Impact of Low-Frequency Ground Motions on Local Site Effect of Guwahati

Abstract

Earthquake-generated ground motions undergo change in their characteristics once propagate from subsoil medium. There will be change in frequency content, duration, and amplitude of ground motion. This phenomenon is called as local site effect. Depending upon the shear strain generated in the soil by the input motion, ground motions can experience very low to very high amplification before it reaches ground surface. As a result, there will be complete change in the ground-motion characteristics between the bedrock and the surface. In the present work, subsoil response for typical locations of Guwahati is assessed. Guwahati lies in seismic zone V and has experience earthquake-induced ground shaking and damages during various earthquakes in the past including 1897 Shillong earthquake. Considering the fact that numerous seismic sources exist in the region, the present work attempts to understand the importance of low-frequency ground motions, which can be experienced from distant seismic sources, on subsoil response of Guwahati. As a first source, typical ground motions from earlier published work are selected. Secondly, synthetic ground motions for past earthquakes are generated using finite-source model. Out of these two dataset collectively, those ground motions which have predominant frequency content less than 5 Hz (to represent distant source-based ground motions) are then selected and utilized for equivalent linear ground response analysis. Such ground motions are selected to ensure that these matches closely with the natural frequency of subsoil medium, to get maximum response. In addition, 45 typical boreholes randomly distributed in Guwahati are selected, and the analyses are performed. Outcomes of the present work will be very helpful in understanding the importance of low-frequency ground motions, which might be actually generated during future earthquakes from distant seismic source in order to arrive at peak ground acceleration values of important structures.KeywordsLocal site effectPredominant frequencyPGAEquivalent linear ground response analysis