Normal Fault Type Research Articles

Rupture process of the 1975 central Oita, Japan, earthquake.

The focal mechanism of the 1975 central Oita, Japan, earthquake (M=6.4) is reexamined, and the rupture process is investigated by comparing synthetic and observed P-waveform data of WWSSN long-period seismograms. The obtained focal mechanism is not a normal fault type as was previously reported, but a strike-slip fault type with dip-slip component. The tensional axis of this solution is consistent with the tectonic stress field in this region. The rupture time, the seismic moment, the fault displacement, and the stress drop are estimated to be 6.7 s, 2.2 x 1025 dyn·cm, 85cm, and 78bar, respectively. However, these only show a general or average feature of this earthquake. Based on a complicated feature of observed seismograms, we found that this event is a multiple shock and has a shallow initial rupture with short source duration. A distinct later phase of seismograms suggests a change of source mechanism during the rupture process, which is consistent with the distribution of the mapped late Quaternary faults in the focal area. The trend and sense of the fault of this earthquake and active faults around there, and strain field of this region agree with the pattern for oblique rifting, proposed by WITHJACK and JAMISON in 1986. It is suggested that oblique rifting, which appears to be related to a right-lateral displacement of the Median Tectonic Line lying east of and subparallel to the inferred rift axis, takes place in this region. The relative displacement between opposite sides of the rift is estimated to be directed in NESW, consistent with the geodetic data.

Shear structures in anhydrite at the base of thrust sheets (Antalya, Southern Turkey)

Minor-scale structures in anhydrite layers and pods within one thrust contact of the Antalya Thrust System are described. Folds whose size range from some centimeters to several meters display a wide variety of attitudes. Their axes are curvilinear and vary from orthogonal to subparallel to the stretching lineation, leading to sheath folds and eyed type sections. Shear bands are frequent. In alternating white (pure) and brown (clay rich) anhydrite, regularly spaced extensional shear bands form typical tilted block patterns. In homogeneously foliated anhydrite they form isolated meter long extensional shear bands with brecciation along their central planes. Reverse-fault type shear bands are associated with meter scale reclined folds with axes subperpendicular to the stretching lineation. Normal and reverse fault type shear bands are self-exclusive, and both indicate a sense of shear toward the south at the scale of thrust system.A model of progressive deformation of the anhydrite layers is presented. Anhydrite layers which have undergone a strong unstable shearing deformation along a thrust, have been boudinaged and now form elongate asymmetric pods. In thinned layers, normal extensional shear bands developed and recumbent fold axes are strongly reoriented toward the direction of the stretching lineation. In the thickened layers, reverse-fault type shear bands developed, particularly within the inverted limbs of asymmetric reclined folds whose axes are at a high angle to the stretching lineation.

Normal fault type events in the upper plane of the double-planed deep seismic zone beneath the Northeastern Japan arc.

The spatial change in the focal mechanism of the intermediate-depth earthquakes in the Tohoku District, the northeastern part of Honshu, Japan, is investigated in detail. In addition to general features of the distribution of focal mechanisms revealed by the previous studies, it is found that normal fault type events are dominant in the upper seismic plane of the double-planed deep seismic zone beneath the volcanic front, where the upper seismic plane is bending downward. The bending of the descending oceanic plate is probably the cause of these normal fault type events.

Seismotectonics around the Mariana trough

Strike-slip type mechanism solutions, with approximately east-west striking tension axes, were obtained for large shallow earthquakes under the central topographic high of the northern Mariana Trough. These solutions are consistent with the model that spreading is in progress in the direction perpendicular to the structural lineations trending N-S in most of the trough and NNW-SSE in the northern part. Weak coupling at the subduction zone between the Pacific plate and the landward plate (Mariana platelet) is suggested by the occurrence of few thrust type and many normal fault type earthquakes near the Mariana Trench. Spreading striking nearly E-W with an approximate total rate of 6 cm/yr in the widest section of the trough is speculatively inferred from the plate motion model of the Philippine Sea plate with respect to the surrounding plates and from the anchored slab model and the collision effect of the Caroline Ridge with the Philippine Sea plate.

