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Volume 118, Issue 7
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SPECIAL ISSUE The 2011 off the Pacific coast of Tohoku Earthquake: Towards an integrated understanding vol. 2
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2012 Volume 118 Issue 7 Pages 395
Published: July 15, 2012
Released on J-STAGE: December 04, 2012
JOURNAL FREE ACCESSDownload PDF (128K)
Articles
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Waveform inversion and dynamic fault weakeningYukitoshi Fukahata, Yuji Yagi, Yuta Mitsui2012 Volume 118 Issue 7 Pages 396-409
Published: July 15, 2012
Released on J-STAGE: December 04, 2012
JOURNAL FREE ACCESSThe 2011 Tohoku-oki earthquake resulted in substantial slip relatively near the coast from Miyagi to Ibaraki prefectures as well as extremely large slip near the trench. The moment-rate function has a shoulder around 35 s and a large peak about 70 s. The former corresponds to a significant moment release in the source area of the 1978 Miyagi-oki earthquake, and the latter was caused by extremely large slip near the trench. The main rupture process is characterized was by large maximum slip (50 m), long slip duration in the large slip area (90 s), and a relatively large stress drop in the up-dip source region (10 MPa). These characteristics mean that dislocation across the plate interface continued to release elastic stress and strain during the earthquake, which strongly suggests significant weakening of frictional strength due to thermal pressurization or other mechanisms on the fault plane. A drastic change in earthquake mechanisms from compression to extension above the large-slip area together with the occurrence of low-angle normal-fault aftershocks at approximately the depth of the plate interface also supports the idea that the earthquake did release roughly all of the accumulated strain on the plate interface owing to dynamic weakening of the fault. The stress accumulated on the plate interface before the earthquake is estimated to have been about 10 MPa from the hypocenter to the trench. If dynamic weakening on a fault plane plays an essential role in large earthquakes, then the periodicity of large earthquakes is questioned, as fault weakening mechanisms are non-linear in nature. Pseudo-cyclical records of the occurrence of large interplate earthquakes can be understood by constant accumulation of elastic strain due to steady plate motion and accidental release of elastic strain due to dynamic fault weakening that strongly depends on the conditions immediately prior to the earthquake.View full abstractDownload PDF (1416K) -
An Example from Core Samples from Maihama 3-chome, Urayasu CityAsahiko Taira, Koichi Iijima, Chiaki Igarashi, Saburo Sakai, Hide Saka ...2012 Volume 118 Issue 7 Pages 410-418
Published: July 15, 2012
Released on J-STAGE: December 04, 2012
JOURNAL FREE ACCESSThe 2011 off the Pacfic coast of Tohoku Earthquake caused an unprecedented and widespread soil liquefaction event especially in the southern part of the Kanto district. An improved understanding of the complex liquefaction processes involved and especially identification of the liquefaction layer, is needed to understand and mitigate against such events. We report here for the first time the application of X-ray computerized tomography (CT) scanning to core samples of the soft and water-saturated near-surface formations subjected to liquefaction. Our results clearly show images of the different sedimentary units and their internal structure, including ripple laminations and load casts. One unit was identified where the original sedimentary structures have been completely obliterated, which we interpret to be the zone of liquefaction. Our results show that X-ray CT scanning is a potentially useful tool for soil liquefaction research in the fields of geology and engineering.View full abstractDownload PDF (4511K) -
Noriyoshi Tsuchiya, Chihiro Inoue, Ryoichi Yamada, Shin-ichi Yamasaki, ...2012 Volume 118 Issue 7 Pages 419-430
Published: July 15, 2012
Released on J-STAGE: December 04, 2012
JOURNAL FREE ACCESSThe Pacific coast of Tohoku District in Northeast Japan (Iwate, Miyagi, and Fukushima Prefectures) was seriously damaged by a violent tsunami resulting from the 2011 off the Pacific coast of Tohoku Earthquake on March 11 2011. The disaster area was covered by tsunami sediments, which are mainly marine sediments. We collected 137 tsunami sediments from Kuji City, Iwate Prefecture through to Minamisoma City, Fukushima Prefecture (40°11′N, 141°58′E to 37°43′N, 140°51′E). Analysis of bulk sediment chemistry and leaching test by pure water and seawater were carried out to access arsenic contamination. In addition, chemical forms of arsenic in different size fractions (<3 kDa, 3 kDa to 0.2 μm, 0.2 μm to 0.45 μm, and ≥0.45 μm) from the leachates were investigated by filtering techniques.
