Kenshiro Otsuki

Comminution and fluidization of granular fault materials: implications for fault slip behavior

Abstract Whilst faulting in the shallow crust is inevitably associated with comminution of rocks, the mechanical properties of the comminuted granular materials themselves affect the slip behavior of faults. Therefore, the mechanical behavior of any fault progresses along an evolutionary path. We analyzed granular fault rocks from four faults, and deduced an evolutionary trend of fractal size frequency. Comminution of fault rocks starts at a fractal dimension close to 1.5 (2-D measurement), at which a given grain is supported by the maximum number of grains attainable and hence is at its strongest. As comminution proceeds, the fractal dimension increases, and hence comminution itself is a slip weakening mechanism. Under the appropriate conditions, comminuted granular materials may be fluidized during seismic slip events. In this paper, we develop a new method to identify the granular fault rocks that have experienced fluidization, where the detection probability of fragmented counterparts is a key parameter. This method was applied to four fault rock samples and a successful result was obtained. Knowledge from powder technology teaches us that the volume fraction of grains normalized by maximum volume fraction attainable is the most important parameter for dynamic properties of granular materials, and once granular fault materials are fluidized, the fault plane becomes nearly frictionless. A small decrease in the normalized volume fraction of grains from 1 is a necessary condition for the phase transition to fluidization from the deformation mechanism governed by grain friction and crushing by contact stresses. This condition can be realized only when shearing proceeds under unconstrained conditions, and this demands that the gap between fault walls is widened. Normal interface vibration proposed by Brune et al. [Tectonophysics 218 (1993) 59] appears to be the most appropriate cause of this, and we presented two lines of field evidence that support this mechanism to work in nature.

Active faults, stress field and plate motion along the Indo-Eurasian plate boundary

Abstract The active faults of the Himalayas and neighboring areas are direct indicators of Recent and sub-Recent crustal movements due to continental collision between the Indian and Eurasian plates. The direction of the maximum horizontal shortening or horizontal compressive stress axes deduced from the strike and type of active faulting reveals a characteristic regional stress field along the colliding boundary. The trajectories of the stress axes along the transcurrent faults and the Eastern Himalayan Front, are approximately N-S, parallel to the relative motion of the two plates. However, along the southern margin of the Eurasian plate, they are NE-SW in the Western Himalayan Front and NW-SE to E-W in the Kirthar-Sulaiman Front, which is not consistent with the direction of relative plate motion. A simple model is proposed in order to explain the regional stress pattern. In this model, the tectonic sliver between the transcurrent faults and the plate margin, is dragged northward by the oblique convergence of the Indian plate. Thus, the direction of relative motion between the tectonic sliver and the Indian plate changes regionally, causing local compressive stress fields. Judging from the long-term slip rates along the active faults, the relative motion between the Indian and Eurasian plates absorbed in the colliding zone is about one fourth of its total amount; the rest may be consumed along the extensive strike-slip faults in Tibet and China.

Empirical relationships among the convergence rate of plates, rollback rate of trench axis and island-arc tectonics: “laws of convergence rate of plates”

Abstract The data on 27 recent arcs which were subducted by normal oceanic lithosphere were compiled. Among the compression/extension rates for island arcs ( V a : compression is defined as positive) and the kinematic components of absolute plate motions parallel to the direction of backarc compression/extension ( V on : oceanward velocity of the overriding plate, V sn : continentward velocity of the subducting plate and V tn : rollback rate of the trench axis), the equations V a = V on + V sn − 7.2 cm/y (1) V tn = 7.2 cm/y − V sn (2) are found to hold as for the convergent zones associated with a Wadati-Benioff zone deeper than 200 km. Equation (1) suggests that backarc spreading and crustal shortening play the role of compensating the deviation of the convergence rate from the critical value of 7.2 cm/y. The equation (2) means that any subducting oceanic lithosphere has the potentiality of making the trench axis rollback at the rate of 7.2 cm/y, and that the virtual rollback rate ( V tn ) is determined by the balance between the potential rollback rate (7.2 cm/y) and V sn . The present author calls the kinematic relationships represented by equations (1) and (2) “laws of convergence rate of plates”. The convergent zones of plates associated with the Wadati-Benioff Zone shallower than 200 km also show a tendency that the greater and smaller values of V on + V sn correspond to the backarc compression and extension. However, the critical value is 3.4 cm/y instead of 7.2 cm/y.

Westward migration of the Izu-Bonin Trench, northward motion of the Philippine Sea Plate, and their relationships to the Cenozoic tectonics of Japanese island arcs

Abstract The essential elements in the understanding of the Cenozoic island arc tectonics of the Japanese Islands came from reconstructing the paleo-position of the plate boundaries and estimating the change in the Philippine Sea Plate motion. By using the 2nd law of the convergence rate of plates, it was estimated that the Izu-Bonin Trench wandered around 400 km east from its present position during the Paleogene and migrated westward thereafter. Island-arc tectonism is related to the convergence rate of plates (1st law of the convergence rate of plates), hence the changes in the Philippine Sea Plate motion was examined by compiling paleomagnetic data. As a result, the main events of the Cenozoic tectonics of Japan were well explained by the change in the position of the plate boundaries and the change in motion of the Philippine Sea Plate.

