Muneyoshi Furumoto

Mechanism of large earthquakes along the eastern margin of the Japan sea

Abstract The source mechanisms of large earthquakes occurring along the eastern margin of the Japan Sea are investigated. The earthquakes studied are those off Oga (May 7, 1964), Sakhalin (Sep. 5, 1971) and Shakotan (Aug. 1, 1940) and, though supplementary, the Niigata (Jun. 16, 1964) earthquake. The focal mechanisms of these four earthquakes are predominantly dip-slip reverse faulting on a plane dipping relatively steeply. The strikes of the fault planes are parallel to the general trend of the eastern margin of the Japan Sea. The parameters for the dislocation are determined on the basis of the long-period surface waves and the tsunami heights at the coasts. The stress drop of 126 bars for the Niigata earthquake is significantly large compared to those for the other three shocks. The smallest is the stress drop of 17 bars for the earthquake off Shakotan. This earthquake was found to be especially interesting in two respects. First, the relative excitation of the short- and long-period seismic waves and the tsunami suggests that the dislocation of 1.1 m took place with an abnormally long duration. Second, the fault plane involves an exceptionally large area of 50 × 170 km 2 . This fault plane cuts through the entire thickness of the lid on the low-velocity zone developing beneath the Japan Sea. The fault plane may manifest a pre-existing fractured zone by which the Japan Sea lithosphere is separated from the northern Japan arc, as far as the west of Hokkaido is concerned.

Hierarchy in earthquake size distribution

Abstract We propose a fault model in which there are a finite number of characteristic lengths between the smallest and the largest scales, e.g., between a grain size and a size of great earthquakes. A fault plane in one scale is segmented into blocks of statistically similar sizes by the barriers of statistically similar heights. Rupture expands by successively “climbing” the barriers. The average barrier height is proportional to the average block size which defines the characteristic length of the relevant scale. Stresses before and after rupturing are assumed to be independent of earthquake size. With these assumptions the probability for rupture to grow further is shown to be independent of the scale of barrier structure. The rupture in one scale, however, tends to grow more easily as it grows so that once the rupture exceeds some critical size, it no longer stops until it encounters the barriers in the next larger scale. Rupture grows by repeating this cycle, which starts with rupturing of a unit block in one scale and ends with one in the next larger scale. Our model involves two parameters which must be determined empirically. One is the length ratio r between two successive scales. The other is the critical number of blocks, n c , which defines the critical size for unstable rupture growth in one scale. We suggest values of ∼ 5 for both r and n c . With these values our model explains a diversity of earthquake phenomena such as the magnitude-frequency relation, the seismic gap and the magnitude gap, the magnitude difference between a main shock and its largest aftershock, the occurrence of multiplet earthquakes, the generation of high frequency seismic waves, the incoherency of the near field acceleration and the recent observation by Furumoto and Nakanishi that the main rupture is often preceded by a precursory rupture whose time duration, in general, scales with seismic moment.

Foreshocks and multiple shocks of large earthquakes

Abstract The rupture process of the Tokachi-Oki earthquake (May 16, 1968) and that of the Kurile Islands earthquake (August 11, 1969) were investigated primarily on the basis of the long-period seismograms of WWSSN. These two large earthquakes are of the similar magnitude ( M = 7.8–7.9) and are only about 460 km apart from each other. The detailed analyses of the long-period P waves and the multiple surface waves of G2, G3, and G4 revealed the following features. 1. (1)The fracture at the earlier stages of the Tokachi-Oki event occurred in the form of multiple-shock activity. The largest two shocks were generated 57 km S and 93 km WSW of the initial hypocentre at 32 and 45 sec after the initial break respectively. 2. (2)The main faulting of the Tokachi-Oki event commenced about 80 km S of the initial epicentre at 40–45 sec after the initial break and propagated in the northerly to northwesterly directions, the rupture velocity being about 4 km/sec. 3. (3)No multiple event nature is seen for the Kurile event. Its initial break represents the commencement of the main-fault propagation. This earthquake was, however, preceded by an intense foreshock activity. The above features indicate that the multiple-shock activity of the Tokachi-Oki event and the foreshock activity of the Kurile event are the same type of precursory phenomena for the main phase of fault propagation. They are understood as the process for the nucleation of cracks at weak spots along the fault plane and their subsequent coalescence to grow larger cracks. This is the self-accelerated process resulting in the climax which marks the initiation of the main-fault propagation. We suggest that the rupture process of other large earthquakes can be described in a very similar way.

Seismological evidence on characteristic time of crack healing in the shallow crust

[1]xa0A continuous observation of shear wave splitting for 17 years reveals a unique temporal variation in seismic anisotropy in the shallow crust induced by a larger earthquake (MW5.7) beneath the Tokai region, Japan. The delay time between the fast and slow wavelets coseismically increased and then decreased back to the pre-event value. The duration of the decreasing stage is about two years. The decrease may indicate crack healing in the upper 10 km of the crust. We approximate the temporal variation in the delay time as a function of logarithm of time, which is concordant with healing phenomena of cracks reported by laboratory experiments. The observation indicates that healing of cracks in crustal rocks is complete in approximately two years.

