著者

秋元 菜摘

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.125, no.4, pp.523-544, 2016-08-25 (Released:2016-09-12)

参考文献数

23

被引用文献数

8 7

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Recently, governments of some cities have adopted urban policies, based on a compact city concept to facilitate accessibility, mainly for mobility-impaired people among Japan's ageing society. The government of Aomori city has planned a popular compact city model, or a centralized urban physical structure, by controlling the urbanized area of the Mid-city zone (population-incentive zone) in the city master plan established in 1999. The effects of measures for providing accessibility to the central railway station as a representative facility of the city center are analyzed, and the implications for urban policy based on a centralized compact city design in Aomori city are investigated. Accessibility, as represented by time-distance to the central station, is measured based on a road network analysis and timetables of train and bus services. As a result, it is observed at a level where most of the population can reach the facility. The simulations are based on the following measures: 1) supporting migration inside of the Mid-city from the Outer-city forming the suburbs; and 2) improving the frequency of bus transportation in the Toyama housing district, a case-study area in the suburbs. The major findings are as follows. 1) Accessibility to the central station by train is improved with migration and the effects are greater for elderly people. It seems to be useful to strengthen these effects if migration is only into the Inner-city. Although variations in migration measures are found under several conditions, accessibility is finally improved to almost the same level. Therefore, it is important to continue migration support measures, even if the pace is slow. 2) Accessibility to the central station by bus from the Toyama housing district is improved substantially under a situation where the frequency of bus services is increased until five-fold. The effect is also improved for elderly people. Some suburban cores such as large housing districts should be set or maintained at least the short-term aims of urban policy, because the population is already distributed in the suburbs and the level of functions of daily life should be maintained there. Lastly, it is important that the city master plan refers to the phasing of centralization from the suburbs, especially, about methods and times. If it is considered that the suburban cores continue independently in the process of compact city policy, the government of the city should include the cores in the urban structure and the city master plan should be revised.

著者

末岡 茂 田上 高広 堤 浩之 長谷部 徳子 田村 明弘 荒井 章司 山田 隆二 松田 達生 小村 健太朗

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.119, no.1, pp.84-101, 2010-02-15 (Released:2010-05-21)

参考文献数

62

被引用文献数

9 8

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Quantitatively estimating denudation is generally difficult because it essentially involves the removal and loss of materials in situ. The denudation rate of mountainous areas in Japan has commonly been studied from the volume of sediment in a basin or catchment. Nonetheless, the availability of these methods is constrained spatially by upstream area and temporally by depositional age. In the last few decades, thermochronometric methods that evaluate thermal history using radiometric-dating methods have been used to evaluate the denudation and tectonic history of orogenic belts around the world. The advantages of thermochronometric methods are that we can calculate the denudation rate at each sampling point and that combining multiple methods and/or target minerals enables us to calculate denudation rates in multiple periods. However, thermochronometric methods have been applied to areas with extraordinarily high denudation rates in island arc areas such as Japan. Thus, the effectiveness of thermochronometric methods for estimating denudation rates in island arc areas has not been demonstrated. We applied apatite and zircon fission-track thermochronometry to granitic rock samples collected from outcrops and a borehole to estimate the tectonic history of the Rokko area, southwest Japan. Previous studies suggested that the Rokko Mountains have been uplifted by active faulting along their northern and southern margins during the Rokko movements, a Quaternary tectonic movement in the Kinki district. However, the tectonic history of the Rokko area prior to 1 Ma has not been revealed due to a lack of prevalent geologic markers. We estimated average denudation rates in various periods based on apatite and zircon FT ages and previously reported radiometric ages. We obtained denudation rates at about 0.04-0.10 mm/yr after about 30 Ma, 0.05-0.7 mm/yr during 50-30 Ma, 0.7-4.0 mm/yr during 70-50 Ma. The denudation rate after the deposition of the Kobe Group (36.9-30.4 Ma) is estimated to be in the 0.01-0.1 mm/yr order, while bedrock uplift rate after about 1 Ma is estimated to be about 0.5 mm/yr. Thus, the tectonic activity of the Rokko Mountains area prior to 1 Ma has been relatively low.