Arsenic concentrations in the pure water leachates of 45 samples had higher arsenic levels than the Japanese Environmental Quality Standards for Soil Pollution (10 μg/l). In particular, tsunami sediments from the Ohya area in Kesen-numa City, Miyagi Prefecture, had arsenic levels 39 times greater than this threshold, reflecting the influence of gold mines in this region. The fraction of arsenic species < 3 kDa in size in leachates of the high arsenic samples was ca. 70%, indicating that most of the arsenic in the leachates is present as fine colloidal particles and/or dissolved species.View full abstractDownload PDF (6383K)
Note
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Kazuhisa Goto, Yuichi Nishimura, Daisuke Sugawara, Shigehiro Fujino2012 Volume 118 Issue 7 Pages 431-436
Published: July 15, 2012
Released on J-STAGE: December 04, 2012
JOURNAL FREE ACCESSGeological investigations of tsunami deposits have proliferated since the 2011 Tohoku-oki tsunami in Japan, and are important in developing tsunami disaster prevention plans. However, studies of tsunami deposits in Japan are localized and do not cover the entire coastal zone. In light of these gaps in knowledge, further detailed studies of tsunami deposits should be encouraged. Most of the existing research needs to be reevaluated and integrated before incorporating the results of these studies into disaster prevention plans. Currently, there is no comprehensive summary of previous research on tsunami deposits and this hinders identification of the best sites to focus new investigations of such deposits in Japan. Here, we summarize previously published papers, books, and reports describing tsunami deposits in Japan, and identify sites that need further geological study. Our review of this literature indicates that existing research has mainly been conducted along the Pacific coast, where the risk of large tsunamis is high. A small number of studies have been carried out in parts of Hokkaido, north of Tohoku, the east coast of Kanto, and Kyushu. Very few studies of tsunami deposits have been carried out along the Japan Sea coast.View full abstractDownload PDF (636K)
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Articles
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Kunimi Yamada, Ken-ichi Yasue, Hideki Iwano, Ryuji Yamada, Koji Umeda, ...2012 Volume 118 Issue 7 Pages 437-448
Published: July 15, 2012
Released on J-STAGE: December 04, 2012
JOURNAL FREE ACCESSFission-track dating and track length analysis were carried out on 11 samples collected from surface outcrops (n = 5) and the Kawaue boring core (n = 6) in the region of the Atera Fault. This fault is a large, active sinistral fault with a dip-slip component of displacement. Mean zircon fission-track ages are ∼76 Ma and ∼69 Ma at sites located southwest and northeast of the fault, respectively. Mean apatite fission-track ages outside and within the fault zone are ∼42 Ma and ∼22 Ma, respectively. Mean track lengths in U-4 zircon, collected from within the fault zone, indicate that only this sample is partially annealed, with an apparent bimodal track length distribution. Forward modeling and considerations of annealing kinetics suggest that this sample was slowly cooled from ∼300 °C since ∼40–60 Ma. We have calculated the excess erosion of the hanging wall northeast of the fault, and true dip displacement of the fault, assuming that these processes control the age differences across the fault. These calculations lead us to three main conclusions: (1) Dip-slip displacement of the Atera fault since 70 Ma is ∼1 km. This calculated displacement is similar to that estimated from basement rocks and/or topography across the fault. (2) The present-day fracture zone along the fault experienced heating post-20 Ma, but prior to the Quaternary. (3) Some minor magmatic intrusions or another heating event, followed by slow cooling, has affected sample U-4 at ∼40–60 Ma. Our first conclusion is consistent with previous studies that concluded the present-day Atera Fault activity commenced in the early Quaternary. Our second conclusion indicates that this fracture zone has existed from ∼20 Ma through to the Quaternary.View full abstractDownload PDF (1845K) -
An application of LA-ICP-MS U-Pb dating on zirconsHisatoshi Ito, Akihiro Tamura, Tomoaki Morishita, Shoji Arai2012 Volume 118 Issue 7 Pages 449-456
Published: July 15, 2012
Released on J-STAGE: December 04, 2012
JOURNAL FREE ACCESSLaser ablation inductively coupled plasma mass spectrometry was used to perform U-Pb dating on zircons from granitic rocks in the Hida Mountain Range, central Japan. Samples were targeted for dating that showed a marked difference in cooling age across the Takasegawa Fault (Ito and Tanaka, 1999). The Okukurobe granite to the west of the fault, and the Oshirasawa granite to the east of the fault, yielded ages of 65.1 ± 1.6 Ma and 64.7 ± 2.3 Ma, respectively. These ages imply that both granites were intruded at the same time (ca. 65 Ma). The Oshirasawa granite has a mylonitic texture, which probably reflects rapid uplift and erosion by faulting in the past 2 Myr. The Kanazawa granodiorite to the east of the Takasegawa Fault was dated as 2.15 ± 0.15 Ma, indicating that this pluton was intruded in the Quaternary. In addition to the Takidani granodiorite, this is the second confirmed occurrence of an outcropping Quaternary pluton in the Hida Mountain Range.View full abstractDownload PDF (2907K)
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