Spatial relations of active faults to inland earthquakes greater than Magnitude 5 and shallower than 20km occurring in 1900-1999 are shown on geotectonic map of Japan, Sheet-maps of 200,000: 1, and Prefecture map. The earthquakes are 230 in number, most of which are M5- and M 6-classes in size. The density distributions of the earthquakes and of the faults are significantly different between areas depending on their position in the island arc system, but there is a good coincidence between the Outer and Inner zones of arcs. The Outer zones of all the arcs are remarkably lower in the distribution density of both earthquakes and faults than the Inner zones This indicates that not only great earthquakes of M>7, but also moderate-size earthquakes of 5<M<7 generate more frequently in areas where faults density is higher. The West Japan Island Arc system has more earthquakes (7.0/104km2) and more faults (10.1/104km2) per unit area than the East Japan Arc system (5.4/104km2and 4.4 104km2, respectively). In Inner zones, the densities of both earthquakes and fault are rem a r kably different by arcs. The zones of high densities more than 5/104km2in the both are zones K, M1, M3and N2of the West Japan Arc system and zones G, H and I of the East Japan Arc system, while zones of low density zones (lower than 5 /10104km2) are Inner zones of the Kuril arc (B, C) and the collision zone (D) of Hokkaido besides Outer zones (A, E and J). Ratios of numbers of earthquakes to faults in an area (Ratio E/F) are also variable: zones G and M3are the largest in Ratio E/F (4.0 and 2.4, respectively), and zones K, M1, M2, and L in the Southwest Japan arc are very low in the ratio(0.4 or smaller). The difference in the ratio may be partly due to variation of seismicity in time, although it basically indicates a defference in tectonic situation of the zones.
Surface-rupturing earthquakes tend to cause larger damage than earthquakes without surface rupture (Takemura,1998). However, the degree of damage during an earthquake is largely influenced by population, surface geology and strength of houses in the epicentral area. I calculated the index valueIby eliminating the effects of population, surface geology and strength of houses from compiled damage data of 37 earthquakes occurred intraplate Japan during the past 115 years:I=D/(j⋅k⋅D7) whereDis number of collapsed houses during an earthquake, D7, is calculated number of collapsed houses when a point source of moment magnitude Mw: 7.0 (depth: Okm) is assumed at the epicenter, j is a coefficient of calibration for population chang e through time, and k is a coefficient of calibration for strength change of houses through time. A damage estimation program by National Land Agency of Japan is used for the damage calculation. Ivalues for surface-rupturing events are expected to be at leas t a factor of ten larger than those for events without surface rupture in the range of Mw: 6.4-6.6 which is an over-lap zone of both types of events. The difference of depth to the upper edge of the seismogenic fault between both types of events possibly explains this observation. However, I interpret it to suggest that source property changes when rupture cuts free surface and this might cause larger damage. All of the earthquakes ofI>0.5 are surface-rupturing events except ones which occurred beneath alluvial plains. Since all of surface-rupturing events occurred by movements of geomorphologically detectable active faults, I can empirically conclude that earthquake with large potential to cause great damage always occurs on geomorphologically detectable active fault except one beneath alluvial plain.
The mixed surveying method of precise mapping by total station-pen computer system and soft sediments sampling system (percussion drilling set; engine type) was applied to the fault displaying major flexural deformation for the purpose of evaluation of fault activity. As results, we clarified that the Asahi fault of the Tokachi Fault Zone, Hokkaido, Japan, has lastly activated ca.200 ka with the single vertical slip of about 1.5 m, and its time recurrence is about 7,000 years, although the activity has diminished since several ten thousands years ago.
A high-resolution P-wave seismic reflection profile was acquired across the toe of the Senya thrust, northern Honshu, Japan, to test the applicability of profiling using a hummer source (3.5 kg). Seismic data was recorded by a portable recording system (60 ch), with 28Hz geophones spaced at 0.5m intervals, and processed by standard Common Mid-Point (CMP) method. The obtained seismic section clearly demonstrates an eastdipping thrust down to 40-m in depth. The geometry of the thrust well fit to the subsurface structure estimated by drill core data.
A new outcrop of the Atotsugawa fault, which is one of the major active faults in central Japan, was observed along the Atotsu River, tributary of the Takahara River. In this outcrop, the total of about 50m wide fractured fault zone, ENE-WSW-striking slicken planes were observed. The slickenlines indicate dextral wrenching of the Atotsugawa fault but the vertical displacement slightly exceeds, with uplifting the southern side of the fault. This result almost same as the previous studies on the other outcrops, showing the continuous distribution of the fault trace with local branching or stepping.
The Nagano Basin is bounded by northsouth-trending reverse faults on the west. The active faults along the eastern margin of the Nagaoka Hills in the northern part of the Nagano Basin displace alluvial surfaces of Holocene age. The average slip rate and history of late Holocene faulting of these faults have not been well constrained by previous studies. We have obtained subsurface geologic information by core sampler across monoclinal scarp 1.9 m high on a Holocene alluvial surface in Nakano City, Nagano Prefecture. The core samples obtained at 11 sites are 1.1 to 6.3 m long. Correlation of strata and radiocarbon dating show that a peat layer dated at about 4200 years BP is warped. The vertical displacement of the top of the peat layer is measured to be at least 5.1 m. From the age and amount of vertical displacement, the vertical slip rate is estimated to be at least 1.2 mm/yr. We found that surface faulting had occurred at least twice since about 4200 years ago. Moreover, we found the monoclinal scarp along the fault at Angenji, Nakano City had grown up by the 1847Zenkoji earthquake.