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.11, no.12, pp.836a-839, 1899-11-25 (Released:2010-12-22)

著者

市橋 伯一

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.129, no.6, pp.871-880, 2020-12-25 (Released:2021-01-18)

参考文献数

34

被引用文献数

2

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All living organisms found so far consist of cells with a micro-water droplet surrounded by a lipid bilayer. Such a compartment structure is necessary for living organisms to repress the amplification of parasitic entities. This role of cellular structures is especially important for the primitive lifeforms that first appeared on ancient Earth. This complex lipid-based cell boundary is considered to have been acquired later in the long evolutionary history of life. Instead, ancient life-forms utilized cell-like structures that could be supplied from the ancient Earth environment. What kinds of structure could be utilized by ancient life-forms? Reviewed here are previous hypotheses regarding ancient cell-like structures, such as compartment structures formed inside a hydrothermal vent or on a rock surface, water droplets in the air that spread from geysers, and vesicles composed of simple amphiphilic molecules. Also introduced are a recent experimental verification of droplets in the air repressing parasite amplification using Spiegelman's RNA replication system. Our understanding of the origins of cellular structures is still limited because of a lack of appropriate experimental examinations based on a deeper understanding of the ancient Earth environment, which can be achieved only by collaborations between geologists and biochemists.

著者

松山 洋

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.132, no.1, pp.17-31, 2023-02-25 (Released:2023-03-09)

参考文献数

31

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Given the lack of studies on the characteristics of precipitation at Haha-jima, Ogasawara (Bonin) Islands, they are investigated using daily precipitation data from AMeDAS Haha-jima (August 30, 2007-December 31, 2020) and the Oki-mura Water Supply Branch of Ogasawara Village (January 1, 1978-December 31, 2017). A regression equation is produced to estimate daily precipitation at AMeDAS using data at Oki-mura for each month; then, they are applied to estimate daily precipitation at AMeDAS from January 1, 1978 to August 29, 2007. The monthly precipitation analysis for the period from January 1978 to December 2020 reveals that precipitation in February at Haha-jima decreased with a 5% significance level. In contrast, precipitation in September at Chichi-jima increased with a 5% significance level. A comparison between the histograms of daily precipitation on both islands in the former (1978-1999) and latter (2000-2020) periods reveals that in February 2000-2020, precipitation of 10-19 mm/day class on both islands decreased, along with a decrease of 20-29 mm/day class at Haha-jima. During the period September 2000-2020, the maximum daily precipitation on both islands surpassed previous records, and precipitation of 70-89 mm/day class increased on both islands. Most of the heavy daily precipitation occurred either during the approach or the passage of a tropical depression or a typhoon.

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.118, no.3, pp.Cover03_1-Cover03_2, 2009-06-25 (Released:2010-03-26)

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泥火山噴出物は粘性が高いため約8mの高さをもち,現在でも成長している.噴出口からは間欠的に泥,地下水,メタンを主とするガス,石油が噴出している.噴出箇所は,北東–南西方向の走向をもつ旗山断層に沿っており,地形的には尾根部に位置している.約200m四方の平坦地には数箇所の噴出口があり,それぞれ噴出物の粘性が異なる. (写真・解説:田中和広,宮田雄一郎 2006年6月27日撮影)

著者

浜田 好弘 田中 和広 宮田 雄一郎

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.118, no.3, pp.408-423, 2009-06-25 (Released:2010-03-26)

