Analisis Kerentanan Tanah Longsor Sebagai Dasar Mitigasi Di Kabupaten Banjarnegara (Vulnerability Analysis as a Basic for Landslide Mitigation in Banjarnegara Regency)

Landslide is a hydrometeorologycal disaster that usually happens in Indonesia. The purpose of this study was to determine the level of landslide vulnerability in Banjarnegara District. This study employed survey and descriptive quantitative methods by using a formula of landslide vulnerability, with variables: natural and management factors. The analysis used in this study was overlaying the predetermined formula and weighting it. The results indicated a variety of vulnerability classes, which were: 1) non-vulnerable zone of 44.88 ha (0.04%), 2) slightly vulnerable zone of 7,800.84 ha (7.29%), 3) fairly vulnerable zone of 88,505.80 ha (82.74%), 4) vulnerable zone of 10,423.32 ha (9.74%), and 5) very vulnerable zone of 196.16 ha (0.18%). The dominant parameters for landslides in Bajarnegara were: rain, geology and regolith. Mitigation techniques employed in those areas should be based on community-self-supporting mitigation through the development of disaster resilient villages. Disaster resilient village is a village that is responsive and can minimize disaster risks through adaptation. Several measures can be done independently autonomously by the community including increase the alertness during rainy period, seal all cracked soil due to the fault movement, and protect the soils through slope (stabilization and protection of slopes)

Application of Filter Cake on Growth of Upland Sugarcanes

Recently planting site of sugarcane was shifted from lowland to upland area. Sugar cane cultivation in upland has many constraints, especially limited water supply and low nutrition availability. The objectives of this research were to study the influence of application of composted filter cake on growth and water use efficiency of upland sugar cane. The research was conducted in Jengkol, Kediri. Treatments consist of three factors: frequency of irrigation (once every 1 week, once every 2 weeks, and once every 3 weeks); sugarcane varieties (PS-862 and PS-864); and compost doses (0, 2.5, 5, and 7.5 ton ha-1). Split plot design with three replications was used in each irrigation treatment, using composted filter cake as main plots and sugarcane varieties as sub plots. The results showed that the highest sugar content was reached at application of 5 ton ha-1 compost and the greatest crystal sugar was reached at 3.09 ton ha-1 compost. Compost application at 5 ton ha-1 on each planted row can reduce frequency of irrigation from once a week to once every 2 weeks

Mantle flow in regions of complex tectonics: insights from Indonesia

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

Teknik Identifikasi Daerah yang Berpotensi Rawan Longsor pada Satuan Wilayah Daerah Aliran Sungai

Kejadian tanah longsor di Indonesia belakangan ini terus meningkat intensitas dan sebarannya. Tanah longsor terjadi jika tahanan geser massa tanah atau batuan lebih kecil dari tekanan geser pada sepanjang bidang longsoran yang disebabkan oleh adanya peningkatan kejenuhan air tanah saat musim penghujan. Tujuan penelitian adalah untuk mendapatkan teknik identifikasi daerah yang berpotensi longsor, agar masyarakat mudah mengenali dan tidak terjadi korban yang tidak perlu. Lokasi penelitian adalah pada lahan yang berada pada wilayah berpotensi longsor di Kabupaten Purworejo, Banjarnegara, dan Karanganyar di Provinsi Jawa Tengah. Metode pengamatan longsor dengan mencatat beberapa parameter penyebab longsor, antara lain: kemiringan lereng, curah hujan, tekstur tanah, regolith tanah, sesar, kepadatan penduduk. Hasil pengamatan daerah yang berpotensi longsor berurutan dari sub Daerah Aliran Sungai (DAS) terberat: Banjarnegara di sub DAS Merawu (12 cm), Purworejo di sub DAS Gesing (8 cm), dan Karanganyar di sub DAS Mungkung-Grompol (0 cm). Semakin tinggi kandungan liat maka semakin berpotensi longsor, selain faktor kemiringan lereng, kedalaman regolit, adanya sesar, dan tingginya curah hujan. Dampak atau manfaat penelitian ini adalah: a) mengantisipasi/meminimalisir terjadinya korban jika terjadi longsor pada daerah yang berpotensi longsor, b) memberi informasi kepada masyarakat untuk mengenal daerah berpotensi longsor dan beradaptasi dengan bencana longsor, c) memberi peringatan dini dengan memasang berbagai alat, antara lain: extensometer, penakar hujan ombrometer, dan mengenalkan berbagai macam tanaman yang tahan longsor

Mekanika teknik (mechanics of materials)

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Deformation and mantle flow beneath the Sangihe subduction zone from seismic anisotropy

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

Systematic variation in anisotropy beneath the mantle wedge in the Java-Sumatra subduction system from shear-wave splitting

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