A crack-like rupture model for the 19 September 1985 Michoacan,Mexico, earthquake

Evidence supporting a smooth crack-like rupture process of the Michoacan earthquake of 1985 is obtained from a major earthquake for the first time. Digital strong motion data from three stations (Caleta de Campos, La Villita, and La Union), recording near-field radiation from the fault, show unusually simple ramped displacements and permanent offsets previously only seen in theoretical models. The recording of low frequency (0 to 1 Hz) near-field waves together with the apparently smooth rupture favors a crack-like model to a step or Haskell-type dislocation model under the constraint of the slip distribution obtained by previous studies. A crack-like rupture, characterized by an approximated dynamic slip function and systematic decrease in slip duration away from the point of rupture nucleation, produces the best fit to the simple ramped displacements observed. Spatially varying rupture duration controls several important aspects of the synthetic seismograms, including the variation in displacement rise times between components of motion observed at Caleta de Campos. Ground motion observed at Caleta de Campos can be explained remarkably well with a smoothly propagating crack model. However, data from La Villita and La Union suggest a more complex rupture process than the simple crack-like model for the south-eastern portion of the fault.     

Relationship between growth of albite porphyroblasts and deformation in a sambagawa schist, Central Shikoku, Japan

Albite porphyroblasts in a basic schist of the Sambagawa metamorphic belt (Besshi—Bashi, Central Shikoku) show zonal variation of inclusions defined by inclusion-free mantles and inclusion-rich cores, though matrix minerals are partly incorporated in the outer zones of the mantles. From comparison of the crystallographic and dimensional fabrics of inclusion and matrix epidote, it has been concluded that the orientation of the principal axes of strain was different for the deformation of matrix fabrics and inclusion fabrics (fabrics produced before formation of cores). Inclusion amphibole in the cores belongs to the actinolite—common hornblende group, with Si content more than 7.0, whereas amphibole in the matrix and outer zones of mantles belongs to the common hornblende group, with Si content less than 7.1. This shows that the metamorphic temperature was higher during the phase of formation of the outer zones of the mantles than during that of the cores. The average direction of maximum growth of the mantles, which has been estimated from differences between the average shapes of porphyroblasts and those of cores, is parallel to the linear orientation of amphibole and epidote in the matrix. The average direction of minimum growth of the mantles coincides with the normal to the schistosity of the matrix. The mantles appear to be of the same generation as the matrix fabrics.     

Thermal expansion of fayalite,Fe2SiO4

Thermal expansion of single-crystal fayalite has been measured by a dilatometric method at temperatures between 25 °C and 850 °C. The results show the presence of anomalous expansion in the b axis, which is correlated to the anomalous variation of elastic moduli with temperature. Grüneisen's parameter is 1.10 and the thermal Debye temperature is 565 K, which is close to the acoustic Debye temperature of 511 K.     

Dynamic response of the KII building to the San Fernando earthquake

The KII Building was subjected to the San Fernando earthquake on 9 February 1971 without suffering any damage. During the earthquake, strong motion accelerations were recorded at the base, middle and top of the building. The simulation analysis of the building was carried out taking into account the interaction between the building and the soil. The internal viscous damping theory considering the different damping coefficients for the building and the soil was adopted. Comparison of the calculated and observed acceleration time histories as well as the floor response spectra showed satisfactory agreement. On the basis of the response spectrum values, the fact that no damage was observed in this building is justified theoretically.     

Subduction of the Woodlark Basin at New Britain Trench, Solomon Islands region

The Woodlark Basin, located south of the Solomon Islands arc region, is a young (5 Ma) oceanic basin that subducts beneath the New Britain Trench. This region is one of only a few subduction zones in the world where it is possible to study a young plate subduction of several Ma. To obtain the image of the subducting slab at the western side of the Woodlark Basin, a 40-day Ocean Bottom Seismometer (OBS) survey was conducted in 1998 to detect the micro-seismic activity. It was the first time such a survey had been performed in this location and over 600 hypocenters were located. The seismic activity is concentrated at the 10–60 km depth range along the plate boundary. The upper limit just about coincides with the leading edge of the accretionary wedge. The upper limit boundary was identified as the up-dip limit of the seismogenic zone, whereas the down-dip limit of the seismogenic zone was difficult to define. The dip angle of the plate at the high seismicity zone was found to average about 30°. Using the Cascadia subduction zone for comparison, which is a typical example of a young plate subduction, suggests that the subduction of the Woodlark Basin was differentiated by a high dip angle and rather landward location of the seismic front from the trench axis (30 km landward from the trench axis). Furthermore, as pointed out by previous researchers, the convergent margin of the Solomon Islands region is imposed with a high stress state, probably due to the collision of the Ontong Java Plateau and a rather rapid convergence rate (10 cm/year). The results of the high angle plate subduction and inner crust earthquakes beneath the Shortland Basin strongly support the high stress state. The collision of the Ontong Java Plateau, the relatively rapid convergence rate, and moderately cold slab as evidenced by low heat flow, rather than the plate age, may be dominantly responsible for the geometry of the seismogenic zone in the western part of the Woodlark Basin subduction zone.     

