Yasutaka Ikeda

Preliminary results of multidisciplinary observations before, during and after the Kocaeli (Izmit) earthquake in the western part of the North Anatolian Fault Zone

On August 17, 1999, a destructive earthquake occurred in the western part of the North Anatolian Fault Zone, Turkey. The earthquake source region has been designated as a seismic gap and an M7-class earthquake has been supposed to occur someday in the future so as to fill this seismic gap. So far we have undertaken various kinds of observations in this area and we could obtain some valuable data before, during and after the mainshock. Here we report some of the preliminary results of our recent studies, which include field work started in late July this year and continued during and after the earthquake occurrence just in the earthquake source region and its vicinity, in addition to seismic observations carried out for several years before the mainshock. Much emphasis is put on magnetotelluric field data acquired during the mainshock; in fact, large variations caused by seismic waves were recorded. Such variations could be interpreted in terms of electromagnetic induction in the conducting crust caused by the velocity field interacting with the static magnetic field of the Earth. In particular, the first motion of seismic wave could be identified in the records and used for precise determination of the hypocenter of the mainshock.

Holocene marine terraces on the northeast coast of North Island, New Zealand, and their tectonic significance

Abstract As many as seven Holocene marine terraces are preserved between Raukokore River and Gisborne on the northeast coast, North Island, New Zealand. Six terraces up to 20 m above present mean sea level (a.m.s.l.) are dated at c. 300, 600–700, 900–1200, 1600–2000, 4500, and 6000 radiocarbon yr B.P. to the west of East Cape. Seven terraces are preserved up to 27 m a.m.s.l. near Pakarae River mouth, and the higher six terraces have radiocarbon ages of c. 1000, 1600, 2500, 3900, 5500, and 7000 yr B.P. The coastal region from Waiapu River to Tolaga Bay has only two to three marine terraces, the highest attaining a maximum height of c.8m. Sponge Bay Terrace is generally the highest preserved marine terrace, and it is underlain by more than 10 m of estuarine deposits that record the rapid rise of postglacial sea level. The terrace surface records the culmination of this sea‐level rise at 5500 yr B.P. or slightly younger in areas of low average uplift rate (<1.5 m/1000 yr) and c. 7000 yr B.P. in areas of high...

Geological evidence for the last two faulting events on the north Anatolian Fault zone in the Mudurnu Valley, western Turkey

Abstract Recurrence intervals of large earthquakes produced by slip on the North Anatolian fault zone have been inferred mainly from historical records, and included much ambiguity. It is therefore important to date individual earthquakes using geological and -geomorphological methods. We excavated an exploratory trench across the surface rupture zone associated with the Mudurnu Valley, western Turkey, earthquake of July 22, 1967, and revealed evidence for the last two faulting events including the 1967 earthquake. On the basis of the radiocarbon dates of sediments in the trench, we estimate most conservatively that the penultimate event occurred after 1480 A.D., but believe it likely that the event occurred shortly after 1650 (±20) A.D. The penultimate event is likely to be correlated with the Anatolian earthquake of August 17, 1668, which caused severe damage along the North Anatolian fault zone for about 600 km from Bolu to Erzincan.

Geologic evidence for two pre-2004 earthquakes during recent centuries near Port Blair, South Andaman Island, India

Coastal stratigraphy near Port Blair, Andaman Islands, where the A.D. 2004 Sumatra-Andaman earthquake was accompanied by ∼1 m of subsidence, provides evidence for two prior earthquakes, perhaps both from the past 400 yr. The first of these (event I) is marked by an abrupt mud-over-peat contact best explained by subsidence similar to that in 2004. Event II is evidenced by an overlying chaotic layer composed of mud clasts in a sandy matrix that is connected with feeder dikes. These mud clasts, probably produced by liquefaction, are capped by laminated sand and mud that we ascribe to an event II tsunami. Radiocarbon ages of plant remains in the peat give discordant ages in the range 100 B.C. to A.D. 1950. Event I probably resembled the 2004 Sumatra-Andaman earthquake in that it was accompanied by subsidence (as much as 1 m) but not by strong shaking near Port Blair. If event II was the A.D. 1762 Arakan earthquake, the laminated sand and mud provide the first evidence that this earthquake was associated with a tsunami.

Strike-slip motion of a mega-splay fault system in the Nankai oblique subduction zone

We evaluated the influence of the trench-parallel component of plate motion on the active fault system within the Nankai accretionary wedge from reflection seismic profiles, high-resolution seafloor bathymetry, and deep-towed sub-bottom profiles. Our study demonstrated that a large portion of the trench-parallel component of oblique plate subduction is released by strike-slip motion along a fault located just landward of and merging down-dip with a mega-splay fault. The shallow portion of the splay fault system, forming a flower structure, seems to accommodate dominant strike-slip motion, while most of the dip-slip motion could propagate to the trenchward décollement. Numerous fractures developed around the strike-slip fault release overpressured pore fluid trapped beneath the mega-splay fault. The well-developed fractures could be related to the change in stress orientation within the accretionary wedge. Therefore, the strike-slip fault located at the boundary between the inner and outer wedges is a key structure controlling the stress state (including pore pressure) within the accretionary prism. In addition, the strike-slip motion contributes to enhancing the continuous mega-splay fault system (outer ridge), which extends for approximately 200 km parallel to the Nankai Trough.