発破によつて誘発される微小破壊

The mechanism generating the micro-fractures in the mine was investigated.The micro-fractures occurring immediately after an explosion were observed for about an hour at the Nakatatsu mine in Fukui prefecture in 1982. The depth of the explosion point was about 700m. The velocity of P-wave near the observation site was 6.32km/s. Three accerelation-type seismometers and ten velocity-type seismometers were placed three-dimensionally surrounding the explosion point. The average span between seismometers was about 50m. The accerelation response of the accerelation-type seiemomenter system is flat up to 10kHz and the velocity response of the velocity-type seismometer system is flat up to about 1kHz.The relative foci to the explosion point were solved from the travel time residuals between the explosion and the micro-fractures. The hypocenters concentrated near one of the corners of the newly excavated face.The mechanism solution of the micro-fractures is normal-fault type, which indicates that the micro-fractures were generated by the compressional stress in the vertical direction.The micro-fractures occuring immediately after an explosion are generated by the stress concentration, that the load of the upper mass which had been supported by the rock which was excavated with the explosion concentrates near the corner of the newly excavated face.

Effects of Clastic Dikes on Roof-Rock Stability in Central Pennsylvania Coal Mine: ABSTRACT

Clastic dikes are common in coal beds of the Appalachian basin. Their presence often causes delays in mine production and poses safety hazards to mine personnel. The U.S. Bureau of Mines is conducting research on the occurrence and origin of clastic dikes and associated roof-rock instability, to gain a better understanding of the effect clastic dikes have on ground pressure around mine openings. Geologic mapping and pressure monitoring of clastic dikes at the Greenwich Collieries North Mine, Indiana County, Pennsylvania, has provided insights to this problem. The North Mine (producing from the lower Freeport coal bed) is experiencing an increase in the number of clastic dikes and associated roof failure as mining advances toward the axis of the Brush Valley syncline. Over 200 individual clastic dikes of claystone matrix with fragments of shale and coal have been mapped at the mine. The dikes range in thickness from a maximum of 2 ft (60 cm) to as small as 0.25 in. (6 mm) and commonly extend vertically from the roof rock 20 ft (6 m) above the coal bed into the coal bed, but rarely through the coal bed to the underclay. Observations of the dikes indicate that they were formed by tensional and compressional forces and may be related to paleostress fields. Characteristically the dikes penetrate the coal bed at high angles to bedding with as mu h as 8 in. (20 cm3) of normal-fault type displacement at the contact of coal bed and roof rock. The measured strike of the clastic dikes shows a strong bimodal distribution of N40°W and N50°E, which is offset 30° to the east of the bimodal distribution of the coal cleat (face N70°W and butt N20°E). Joints in the shale roof rock show a dominant peak of N 40°W, which is coincident with one of the major peaks of the dike distribution. The depositional environment may have been an important factor in clastic dike formation, as the abundance of preserved casts of tree stumps in the shale roof is inversely proportional to the number of clastic dikes. Hydraulic pressure-monitoring devices were installed near the clastic dikes to monitor the development of associated roof failure. The results of this monitoring show that the roof behaved as two cantilever beams where a clastic dike was present and that the roof must be supported immediately after mining. Analysis of the anisotropic qualities of the roof strata and rock-pressure conditions suggests that changes in the in-situ stress field near clastic dikes have aided in the propagation of roof failure. End_of_Article - Last_Page 1920------------