参考文献数

38

被引用文献数

9 8

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The geology and geochemistry of mud volcanoes in Taiwan was investigated to elucidate the relationship between their distribution and geological structure and the mechanism of ascending fluid migration from deep underground regions caused by abnormal pore water pressure. A detailed geological survey was carried out to describe the geological structure and the stratigraphy of mud volcanoes in the Pliocene Gutingkeng Formation in the Hsiaokunshui area. Groups of several to tens of mud volcanoes are distributed along the anticline axis within an area of 400 m in diameter. Mud volcanoes are classified into three types on the basis of differences in their morphological features (pudding type, crater type and pool type) corresponding to three types of erupted groundwater having different viscosities. As a result of geochemical studies on groundwater that erupted from mud volcanoes, it is shown that the geochemistry of groundwater that erupted from mud volcanoes distributed along the anticline, such as the Hsiaokunshui mud volcanoes, is characterized by lower δ18O ratios and high concentrations of soluble ions compared to those distributed along the Chishan Fault. Also, it is concluded that pressurized groundwater diluted by water produced during the dehydration of clay minerals ascended through the Chishan Fault and along the Hsiaokunshui anticline. On the other hand, on the basis of high δ18O ratios, it is suggested that the groundwater of mud volcanoes along the Chishan Fault was quickly expelled from underground regions deeper than those along the Hsiaokunshui anticline. Also, the groundwater of mud volcanoes along the Hsiaokunshui anticline ascended through a variety of paths from the mud chamber to the ground surface, and consequently various types of mud volcanoes were formed on the ground surface.

著者

中田 英二 千木良 雅弘

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.118, no.3, pp.511-532, 2009-06-25 (Released:2010-03-26)

参考文献数

40

被引用文献数

10 9

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Understanding erosion processes is important to prevent natural disasters such as slope failure and bedrock erosion in immature sedimentary rocks. Pliocene–Pleistocene illite-rich, non-smectite mudstone of the Gutingkeng Formation is distributed over 250 km2 in southern Taiwan, forming badlands (locally called moon-world) with mud volcanoes nearby. These volcanoes erupt saline water and natural gas, and producing a Na+, Ca2+, Cl-, and SO42- rich unsaturated Popcorn crust, which covers the mudstone slope surfaces in the moon-world area. In the crust porewater, ion strength reaches about 10 mol/L; zeta potential on particle surfaces shows a highly positive voltage. Repulsion occurs between particles under this high voltage in the crust, which is rapidly slaked to form mud by heavy precipitation. The zone that is rapidly slaked by precipitation reaches 10-20 cm beneath the crust surface. Ion strength of porewater of fresh mudstone is 0.5 mol/L approaches 0 mV (range 0.2-0.5 mol/L, pH 4-6). The surface charge of particles decreases with the infiltration of precipitation into the crust and fresh rock, with a minus surface charge occurring with increased rain infiltration. This leads to many cracks forming on the surface of mudstone, which is different from the mechanism of rapid slaking. Evaporation from the 10-20 cm-thick zone between the crust and the underlying fresh mudstone would stop if water was not supplied from depth, which is supported by in-situ measurements of water evaporation in the field. These mudstones erode readily under high precipitation because of the repulsion caused by the high ion strength of porewater. High-salinity porewater including mudstone is distributed near the active mud volcanoes where saline water rises and there is a rapid uplift rate. Rapid slaking occurred with some elements in the concentrated crust and near the surface drying zone.

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.14, no.4, pp.268, 1902-04-15 (Released:2010-12-22)

著者

新井 健司

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.118, no.6, pp.1265-1274, 2009-12-25 (Released:2010-03-23)

