Shallow shear-wave velocity beneath jakarta, indonesia revealed by body-wave polarization analysis

Noting the importance of evaluating near-surface geology in earthquake risk assessment, we explored the application to the Jakarta Basin of a relatively new and simple technique to map shallow seismic structure using body-wave polarization. The polarization directions of P-waves are sensitive to shear-wave velocities (Vs), while those of S-waves are sensitive to both body-wave velocities. Two dense, temporary broadband seismic networks covering Jakarta city and its vicinity were operated for several months, firstly, from October 2013 to February 2014 consisting of 96 stations, and secondly, between April and October 2018 consisting of 143 stations. By applying the polarization technique to earthquake signals recorded during these deployments, the apparent half-space shear-wave velocity (Vs ahs) beneath each station is obtained, providing spatially dense coverage of the sedimentary deposits and the edge of the basin. The results showed that spatial variations in Vs ahs obtained from polarization analysis are compatible with previous studies, and appear to reflect the average Vs of the top 150 m. The low Vs that characterizes sedimentary deposits dominates most of the area of Jakarta, and mainly reaches the outer part of its administrative margin to the southwest, more than 10 km away. Further study is required to obtain a complete geometry of the Jakarta Basin. In agreement with previous studies, we found that the polarization technique was indeed a simple and effective method for estimating near-surface Vs that can be implemented at very low-cost wherever three-component seismometers are operated, and it provides an alternative to the use of borehole and active source surveys for such measurements. However, we also found that for deep basins such as Jakarta, care must be taken in choosing window lengths to avoid contamination of basement converted phases

Borehole Microseismic Imaging of Hydraulic Fracturing: A Pilot Study on a Coal Bed Methane Reservoir in Indonesia

Over the last decade, microseismic monitoring has emerged as a considerable and capable technology for imaging stimulated hydraulic fractures in the development of unconventional hydrocarbon resources. In this study, pilot hydraulic-fracturing treatments were operated at a coal-bed methane (CBM) field in Indonesia to stimulate the flow and increase the reservoir's permeability while the monitoring system was set in a single near-vertical borehole. Locating event sources accurately is fundamental to investigating the induced fractures, but the geometry of a single downhole array is a challenging data processing task, especially to remove ambiguity of the source locations. The locating procedure was reviewed in 3 main steps: (i) accurate picking of P- and S-wave phases; (ii) inclusion of P-wave particle motion to estimate the back azimuth; (iii) guided inversion for hypocenter determination. Furthermore, the seismic-source moment magnitudes were calculated by employing Brune's model. Reliable solutions of locations were obtained as shown statistically by uncertainty ellipsoids and a small misfit. Based on our results, both induced and triggered seismicity could be observed during the treatments and therefore conducting intensive monitoring is important. The triggered seismicity is an undesired activity so disaster precautions need to be taken, in particular for preventing reactivation of pre-existing faults

Horizontal-to-Vertical Spectral Ratio (HVSR) Method for Earthquake Risk Determination of Jakarta City with Microtremor Data

Jakarta is the capital of the Republic of Indonesia which lies above a thick sedimentary basin. Geographically, Jakarta is 200 km away from the Indo-Australian subduction zone that sinks under the island of Java. There are many vital buildings and with the thick sediments underlying the city of Jakarta. Therefore, this region has a considerable seismic vulnerability. This will be dangerous if there is an earthquake that has the same frequency as the natural frequency of the building. It will cause a resonance resulting in amplification of seismic waves in the area. Each building has a different natural frequency, one that affects is the height of the building. To characterize the subsurface structure of the Jakarta Basin, microtremor data processing was obtained from the recording of 95 stations which was operated in October 2013 - February 2014 using Horizontal-to-Vertical Spectral Ratio (HVSR) method. HVSR is a method for obtaining subsurface information from single station measurements by comparing the Fourier spectrum of horizontal components to its vertical components. This ratio is a function of the frequency that will produce the H/V curve. The dominant frequency value on the HVSR curve represents the natural frequency of the area. The Seismic Vulnerability Index (Kg), which serves to determine the soil weak zone, can be calculated from the H/V curve. The dominant frequency value maps generated for the Jakarta area range from 0.2-0.22 Hz for low frequencies and 1-8.6 Hz for high frequencies. The large dominant frequency correlates with the thin sediment layer. Based on the frequency range, the south and northwest regions of Jakarta have relatively shallow basement depths compared to other regions. The resulting amplification value map can be divided into 4 maps with different period ranges. From the four maps, the North Jakarta area or the area around Jakarta's coastline is most at risk of amplification with an H/V value up to 11 because the area is associated with alluvial deposits and coastal sediments. The seismic vulnerability distribution map in Jakarta City ranges from 15-850 relatively high in northeast and north of Jakarta

Seismic source analysis of the destructive earthquake November 21, 2022, Mw 5.6 Cianjur (Indonesia) from relocated aftershock

Acknowledgements: We thanks to Research and Community Service Institution of Bandung Institute of Technology to support this project. This project partially funded by Riset Pengembangan ITB awarded to ZFZ, Riset ITB awarded to ADN and Riset BIMA Dikti awarded to NTP. We used PyGMT27 for producing maps in Figs. 1, 4 and 5.AbstractA destructive shallow earthquake with a magnitude of 5.6 struck Cianjur, West Java, Indonesia on November 21, 2022. This earthquake resulted in 602 casualties and the collapse of over 67,504 residences. The day after the mainshock, we deployed 19 temporary seismic stations to monitor aftershocks for a period of 30 days. We manually picked arrival times for 4499 P-waves and 3419 S-waves and determined locations for 514 events. Following the velocity model update, phase refinement through waveform cross correlation, and relocation using double-difference methods, we were able to determine 442 well-defined hypocenters of the aftershocks. We identified two clusters of aftershocks: one in the NNW-SSE direction, with a length of about 8 km, and another in the WSW-ENE direction, with a length of around 6 km. The seismogenic zone of these clusters ranges from a depth of 3 to 13 km. Our interpretation suggests that these clusters may indicate a conjugate fault. It is possible that the mainshock (Mw5.6) Cianjur earthquake on November 21, 2022 occurred on the WSW-ENE direction with sinistral movement.</jats:p