Study of Microearthquake Activity in Four Geothermal Areas of Taiwan: ABSTRACT

Detailed microearthquake surveys were conducted in four geothermal areas of Taiwan, namely, Tatun volcanic region, Chingshui- Tuchang geothermal area, Lushan hot spring area, and Hungyeh hot spring area, during the past few years. It is found that most of them have microearthquake activity. Based on the microearthquake data, fracture zones permitting deep circulation of water are inferred. Microearthquakes in the Tatun volcanic region are concentrated in a 1.9 mi (3 km) wide and 4.3 mi (7 km) long, northeast-striking elongated zone, where conspicuous geothermal manifestations are present. The focal depths of these events are mostly less than 0.6 mi (1 km). The occurrence of these microearthquakes may be related to the minor normal faults in that region. In the Chingshui- uchang geothermal area, microearthquakes are located in a northeast-striking zone, 25 mi (40 km) long and 4.3 mi (7 km) wide. The foci clearly define a northwest-dipping fracture zone with a thickness of about 1.9 mi (3 km). Three composite fault plane solutions are all of the normal fault type. No microearthquake activity was found within a 3 mi (5 km) radial range around the Lushan hot spring area, during a 34-day recording period. A highly active microearthquake zone was found in a region 1.25 mi (2 km) to the west of the Hungyeh hot spring area and the foci define an east dipping fracture zone. However, the causative faults of these events are of the thrust fault type. End_of_Article - Last_Page 987------------

ACTIVE FAULT SYSTEMS OF THE QUATERNARY VOLCANIC REGION IN THE CENTRAL PART OF OITA PREFECTURE, KYUSHU DISTRICT, SOUTHWEST JAPAN

Recent studies have revealed regional variations in types and intensities of the active faulting in Japan. Normal faults are developed exceptionally well in Central Kyushu, while strike-slip or reverse faults are predominant in the main part of Japan (Matsuda, 1973). The focal mechanisms of several shallow earthquakes in Central . Kyushu showed normal fault type (Yamashina and Murai, 1975), such as the Oita earthquake of April 21, 1975 (M=6.4). The purpose of this paper is to present new data on the active fault system of the Quaternary volcanic region in the central part of Oita prefecture, Central Kyushu, where normal faults are densely developed, and tectonic implications of graben formations are also discussed. The results obtained are summarized as follows: (1) Normal faults with short and irregular traces are densely developed in the central part of Oita prefecture, most of which are integrated into several grabens, such as the Haneyama graben and the Hayami graben (Fig. 3). (2) Judging from the wide-spread distribution of Tsukushi lavas across the present rims of grabens, the Haneyama and Hayami grabens had not yet been formed before the eruptions of Tsukushi lavas. But the faulting had already taken place before the eruptions of Haneyama lavas in the Haneyama graben and before the deposition of Yabakei pyroclastic flow in the Hayami graben. Based on the absolute ages of these volcanic rocks, the faulting of these grabens is estimated to have begun during the period from 800, 000 to 500, 000 years B. P. (3) Haneyama lavas erupted from inside the Haneyama graben and hornblende andesites of San'in volcanic series erupted from inside the Hayami graben. These volcanisms occurred along with faulting. (4) The Hayami graben is developed on the crest of a broad up-warping as indicated by general relief and drainage pattern, and the altitude of its floor is considerably high in spite of local subsidence caused by the normal faulting. Such marginal uplifting is recognized also around the Haneyama graben, though it is less distinct. (5) Among the faults near the northwest rim of the Hayami graben, the faults located on the north (Fault 1-5 in Fig. 6) had finished their activities before the formation of Tateishi-yama volcano. However, the faults located on the south (Fault 8 and 9 in Fig. 6) have been active after the formation of the volcano. This indicates that the location of faulting shifted from north to south, namely inward the graben. The distance of about 2 km between Fault 1 and Fault 8 represents the amount of shifting. (6) The arrangement of volcanic vents inside the grabens suggests the existence of exten. sion fractures underneath the grabens. If we assume the existence of extension fractures, , it becomes considerably easier to understand the phenomena (4) and (5): marginal uplifting of the grabens can be explained as a deformation due to opening of extension fractures, and the opening causes extension of a graben, so that locations of faulting will shift relatively inward the graben (Fig. 9). Faulting itself seems to be a surface effect associated with opening of extension fractures. (7) Surface displacements due to opening of a crack with various depth in an elastic half space are calculated (Fig. 10). By comparing the observed pattern of marginal uplifting with the calculated curves of surface displacements, the depth and the amount of opening of the extension fracture underneath the Hayami graben are estimated to be 8-12 km and 1.7-3.2 km, respectively. The latter value is in well-agreement with the extension of about 2 km inferred from the inward shift of faulting. (8) Based on the arrangement of surface faults and volcanoes, a possible model of subsurface structure of the Hayami graben is presented (Fig. 11).