Frictional melting can terminate seismic slips: Experimental results of stick‐slips

[1] The appropriate setting of sensors and the use of a high speed data acquisition system enabled us to detect the exact time of melting of the slip surfaces during a stick-slip event (fault displacement of 0.32 mm, stress drop of 230 MPa, rise time of 23 μs and maximum slip velocity of 40 m/s) in triaxial apparatus at 150 MPa confining pressure. Once slip started, the friction coefficient decreased continuously from 0.55 to 0.3, after which the frictional resistance recovered promptly and remarkably. This strengthening can be attributed to the highly resistive initial melt, a potential mechanism for arresting seismic slips.

Neogene tectonic stress fields of northeast Honshu Arc and implications for plate boundary conditions

Abstract More than 3300 metalliferous veins were analyzed to reconstruct the Neogene tectonic stress field of the northeast Honshu Arc using basic fracture mechanics. The veins are grouped into a dominant NE system and subordinate E-W, N-S and NW systems. The NE system is associated with assemblages of conjugate strike-slip faults and extension joints or normal faults which were formed during a period from 15 to 5 Ma. The fractures suggest that σ1orσ2 was oriented ENE and σ3 was oriented NNW. σ3 is thought to have been tensional, because no veins are associated with thrust faults. This stress orientation is neither perpendicular nor parallel to the axis of the Japan Trench where the Pacific plate was moving west-northwest and was subducted under the northeast Honshu Arc. The inconsistency between the stress orientation and the plate kinematics can be explained by the dynamic effects related to other plate boundaries surrounding the northeast Honshu Arc, namely compression on the Hokkaido Axial Zone and the Itoigawa-Shizuoka Tectonic Line. In addition, the triple junction of the Japan Trench, Izu-Bonnin Trench and the Sagami Trough was probably located 300–400 km northeast from its present position and the subduction zone of Nankai Trough-Sagami Trough is assumed to have been a tensional plate boundary. The reconstructed stress field is consistent with transtensional backarc rifting at about 15 Ma.

Local Behavior of Water Molecules on Brucite, Talc, and Halite Surfaces: A Molecular Dynamics Study

The structural and dynamic properties of water between brucite (0001), talc (001), and halite (100) surfaces have been calculated by classical molecular dynamics (MD) simulations at ambient conditions. The interaction potential models between water and the minerals have been developed by the energy curves obtained from the ab initio electronic state calculations. Orientational anisotropy of water molecules is almost limited in the vicinity of all the surfaces. The significant different properties of water between the surfaces are observed in the density profiles and self-diffusion coefficients. The density profile of water between talc surfaces is flat and the density is equivalent to the bulk one, while those of water between brucite and halite surfaces strongly oscillate with the distance from the surfaces. The self-diffusion coefficients parallel to the surfaces are enhanced in the vicinity of brucite and talc surfaces, and reduced on halite surface compared with that in bulk.

An empirical evolution law of fractal size frequency of fault population and its similarity law

Size (displacement) frequencies of 10 fault populations follow a power-law associated with cut-off in the range smaller than a characteristic size. They are depicted by five geometrical parameters of fault population; cut-off size, sharpness of cut-off, fault number density, fault displacement density and the power-law exponent. The former three are rock properties related to dissipative energy density and constitute the similarity law of the size frequency. The fault displacement density is a measure of total input energy density. The power-law exponent decreases as the − 1/4 power of total input energy density normalized by dissipative energy density.

Quasi‐static slip patch growth to 20 m on a geological fault inferred from acoustic emissions in a South African gold mine

Three months of acoustic emission (AE) monitoring in a South African gold mine down to Mw −5 revealed a newly emergent planar cluster of 7557 events −3.9 ≤ Mw ≤ −1.8 (typical rupture radius of 6–70 cm) that expanded with time to reach a size of 20 m on a preexisting geological fault near an active mining front 1 km beneath the ground. It had a sharply defined, planar configuration, with hypocenters aggregated within a thickness of only several decimeters. We infer that the zone defines an aseismic slip patch on the fault, wherein the individual AEs represent failures of very small asperities being loaded by the aseismic slip. Additional support for the interpretation was obtained by analyzing composite focal mechanisms and repeating events. The patch expansion over 2 months was likely quasistatic because all individual AEs ruptured much smaller areas than the cluster size at the corresponding time. The b values dropped gradually from 2.6 to 1.4, consistent with a significant increase in shear stress expected of the mining style. Another cluster with similar characteristics emerged later on a neighboring part of the same fault and grew to a 10 m extent in the last weeks of the study period. The quasi-static expansion of inferred localized slow-slip patches to sizes of 10–20 m suggests that the critical crack length on natural faults can be at least as large, much exceeding the decimeter range derived from laboratory stick-slip experiments on saw-cut rocks.

New data which support the “laws of convergence rate of plates” proposed by Otsuki

Abstract The VLBI geodetic networks have recently detected a shortening rate of about 6.2 cm/y in the baseline lengths between Kashima station on the Northeast Honshu Arc and the Pacific stations. In the northern part of the Mariana Trough, a change of the backarc spreading rate has been demonstrated through magnetic anomaly analysis. The backarc spreading commenced at about 3.5 Ma with a half spreading rate of 2.5 cm/y, and it has decreased to about 1 cm/y toward the present. These new data are consistent with the first law of the convergence rate of plates proposed by Otsuki (1989), thus supporting its validity.