Seismic reflection profiling across the source fault of the 2003 Northern Miyagi earthquake (Mj 6.4), NE Japan: basin inversion of Miocene back-arc rift

The Northern Miyagi earthquake (Mj 6.4) on 26 July, 2003, was a shallow crustal earthquake produced by high-angle reverse faulting. To construct a realistic geologic model for this fault system from depth to the surface, seismic reflection profiling was carried out across the northern part of the source fault of this earthquake. The common mid-point seismic reflection data were acquired using a vibrator truck along a 12 km-long seismic line. The obtained seismic profile portrays a Miocene half-graben bounded by a west-dipping fault. Consistent with gravity anomaly data, the maximum thickness of the basin fill probably reaches 3 km. From the regional geology, this basin-bounding normal fault forms the eastern edge of the northern Honshu rift system and was produced by rapid extension during 17–15 Ma. The deeper extension of the fault revealed by seismic profiling coincides with the planar distribution of aftershocks. The hypocentral distribution of the aftershocks shows a concentration on a plane dipping 55 degrees to the west with listric geometry. Thus, the basin inversion has been performed using the same fault; the 2003 Northern Miyagi earthquake was generated by fault reactivation of a Miocene normal fault.

The slip-rate along the northern Itoigawa-Shizuoka tectonic line active fault system, central Japan

The slip-rates on the northern extent of Itoigawa-Shizuoka tectonic line (ISTL) are estimated based on seismic reflection profiles, drill core data and analysis of tectonic geomorphology. The ISTL is a major tectonic line that passes through the Honshu Island of Japan, and its northern and central segments form an active fault system characterized by high slip-rates. In the Kamishiro basin, near the northern end of the ISTL active fault system, the rate of net slip is estimated to be 4.4–5.4 m/kyr over the last 28 ka, with a vertical-separation-rate of 2.2–2.7 m/kyr. In the Omachi area, south of the Kamishiro basin, the Quaternary slip-rate is estimated to be at least 2.9 m/kyr based on the balanced cross-section derived from reflection profiles and surface geology. The dip angle of 30° determined from the Omachi seismic profile suggests a vertical-separation-rate of at least 1.5 m/kyr. Based on compiled evidence from the available geomorphological and paleo-seismological data, vertical-slip-rates of 1.0–2.9 m/kyr are inferred for the region between Hakuba and Toyoshina over the past 3 ka. The northern ISTL exhibits dip-slip-rate of at least 2.9 m/kyr, with a constant average slip-rate of 2.0–5.8 m/kyr since the Early Quaternary. A paleoseismological data and long-term slip-rate along the northern ISTL has potential for a large earthquake.

Origin and age of erosion surfaces in the upper drainage basin of Waiapu River, northeastern North Island, New Zealand

Abstract In the upper drainage basin of Mata River, a tributary of the Waiapu River, hilltop height accordances at two levels, 500–580 m and 560–640 m above sea level, are observed. Both hilltop height accordances are inferred to represent erosion surfaces developed over folded Neogene rocks. The lower erosion surface is estimated from geomorphological position relative to the Mangamaunu Upland and tephrochronology to have been formed during a period of 25 000–30 000 years from prior to c. 35 000 years ago to c. 12 400 years ago (late Otiran). This lower erosion surface is not an elevated peneplain but was originally formed at a high level similar to its present altitude. The lower erosion surface was probably formed by cryoplanation under cooler and slightly drier climatic conditions than present with a sparse vegetation cover. The age of the higher erosion surface is not known but it must be much older than c. 35 000 years.

Seismological and geological characterization of the crust in the southern part of northern Fossa Magna, central Japan

The northern Fossa Magna (NMF) is a Miocene rift basin formed in the final stages of the opening of the Sea of Japan. The northern part of Itoigawa-Shizuoka Tectonic Line (ISTL) bounds the western part of the NMF and forms an active fault system that displays one of the largest slip rates in the Japanese islands. Reflection and refraction/wide-angle reflection profiling and earthquake observations by a dense array were undertaken across the northern part of ISTL in order to delineate structures in the crust, and deep geometry of the active fault systems. The ISTL active fault system at depth (ca. 2 km) shows east-dipping low-angle in Omachi and Matsumoto and is extended beneath the Central Uplift Zone and Komoro basin keeping the same dip-angle down to ca. 15 km. The upper part of the crust beneath the Central Uplift Zone is marked by the high Vp and high resistivity zone. Beneath the folded zone of the NMF, the middle to lower crust shows low Vp, low resistivity and more reflective features. The balanced geologic cross-section based on the reflection profiles suggests that the shortening deformation since the late Neogene was produced by the basin inversion of the Miocene low-angle normal fault.

Seismic reflection profiling across the Itoigawa-Shizuoka Tectonic Line at Matsumoto, Central Japan

The Itoigawa-Shizuoka Tectonic Line (ISTL) in Central Japan is a fault zone with a very high slip rate during Pliocene-Quaternary time. Our seismic reflection and gravity surveys across the northern segment of the ISTL at Matsumoto have revealed its geometry to a depth of ∼5 km. The fault plane was found to be of fairly low angle, in spite of the surface geologic observations that late Quaternary movements on this fault zone are dominantly strike slip. Partitioning of slip is taking place between the East Boundary Fault (thrust) and the Gofukuji Fault (left-slip), which constitute the fault zone and are parallel to and a few km apart from each other. However, these two faults are found to merge down-dip at a depth as shallow as 1.5 km below the surface. The geometry of the ISTL is significantly discordant with the orientation of the maximum shear stress acting regionally on Central Japan, indicating that the fault plane is of very low strength.