参考文献数

12

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Investigations of disasters caused by past earthquakes are useful when taking measures against earthquake disasters that may occur in the near future. In western Saitama Prefecture, there have been few reports so far on damage caused by earthquakes occurring beneath the Tokyo metropolitan area; however, a remarkable description was found in the annual report of the Central Meteorological Observatory of Japan. According to the report, a slope in Hanno town failed over a width extending 640-660 m caused by the 1894 Meiji Tokyo earthquake. The author investigated the location of the failed slope and details of damage. A family in Kusumi, Hanno city (Hanno town at that time), suffered from a landslide caused by a past earthquake which is considered to be the 1894 Meiji Tokyo earthquake. The slope beside their residence extends for about 700 m to the east along the Hirayama ridge, and descends steeply toward the Iruma River. Many scars from small landslides are left on the entire surface of the slope. These topographical features suggest that several landslides occurred on this slope following the 1894 Meiji Tokyo earthquake. As a result, the author has come to the conclusion that the failed slope reported by the Central Meteorological Observatory of Japan was located along the Hirayama ridge, and several landslides occurred on this slope. It is possible to recognize that a strong quake shook Hanno town, which is located far from the epicenter.

著者

田中

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.34, no.2, pp.107, 1922-02-15 (Released:2010-12-22)

著者

藤原 治 太田 耕輔 青島 晃

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.132, no.4, pp.309-325, 2023-08-25 (Released:2023-09-08)

参考文献数

32

被引用文献数

2

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Array coring survey at the site of Yonezu Pond, which is depicted on a 1680s map, revealed a sand bed deposited by a tsunami caused by the 1498 CE Meio Earthquake occurred in the eastern Nankai Trough. This sand bed consists mainly of medium-grained sand, 10-15 cm thick, forming large ripples or dunes that record the reversal of tsunami inundation and return flows. Our age model based on radiocarbon dating limits the depositional timing of the sand bed to c. 1440-1600 CE. Only a tsunami could have generated a flow fast enough and long enough in duration to deposit a large amount of sand in Yonezu Pond, which at that time was more than 1.2 km inland from the coast and river. The facies change from peat to clay and pollen composition before and after the Meio tsunami suggest that the tsunami had a significant impact on the vegetation around the pond, especially herbaceous vegetation. Plant opal analysis revealed that paddy field devastation occurred with the formation of the Meio tsunami deposit.

著者

砂村 倫成 野口 拓郎 山本 啓之 岡村 慶

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.118, no.6, pp.1160-1173, 2009-12-25 (Released:2010-03-23)

参考文献数

59

被引用文献数

3 2

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Hydrothermal circulations supply a huge amount of chemical species into the deep sea. More than 99% of chemical species emitted from high-temperature hydrothermal fluids flow into the deep sea and construct deep-sea hydrothermal plumes. Observations of hydrothermal plumes have led studies of deep-sea hydrothermal vents, such as locating deep-sea hydrothermal vents, locating deep-sea volcanic eruptions, and calculating geochemical fluxes from sub-seafloor to deep ocean. Hydrothermal plumes affect the microbial community in deep seas by supplying many reduced chemicals, which are possible energy sources of chemolithotrophic microbes. This paper (1) reviews physical, chemical, biological studies of hydrothermal plumes and (2) discusses novel field survey technology and ecological infection of sub-seafloor to the deep-sea environment.

著者

大渡 忠太郎

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.11, no.5, pp.355-358, 1899-05-25 (Released:2010-10-13)

著者

安江 恒 久保 典子 赤尾 実紀子 佐野 雅規 中塚 武

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.128, no.1, pp.49-59, 2019-02-25 (Released:2019-04-03)

参考文献数

31

被引用文献数

3 1

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Ring width, maximum density, and δ18O chronologies of Tsuga diversifolia and Picea jezoensis var. hondoensis growing in a sub-alpine forest at Mt. Senjo in the Akaishi Mountains were developed. Ring width and maximum density were measured with X-ray densitometry. Tree-ring δ18O was measured using a mass spectrometer after cellulose extraction. The chronologies developed have significant positive correlations with monthly temperatures in July, August, and September, with the exception of the ring width of T. diversifolia. The transfer functions for July-September temperature were developed using the four chronologies and were verified statistically. The transfer functions reveal a high coefficient of determination, whereas statistical verifications were not successful with rather low RE, CE, and sign test. The estimated temperatures since 1774 partially agreed with reported climate changes based on historical records. The results indicate that estimated temperature is weak for reconstructing increasing trends and low-frequency variations of temperature, although it is potentially useful for higher frequency temperature changes in local areas.

