Spatial Distribution Of Earthquakes Research Articles

Mechanism of earthquakes and nature of faulting on the mid-oceanic ridges

The mechanisms of 17 earthquakes on the mid-oceanic ridges and their continental extensions were investigated using data from the World-Wide Standardized Seismograph Network of the U. S. Coast and Geodetic Survey and from other long-period seismograph instruments. Mechanism solutions of high precision can now be obtained for a large number of earthquakes with magnitudes as small as 6 in many areas of the world. Less than 1% of the data used in this study are inconsistent with a quadrant distribution of first motions of the phases P and PKP; in many previous investigations 15 to 20% of the data were often inconsistent with the published solutions. Ten of the earthquakes that were studied occurred on fracture zones that intersect the crest of the mid-oceanic ridge. The mechanism of each of the shocks that is located on a fracture zone is characterized by a predominance of strike-slip motion on a steeply dipping plane; the strike of one of the nodal planes for P waves is nearly coincident with the strike of the fracture zone. The sense of strike-slip motion in each of the ten solutions is in agreement with that predicted for transform faults; it is opposite to that expected for a simple offset of the ridge crest along the various fracture zones. The spatial distribution of earthquakes along fracture zones also seems to rule out the hypothesis of simple offset. Two well documented solutions for earthquakes that are located on the mid-Atlantic ridge but that do not appear to be located on fracture zones are characterized by a predominance of normal faulting. The mechanisms of four earthquakes on extensions of the mid-oceanic ridge system—one near northern Siberia and three in East Africa—are also characterized by a predominance of normal faulting. The inferred axes of maximum tension for these six events are approximately perpendicular to the strike of the mid-oceanic ridge system. The results are in agreement with hypotheses of sea-floor growth at the crest of the mid-oceanic ridge system.

The seismicity of the Caribbean region

The hypocenters of approximately 500 Caribbean earthquakes were relocated using a digital computer. Since a large number of epicenters can now be located with an accuracy of about 10 km, the spatial distribution of earthquakes can be used in detailed investigations of the tectonics of deep-sea trenches, fault zones, and island arcs. A nearly continuous belt of shallow-focus seismicity can be traced from Central America to the Greater and Lesser Antilles and thence to northeastern Venezuela. In Venezuela intense sources of seismic activity are apparently associated with the El Pilar and Bocono fault zones. This study indicated a gap in the seismicity of northern South America between longitudes 64.5°W and 69°W. In addition to this area, several other segments of island arcs have acted as distinct tectonic units. These units are often bounded by major transverse features. About 30% of the events reported in this study were assigned focal depths greater than 70 km; earthquakes having depths as great as 200 km were detected. A zone of intermediate-depth earthquakes can be traced along the entire length of the Lesser Antilles. Most of the hypocenters in the Lesser Antilles are confined to a zone about 50 km wide which dips about 60° west. An intense source of intermediate earthquakes is located beneath the eastern end of Hispaniola. Much of the shallow-focus activity in the vicinity of the Puerto Rico trench is located beneath the south wall of the trench. Some activity was detected in the Puerto Rico trench itself, and several epicenters are associated with structural elements which strike WNW. Earthquake epicenters near the Virgin Islands indicate that the Anegada faul zone may be at least 400 km long.

地震の規模別頻度の地域性について (I)

The b's in magnitude-frequency relation n(M)dM=const.×10-bM dM are obtained for various regions in Japan and discussed in some detail. For the above purpose, the magnitude of earthquakes of hypocenter deeper than 60km and earthquakes of smaller magnitude which are not in “The Seismological Bulletin” are obtained and added to the present data. The range of regions for which the parameter b's are obtained, are determined from spatial distribution of earthquakes. In other words, the so-called “nest of earthquakes” are taken as a unit region.The values of b's are 1.0-1.1 for the magnitude 6 and larger off the Pacific coast of Tohoku-Kwanto districts which is the most active seismic region in Japan for corresponding magnitudes and the b values for the magnitude range of 4 to 5 in other smaller regions, that is, “smaller nests” are around 0.9-1.0.This result shows that at least for the active seismic regions, there are no remarkable difference of the b values among seismically active regions, also there is no significant difference of values between crust and upper mantle.Also, the relation of b, m and β are discussed, where m is the parameter in the amplitude-frequency relation n(a)da=const. a-m da and β is the parameter in the intensity-frequency relation n(I)dI=const, ×10-βI dI.