Western Java Ambient Noise Tomography: A Preliminary Result

Western Java, Indonesia, has at least three important active faults: the Cimandiri, Lembang, and Baribis Faults, which pose a great danger for the cities near them. It is crucial to have a better understanding of shallow crustal structure to delineate active faults and deep basins in order to support seismic hazard and disaster mitigation efforts in Indonesia. In this study, we perform ambient seismic noise tomography which can give better resolution of the shallow structure beneath western Java. We have deployed a seismometer network in the western to central Java region through a research collaboration program between the Bandung Institute of Technology (ITB) and the Australian National University (ANU). We deployed 70 seismometer stations from June to January 2017 to acquire ambient seismic noise data. As the first stage of the data processing, we will focus on conducting single data preparation and cross-correlation to retrieve an estimate of Green's functions between station pairs. We also use the frequency-time analysis technique to obtain dispersion curves to measure the interstation group velocity. The group velocity is use as an input in tomography inversion. Our preliminary results show low velocity anomalies associated with sedimentary basins and a high velocity anomaly associated with the southern mountains

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

Group velocity maps using subspace and transdimensional inversions: ambient noise tomography in the western part of Java, Indonesia

In this paper, we compare two different methods for group velocity inversion: iterative, least-squares subspace optimization and probabilistic sampling based on the transdimensional (Trans-D) Bayesian method with tree-based wavelet parametrization. The wavelet parametrization used a hierarchical prior for wavelet coefficients which could adapt to the data. We applied these inversion methods for ambient noise tomography of the western part of Java, Indonesia. This area is an area prone to multiple geological hazards due to its proximity to the subduction of the Australia Plate beneath Eurasia. It is therefore important to have a better understanding of upper crustal structure to support seismic hazard and disaster mitigation efforts in this area. We utilized a new waveform data set collected from 85 temporary seismometers deployed during 2016–2018. Cross-correlation of the waveform data was applied to retrieve empirical Rayleigh wave Green’s functions between station pairs, and the spatial distribution of group velocity was obtained by inverting dispersion curves. Our results show that, although computationally expensive, the Trans-D Bayesian approach offered important advantages over optimization, including more effective explorative of the model space and more robust characterization of the spatial pattern of Rayleigh wave group velocity. Meanwhile, the iterative, least-square subspace optimization suffered from the subjectivity of choice for reference velocity model and regularization parameter values. Our Rayleigh wave group velocity results show that for short (1–10 s) periods group velocity correlates well with surface geology, and for longer periods (13–25 s) it correlates with centres of volcanic activity.This work was partially funded by the Ministry of Public Works and Housing, and the Indonesian Ministry of Research, Technology and Higher Education under WCU Program 2019 managed by ITB awarded to S. Widiyantor

Seismic microzonation of Bandung basin from microtremor horizontal-to-vertical spectral ratios (HVSR)

Bandung is located on a thick sedimentary basin, which is mainly composed of volcanic rocks and the depositional remnants of an ancient lake. The high population density and vital infrastructure, surrounding by potential sources of earthquakes make Bandung vulnerable to earthquake impact. To study the seismic response of Bandung, a microzonation study is needed so as to facilitate disaster risk assessment and mitigation. The parameters which are mapped for the purpose of microzonation are the distribution of the dominant frequency (F0), the amplification factor (Am) and seismic susceptibility (Kg). We use the HVSR analysis method with microtremor/ambient seismic noise data. The microtremor data were taken from 58 measurement points from the Bandung Seismic Experiment network run from March to October 2014 in the Bandung basin and surrounding areas. The results show that the dominant peak frequency in the studied area ranges from 0.195 Hz to 7.016 Hz, amplification (A) spans from 1.6 to 11.3, and the value of the seismic susceptibility index (Kg) ranges from 0.6 to 245.6. The spatial distribution of seismic susceptibility index indicates that almost all areas of the Bandung basin have high susceptibility to earthquake hazard. The highest susceptibility is found in the eastern part of the Bandung basin that includes Bojongsoang, Rancaekek, Ciparay, Rancasari, and Majalaya, while the lower susceptibility zones are scattered in the hills and mountains around the basin of Bandung

Shear wave velocity structure beneath Bandung basin, West Java, Indonesia from ambient noise tomography

We investigated the seismic shear wave velocity structure of the upper crust beneath the Bandung area in West Java, Indonesia, using ambient seismic noise tomography. We installed 60 seismographs to record ambient seismic noise continuously in the city of Bandung and its surrounding area for 8 months. After interstation cross-correlation of recordings of ambient seismic noise, we obtained empirical Green's functions for Rayleigh waves. Group velocity dispersion curves for Rayleigh waves between periods of 1 and 8 s were measured on each interstation path by applying the multiple filter analysis method with phase-matched processing. The spatial variation of group velocities shows a good correlation with the geological structure of the Bandung Basin. The Rayleigh wave dispersion maps were inverted to obtain the 1-D shear wave velocity profiles beneath each station, which were interpolated to infer a pseudo-3-D structure under the study region. The results show that the Bandung Basin has a thick layer of sediment. Along the northern, eastern and southern mountains surrounding the Bandung Basin there is high-velocity structure, except to the west of the Tangkuban Parahu volcano, where a massive low-velocity structure extending throughout the upper crust might indicate the presence of fluids or partial melts.BP is very grateful to BMKG for a doctoral scholarship during his study at the Institut Teknologi Bandung (ITB). The data used in this study were acquired using the research funding from the Australian Research Council Linkage Project LP110100525, partially supported by the Australian Aid program of the Australian Dept. Foreign Affairs and Trade. This study was also partially funded by the Indonesian Ministry of Research, Technology and Higher Education under WCU Program 2019 managed by ITB awarded to S. Widiyantoro

The crustal structure beneath The Netherlands derived from ambient seismic noise

This work presents the first comprehensive 3-D model of the crust beneath The Netherlands. To obtain this model, we designed the NARS-Netherlands project, a dense deployment of broadband stations in the area. Rayleigh and Love wave group velocity dispersion was measured from ambient noise cross-correlations. Azimuthally anisotropic group velocity maps were then constructed and the isotropic part was used to determine a shear wave speed model that includes the effects of radial anisotropy. Employing the Neighbourhood Algorithm for the depth inversion, we obtained probabilistic estimates of the radially anisotropic model parameters. We found that the variations in the thickness of the top layer largely match the transition from sediments of Permian age to those of Carboniferous age. Regions of high faulting density such as the West Netherlands Basin and Roer Valley Graben are recognized in our model by their negative radial anisotropy (V SH − V SV < 0). The model has a mid-crustal discontinuity at a depth of around 13 km and the average Moho depth is 33 km, with most of its depth variations within 2 km. Specifically, a localized Moho uplift to a depth of 29 km is found within Roer Valley Graben, in the Campine region in Belgium. Furthermore, our Rayleigh and Love wave group velocity data at periods of around 20 s show evidence for azimuthal anisotropy with a NW-SE fast direction. This anisotropy is likely related to NW-SE rock fabric in the lower crust thought to originate from the Caledonian deformation