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Jakarta, Indonesia

Hammond J.O.S.,University of Bristol | Hammond J.O.S.,Kyushu University | Wookey J.,University of Bristol | Wookey J.,University College London | And 4 more authors.
Physics of the Earth and Planetary Interiors | Year: 2010

The tectonic context of south-east Asia is dominated by subduction. One such major convergent boundary is the Java-Sunda trench, where the Australian-Indian plates are being subducted beneath the Eurasian plate. We measure shear-wave splitting in local and teleseismic data from 12 broadband stations across Sumatra and Java to study the anisotropic characteristics of this subduction system, which can provide important constraints on dynamical processes involved. Splitting in S-waves from local earthquakes between 75 and 300 km deep show roughly trench parallel fast directions, and with time-lags 0.1-1.3 s (92% ≤0.6 s). Splitting from deeper local events and SKS, however, shows larger time-lags (0.8-2.0 s) and significant variation in fast direction. In order to infer patterns of deformation in the slab we apply a hybrid modelling scheme. We raytrace through an isotropic subduction zone velocity model, obtaining event to station raypaths in the upper mantle. We then apply appropriately rotated olivine elastic constants to various parts of the subduction zone, and predict the shear-wave splitting accrued along the raypath. Finally, we perform grid searches for orientation of deformation, and attempt to minimise the misfit between predicted and observed shear-wave splitting. Splitting from the shallow local events is best explained by anisotropy confined to a 40 km over-riding plate with horizontal, trench parallel deformation. However, in order to explain the larger lag times from SKS and deeper events, we must consider an additional region of seismic anisotropy in or around the slab. The slab geometry in the model is constrained by seismicity and regional tomography models, and many SKS raypaths travel large distances within the slab. Models placing anisotropy in the slab produce smaller misfits than those with anisotropy outside for most stations. There is a strong indication that inferred flow directions are different for sub-Sumatran stations than for sub-Javanese, with >60° change over ∼375 km. The former appear aligned with the subduction plate motion, whereas the latter are closer to perpendicular, parallel to the trench direction. There are significant differences between the slab being subducted beneath Sumatra, and that beneath Java: age of seafloor, maximum depth of seismicity, relative strength of the bulk sound and shear-wave velocity anomaly and location of volcanic front all vary along the trench. We speculate, therefore, that the anisotropy may be a fossilised signature rather than due to contemporary dynamics. Crown Copyright © 2009. Source

News Article
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A tractor drives past at a rice drying yard at an export rice plant in the central Chainat province in Thailand, December 16, 2015. Last year at this time he was farming rice on his two-hectare farm 40 kilometers northeast of Bali’s airport. But a long spell of dry weather, which has lasted since last July, dried up irrigation channels in his village of Tegal Mengkeb. To survive, the 33-year-old walked away from his farm last December and began driving a taxi in Nusa Dua, a tourist hub with dozens of luxury resorts. He dreams of returning home. "We need regular good showers, but there is mostly drizzle. Unless the subaks (water channels) are full again, I can’t plant any crop,” he said. Aslam may have found a way to stay on the farm next year, however. He recently signed up for new government-backed crop insurance, one of 100 farmers to do so in Bali, where rice is grown on about 80,000 hectares of land. The Bali insurance program, launched last October, promises to pay farmers up to six million rupiah ($480) for a crop failure caused by disasters such as drought, flooding or pest attacks. The premium is 180,000 rupiah ($13) per hectare, but the state has agreed to pay 80 percent of the cost. That means a farmer like Aslam only has to come up with 36,000 rupiah, or about $2, per hectare. The program is part of a larger Indonesian crop insurance scheme introduced in 2012-2013 with financial support from the Japan International Cooperation Agency (JICA). In the first season of the program, 470 hectares of rice fields were insured in East Java and Sumatra. This year, the government has moved to include Bali and a few other provinces, though delays in the expansion have limited the number of farmers signed up. "Our previous target this year was 11,000 hectares of rice fields (insured), but only 4,000 hectares can be insured due to limited time," said Ida Bagus Wisnuardhana, head of the Bali provincial Agriculture and Foodstuffs Affairs office. Currently, the scheme targets only small-scale farmers growing rice, but the federal government hopes to bring in all 27 million farmers in Indonesia’s 33 provinces by 2019, according to a paper published by Japan’s Ministry of the Environment. In Indonesia, the dry season runs from May to August. But Badan Meteorologi, Klimatologi dan Geofisika (BKMG) - the local official weather mapping organization – says the island of Bali has seen “extreme” weather since the end of August. The agency attributes the unusually hot weather to the El Nino phenomenon. In November, the temperature in the area around Denpasar, Bali’s provincial capital, rose as high as 37 degrees Celsius above the average daily temperature of 31.4 degrees Celsius, said Nyoman Gede Wiryajaya of BKMG. Bali's provincial agricultural department says nearly a thousand hectares of farmland are suffering some degree of drought, which threatens the coming harvest. With crops drying, local media have already reported food shortages in several villages. Buleleng, a north Bali district which has recorded crop failure on 160 hectares, has been declared under "severe drought". With no rice available, "we have been living off dried cassava for several weeks," said Palembang Kaka, a small-scale farmer from Buleleng who now works as a porter in Pasar Badung, Denpasar’s largest community market. Although globally El Nino is expected to start weakening soon, meteorological service officials in Bali are advising farmers to brace for more dry weather. Sutopo Purwo Nugroho, of the National Disaster Management Agency, predicted that "rainfall will be extremely low until the end of this year". For Aslam, the prediction could mean another missed crop and another season at the wheel – unless his new insurance policy works. "We will see how the insurance (money) is paid. I hope it is enough to recover my losses,” he said.

