Site effects in Mexico City eight years after the September 1985 Michoacan earthquakes

The purpose of this paper is to take a comprehensive look at site effects in Mexico City for the 1985 Michoacan earthquake. We examine, successively, 1D and 2D models. For the latter, we consider in detail both large scale and small scale heterogeneities, using extensively the Aki-Larner wave propagation method, in the version given by Bard and Gariel. In particular, we make a critical review of the different explanations proposed for the large duration of strong ground motion in the lake zone. Our purpose is two-sided. We first outline the difference between what is well established and what remains still unexplained regarding the seismic response of Mexico City basin. On the other hand, we wish to make explicit the conditions that the proposed models require to explain strong motion duration. Our results allow us to qualify the models proposed to date and to point out what could be the experiments and the new data required to find a truly satisfactory explanation of strong ground motion at Mexico City.     

Acebuches amphibolites in the Aracena hercynian metamorphic belt (southwest Spain): Geochemical variations and basaltic affinities

A 1500–2000 m sequence of amphibolites exhibits progressively increasing metamorphic grade from greenschist to the amphibolite/granulite facies transition. The chemical variations are considered to be principally inherited from the basaltic parent. The appearance of clinopyroxene is related to P,T and to bulk-rock composition during metamorphism. Excluding some basal rocks in the orthopyroxene zone, chemical variations related to metamorphism are minor. The small decrease in the K₂O content from 0.3% to 0.1% with increasing grade suggests that the low K content was a characteristic of the basaltic precursor. These amphibolites have affinities with abyssal tholeiites.     

Strong motion dataset of Turkey: data processing and site classification

Strong motion records of Turkey are studied in order to prepare a catalog to be used as a database for further studies (for instance empirical attenuation laws). The network started to be installed in 1973, and the first record was obtained in 1976. The instruments are of SMA-1 analog recorders and SIG SM-2 and GeoSys GSR-16 digital types. Out of a total of 426 records released on the web sites of the General Directorate of the Disaster Affairs and of Kandili Observatory, a set of 210 records was selected with a satisfactory quality, for which it was possible to associate correctly determined source parameters (source magnitudes and epicentral distances). Most of the records are obtained from around North and East Anatolian Fault zones, as well as from western and southwestern parts of Turkey. The main outcome of this paper is a strong motion catalog of Turkey, with the indication of site conditions, of the frequency band of the reliability of the records, peak values of acceleration, velocity and displacements, source parameters (magnitude, epicentral and macroseismic distances), intensity and finally the fault plane solutions whenever possible. The aim is to have, with other regional dataset, a homogenous and good quality dataset.     

Hydrological conditions over the western Mediterranean basin during the deposition of the cold Sapropel 6 (ca. 175 kyr BP)

A new oxygen isotope record is reported from a stalagmite collected in the Argentarola Cave located on the Tyrrhenian coast of Italy. As shown from observations and numerical modeling of δ¹⁸O in modern precipitation, the recorded δ¹⁸O variability for this zone is dominated by the amount of precipitation (so-called ‘amount effect’). The δ¹⁸O profile measured in the stalagmite is characterized by a prominent negative excursion (ca. 2-3‰) between 180 and 170 kyr BP. This paleoclimatic feature is interpreted as being due to a relatively wet period which occurred during the penultimate glacial period, more precisely, during Marine Isotope Stage 6.5. This pluvial phase is shown to correspond chronologically to the deposition of the sapropel event 6 (S6). Although this particular sapropel event occurred during a cold phase, the δ¹⁸O excursion is similar to those corresponding to other sapropels (S4, S3 and S2). The evidence for humid conditions during S6 in the western Mediterranean basin agrees with previous studies based on deep-sea sediment cores. Taken collectively, the data suggest that during sapropel events dilution of ocean surface waters was not restricted to the output of the river Nile but was rather widespread over the entire Mediterranean Sea due to increased rainfall.     

Computation of amplification factor of earthquake ground motion for a local sedimentary structure

We present methodology of calculating acceleration and corresponding earthquake ground motion characteristics at a site of interest assuming acceleration at a reference site for two basic configurations. In one configuration we assume that the reference ground motion is not affected by the local structure beneath the site of interest. In the other configuration we assume that the reference ground motion is affected by the local structure. Consequently, the two configurations differ from each other by the presence of the reference site within the computational model. For each of the two configurations we assume two wavefield excitations: a vertical plane-wave incidence and a point double-couple source. We illustrate the methodology on the example of the Grenoble valley. The extensive investigation of effects of local surface sedimentary structures based on the developed methodology is presented in the accompanying article by Moczo et al. (Bull Earthq Eng, 2018) in this volume.     