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.132, no.3, pp.Cover03_01-Cover03_02, 2023-06-25 (Released:2023-07-07)

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紀伊半島南部に分布する古第三系~新第三系の牟婁(むろ)層群佐本川(さもとがわ)層は,四万十帯南帯の付加体を構成する砂岩・泥岩互層である.そのなかには堆積物がまだ未固結時に形成されたさまざまな規模の褶曲構造が見られる.写真は和歌山県西牟婁郡すさみ町口和深(くちわぶか)付近の海岸で観察される褶曲の好例である.かつて重力性のスランプ褶曲と考えられていたが,場所によって褶曲軸の倒れる方向が正反対のものもあることから,現在では付加体形成時の高間隙水圧下で形成された構造と判断されている.とくに目を引くこの褶曲露頭は,付近の小字(こあざ)の地名である天鳥(あまどり)にちなむ「フェニックス褶曲」という愛称をもつ.中央の「>」状褶曲上部の水平部分の長さは約5 mあり,迫力のある景観を提供している.ちなみに,この露頭を観察するためには,事前に南紀熊野ジオパークセンターを通じてガイドツアーに申し込む必要がある.(写真・説明:小松原純子 2023年3月28日撮影)

著者

李 盛源 田瀬 則雄

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.132, no.3, pp.231-246, 2023-06-25 (Released:2023-07-07)

参考文献数

41

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The remediation effects of a permeable reactive barrier (PRB) using sulfur-limestone are investigated. The results show that highly concentrated NO3− (133-180 mg/L) in shallow groundwater decreased below the detection limit within the PRB. This phenomenon was maintained for approximately 300 days; therefore, the remediation effects within the PRB and its durability are demonstrated. It is suggested that the remediation effects due to the PRB are not solely restricted to the area within the barrier, because the decrease in NO3− extended 4.0 m downstream. Furthermore, the results of ORPSHE, δ15N, and isotopic fractionation factor (ɛ) indicate that a reductive condition forms within the PRB, where sulfur-oxidizing denitrifying bacterium preferentially exists. Before installing the PRB, three-dimensional (3-D) groundwater flow systems around the study site and detailed hydrogeological setting should be investigated first.

著者

中村 高志 尾坂 兼一 CHAPAGAIN Saroj Kumar 西田 継

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.132, no.2, pp.183-196, 2023-04-25 (Released:2023-05-15)

参考文献数

47

被引用文献数

2 2

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ネパール・カトマンズ盆地における硝酸イオン(NO3−)およびアンモニウム(NH4+)による高濃度の地下水汚染について,窒素の起源とその分布の要因について調査を行なった。2009年と2010年の雨季(8月)に,丸井戸と掘り抜き井戸から計36の浅層地下水の試料を採取した結果,高濃度のNO3−(最大:63.9 mg/L)ならびにNH4+(最大:36.7 mg/L)が検出された。多くの丸井戸から採取した地下水については全溶存無機窒素に対するNO3−の含有量が多く,不飽和帯から流入するNO3−または酸素の供給により井戸内部で硝化が生じている可能性が示唆された。NO3−の窒素および酸素の安定同位体比(δ15Nとδ18O)の観測の結果,浅層地下水の主要な窒素汚染源は下水の漏水による窒素負荷であることが示された。また,NO3−濃度の減少に伴うNO3−-δ15N値の指数関数的な増加がみられたことや,δ18O値とδ15N値の間に得られる相関の近似式の傾きがおよそ0.5を示したことから,浅層地下水中において脱窒反応が生じでいることが考えられた。加えて,NO3−濃度と溶存有機炭素の間には負の相関関係がみられ,溶存有機炭素が脱窒反応の進行に強く寄与していることを示した。

