S33A-2745
Seismic Characterization of the Jakarta Basin

Wednesday, 16 December 2015
Poster Hall (Moscone South)
Athanasius Cipta1,2, Erdinc Saygin1, Phil R Cummins1, Masturyono Masturyono3, Ariska Rudyanto3 and Masyhur Irsyam4, (1)Australian National University, Canberra, ACT, Australia, (2)Geological Agency of Indonesia, Earthquake Hazard, Bandung, Indonesia, (3)Indonesian Agency for Meteorology, Climatology and Geophysics, Jakarta, Indonesia, (4)Bandung Institute of Technology, Civil Engineering, Bandung, Indonesia
Abstract:
Jakarta, Indonesia, is home to more than 10 million people. Many of these people live in seismically non-resilient structures in an area that historical records suggest is prone to earthquake shaking. The city lies in a sedimentary basin composed of Quaternary alluvium that experiences rapid subsidence (26 cm/year) due to groundwater extraction. Forecasts of how much subsidence may occur in the future are dependent on the thickness of the basin. However, basin geometry and sediment thickness are poorly known.

In term of seismic hazard, thick loose sediment can lead to high amplification of seismic waves, of the kind that led to widespread damage in Mexico city during the Michoacan Earthquake of 1985. In order to characterize basin structure, a temporary seismograph deployment was undertaken in Jakarta in Oct 2013- Jan 2014. A total of 96 seismic instrument were deployed throughout Jakarta were deployed throughout Jakarta at 3-5 km spacing. Ambient noise tomography was applied to obtain models of the subsurface velocity structure. Important key, low velocity anomalies at short period (<8s) correspond to the main sedimentary sub-basins thought to be present based on geological interpretations of shallow stratigraphy in the Jakarta Basin.

The result shows that at a depth of 300 m, shear-wave velocity in the northern part (600 m/s) of the basin is lower than that in the southern part. The most prominent low velocity structure appears in the northwest of the basin, down to a depth of 800 m, with velocity as low as 1200 m/s. This very low velocity indicates the thickness of sediment and the variability of basin geometry.

Waveform computation using SPECFEM2D shows that amplification due to basin geometry occurs at the basin edge and the thick sediment leads to amplification at the basin center. Computation also shows the longer shaking duration occurrs at the basin edge and center of the basin.

The nest step will be validating the basin model using earthquake events recorded by the Jakarta array. The Bohol 2013 earthquake is one good candidate event for model validation. This will require using a source model for the Bohol earthquake and a plane wave input to SPECFEM3D.

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