Spacio-temporal history of the deep Colombia earthquake of 1970

Abstract The deep Colombia earthquake of July 31, 1970, showed a very high multiple-shock activity. The hypocentres of the seven events forming the multiple shock were determined by a relative location method. They cluster on a plane which coincides fairly well with the westerly-dipping nodal plane and show a southwards and upwards migration with time. The hypocentre distribution suggests that the earthquake occurred in the form of shear faulting. The primary P waveforms are computed from the long-period P-wave signals by eliminating the responses of the instrument, crust and mantle. The fault process is inferred from the spacio-temporal relation of the seven events and from the primary P waveforms: the rupture initiated in the lower part of the source region and spread out southwards and upwards. The whole process was almost finished when the intense multiple-shock activity terminated (about 50 s). The mechanism change of this earthquake can be described mainly by the change of the orientation of the fault plane rather than by slip vector change. The strike of the fault plane rotated clockwise about 10° during the events which occurred in the lower part of the source region. The further rotation of about 10° occurred at the subsequent stage. The rupture spread out with a jerk over a curved surface. The difference between the rupture processes in shallow and deep large earthquakes is discussed.

Asymmetrical distribution of rayed craters on the Moon

The synchronous rotation of the satellite ought to cause a spatial variation in the cratering rate over its surface. The crater density is expected to be maximum at the apex of the orbital motion and decrease with the increase of the angular distance from the apex. The ratio of the density at the apex (maximum) to that of the antapex (minimum) depends on the average encounter velocity of impactors to the satellite. Although the Moon is also in a state of the synchronous rotation, it has been supposed that the asymmetry in the crater density on the Moon can be hardly observed. We report here a spatial variation in the density of rayed craters on the Moon, which may be associated with the synchronous rotation. Since the lifetime of a ray is relatively short (<0.8 billion years), the results provide information on recent impacts. Rayed craters are identified on Clementine 750-nm mosaic images. We investigate craters in a lower latitude zone from 42°N to 42°S. To avoid an effect of material difference on the ray preservation, we analyze craters on the highland from 70°E to 290°E in east longitude. A total of 222 rayed craters larger than 5 km in diameter are identified in the study area of about 1.4×107 km2. The average density of rayed craters on the leading side is substantially higher than that on the trailing side. The crater density decreases as a sinusoidal function of the angular distance from the apex. The observed ratio of the density at the apex to that at the antapex is about 1.5. The ratio suggests that recent craters on the Moon are formed mainly by near-Earth asteroids rather than comets with higher encounter velocities.

Is the Ryukyu subduction zone in Japan coupled or decoupled? —The necessity of seafloor crustal deformation observation

The 2004 Sumatra-Andaman earthquake of Mw 9.3 occurred in a region where a giant earthquake seemed unlikely from the point of view of tectonics. This clearly implies that our current understanding of strain accumulation processes of large earthquakes at subduction zones needs to be reexamined. The Ryukyu subduction zone is one such zone since no large earthquake has been anticipated there for reasons similar those pertaining to the Sumatra-Andaman arc. Based on our analysis of historical earthquakes, plate motion, back-arc spreading, and GPS observation along the Ryukyu trench, we highly recommend monitoring seafloor crustal deformation along this trench to clarify whether a large earthquake (Mw>8) could potentially occur there in the future.

A mechanism of respiration-dependent water uptake enhanced by auxin

SummaryThere are many contradictory observations on the mechanohydraulic relation of growing higher plant cells and tissues. Graphical analysis of the simultaneous equations which govern irreversible wall yielding and water absorption has made more comprehensive the understanding of this relation when relative growth rate is plotted against turgor pressure. It suggests that some respiration-dependent and auxin sensitive process might regulate the difference of osmotic potential between cells and water source. Based on anatomical and electrophysiological knowledge of the pea stem xylem, we propose the wall canal system as the mechanism of respiration-dependent water uptake which is sensitive to auxin. This system consists of the xylem apoplastic walls, the xylem proton pumps, active solute uptake system and cell membranes. In the simplest case, third-order simultaneous differential equations are involved. Numerical analysis showed that net uptake of solutes enables water to be taken up against an opposing gradient of water potential. The behaviour of this wall canal system describes well the mechano-hydraulic relation of enlarging plant cells and tissues. Recent typical, but incompatible, interpretations of this relation are critically discussed based on our model.

Seismic moment of great deep shocks

Abstract The long-period Rayleigh waves were investigated for the largest four deep shocks in 1963–1973 to determine the seismic moment by the same technique as used for shallow earthquakes. The results could be used for a quantitative comparison of source parameters between shallow and deep events. Three of the four shocks occurred beneath the South American continent (the Colombia earthquake, 1970; the western Brazil earthquake, 1963; the Peru—Bolivia border earthquake, 1963) and the other beneath the Japan Sea (1973). The focal depths are 653, 576, 593 and 575 km, respectively. The largest value of seismic moment was obtained as 2.1 · 1028 dyncm for the Colombia earthquake. This value is still about forty times smaller than that for the great Alaskan earthquake. A slight inconsistency was found between the first-motion diagram and the Rayleigh wave radiation pattern for the Colombia earthquake and the Peru—Bolivia border earthquake.

A mechanism of respiration-dependent water uptake in higher plants

where V is the volume o f the symplasts (m3), L is the relative hydraulic conductance o f the tissue (Pa -1 s 1), is the solute reflection coefficient o f the membrane, C i and C X are the osmotic concentra t ion within the symplast and the xylem vessels respectively (mole m 3), R is the gas constant ( Jmole l K--l), T is absolute temperature (K), and pi and P• are cell turgor pressure and hydrostat ic pressure in the xylem vessels respectively (Pa). Compar i son of water permeabilities between synthetic and natural membranes suggests that o f natural membranes must be unity for physiological solutes (OscHMAN et al. 1974). On the other hand, the relative rate o f tissue enlargement has been shown to depend on irreversible wall yielding;