The Kamishiro fault is one of the major active faults constituting the Itoigawa-Shizuoka Tectonic Line (ISTL) in central Japan. The Kamishiro fault is an east-dipping reverse fault. This fault cuts and warped the young lake deposits (late Pleistocene and Holocene in age) in the Kamishiro basin. The slip rate on the Kamishiro fault near the surface has been estimated by Geo-Slicer and shallow drillings survey, but was found to be significantly smaller than the vertical slip-rate that was estimated from the displacement of the AT volcanic ash. We carried out a 55 m deep drilling and a seismic reflection profiling using S-waves in this basin to clarify the subsurface structure of the Kamishiro fault. As a result, it was clarified that the Kamishiro fault is associated with drag folding near the surface. If we take the drag folding into account, the overall rate of slip on the fault would be as high as 4.4-5.2mmlyr during the past 28ka.
The Tottorii-ken Seibu earthquake (114JMA7.3) occurred on 6th October 2000 on the border between Tottori and Shimane prefectures, western Japan, an area that contains no significant active faults according to active fault catalogues. No distinct surface rupture became evident as a result of this earthquake, even though it is generally assumed that earthquakes of magnitude 7.0 or larger are almost associated with surface rupture along associated geomorphologically mapped active faults. This means that, despite its large magnitude, the Tottorii-ken Seibu earthquake was not a major intraplate earthquake that took place on geomorphologically mapped active faults, but one related to significant background seismicity. This might explain the minimal damage that it caused, as compared to damage from the Kobe earthquake(MJMA7.2) by the Rokko and Nojima faults. If an occurrence probability of an M7.3 earthquake taking place in this region in 50 years had been calculated using the seismicity catalogue and the Gutenberg-Richter magnitude-frequency law, it would have been 7-12%, because several moderate size earthquakes took place in the previous decade. However, using the seismicity catalogue and the G-R law to evaluate maximum earthquake magnitude and earthquake probability clearly remains problematic for background earthquakes.
The Fukuchiyama fault system consists of two faults named the Tonda fault at the northern part and the Fukuchiyama fault at the southern part, The Fukuchiyama fault system has been activated as a reverse fault with relative subsidence of the eastern mountain area. This fact shows the activity of the Fukuchiyama fault system has been the reactivity along the scar of the“Geologic Boundary Fault”between Paleogene and Paleogene or Mesozoic basements. The reverse activity has occurred after the formation of higher terrace in the middle Pleistocene. As a result of the trenching study, the latest activity of the fault system occurred at 11,000 - 25,000y B. P. with 0.6m vertical offset.
Kamegawa fault locates at the western part of Beppu Bay Graben. Based on the trenching study of the fault, we obtained some new evidences on the activity of Kamegawa fault. The fault plane in trench 1 shows the strike of N86°W and dip of 75°-85°S. Tephra and gravel layers are deformed along the fault plane by faulting. The faulting of Kamegawa fault is estimated to have occurred at three times since the deposition of AT(Aira Tn) volcanic ash (23,000 years BP). The latest faulting is considered to have occurred during 2,020∼3,400 years BP, before accumulation of the Yufudake volcanic ash. The reccurence interval is about several thousands of years with 60∼80 cm vertical slip.
At the southwest margin of the Lake Efteni, we extracted Geoslicer samples with the interval of 5-10 m in order to reveal the recurrence of the past large earthquakes produced by the 1999 Duzce earthquake (M7.1)segment. The geologic section across the earthquake rupture zone indicates that the mud flow deposits (Unit 2)drop toward the north more than 15 m, and that Unit 2 is covered by younger fine sediments with several angular unconformities. These unconformities, which are probably associated with surface faulting events, indicate two probable and two possible faulting events after the deposition of Unit 2. Considering that the radiocarbon dates of the peat layer just above Unit 2 are about 2 ka, we can estimate that the average recurrence interval of faulting events is about 400 to 500 years and that the vertical offset of each individual faulting event is 2.5 to 3 m. Moreover, we can point out the possibility that the timing of the penultimate faulting event is younger than AD 1500. The Duzce fault shows very high activity during the past 2000 years.
The Kachchh falls under seismically active zone-V outside the Himalayan seismic belt, and forms a part of Stable Continental Region (SCR). In the span of 50 years the Kachchh has experienced two large magnitude earthquakes, i. e. the July 21,1956 Anjar with ML 7 and the 2001 event with Mw 7.6. No coseismic surface rupture was associated with the Bhuj earthquake, suggests movement along a blind thrust has resulted into extensive lateral spreading of water saturated near surface soil horizon giving rise to formation of numerous EW striking extensional cracks or normal faults and the longitudinal pressure ridges in the epicentral area around Budharmora and Morgar villages. Liquefaction in the low-lying areas comprising fine sediments was the common phenomenon observed during this earthquake. By satellite photo interpretation sever a l traces of active faults were identified for the first time occurring in the pediment zones along the northern margins of Katrol Hill Range and Northern Hill Range respectively. The active faults have displaced along them the Late Quaternary alluvial fan deposits and colluvial debris, resulting into formation of the north facing fault scarplets. Our field observations reveal that none of the active faults have moved this time. Thus, it is suggested that these faults still have high potential to break large magnitude earthquake in future.
On the gentle north-facing warping surface extending E-W in the epicentral area around Budharmora and Morgar villages extensive lateral spreading took place by the Bhuj earthquake. The deformation was generated in water saturated surface soil horizon resulting in numerous E-W striking extensional cracks or normal faults and rises to the surface forming the longitudinal pressure ridges. Deformational features identified during our field and aerial survey are mostly gravitational features resulted by strong ground shaking over a gently sloping surface.