Toshihiko Shimamoto

High‐velocity frictional properties of gabbro

High-velocity friction experiments have been performed on a pair of hollow-cylindrical specimens of gabbro initially at room temperature, at slip rates from 7.5 mm/s to 1.8 m/s, with total circumferential displacements of 125 to 174 m, and at normal stresses to 5 MPa, using a rotary-shear high-speed friction testing machine. Steady-state friction increases slightly with increasing slip rate at slip rates to about 100 mm/s (velocity strengthening) and it decreases markedly with increasing slip rate at higher velocities (velocity weakening). Steady-state friction in the velocity weakening regime is lower for the non-melting case than the frictional melting case, due perhaps to severe thermal fracturing. A very large peak friction is always recognized upon the initiation of visible frictional melting, presumably owing to the welding of fault surfaces upon the solidification of melt patches. Frictional properties thus change dramatically with increasing displacement at high velocities, and such a non-linear effect must be incorporated into the analysis of earthquake initiation processes.

Onshore Tsunami Deposits Caused by the 1993 Southwest Hokkaido and 1983 Japan Sea Earthquakes

Onshore tsunami deposits resulting from the 1993 Southwest Hokkaido and 1983 Japan Sea earthquakes were described to evaluate the feasibility of tsunami deposits for inferring paleoseismic events along submarine faults. Tsunami deposits were divided into three types, based on their composition and aerial distribution: (A) deposits consisting only of floating materials, (B) locally distributed siliclastic deposits, and (C) widespread siliclastic deposits. The most widely distributed tsunami deposits consist of the first two types. Type C deposits are mostly limited to areas where the higher tsunami runup was observed. The scale of tsunami represented by vertical tsunami runup is an important factor controlling the volume of tsunami deposits. The thickest deposits, about 10 cm, occur behind coastal dunes. To produce thick siliclastic tsunami deposits, a suitable source area, such as sand bar or dune, must be available in addition to sufficient vertical tsunami runup. Estimation of the amounts of erosion and deposition indicates that tsunami deposits were derived from both onshore and shoreface regions. The composition and grain size of the tsunami deposits strongly reflect the nature of the sedimentary materials of their source area. Sedimentary structures of the tsunami deposits suggest both low and high flow régimes. Consequently, it seems very difficult to identify tsunami deposits based only on grain size distribution or sedimentary structure of a single site in ancient successions.

An argument against the crush origin of pseudotachylytes based on the analysis of clast-size distribution

Abstract The origin of pseudotachylytes has been controversial since Wenk cast doubt on the melt origin of the matrix of pseudotachylytes, in 1978. The matrix of this rock is so fine grained that the crush origin of pseudotachylytes, revived by Wenk, cannot easily be denied. This paper presents a new line of argument based on the size analysis of clasts contained in pseudotachylytes in felsic granulite from the Musgrave Range, central Australia. These clasts are definitely crush products produced during the pseudotachylyte generation. Their sizes, as measured on photomicrographs of thin sections, obey the size distribution: N = N′r −D , where N is the cumulative number of clasts with sizes greater than r, D is the fractal dimension, and N′ is a constant that depends on the number of measurements. D was found to be 1.5 ± 0.05 for the size ranges of 10–2000μm. If the matrix of the pseudotachylytes consists mostly of ultrafine crush products, they must have formed simultaneously with those coarse crush products. The proportion of fine products relative to the coarse clasts can be estimated, assuming that a similar size distribution also holds for the fine products. The estimated area occupied by fine products in a thin section is of the order of only several percent, whereas the measured area of the matix is about 60%. Thus the major part of the matrix of the pseudotachylytes cannot be regarded as crush products. It is also shown that the number of clasts smaller than about 5 μm becomes very small, perhaps as a result of nearly complete dissolution of fine clasts in a melt. However, if the ultrafine-grained matrix of the pseudotachylytes had formed by crushing during seismogenic fault motion, the grain-size refinement during the crushing should have occurred jumping the size range of at least 1–5 μm. This is quite unreasonable and disproves the crush origin for the matrix of pseudotachylytes.

The strength profile for bimineralic shear zones: an insight from high-temperature shearing experiments on calcite–halite mixtures

Abstract Shearing experiments on mixed halite–calcite layers (0.7 mm thick) have been performed to understand the behavior of bimineralic fault zones, using a high-temperature biaxial testing machine at a slip rate of 0.3 μm/s and shear strains to about 30. Temperature was increased to 700°C in linear proportion to the normal stress, with the experimental geotherm of about 22°C/MPa, simulating the natural geothermal gradient. The experimental data clearly demonstrate that the effect of mineral composition on the ultimate frictional strength is distinctly different from that on the residual frictional strength. Tharps framework model (1983) quantitatively accounts for the ultimate frictional strength, whereas the residual frictional strength at large shear strains can be predicted by Jordans two-block model (1988). Thus, the frictional behavior of a bimineralic shear zone changes from the framework model to the two-block model with increasing displacement. Inclusion of halite, the weaker member, in quantities as small as 5% by volume, reduces the friction almost to the level of pure halite and suppresses stick-slip at large shear strains, because halite grains are extremely sheared at the zone of strain concentration. The strength profile and the slip mode, including the lower limit of seismic behavior, of bimineralic shear zones at large shear strains are controlled primarily by the weaker member. The present results disprove Strehlaus fault model (1986). The role of individual constituent mineral is not clarified in Scholzs fault model (1988), and his model disagrees with existing experimental data.

