Probably one of the most significant heterogeneities of a continental lithosphere is the noticeable difference in the thickness and properties of its cratonic and mobile parts. The trans-continental mobile belts (Paleoorogens/paleo-sutures) represent a relatively thinner, warm, wet and weak lithosphere, which makes it more vulnerable to episodic mantle (or plume) upwellings and compressional forces. Here evidence is presented from the Indian continental lithosphere to show that these properties of mobile belts (MB) facilitate channeling of thermomagmatic fluxes (TMF) in both lateral as well as vertical directions. This, to a major extent, can account for the observed concentration (or focusing) of geophysical anomalies, tectonomagmatic features and strain along these MBs. In addition, a closer examination of the three continental breakups of Greater India since the Cretaceous reveals that the combination of a sufficiently weakened MB and mantle plume could become ‘fatal’ for the supercontinental stability. On the other hand, the thick (>200 km) or deep-rooted continental lithosphere beneath cratons is characterized by a relatively cold and dry lithosphere, which resists remobilization. From this relative impenetrability, TMFs are channeled mostly through the MBs following the long-term stability of the cratonic lithosphere. This difference in cratonic and MB regimes results in strongly heterogeneous thermal blanketing. However, in certain situations, the edge of a cratonic region may also get chipped-off following a number of thermotectonic rejuvenations of adjoining MBs - as exemplified by the breakup of the Antongil and Masora cratonic blocks (now lying on the Madagascar) from the Western Dharwar craton (India) during the India-Madagascar separation. From the study of supercontinental dispersals, it seems that the processes of breakup along pre-existing mobile belts may be globally applicable. This is, at times, also accompanied by chipping of cratons.

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In 1996, an airgun-ocean bottom seismometer survey was carried out in the northern part of the central Japan Basin. The crustal thickness in the central part is about 9 km, including a sedimentary layer with thickness of 1.5 km, and increases eastward. The obtained crustal structure is slightly different from those of typical ocean basins. The thickness and velocity of less than 6.5 km/s in the upper part of the crust do not correspond to that of a typical oceanic crust and the clear linear geomagnetic anomaly around this survey line has been unconfirmed. Although, this crust could be interpreted to be either anomalous thick oceanic crust formed at spreading centers influenced by a mantle plume or thinned continental crust at ocean-continental boundaries in passive margins, we prefer the latter as a conclusion, that is, it may be formed by thinning of a continental crust rather than by the melt of mantle plumes during the opening of the Japan Sea. In addition, the difference of the crustal structures in the study area and the northeastern Japan Basin where the crust is typical oceanic, indicates that the process of crustal formation may differ in the northern part of the central Japan Basin from in the northeastern Japan Basin.

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We observe a correlation between the Earth tide and earthquake occurrence that is closely related to the regional tectonic stress. We investigate the direction of the tidal compressional stress using shallow earthquakes occurring in 100 subregions of Japan for nearly five years. The azimuthal distribution of the compressional stress obtained for the observed earthquake data is compared with that synthesized for random earthquake occurrence. Statistical analysis confirms a significant difference between the observed and random catalogs for 13 subregions, which include the areas where unusual seismic activities took place recently, and where the possibility of future large earthquakes has been argued. For these subregions, earthquakes preferentially occur when the tidal compressional stress is near the dominant direction of P-axes of focal mechanisms obtained in the corresponding subregions. This suggests that the tidal stress may encourage earthquake occurrence when it acts in the direction to increase the regional tectonic stress.

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The Yamakogawa Rhyolite, which erupted in the early Quaternary period in central Kyushu, Japan, comprises seven units, three contain of which spatter and stretched pumice. Our fieldwork shows that these are the deposits of strombolian fire-fountains and rheomorphic tuff. Such deposits derived from silicic magma have been previously described and still are controversial. Some of the reasons given for their formation were exclusively peralkaline composition and high-magmatic temperature. The chemical analyses of the Yamakogawa Rhyolite show nonperalkaline composition and low-magmatic temperature. Moreover, the mineral assemblage of the Yamakogawa Rhyolite suggests that its water content was indistinguishable from other rhyolitic deposits. This is the first report that demonstrates that eruption of silicic magma as fire-fountain and pyroclastic flow with rheomorphism is not, necessarily, restricted to peralkaline composition, high-magmatic temperature and low-water content rhyolite.

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At coastal sites, geomagnetic variations for periods shorter than a few days are strongly distorted by the conductivity of the nearby sea-water. This phenomena, known as the ocean (or coast) effect, is strongest in the magnetic vertical component. We demonstrate the ability to predict the ocean effect of geomagnetic storms at geomagnetic observatories. The space-time structure of the storm is derived from the horizontal components at worldwide distributed observatories from which we predict the vertical component using a model of the Earth's conductivity that a) only depends on depth, and b) includes the conductivity of the sea-water. The results for several strong geomagnetic storms (including the “Bastille Day” event of July 14-15, 2000) show much better agreement (improvement by up to a factor of 2.5) between the observed and the modeled magnetic vertical component at coastal sites if the oceans are considered. Our analysis also indicates a significant local time asymmetry (i.e., contributions from spherical harmonics other than P01 ), especially during the main phase of the storm.