1976年10月31日日高山脈地震

On October 31, 1976 an earthquake of magnitude 5.0 occurred under the west slope of the Hidaka range, southern Hokkaido.It is the first remarkable event since a seismic observation through a telemetering system at the Research Center for Earthquake Prediction (RCEP), Hokkaido University started in July, 1976. Using data obtained from three stations of RCEP near the epicentral region, a seismic investigation was made on the above event.This earthquake occurred at a depth of about 55km in the uppermost mantle, however, its aftershock activity is very similar to that of earthquake in the crust elsewhere.Background seismicity around the focal region of this shock was estimated as 0.04 shocks (M≥0.5) per day.Frequency of aftershocks per day has decreased to this level about 140 days after the main shock. Aftershocks are confined in a region of about 6×6×4km3 including the main shock. No distinct seismic activity change is found prior to the main shock.Focal mechanisms of the main shock and of the largest aftershock are of normal fault type. Maximum tension axes lie in nearly horizontal plane and their directions are perpendicular to the strike of the Hidaka range. A large earthquake (M=6.7) which occurred under the east slope of the Hidaka range on January 21, 1970, showed focal mechanism of reverse fault type. But all of B-axes of above three events are parallel to the strike of the Hidaka range.These imply some complexity of focal process beneath both sides of the Hidaka range.

Tectonic implications of the large shioya-oki earthquakes of 1938

The tectonic processes taking place along the southern part of the Japan trench are discussed on the basis of the focal mechanism of the 1938 Shioya-Oki event which consists of the five large earthquakes of M s = 7.4, 7.7, 7.8, 7.7 and 7.1 . Detailed analyses of seismic waves and tsunamis are made for each of these earthquakes, and the dislocation parameters are obtained. The total seismic moment amounts to 2.3 · 10 28 dyn.cm. The five earthquakes are grouped into either a low-angle thrust type or a nearly vertical normal-fault type. These mechanisms are common with other great earthquakes of the northwestern Pacific belt, and can be explained in terms of the interaction between the oceanic and continental plates. The vertical displacement inferred from the seismic results is in approximate agreement with the precise level data over the period from 1939 and 1897. This agreement suggests that the rate of the strain accumulation at the preseismic time is very small in the epicentral area. Repeated levelings at the postseismic time reveal a large-scale recovery of the coseismic subsidence. The postseismic deformation is one-third to one-half of the coseismic displacement. The time constant of the recovery is estimated to be 5 years or less. This type of deformation may be a manifestation of viscoelasticity of a weak zone underlying the continent. The amount of dislocation, together with the longterm seismicity, suggests a seismic slip rate of about 0.4 cm/year, which is one order of magnitude smaller than that for the adjacent regions. This suggests that a large part of the plate motion is taking place aseismically in this region. The tectonic process now taking place in the southern Japan trench can be considered to represent a stage just prior to a complete detachment of the sinking portion of the oceanic plate.

