No abstract

Jakarta has been selected as a study area for seismic microzonation by considering its population and its infrastructure growth, seismicity and geological setting. One of the important factors in seismic hazard analysis is the characteristics of the ground overlying bedrock, and, as yet, this has not been studied for Jakarta. This study was intended to estimate the depth to bedrock by applying microtremor array exploration. The phase velocity of microtremors was estimated by the spatial autocorrelation (SPAC) method, whereas the S-wave velocity profile was estimated by inversion using a genetic algorithm. Microtremor array analysis has been conducted at 55 sites that covered the whole of Jakarta. The result of one-dimensional (1D) S-wave velocity profile estimation indicates that the engineering bedrock depth has pronounced differences between northern (>700 m) and southern Jakarta (about 300 m). A 3D S-wave velocity model was constructed from 1D profiles resulting from a second inversion in which the depth of constant-velocity layers, with velocities of 500, 700 and 900 m s−1, was determined. The constructed 3D velocity structures show a bedrock morphology that has a depth range of 350–725 m, with depths increasing towards northern Jakarta. The implication of differences in bedrock depth is that estimated seismic amplification for the thicker sediment will be higher.

A tsunami earthquake (M w = 7.7) occurred south of Java on 17 July 2006. The event produced relatively low levels of highfrequency radiation, and local felt reports indicated only weak shaking in Java. There was no ground motion damage from the earthquake, but there was extensive damage and loss of life from the tsunami along 250 km of the southern coasts of West Java and Central Java. An inspection of the area a few days after the earthquake showed extensive damage to wooden and unreinforced masonry buildings that were located within several hundred meters of the coast. Since there was no tsunami warning system in place, efforts to escape the large waves depended on how people reacted to the earthquake shaking, which was only weakly felt in the coastal areas. This experience emphasizes the need for adequate tsunami warning systems for the Indian Ocean region.

Summary The inversion of traveltimes from refraction experiments sometimes encounters the ray coverage problem. Irregular station distribution and ray shadow zones may lead to poor ray coverage in some regions of a complex underground structure. To overcome this problem, we propose a joint inversion of refraction and gravity data since gravity surveys are carried out densely and homogeneously. An interface separating homogeneous sediments and basement rocks is parametrized by a set of knot points with Lagrange interpolation. Densities are related to slownesses through experimental formulae. Model regularization is used to avoid oscillatory artefacts in the solution and to minimize the non‐uniqueness of the solution. The validity of the formulation is verified using synthetic data for a valley‐like basin with pseudo‐random noise. The proposed method is successfully applied to actual data obtained in and around the Osaka Basin, Japan. To prevent solution instabilities owing to a lack of data beneath Osaka Bay, the interface under the bay is constrained to the depth interpretation of seismic reflection profiles. The result shows good agreement with the geological setting in this region.

SUMMARY Some special operators and algorithms have been introduced to the finite difference (FD) traveltime calculation to overcome numerical instability related to head waves. However, further modifications are still required for head waves travelling along an irregular interface, because the interface often generates multiple arrivals of the head waves, causing another instability in the traveltime calculation. We solve this multipathing problem by defining new 3‐D operators and implementing them with illumination conditions. Podvin & Lecomte operators are adopted for 2‐D models. We then apply these operators to 2‐ or 3‐D synclinal structures for checking their feasibility.

The Lembang fault located at northern part of populated Bandung basin is the most conspicuous fault that potentially capable in generating earthquakes. The first seismic investigation around Lembang fault has been done by deploying a seismic network from May 2010 till December 2011 to estimate the seismic activities around that fault. Nine events were recorded and distributed around the fault. Seven events were likely to be generated by the Lembang fault and two events were not. The events related to the Lembang fault strongly suggest that this fault has left-lateral kinematic. It shows vector movement of Australian plate toward NNE might have been responsible for the Lembang fault kinematic following its initial vertical gravitational movement. The 1-D velocity model obtained from inversion indicates the stratigraphy configuration around the fault composed at least three layers of low Vp/Vs at the top, high Vp/Vs at the middle layer and moderate Vp/Vs at the bottom. In comparison with general geology of the area, top, mid and bottom layers may consecutively represent Quaternary volcanic layer, pre-Quaternary water-filled sedimentary layer and pre-Quaternary basement. Two eastern events related to minor faults and were caused by a gravitational collapse.

The bedrock depth is one of the most important parameters in seismic hazard analysis. This parameter has not been identified well in Jakarta. This study was conducted to determine the bedrock depth in Jakarta based on S-wave velocity parameters. Microtremors array method was applied in this study to obtain 1D and 2D S-wave velocity profiles. The spatial autocorrelation (SPAC) method was used to estimate dispersion curves, while S-wave velocity structure was derived by genetic algorithm. Reffering to the geological condition in the study area, microtremors array measurements were conducted for two East-West lines in the northern and southern parts of Jakarta. The result of 2D construction of S-wave velocity structure shows stratigraphy cross sections that consists of four layers, where the bedrock depths in northern Jakarta can be depicted in the range from 519 to 662 m and in the southern part in the range from 353 m to 399 m. It has a positive correlation with the local geological condition i.e the sediment thickness increases to the north.

The Lembang fault considering vulnerable because of the site is near downtown Bandung. The west Lembang segment is the rural area of Cibodas. The two opinions is emerging, become normal fault or strike slip fault, based on recent earthquake (2010 onward). The geology data show the structure is observed by the outstanding topographic extended about 20 km, with volcanic rock very dominant. The seismology data is analysed for year 2011 in Mw less than 3, and classical fault plane solution is obtained. The gravity data is better acquired than 0.1 mGal, with elevation precision better than 60 cm. The upward Continuation, and second vertical derivative, is applied on data, as well as modelling 2.5D. The section is presenting in one section only, while data is getting in larger area. The seismology data show mostly for event is strike slip although in several parts having event normal. The gravity data is not ideal as graben like structure but rather locally low about 10 mGal amplitude. The combining the two methods suggested the slip zone or sag pond is more possible for the local anomaly. Besides, the Lembang fault is extended toward north rather to west or south.