SEISMIC MICROZONATION AND SITE EFFECTS AT AL HOCEIMA CITY,MOROCCO

Measurements of background noise at Al Hoceima city and its extension zones allowed us to elaborate a seismic microzonation map. The analysis of this map shows existing relationships between the observed dominant periods and the local geological conditions. In particular, a strong correlation between the dominant periods and the topography is apparent; indeed, we find larger dominant periods in areas with higher topography such as in the Ras El Abed sector to the north of the city and in the area of Cala Bonita, south of the city. Furthermore, the field work conducted in the region revealed that the karstic phenomenon is well developed in the Ras El Abed sector, and affects the dominant periods and a hence the site response.     

STRONG MOTION VERSUS WEAK MOTION: IMPLICATIONS FOR MICROZONATION STUDIES

A set of mainshock and aftershock data following the Chamoli earthquake of March 29, 1999, recorded at a single station viz. Gopeshwar, has been studied. Particularly, the utility of the use of aftershock/weak motion data for site characterisation in seismic microzonation studies is investigated. The analysis of aftershock and mainshock data indicates that the spectral shape and amplification is quite different during the main-shock and the aftershock. This, in turn, implies that the use of weak motion/aftershock records may lead to erroneous conclusions regarding the expected ground motion during a strong earthquake. Further, it has been shown that the site characteristics estimated from H/V ratios are not stable in the near field conditions, even for weak motion data.