3 research outputs found
Complex Shear Partitioning Involving the 6 February 2012 MW 6.7 Negros Earthquake Ground Rupture in Central Philippines
No full textA 75 km-long, generally NE-striking ground rupture associated with the 6 February 2012 MW 6.7 (Mb 6.9) Negros earthquake was mapped on the eastern side of Negros Island, Philippines. It closely follows a previously unmapped, pre-existing fault trace along the coast which is marked mostly by terrace-forming scarps. The dominance of vertical separation (west side up) is consistent with a west-dipping reverse fault, as indicated by focal mechanism solutions. The ground rupture map eliminates the ambiguity in the focal mechanism solution regarding the orientation, sense of motion, and location of the seismogenic fault plane, which are indispensable in the assessment of seismic hazards and the nature and distribution of deformation. This study uses the ground rupture map of the 2012 Negros earthquake in sorting out the mechanism of deformation in the Visayas Islands region. The ground rupture’s length is well within the aftershock area while its scarp heights are consistent with an earthquake of its magnitude and nature of movement. The 2012 Negros earthquake rupture’s pattern, scarp types, and offset of man-made structures are similar to those of recent reverse/thrust ground ruptures mapped globally and are distinct from those associated with erosion, landslide, and liquefaction. The onshore coseismic reverse fault of the Negros earthquake, which contradicts a model of coseismic slip on an offshore blind thrust fault by previous workers, represents the first thoroughly mapped ground rupture of its kind in the Philippines. The ground ruptures of the 2012 Negros and 2013 Bohol earthquakes, along with the Philippine Trench and the Philippine Fault Zone (PFZ), represent a complex shear partitioning mechanism in the Visayas Islands region. This departs from the current simple shear partitioning model for the region and is distinct from those for other regions along the PFZ and adjacent subduction zones. This study shows how an appreciation of morphotectonic features can lead to a better understanding of the distribution of deformation and the nature of earthquake hazards
フィリピン，ルソン島中央部のパイタン湖のコア試料の層序と古環境
No full text2016-03Paitan Lake located on the northern part of Luzon Central Plain, is a tuff ring in the Amorong monogenic volcanoes. The lake has 0.8km^2-surface area, 38.5km^2-drainage basin, and one outflow-river with a few meter of width in the northern side. In order to establish the tephrostratigraphy in Luzon Island, the Philippines, two sediment cores, PPL-1 (13m in depth) and PPL-2 (28m in depth), were obtained from this lake. These cores consisted of two units: an upper alternating bed of mud layers and peaty layers in a depth range from 0 to 6m, and a lower dark mud bed from 6 to 28m. Seven tephra layers were observed in the cored sediments. Phenocryst assemblage implies that these are originated from Pinatubo volcano located ca. 90km SW from this lake. To detect cryptotephra, initial magnetic susceptibility was measured at 2-cm interval. Soft X-ray photographs were taken to interpret deformation structures during the drilling process. Radiocarbon dates of seven plant fragments and twelve organic sediments were measured by the AMS facility of KlGAM, the Institute of Accelerator Analysis Ltd. and ISEE of Nagoya University, respectively. Inferred age for four tephra horizons is consistent with previous dates for Pinatubo eruptions. To discuss paleoenvironment changes of this lake, diatom analysis with 20-cm intervals is conducted. Attached diatoms indicating marsh or peat land have emerged in cored sediments of 0 to 3m in depth with high CN ratio. From 3.2 to 28m in depth, the freshwater planktonic diatoms become dominant species and indicete freshwater lake environment. Aulacoseira granulate is dominant in 3.2 to 6m in depth, on the other hand, deeper part than 6.6m, Aulacoseira muzzanensis is dominant. Both species may indicate changes of water depth of the lake. Furthermore, only around 25m in depth, Cyclotella radiosa is dominant. At this horizon (ca. 20 cal kBP), organic carbon content and CN ratio also increase. This signal may indicate some event at Last Glacial Maximum (LGM).フィリピン，ルソン島のパイタン湖はAmorong単成火山群のタフリングで，閉鎖的な環境が持続されてきた．筆者らは，湖畔で深度28mのコア試料を採取し，珪藻群集分析，軟X線写真撮影，14C年代測定，火山灰分析，初磁化率測定，炭素・窒素含有量測定を行った．植物片7点と有機質堆積物12点の14C年代から，得られたコア試料は，最近約24,000年間を記録し，その堆積速度は 0.8～2.4m/kyrである．6 枚のテフラ層が検出され，岩石記載学的特徴からピナツボ火山が起源である．珪藻群集では，深度0～3mで付着性珪藻が出現し，沼沢もしくは泥炭地と推定された．3m以深は淡水浮遊性珪藻へと優占種が変化し，淡水湖沼環境であったことを示す．さらに深度6m付近でAulαcoseira granulateからAulacoseira muzzanensisへ優占種が変化した．最終氷期極相期(LGM)に当る深度25mでのみCyclotella radiosaが優占種となり，何らかのイベントを示すと考えられる．タンデトロン加速器質量分析計業績報告departmental bulletin pape