Guidelines for the good practice of surface wave analysis: a product of the InterPACIFIC project

Surface wave methods gained in the past decades a primary role in many seismic projects. Specifically, they are often used to retrieve a 1D shear wave velocity model or to estimate the V_S,30 at a site. The complexity of the interpretation process and the variety of possible approaches to surface wave analysis make it very hard to set a fixed standard to assure quality and reliability of the results. The present guidelines provide practical information on the acquisition and analysis of surface wave data by giving some basic principles and specific suggestions related to the most common situations. They are primarily targeted to non-expert users approaching surface wave testing, but can be useful to specialists in the field as a general reference. The guidelines are based on the experience gained within the InterPACIFIC project and on the expertise of the participants in acquisition and analysis of surface wave data.     

Towards a microzonation of the Greater Beirut area: an instrumental approach combining earthquake and ambient vibration recordings

Lebanon is situated on the 1000 km long Levant transform fault that separates the Arabic from the African tectonic plates. In Lebanon, the Levant fault splits up into a set of ramifications that had, in the past, generated major destructive earthquakes causing a lot of destruction and thousands of casualties. The most devastating one was the 551 A.D. offshore earthquake that destroyed Beirut, the capital of Lebanon. This paper presents a site effect study in Beirut, aimed at proposing a framework for future microzonation works in the city. It includes two complementary parts. A 6-month, temporary seismological experiment was first conducted to estimate the site response at 10 sites sampling the main geological units of Beirut on the basis of local and regional earthquake recordings. This spatially sparse information was then complemented by a large number (615) of microtremor measurements covering the Beirut municipality and part of its suburbs with a 400 m dense grid. The recordings were analysed with the standard site-to-reference and horizontal-to-vertical spectral ratio methods for earthquake recordings, and the horizontal-to-vertical ratio for ambient noise recordings. Significant ground motion amplification effects (up to a factor of 8) are found in a few areas corresponding to recent deposits. The consistency between results from earthquake and microtremor recordings allows proposing a map of the resonance frequencies within the city and its suburbs, with frequencies ranging from 0.5 to 5 Hz for the deepest deposits, and 5–10 Hz for shallow areas. Finally, the results are discussed and a way to combine the results obtained from the temporary stations to the great number of recordings coming from the permanent Lebanese network is proposed.     

Ground motion prediction with the empirical Green's function technique: an assessment of uncertainties and confidence level

An assessment of uncertainties for ground motion predictions with the aid of the empirical Green's function (EGF) technique is presented. The main input parameters were identified, and their respective uncertainties were assessed by means of an international expert inquiry. The repercussion of these input uncertainties on the final ground motion estimates were investigated by means of the Latin Hypercube Sampling technique. The mean ground motion estimates (response spectra) and their standard deviations were compared with results obtained from empirical attenuation laws. The most sensitive input parameter turned out to be the seismic moment corresponding to the EGF. In general, if the source parameters are well determined, equivalent uncertainties, statistically speaking, can be expected from the EGF technique and from the application of attenuation laws. Therefore, if EGFs with well known source parameters are available, the EGF technique seems to be preferable: site effects are automatically taken into account, and physically realistic acceleration time histories can be obtained. However, further investigations on the reliability of the EGF technique should be performed, and finally, it is recalled that the EGF technique is based on the assumption of linearity. If conditions are such that this assumption cannot be maintained, the EGF technique should be combined with non-linear geotechnical methods.     

Evidence of nonlinear site response in HVSR from SMART1 (Taiwan) data

Deamplification of strong motion and the increase of the effective period of soil deposits are typical nonlinear effects; we seek them in SMART1-array data by applying the horizontal-to-vertical spectral ratio (HVSR) technique. The recordings, from four soil and one rock stations, represent 23 earthquakes (M_L 4.9–7.0); PGA varies between 20–260 cm/s². For each station, mean HVSR curves are calculated for two PGA ranges: <75 cm/s² and >100 cm/s² (weak and strong motion). At the soil stations, the “weak” (linear) and “strong” (nonlinear) responses are significantly different. Below 1–1.8 Hz, the nonlinear response exceeds the linear one. Above 2 Hz, the nonlinear response drops below the linear one and above 4–6 Hz below unity (deamplification). From 10 to 16 Hz, the two responses converge. One soil site shows significant negative correlation between resonance frequency and ground acceleration. Such behaviour agrees with other empirical studies and theoretical predictions. Our results imply that the HVSR technique is sensitive to ground-motion intensity and can be used to detect and study nonlinear site response.