1. Regional Study on the Characteristic Seismicity of the World : Part 3. New Hebrides Islands Region
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概要
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Characteristics of the seismicity of the New Hebrides Islands region was investigated. Shallow earthquake swarms were found to be quite active. Other shallow earthquakes also contained many inductive events which follow one, two or sometimes several events closely related with the previous one both in time and space. A typical arrangement of foci which is commonly observed in the Islandsarc-trench system was disturbed in the northern part near the Torres trench, where an upper mantle earthquakes zone with the depth range from 200 km to 300 km turned eastward departing from the trend of the Islands arc. Three earthquake nests were discovered in the upper mantle. Two of them, N1 and N2, located around 15°S near the main Islands with the depth range from 100km to 150km beneath the sea. N1 has several inductive events. The third one was centering at 18.6°S and 169.1°E with the depth range from 200km to 250km. Foci near N1 distributed around a plane with the dip of about 45° northward, while those near the other two nests N2 and N3 inclined southward at the same angle. Along an extended profile of NNW-SSE direction following the trend of the Islands arc, foci in the upper mantle, in general, have the tendency to increase their depths both northward and southward, where trenches appear respectively at the western sides. Foci distribution along a few profiles perpendicular to the trend of the Islands arc were examined. The dips of the focal zone were found to be in all cases around 70°, which are of exceptionally large values compared with those obtained along similar profiles for other Islands arc regions. The focal zone extended to 300 km in two profiles only containing the New Hebrides trench and the southern part of the Torres trench. Less active depth ranges were also found to exist from 160km to 200km in these two focal zones. The deep earthquakes distributed not parallel to the trend of the Islands arcs but in the direction of the Fiji Islands. A shallow seismic zone also turned toward the Fiji Islands in its southern end around 23°S, 172°E. These tendencies were compared with the epicentral distributions around Fiji Islands already studied, and, it was cleared that the trends above mentioned were well connected with the seismic zones of the Tonga-Kermadec region by way of Fiji.|New Hebrides諸島北東部の海洋底に分布する深発地震の震央分布は,島弧にも海溝にも平行せずに,Fiji島に向つて斜めに分布している.これがこの周辺を特殊な地震地域の一つに取り上げた理由である.
- 1970-03-31
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