著者

高波 紳太郎

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.131, no.3, pp.317-338, 2022-06-25 (Released:2022-07-08)

参考文献数

59

被引用文献数

2

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Ata welded ignimbrite (110 ka) lies beneath Ito non-welded ignimbrite and Osumi pumice fall deposit (30 ka) in the Kimotsuki Plain, southern Kyushu. Previous geomorphological studies of the Kimotsuki Plain focused on landform development after deposition of Ito ignimbrite. Landform development of Kimotsuki Plain since the last interglacial, especially until just before deposition of the Osumi pumice fall, is reconstructed using geological data collected from outcrop observations and borehole records. The basal-surface of the Osumi pumice fall deposit obtained shows that Ata welded ignimbrite had been dissected by the Kimotsuki River and its tributaries in response to the last glacial sea-level drop before the Osumi pumice fall was deposited. Longitudinal profiles along the Kushira River in 110 ka and 30 ka indicate recession of the Tanida waterfall, which formed at the edge of the Ata welded ignimbrite plateau. These profile changes imply that the Tanida waterfall retreated 2.4-6.0 km upstream between 110 ka and 30 ka. The Kushira formation, Marine oxygen Isotope Stages (MIS) 5e marine deposits under Ata welded ignimbrite, was found below the present sea-level at multiple locations in the Kimotsuki Plain. This vertical distribution of the Kushira formation indicates that the Kimotsuki Plain has been in a tectonically stable or subsidence area since the MIS 5e, in contrast with the Onejime and Natsui areas, which have been tectonically uplifting. The depositions of the two ignimbrites had significant impacts on filling the Paleo-Shibushi Bay (Sea) and the development of the Kimotsuki Plain under sea-level lowering during the last glacial period. The top-surface of basement rocks is more than −120 m below sea level at the floors of paleo valleys, even though it is adjacent to mountains composed of the basement. Further investigation of lower alluvium and its basal-surface is required for an understanding of valley incision and delta evolution of the Kimotsuki Lowland after deposition of the Ito pyroclastic flow.

著者

宮縁 育夫 星住 英夫

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.126, no.5, pp.581-593, 2017-10-25 (Released:2017-11-10)

参考文献数

34

被引用文献数

2 1

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Several volcaniclastic deposits discovered at the foot of the eastern wall of Aso caldera, central Kyushu, southwestern Japan, are divided into two types: lahar deposits (Lh1-Lh5 in descending order), which contain abundant subangular to subrounded lithic clasts ( 3.5 m in diameter) set in a sandy to silty matrix, and debris avalanche deposits (DA1 and DA2), which include numerous plastically deformed fragments of tephra (ash and scoria) and soil layers in a homogenous silty to clay matrix. DA2, which underlies a paleosol dated at 5.4 ka (calibrated 14C age), is the largest volcaniclastic deposit observed in the section (more than 2.5 m thick and about 70 m wide). Because the debris avalanche deposits display no evidence that they were transported by water, they are likely to have originated from landslides triggered by intense earthquakes. Tephra chronology and 14C-dating on paleosols along the succession suggest that lahars occurred once over 900 years (6.3-5.4 ka), three times over 1400 years (5.4-4 ka), once over 400 years (4-3.6 ka) and twice (including the 2012 lahar) in the last 3600 years. This evidence indicates that the lahars occurred at an interval of 400-1800 years. In contrast, two debris avalanche deposits exist in the same succession spanning the last 6300 years. Including landslides and associated debris avalanches triggered by the 2016 Kumamoto earthquake (Mj 7.3), landslides generating debris avalanches in the Aso caldera occurred at least three times in the last 6300 years. This may suggest the frequency of large earthquakes triggering debris avalanches in the central Kyushu region, which has many active faults.