Ground Penetrating Radar imaging of two large sand blow craters related to the 2001 Bhuj earthquake, Kachchh, Western India

The 2001 Bhuj earthquake (Mw 7.7) formed several medium to large sand blow craters due to extensive liquefaction of the sediments comprising the Banni plain and Great Rann of Kachchh. We investigated two large closely spaced sand blow craters of different morphologies using Ground Penetrating Radar (GPR) with a view to understand the subsurface deformation, identify the vents and source of the vented sediments. The study comprises velocity surveys, GPR surveys using 200 MHz antennae along three selected transects that is supplemented by data from two trenches excavated. The GPR was able to provide good data on stratigraphy and deformation up to a depth of 6.5 m with good resolution. The GPR successfully imaged the subsurface characteristics of the craters based on the contrasting lithologies of the host sediments and the sediments emplaced in the craters. The GPR also detected three vertical vents of ∼ 1 m width continuing throughout the profile which are reflected as high amplitude vertical events. We conclude that the large sand blows during the 2001 Bhuj earthquake were produced due to liquefaction of sediments in the subsurface at > 6.5 m depth and that the clay-rich sediments of the Banni plain have behaved as the fine grained cap over it. The present study provides a modern analogue for comparing the liquefaction features of past great earthquakes (for example, the 1819 earthquake) that have occurred in the Kachchh region to understand the phenomena of liquefaction.