Di Leo J.F.,University of Bristol | Wookey J.,University of Bristol | Hammond J.O.S.,Imperial College London | Kendall J.-M.,University of Bristol | And 4 more authors.
Physics of the Earth and Planetary Interiors | Year: 2012

Subduction of oceanic lithosphere is the most direct feedback between the Earth's surface and deep interior. However, the detail of its interaction with the broader convecting mantle is still unclear. Mantle flow around subduction zones can be constrained using seismic anisotropy, but despite many such studies, a simple global picture is lacking. The Sangihe subduction zone (where the Molucca Sea microplate is subducting westward beneath the Eurasian plate) is part of the tectonically complex Sulawesi-Philippine region, and an ideal natural laboratory to study complex subduction processes. We investigate the anisotropic structure of the Sangihe subduction zone with shear wave splitting measurements of local S and SKS phases at two stations (MNI in Sulawesi, DAV in the Philippines), as well as downgoing S phases at five stations at teleseismic distances. Combining different phases allows a better vertical resolution of anisotropic fabrics than is possible with a single phase. The broad depth distribution of local events (∼60-630. km) allows us to observe a change in splitting behaviour at ∼380. km depth: above, fast directions (φ) are trench-parallel and delay times (δt) are ∼0.34-0.53. s with no increase with depth. We suggest this anisotropy is caused by aligned cracks, possibly melt-filled beneath the volcanic arc, and fossil anisotropy in the overriding plate. Below ∼380. km, φ is predominantly trench-normal and δt are slightly higher (∼0.53-0.65. s). As no correlation is observed with inferred distance travelled inside the slab, we attribute this anisotropy to shear layers atop the slab, which are coherent from ∼200 to 400. km depth and perhaps extend into the transition zone. SKS and source-side measurements show larger δt (∼1.53 and 1.33. s, respectively) and trench-parallel φ. Since these phases predominantly sample sub-slab mantle, we consider along-strike lateral flow associated with the double-sided subduction of the Molucca Sea microplate to be the most likely explanation. We thus infer three dominant regions of anisotropy at the Sangihe subduction zone: one within the overriding lithosphere, one along the slab-wedge interface, and one below the subducting Molucca Sea slab. The mantle wedge above 200. km depth and the slab itself do not seem to contribute notably to the measured anisotropy. This study demonstrates the insight seismic anisotropy can provide into mantle dynamics even in tectonically complex subduction systems. © 2012. Source

Di Leo J.F.,University of Bristol | Wookey J.,University of Bristol | Hammond J.O.S.,Imperial College London | Kendall J.-M.,University of Bristol | And 4 more authors.
Geochemistry, Geophysics, Geosystems | Year: 2012

Indonesia is arguably one of the tectonically most complex regions on Earth today due to its location at the junction of several major tectonic plates and its long history of collision and accretion. It is thus an ideal location to study the interaction between subducting plates and mantle convection. Seismic anisotropy can serve as a diagnostic tool for identifying various subsurface deformational processes, such as mantle flow, for example. Here, we present novel shear wave splitting results across the Indonesian region. Using three different shear phases (local S, SKS, and downgoing S) to improve spatial resolution of anisotropic fabrics allows us to distinguish several deformational features. For example, the block rotation history of Borneo is reflected in coast-parallel fast directions, which we attribute to fossil anisotropy. Furthermore, we are able to unravel the mantle flow pattern in the Sulawesi and Banda region: We detect toroidal flow around the Celebes Sea slab, oblique corner flow in the Banda wedge, and sub-slab mantle flow around the arcuate Banda slab. We present evidence for deep, sub-520 km anisotropy at the Java subduction zone. In the Sumatran backarc, we measure trench-perpendicular fast orientations, which we assume to be due to mantle flow beneath the overriding Eurasian plate. These observations will allow to test ideas of, for example, slab-mantle coupling in subduction regions. © 2012. American Geophysical Union. All Rights Reserved. Source

Saygin E.,Australian National University | Cummins P.R.,Australian National University | Cipta A.,Australian National University | Hawkins R.,Australian National University | And 6 more authors.
Geophysical Journal International | Year: 2016

In order to characterize the subsurface structure of the Jakarta Basin, Indonesia, a dense portable seismic broad-band network was operated by The Australian National University (ANU) and the Indonesian Agency for Meteorology, Climatology and Geophysics (BMKG) between October 2013 and February 2014. Overall 96 locations were sampled through successive deployments of 52 seismic broad-band sensors at different parts of the city. Oceanic and anthropogenic noises were recorded as well as regional and teleseismic earthquakes. We apply regularized deconvolution to the recorded ambient noise of the vertical components of available station pairs, and over 3000 Green's functions were retrieved in total. Waveforms from interstation deconvolutions show clear arrivals of Rayleigh fundamental and higher order modes. The traveltimes that were extracted from group velocity filtering of fundamental mode Rayleigh wave arrivals, are used in a 2-stage Transdimensional Bayesian method to map shear wave structure of subsurface. The images of S wave speed show very low velocities and a thick basin covering most of the city with depths up to 1.5 km. These low seismic velocities and the thick basin beneath the city potentially cause seismic amplification during a subduction megathrust or other large earthquake close to the city of Jakarta. © The Authors 2015. Published by Oxford University Press on behalf of The Royal Astronomical Society. Source

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