A review of the microtremor horizontal-to-vertical spectral ratio (MHVSR) method.

The single-station microtremor horizontal-to-vertical spectral ratio (MHVSR) method was initially proposed to retrieve the site amplification function and its resonance frequencies produced by unconsolidated sediments overlying high-velocity bedrock. Presently, MHVSR measurements are predominantly conducted to obtain an estimate of the fundamental site frequency at sites where a strong subsurface impedance contrast exists. Of the earthquake site characterization methods presented in this special issue, the MHVSR method is the furthest behind in terms of consensus towards standardized guidelines and commercial use. The greatest challenges to an international standardization of MHVSR acquisition and analysis are (1) the what - the underlying composition of the microtremor wavefield is site-dependent, and thus, the appropriate theoretical (forward) model for inversion is still debated; and (2) the how - many factors and options are involved in the data acquisition, processing, and interpretation stages. This paper reviews briefly a historical development of the MHVSR technique and the physical basis of an MHVSR (the what). We then summarize recommendations for MHVSR acquisition and analysis (the how). Specific sections address MHVSR interpretation and uncertainty assessment.

Gravity perturbed waves in elastic solids and in viscous fluids. Can they help explain strong motion records at Mexico City?

Local site effects on seismic ground motion during the September 1985 earthquakes were at the origin of significant damage and the loss of thousands of lives in Mexico City. In this paper we investigate the importance of site effects for these earthquakes. First, a brief review is made of the different large scale elastic models proposed to explain site effects. In a second place we investigate whether gravity may have influenced strong motion duration during the 1985 earthquakes, as suggested by Lomnitz (1990). Two possibilities are envisaged: gravity effects in elastic wave propagation in an extremely soft layer; and second, the more remote possibility that non linear effects bring the rheology of the surficial clay layer nearer to a fluid than a solid, with the consequent propagation of gravity waves. Our results allow to quantify the influence of the large scale structure of the valley and to conclude that, very likely, gravity effects are not a significant factor in the 1985 strong motion records in Mexico City (AU)

Seismic response of buildings during earthquakes : Experimental results from strong motion data

The kinematic behaviour of 25 "standard" buildings is derivet from an analysis of 29 accelerometric record sets obstained by the California Strong Motion Instrumentation Program during recent earthquakes. Measurements of the main natural frequencies, modal shapes, and corresponding dampings, are abstained by means of a multi-sensor, multi-degreeof freedom linear system identification technique, while various spectraltechniques (Fourier spectral rations, coherence analysis) provide reliable, quantitative estimates of the amount of torsional and rocking motions (where there is adequate instrumentations) (AU)