Accretion, core formation, H and C evolution of the Earth and Mars

Recent understandings of planetary accretion have suggested that accumulation of a small number of large planetesimals dominates intermediate to final growth stages of the terrestrial planets, with impact velocity high enough to induce extensive melting of the planetesimal and target materials, resulting in formation of a large molten region in which gravitational segregation of silicate and metal, that is, core formation proceeds. In case of homogeneous accretion, volatiles contained in each planetesimal are likely subjected to partitioning among gas, silicate melt, and molten metallic iron at significantly high temperatures and pressures in such a massive molten region. Each phase would subsequently form the proto-atmosphere, -mantle, or-core, respectively. Such chemical reprocessing of H and C associated with core formation, which is followed by both degassing from mantle and atmospheric escape, may result in a diverse range of H₂O/CO₂ in planetary surface environments, which mainly depends on the H and C content relative to metallic iron in planetary building stones. This may explain inferred difference in volatile distribution between the Earth's (relatively H₂O-rich, CO₂-poor) and the martian (H₂O-poor, CO₂-rich) surface environments. Such volatile redistribution may be systematically described by using the retentivity of H₂O, ξ, defined as follows: ξ = 1 − ([CO]⁰ + 2[CH₄]⁰ + 2[C(gr)]⁰)/[H₂O]⁰, where [i]⁰ represents mol number of species i partitioned into non-metallic phases, that is, gas and silicate melt in impact-induced molten region. When ξ > 0.5, relatively H₂O-rich and CO₂-poor surface environment may eventually evolve, although a small portion of H₂O partitioned into the NON=metallic phases are possibly consumed by subsequent chemical reactions with reduced C-species with producing CO₂ and H₂. When ξ< 0.5, on the contrary, H₂O consumption by the above reactions and selective loss of H₂ to space may result in relative H₂O-depleted and CO₂-rich surface environment. Given the building stone composition by the two-component model by Ringwood (1977) and Wänke (1981), ξ is found to decrease with increasing the mixing fraction of the volatile-rich component: ξ > 0.5 for the mixing fraction smaller than about 15–20% and ξ < 0 for the mixing fraction larger than about 20–30%. This is not significantly dependent on temperature and pressure in molten region and H/C ratio in the building stone. The estimated mixing fraction of the volatile-rich component, about 10% for the Earth and 35% for Mars, is consistent with the observed difference in volatile distribution between the surfaces of both planets.     

Analysis of saturated dense sand-structure system and comparison with results from shaking table test

An effective stress method is presented for the analysis of liquefaction of ground including soil-structure interaction, based on an explicit-implicit finite element method. A simple constitutive model is developed to be incorporated in the effective stress method. The constitutive model consists of the Ramberg-Osgood model extended to two-dimensional problems and a new dilatancy model. The effectiveness of the constitutive model is examined with results of a simple shear test. Besides, the effective stress method is verified by comparing its numerical results with results of a shaking table test. It is found that the present method can simulate well the response of a saturated dense sand-structure system. The difference of the response computed by the effective stress method and the total stress method is discussed. It is found that the total stress method can simulate the response of the saturated sand within an accumulating excess pore water pressure of less than 70 per cent of the initial overburden stress.     

The Japan Trench and its juncture with the Kuril Trench: cruise results of the Kaiko project, Leg 3

This paper presents the results of a detailed survey combining Seabeam mapping, gravity and geomagnetic measurements as well as single-channel seismic reflection observations in the Japan Trench and the juncture with the Kuril Trench during the French-Japanese Kaiko project (northern sector of the Leg 3) on the R/V “Jean Charcot”. The main data acquired during the cruise, such as the Seabeam maps, magnetic anomalies pattern, and preliminary interpretations are discussed. These new data cover an area of 18,000 km² and provide for the first time a detailed three-dimensional image of the Japan Trench. Combined with the previous results, the data indicate new structural interpretations. A comparative study of Seabeam morphology, single-channel and reprocessed multichannel records lead to the conclusion that along the northern Japan Trench there is little evidence of accretion but, instead, a tectonic erosion of the overriding plate. The tectonic pattern on the oceanic side of the trench is controlled by the creation of new normal faults parallel to the Japan Trench axis, which is a direct consequence of the downward flexure of the Pacific plate. In addition to these new faults, ancient normal faults trending parallel to the N65° oceanic magnetic anomalies and oblique to the Japan trench axis are reactivated, so that two directions of normal faulting are observed seaward of the Japan Trench. Only one direction of faulting is observed seaward of the Kuril Trench because of the parallelism between the trench axis and the magnetic anomalies. The convergent front of the Kuril Trench is offset left-laterally by 20 km relative to those of the Japan Trench. This transform fault and the lower slope of the southernmost Kuril Trench are represented by very steep scarps more than 2 km high. Slightly south of the juncture, the Erimo Seamount riding on the Pacific plate, is now entering the subduction zone. It has been preceded by at least another seamount as revealed by magnetic anomalies across the landward slope of the trench. Deeper future studies will be necessary to discriminate between the two following hypothesis about the origin of the curvature between both trenches: Is it due to the collision of an already subducted chain of seamounts? or does it correspond to one of the failure lines of the America/Eurasia plate boundary?