Field survey report on tsunami disasters caused by the 1993 Southwest Hokkaido earthquake

Detailed field work at Okushiri Island and along the southwest coast of Hokkaido has revealed quantitatively (1) the advancing direction of tsunami on land, (2) the true tsunami height (i.e., height of tsunami, excluding its splashes, as measured from the ground) and (3) the flow velocity of tsunami on land, in heavily damaged areas. When a Japanese wooden house is swept away by tsunami, bolts that tie the house to its concrete foundation resist until the last moment and become bent towards the direction of the house being carried away. The orientations of more than 850 of those bent bolts and iron pipes (all that can be measured, mostly at Okushiri Island) and fell-down direction of about 400 trees clearly display how tsunami behaved on land and caused serious damage at various places. The true tsunami height was estimated by using several indicators, such as broken tree twigs and a window pane. The flow velocity of tsunami on land was determined by estimating the hydrodynamic force exerted on a bent handrail and a bent-down guardrail by the tsunami throughin situ strength tests.Contrary to the wide-spread recognition after the tsunami hazard, our results clearly indicate that only a few residential areas (i.e., Monai, eastern Hamatsumae, and a small portion at northern Aonae, all on Okushiri Island) were hit by a huge tsunami, with true heights reaching 10 m. Southern Aonae was completely swept away by tsunami that came directly from the focal region immediately to the west. The true tsunami height over the western sea wall of southern Aonae was estimated as 3 to 4 m. Northern Aonae also suffered severe damage due to tsunami that invaded from the corner zone of the sand dune (8 m high) and tide embankment at the northern end of the Aonae Harbor. This corner apparently acted as a tsunami amplifier, and tide embankment or breakwater can be quite dangerous when tsunami advances towards the corner it makes with the coast. The nearly complete devastation of Inaho at the northern end of Okushiri Island underscored the danger of tsunami whose propagation direction is parallel to the coast, since such tsunami waves tend to be amplified and tide embankment or breakwater is constructed low towards the coast at many harbors or fishing ports. Tsunami waves mostly of 2 to 4 m in true height swept away Hamatsumae on the southeast site of Okushiri Island where there were no coastal structures. Coastal structures were effective in reducing tsunami hazard at many sites. The maximum flow velocity at northern Aonae was estimated as 10 to 18 m/s (Tsutsumiet al., 1994), and such a high on-land velocity of tsunami near shore is probably due to the rapid shallowing of the deep sea near the epicentral region towards Okushiri Island. If the advancing direction, true height, and flow velocity of tsunami can be predicted by future analyses of tsunami generation and progagation, the analyses will be a powerful tool for future assessment of tsunami disasters, including the identification of blind spots in the tsunami hazard reduction.

Cathodoluminescence observations on low-temperature mylonites: potential for detection of solution-precipitation microstructures

Abstract This note represents cathodoluminescence (CL) images, obtained using a scanning electron microscope, of a low-temperature granitic mylonite from central Japan. It is argued that CL observations offer a simple but powerful tool for elucidating the importance of solution-transfer processes during mylonitization. Cataclastic deformation such as fracturing of feldspars, accompanied by solution-precipitation processes, were found to be much more clearly observable in CL images than using other techniques. It is also shown that quartz grains precipitated from solution can be distinguished from original quartz grains in the granite, even after being deformed plastically. More detailed CL work on mylonites is needed in the future, since quantitative evaluation of the role of solution-precipitation processes in deep fault zones is essential for constraining the rheological properties of faults at depth.

On newtonian viscoplastic dislocation creep and the effect of the Peierls stress

Abstract Single crystals of synthetic quartz have been deformed in a uniaxial rig without a confining pressure, and a mechanical steady state has been observed. Using the new results on quartz, together with those on metals and ceramics published in the literature, it is demonstrated that crystalline materials may deform by newtonian dislocation creep with a threshold stress σc under certain conditions. This newtonian viscoplastic (σc ≠ 0) behavior may not be recognized if the conventional power law creep equation under the assumption of σc = 0 is applied to describe the experimental data. Observations on metals, ceramics and silicates suggest that the identified newtonian viscoplastic dislocation creep operates at stresses larger than the Peierls stress and that the threshold stress may originate from the lower limiting stress required to cause dislocation glide. The newtonian viscoplastic dislocation creep proposed herein is different from the classical Harper-Dorn creep which operates at stresses lower than the Peierls stress.

Hidden Fault Scarp Inferred from Gravity Analysis and Disaster Belt of the 1995 Hyogo-Ken Nanbu Earthquake

Dense gravity measurements were carried out together with the GPS positioning along five survey lines across the Rokko fault system. The subsurface structure relevant to the 1995 Hyogo-ken Nanbu earthquake was inferred from the Bouguer anomaly. A hidden fault was discovered on the southwestward extension of the Koyo fault underneath the sedimentary layer. The extension runs just on the edge of the earthquake disaster belt. The thickness of the sedimentary layer was estimated to decrease gradually toward the mountain side. The wedge-like structure of the soft layer and the hidden fault scarp under the Kobe plain may serve as a focusing lens of seismic rays during the earthquake.