Imaging Subsurface Structures at Fast Eroding Coastal Areas in Northern Bengkulu Using 2D Seismic MASW Method

Coastal erosion is a global problem, affecting many countries around the world. On cliffed coastlines, erosion can be intermittent and sudden, posing an additional hazard to coastal communities. To mitigate such problems, it is necessary to understand the processes leading to cliff erosion. Here, we focus on the physical properties of the subsurface structure that might contribute to cliff instability. As a specific study site, we use the coastal region of the northern part of Bengkulu Province, which is well known as being prone to coastal abrasion/erosion. Seismic field surveys were conducted at five locations, using a 92 m seismic line with geophones spaced every 2 m. The measurements were processed using the Multichannel Analysis of Surface Wave (MASW) methodology with commercially available software. Our results suggest that the subsurfaces are divided into three layers where the top layers lie at thickness between 10 and 15 m with S-wave velocity (Vs) < 200 m/s (consistent with diluvial clay). The second layer has a thickness > 10 m with Vs between 600 and 800 m/s (consistent with loose sand), and overlays a bedrock layer for which Vs is much higher. We found between the first and second layers are a narrow ‘discontinuity region’ with Vs of 300–500 m/s (consistent with soft sand/silt). The configuration/composition/orientation of layers indicate stable permeable layers overlying saturated and impermeable layers at or below the sea level. The upper permeable layers are vulnerable to instability when subjected to external disturbances such as heavy precipitation, storm waves or earthquakes. The relatively mild wave climate and absence of notching around the water line leads us to conclude that variation in saturation levels is the primary driver of the observed cliff erosion.