Pyroclastic Deposits Identification using Near-Surface Seismic Refraction Tomography in Rawa Dano Volcanic Complex, Banten, Indonesia

Rawa Dano is a caldera lake which resulted from Dano Purba Volcano’s massive eruption, and it produced a huge amount of pyroclastic deposits that typically formed complex volcaniclastic series. Due to the lack of information regarding the subsurface properties of Rawa Dano area, therefore in this study, a low-energy seismic refraction survey was carried out to identify the distribution of pyroclastic deposits resulted from intensive volcanic eruptions. The data were acquired from two lines in two different sites. Variations of longitudinal velocity in the seismic vertical cross-section suggest that there are more than one type of deposits existed in the area. The results show two main refractors which are related to the deposition of different facies. The seismic velocity shown in the upper part of the seismic tomography model indicates that the pyroclastic deposit has a great thickness. This finding suggests that the eruptions happened massively. By combining the results from both sites, it could be inferred that the preceding one is even bigger in magnitude. The result is in agreement with the earlier surface geological study, which explains a similar conclusion. This research demonstrates the capability of seismic refraction tomography to map the distribution and condition of volcanic deposits around Rawa Dano Volcanic Complex.

The significance and tectonic evolution of Huatung basin

<p>The Huatung basin (HB), located between the Philippine Sea plate (PSP) and the South China Sea (SCS), has likely existed near tectonically-active plate boundaries since the early Cenozoic. It may record SCS evolution from the SCS rifting phase to today, and is a key region to understand the broad geodynamic interactions between the SCS and PSP. A left-lateral shear plate boundary between the SCS and PSP followed the Gagua ridge and was active before 56 Ma. A slight compressive component along the Gagua ridge might have occurred from 40 to 30 Ma, giving rise to the topographic uplift of Gagua ridge and adjacent ridges with possibly some underthrusting of the PSP below the HB. A significant compressive episode also occurred along a second fracture zone around 23 Ma ago. The Manila trench inception occurred along the PSP-SCS plate boundary before the end of SCS spreading, involving the subduction of the younger SCS beneath the older HB. Later the intra-oceanic Luzon arc formed and collided in a sub-parallel fashion with the Eurasian continent around 5-6 Ma ago to form Taiwan. The PSP/EU motion was oblique with respect to this plate boundary during SCS opening. However, we have no direct evidence of the HB age (early Cenozoic or early Cretaceous) and if the PSP underthrusted below the HB. We propose to carry a deep seismic refraction survey and dredge sampling of basement units to clarify this problem. This work is supported by the Chinese National Natural Science Foundation (contracts 91958212, 41730532, 41576070 and 41676043).</p>

The Seismic Refraction Survey to Determine the Depth of Bedrock at the Damansara Area for Horizontal Directional Drilling Method Application

The seismic reflection survey conducted along the road at Damansara to determine the depth of bedrock in order to justify whether HDD method can be utilize to store the fiber optic cable. 10 line seismic survey performed along 1.2 km roadside. The result show that the subsurface profile represent by two layer of earth materials that is topsoil and bedrock granite. Determination between topsoil and granite based on the values of seismic velocity. The boundary between granite and soil interpreted by a velocity value 1,200 m/s. If the velocity values is less than 1,200 m/s, it interpreted as soil or highly weathered rock. Meanwhile the velocity value more than 1,200 m/s is refer as rock and hard to excavate especially using HDD method. The study shows that the general thickness of topsoil along the road in Damansara is around 2.0 to 4.0 m. The minimum thickness of topsoil is 1.0 m and maximum found around 6.0 m. The bedrock observed very shallow and not suitable for HDD method to implement.