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Consecutive bursts of Pi2 pulsations are examined with magnetic field data obtained by the SMALL array in 1999. With reference to the H-component magnetic bays in the high-latitude magnetograms at CPMN, 10 events consisting of two consecutive Pi2 bursts simultaneously observed by the Beijing (BJI, L = 1.46) and Wuhan (WHN, L = 1.20) stations are identified as associated with substorm onsets. Owing to the same waveform seen by the CPMN and IGPP/LANL arrays, they are the global phenomena. Their occurrences are mostly in the 2100-2300 LT (local time) sector in which the dominant frequencies at WHN are higher than the mean frequency, but those at BJI are lower and close to the frequency of the surface wave at the plasmapause. Moreover, the LT dependence of azimuth and polarization of two consecutive Pi2 bursts at BJI and WHN are analyzed and consistent with the ULF waves theory by Itonaga and Yumoto (1998). GOES 8 and GOES 10 confirm the formation of the substorm current wedge after the onsets of two Pi2 bursts. Thus during substorm onsets, Pi2 pulsations at low latitudes may result from hydromagnetic waves driven by an impulsive source in the magnetotail which could commence in the longitude of 2230 LT and later propagate westward and eastward as well. Low-latitude Pi2 waves near the source site may be affected by several factors as they propagate by the stimulation of a surface wave at the plasmapause, by a localized field line oscillation inside the plasmapause, and by the magnetospheric/plasmaspheric cavity (resonance) mode.

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Earthquake swarms have been observed since 1976 in the southeastern region of Mt. Ontake, where the 1984 Western Nagano Prefecture earthquake (M6.8) occurred. Most earthquake swarm activities show a dense distribution in a narrow area and at shallow depths. We have carried out forward 3-D magnetotelluric modeling for the data set observed around the focal region of the earthquake swarms. An initial 3-D image was constructed based on previous 2-D magnetotelluric analysis results. The resistivity structure shows remarkable lateral inhomogeneity: a low resistivity zone is located at shallow depth around the center of the study area and the low resistivity zone is surrounded by a high resistive structure and its shape is about a 4-km cube. This feature agrees well with the distribution of the degree of water saturation deduced from seismic information. Hypocenters of recent seismicity are distributed around an area between the resistivity boundary and the relatively high-resistance region. This fact strongly suggests that earthquakes in the study area are triggered by fluid (water) in the crust.

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Tectonic movement may disturb lateral continuities of crustal structures at weak zones. These lateral discontinuities of density structure cause undulations of the gravity anomaly field whose complexity can be an indicator of past crustal instability. On the other hand, topographic lineaments are formed along weak crustal zones. We would expect therefore that gravity anomaly roughness bears some relation to the distribution of surface lineaments. We develop a statistical method by employing the Bouguer anomaly standard deviation as an index of gravity anomaly roughness in terms of which we interpret the spatial distribution of topographic lineaments in Chugoku district, Japan. Locations of the lineaments, which could be diagnostic of the heterogeneity of the previously damaged crust, tend to coincide with those of high roughness areas of gravity anomaly fields.

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Aeromagnetic data analysis over the Narmada-Son Lineament (NSL) shows that prominent anomalies are located over dyke swarms and trap flows and the major formations present have distinct magnetic signatures. A major extensive ENE fault is identified which coincides with the course of the Narmada river and which appears, as earlier suggested, to control Gondwana sedimentation to the south and Vindhyan sedimentation to the north. By isolating the sources, it is found that the signatures of the Deccan traps have a distribution that is possibly related to their time of eruption and subsequent evolution. The region between Hoshangabad (HBD) and Narsimapura (NSR) are made up of two structural units: a shallow ENE structure superposed on a deeper NW-SE feature which is possibly related to the continuation of the Godavari Gondwana Graben towards the northwest. The region north and south of the deep-seated fault F1 has undergone different evolutionary history. The thickness of the magnetic crust in the Narmada-Son region lies between 25 to 31 km, implying a lithological change below this depth. This is borne out by the high velocity layer (7.2 km/s) within the lower crust identified by DSS studies.

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Fractal fluctuation analysis is applied to ground-based SYM-H data during quiet times and during magnetic storm times spanning two solar cycles between 1981-2002. On the basis of Kp, intervals were selected that corresponded to quiet and active magnetospheric dynamics. A nonlinear detrended fluctuation analysis (DFA) was applied to monitor nonlinear variability over the solar cycles. We find significant variations in nonlinear statistics between quiet and active intervals, which indicates a difference in statistical variability for quiet times, and storm times.

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