCordillera and the Alboran Sea, and eventhe present compression between the Iberianand European plates.

Liquefaction potential in the Lower Segura river basin (south-east Spain)

Financiado por el proyecto PETRI (PTR94-0046) y por una beca de la Conselleria de Educacion y Ciencia de la Generalitat Valenciana.