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Seismic profiling of deep geological structure in the Kobe and Ashiya areas
Takanobu YOKOKURA(Geophysics Department, GSJ), Kazuo Yamaguchi(Geophysics Department, GSJ), Naomi KANO(Geophysics Department, GSJ), Teruki MIYAZAKI(Research Planning Office, GSJ), Takeshi IKAWA(R & D Department, JAPEX Geoscience Institute Inc., ), Yohichi OHTA(R & D Department, JAPEX Geoscience Institute Inc., ), Taku KAWANAKA(R & D Department, JAPEX Geoscience Institute Inc., ) and Susumu ABE(R & D Department, JAPEX Geoscience Institute Inc., )
1999
vol. 50 (4), P. 245-267
12 figs., 1 appendix-table
Keywords: 1995 Hyogo-ken Nanbu Earthquake, Kobe, Ashiya, seismic reflection method, Osaka-wan Fault, Wada-misaki Fault, Maya Fault, Koyo Fault, Ashiya Fault, deep structure, basement, vibrator, airgun, geophone, baycable
Abstract: On January 17, 1995 the Kinki district, southwest Japan, was struck by a disastrous earthquake of M7.2, named as the 1995 Hyogo-ken Nanbu Earthquake. In order to clarify deep structures of this region, we conducted seismic surveys along 12 survey lines. Various types of evidence suggest that there exist many faults at the boundary regions between land and sea. To obtain integrated data in scrutinizing deep structure of these faults, we conducted continuous and uniform surveys traversing land, shallow water and sea, which are about 260 km long in total. In this paper we will discuss four survey lines of them in the Kobe and Ashiya areas. Sources used in the survey were one to four vibrators (Y-2400) on land, and two airguns at shallow water. Receivers were over 240-ch geophones (natural frequency: 10 Hz) at the interval of 25 m on land, and maximum 96-ch baycables at the interval of 25 m at shallow water. Sources were shot at standard intervals of 50 m on land, and 25 m at shallow water. Common-mid points (CMP) were set at 12.5 m intervals. The standard CMP fold number was 48 both on land and at shallow water. Signals generated from vibrators and airguns were simultaneously received by geophones and baycables to integrate the profiling lines both on land and at shallow water. After applying many noise-suppression methods, we have relatively clear images. The processing results indicate: (1) The Ashiya Fault is a reverse fault with relatively low angle. (2) A wide fracture zone exists between the Ashiya Fault and the Ashiya river mouth, where several concealed faults have been found . The northern end of the fracture zone corresponds to the extension of the Koyo Fault. (3) The Nishinomiya Flexure corresponds to a low-angle and small-scale fault near earth's surface which branches from the Koyo Fault. (4) Two new reverse faults have been found between the JR Kobe station and the Okura Hill. These faults together with the Egeyama Fault have lifted the Okura Hill. One of these is "the Motomachi Fault (a tentative name)" which is located beneath the Motomachi Flexure. (5) At the south of the Kobe Port, the Osaka-wan Fault branches off into three: the Wada-misaki Fault, the Maya Fault and "the Rokko Island Fault(a tentative name)". (6) The Wada-misaki Fault runs northward from 4km south of the Cape Wada-misaki via the Kobe Port. Then it gradually changes the strike, and runs northeastward via the Port Terminal and the Ikuta river mouth. The major branch of the Wada-misaki Fault runs further northeastward. Another small branch runs from the Ikuta river mouth toward the Gosukebashi Fault. (7) The Maya Fault runs northward from 2km south of the Port Island via east end of the Port Island, and changes the strike northeastward off the southeasteren Maya Wharf. It runs further northeastward via the Rokko-ohashi Bridge. (8) The Rokko Island Fault runs northward off the southeastern Port Island, changes the strike northeastward off the southwestern Rokko Island, and then may cross the Rokko Island. (9) These three branches of the Osaka-wan Fault are probably connected to the pre-exsisting active faults in the Rokko Mountains, and are probably boundaries of basement blocks beneath the Kobe and Ashiya area. (10) Beneath the so-called "damage belt", there is no concealed fault which runs paralel with the belt. The major cause of the damage belt may be due to some kind of interference of seismic waves produced by non-symmetrical distribution of basement depth. However some faults with a NE-SW trend may have some role in forming the damage belt.
Diatom biostratigraphy of the Middle Miocene Hatatate Formation, Sendai City, Miyagi Prefecture
Yukio YANAGISAWA (Geological Museum, GSJ)
1999
vol. 50 (4), P. 269-277
4 figs., 1 table
Keywords: diatom, sedimantary rocks, biostratigraphy, biochronology, Hatatate Formation, Moniwa Formation, Sendai, Miyagi Prefecture, Japan, Neogene, Miocene
Abstract: Diatom biostratigraphy is established for the Middle Miocene Hatatate Formation exposed along the Iwanosawa section, Sendai City, Miyagi Prefecture, northeastern Japan. The formation is composed mainly of muddy fine-grained sandstone intercalating a number of thin tuff beds. The lower Hatatate Formation above the tuff bed Ht 10 is assigned to the Denticulopsis praedimorpha Zone (NPD 5B) of Akiba (1986), and contains two diatom biohorizons, D52.5 (the first acme of Denticulopsis crassa) and D53 (the first occurrence of D. praedimorpha var. praedimorpha). The rapid decrease of radiolarian Cyrtocapsella tetrapera lies just below the diatom biohorizon D53. Remarkable diachronism across latitude is suggested for the last occurrence of calcareous nannofossil Cyclicargolithus floridanus which lies between the diatom biohorizons D52.5 and D53 in the Hatatate Formation. The glauconite bed in the lowest Hatatate Formation is characterized by a very slow sedimentation rate and was deposited during the interval from 15 to 13 Ma
Abstracts for Geological Survey Seminar, no. 258
Geological research works in Mongolia
1999
vol. 50 (4), P. 279-289