1938年福島沖群発地震による津波の発生機構

The source mechanisms of eight tsunamis generated off Fukushima, the Pacific side of northeast Japan, in May to November 1938 are re-studied, making use of tide gauge records. The source areas for each tsunami inferred from the inverse refraction diagrams are shown in the bathymetric charts. The source locations moved to the north direction along the continental shelf, and the north end was in contact with the source area of the tsunami at Nov. 3, 1936. Among these tsunamis, three tsunamis were recorded in a downward direction at Onahama located near the sources, but other tsunamis began with an upward motion. According to the seismic wave data, the tsunamis of downward and upward initial motions correspond to the normal and reverse fault earthquakes, respectively. Tsunamis accompanied by earthquake of the normal fault type were generated later than those for the reverce fault earthquakes, and these source areas are located in the deep sea-bottom side. The source lengths of tsunami are 60-130km, corresponding to earthquake magnitude. A whole length from north to south ends of the sources is 200km.The calculated sea-level disturbances at the sources for each tsunami are in the range of 10-50cm which the heights nearly agree with the seismic fault models. The whole volume of the displaced water in the tsunami sources is 13×1015cm3 which is comparable to that of the 1968 Tokachi-oki tsunami (tsunami magnitude of Imamura-Iida scale m=2.5).

Effects of Earthquakes on Ground (I)

An empirical formula for the hypocentral distance of damaged railroad subgrade in earthquakes is given. Ground cracks caused by earthquakes are classified into those of tensile type, normal-fault type, and strike-slip type. After description of soil-liquefaction phenomena in earthquakes, the mechanism of spontaneous liquefaction of soils is reviewed critically. The solid-mass sliding and flow slide of soil slopes, and the fragment failure and rock-mass sliding of rock slopes are described. The effect of pore-water pressure on the stability of slopes in cases of the solid-mass sliding and the flow slide is discussed.

Seismicity and tectonics of the western Pacific: Izu-Mariana-Caroline and Ryukyu-Taiwan regions

The hypocenters of about 1000 earthquakes in the Izu-Bonin, Mariana, Palau, Caroline, and Ryukyu-Taiwan regions were relocated by computer, and earthquake mechanism solutions based on the first motions of P, pP, and S were determined for 26 earthquakes. The spatial distribution and the mechanisms were compared with major tectonic features such as volcanic zones, island arcs, and trenches. Although a nearly continuous zone of shallow earthquakes can be traced from Honshu to Palau, deep shocks were not detected south of 16°N; intermediate-depth events were not found south of the Marianas. A region of low seismicity for shallow earthquakes and a discontinuity for deeper events occurs in the Volcano Islands adjacent to a prominent gap in the trench where the Marcus-Necker ridge intersects the arc. In some profiles of this series of arcs, the deep seismic zone is nearly planar, but in other cases it is more contorted and the thickness of the zone appears to vary from place to place. Maxima and minima in seismic activity occur for deep and intermediate depths, and these features migrate in depth similar to that found in the Tonga arc. Activity as deep as about 680 km occurs in the northern Marianas where the deep focal zone is nearly vertical for depths greater than about 200 km. This nearly vertical distribution suggests that gravitational tectonics may play an important role as a motive force for global tectonics. Seismicity in the Ryukyu arc is concentrated in a thin planar zone that dips 35° to 45° northwest to a depth of about 280 km. Near the northwest coast of Taiwan all tectonic elements change strike abruptly, and the mechanisms change from thrusting of the Philippine Sea plate beneath the Ryukyu arc to strike slip near the east coast of Taiwan. South of this point, most of the events are shallower than 100 km, and the focal zone is not well defined in vertical sections. No events were detected from the Kyushu-Palau ridge, from the Central Basin fault, or from the rest of the interior of the Phillipine Sea. Thus, this region appears to be a single lithospheric plate. Normal faulting is found for some earthquakes either in the deepest parts of the trench or along the seaward wall. A zone of shallow earthquakes also can be identified on many of the profiles near the volcanic axis. Mechanism solutions for two events near this zone are also of the normal-fault type. Mechanism solutions for shallow earthquakes beneath the islandward margins of the Izu-Bonin and Mariana trenches indicate that the Pacific plate is underthrusting the Philippine Sea plate in a westerly direction. The results of the relocations and other pertinent data are given in a separate appendix, which is available on microfiche along with the entire article. Order from the American Geophysical Union, Suite 435, 2100 Pennsylvania Ave., N.W. Washington, D.C. 20037. Document J69-002; $1.00. Payment must accompany order.