Geophysical and geological interpretation of discrepancies in site residuals in northern Morocco: a simple qualitative approach

Abstract

Surface geology, gravimetric, and heat flow data are used to explain discrepancies between MD site residuals, using the average magnitude site residuals (AMSRi) which are opposite of the station corrections (cStaj) calculated for the 25 selected short-period seismic stations of the Moroccan seismological network. MD site residuals range from -0.29 to +0.32, and appear to be the result of the complex interaction of the predominant near-station geophysical and geological parameters. With this simple qualitative approach, we found that high negative residuals are obtained for older well-consolidated Precambrian crystalline rocks with low surface heat flow, and high positive residuals are attributed to recent unconsolidated sediments with high heat flow. The explanation for the intermediate residual values remains more complex and requires more accuracy and better quality data, and may not be fully justified in this study.Calibration of the northern Morocco seismic stations (newly installed, from 1989 to 1994) allowed us to set a relationship for duration magnitude in the form Moq = -0 . 1 4 + 1.63 log10 ('rid)

The MD Scale in Northern Morocco: A Comparative Study of Two Empirical Approaches

“…geophys., This is an interesting result, because it is known that, when calibrating similar instruments (in this study: Kinemetrics SS-1 Ranger Seismometers, 1-Hz, vertical component, 36 ± 6 dB attenuator setting) for duration-magnitude scales, there is no obvious change in the residuals (durations) when amplification gain changes slightly (±6 dB) at station (LEE et al, 1972;BAKUN, 1984), and therefore discrepancies between residuals which nearly result in the site effect may be correlated to geological and geophysical conditions of the site in the immediate vicinity of the stations (BORCHERDT and GIBBS, 1976;ROGERS et al, 1979;BAKUN, 1984;MICHAELSON, 1990;SU et al, 1992;EATON, 1992;MOUAYN et al, 2004). This last reflection has been developed in a recent work soon to be published (MOUAYN et al, 2006).…”