Summary
The only instance of a confirmed deep lower crustal earthquake occurrence in the Indian shield region has been that of the 1938 Satpura earthquake (M 6.3) of central India, reportedly at a depth of about 40 km. Moment tensor inversion of regional broadband waveform data of the 1997 May 21 Jabalpur earthquake (Mw 5.7) confirms yet another such earthquake at about 35 km depth in the central part of the Narmada‐Son lineament (NSL) zone. The study is based on a refined velocity model obtained using a traveltime grid search method. A reverse fault mechanism is obtained which, for a palaeo‐rift valley zone, indicates the possibility of reactivation of a pre‐existing fault under the influence of the ambient stress field due to the India–Eurasia plate collision forces. The occurrence of earthquakes at lower crustal depths, quite unusual for the Indian shield region, indicates a possible causative mechanism related to crust–mantle interaction. Based on the close proximity of the two deep earthquakes and their disposition with respect to the local trend of the central part of the NSL, we suggest a model of stress accumulation due to horizontally elongated or elliptical, possibly serpentinized mafic intrusives in the lower crust, to explain the occurrence of deep earthquakes in the heart of the Indian shield.
Sources of luminescence and their buildup processes accompanying the 1995 Hyogo-ken Nanbu earthquake of M 7.2 are studied based on pieces of information obtained mainly by interviewing eyewitnesses. Gross forms of relatively large-scale luminous sources are roughly classified into four types: lightning with zigzag lines, a swelling shield-shaped source, an upward-extending fan-shaped source, and a belt of lights. The last one includes an arc-like source. Each source is predominantly in tones of either colorless-white, blue or orange-color. This paper presents 23 spottings, distributed as wide as 50 km from the epicenter of the mainshock near Kobe City. Along with these spottings, some local flashing events were reported. The upper limit of the height of several sources was able to be estimated as less than 200 m above the ground. The linear dimension of the horizontal extent ranged from about 1 to 8 km. The luminance was estimated to be more than an order of 103 cd/m2 for an arc-like orange colored source at the eastern part of the aftershock area. Not a few sources were glittering more intensely than this case. According to most of the eyewitnesses, the luminosity started from ground level on land, suggesting that discharge processes of the polarized electricity in near-surface rocks may be the primary driving force of the luminescence. However, electricity charged in the air should be also responsible to some luminous phenomena, especially a kind of lightning above the sea. Fog or dust was observed in the air in the region around Nishinomiya City, east of Kobe, preceding the quake, which might have played the role of an effective electrical conductor in glow discharge.
Shear‐wave splitting from local deep earthquakes is investigated to clarify the volume and the location of two anisotropic bodies in the mantle wedge beneath central Honshu, Japan. We observe a spatial variation in splitting parameters depending on the combination of sources and receivers, nearly N–S fast in the northern region, nearly E–W fast in the southern region and small time delays in the eastern region. Using forward modelling, two models with 30 and 10 per cent anisotropy are tested by means of a global search for the locations of anisotropic bodies with various volumes. The optimum model is obtained for 30 per cent anisotropy, which means a 5 per cent velocity difference between fast and slow polarized waves. The northern anisotropic body has a volume of 1.00° (longitude) × 0.5° (latitude) × 75 km (depth), with the orientation of the symmetry axis being N20°E. The southern anisotropic body has a volume of 1.25° × 1.25° × 100 km with the symmetry axis along N95°E. Our results show that the anisotropic bodies are located in low‐velocity and low‐Q regions of the mantle. This, together with petrological data and the location of volcanoes in the arc, suggests that the possible cause of the anisotropy is the preferred alignment of cracks filled with melt.
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