Herdmechanische Vorgä‘nge bei einigen zirkumpazifischen Erdbeben

Abstract

Earthquake mechan'sms can be determined by a method given by P. Byerly and later developed by J. H. Hodgson and his collaborators. By a comparison with the tectonics of the earthquake region, it is possible in most cases to decide which of the two nodal planes represents the fault plane. As a certain arbitrariness cannot be avoided, also the stikes of the auxiliary planes have been given. If the arrows, indicating the direction of motion, have been drawn perpendicularly to the fault plane, this may mean that no auxiliary plane could be determined. The mechanisms have been determined for 11 earthquakes in the years 1931–1950 in the Pacific area. It results that there is no general rule for the mechanisms, although the strikes of the fault planes in general coincide with the Circum‐Pacific structural lines. The dip of the fault plane is as small as 31° in one case (earthquake no. 7), whereas the dip for the other cases is at least twice as large. Most of the earthquakes have a very large dip of the fault plane, in the range 70–88°. Dip values above 80° are especially frequent. The cases with smaller dip are localized to the north and northwest part of the Circum‐Pacific belt. The coastal earthquake belts of Central and South America exhibit a normal dip of the fault plane under the continent; this is also true for the northwest part of the Circum‐Pacific belt. The conditions are similar at the southeast coast of Kamchatka, in Alaska, in western part of the Aleutians, and in the area between the Loyalty Islands and New Hebrides. In the other cases the fault plane dips toward the Pacific. In northwestern Pacific belt the motions are often perpendicular to the strikes of the fault planes, whereas in the other areas the vertical motions are combined with horizontal displacements. The number of observations for a single earthquake varies between 22 and 47. A few earthquakes in the Alpide belt have also been investigated, and comparisons with the results of other authors (A. R. Ritsema, 1955; A. N. Tandon, 1955) showed good agreement in spite of the fact that these authors have used 4–5 times as many observations. In a particularly favourable case (Hindu Kush, Jan. 9, 1933), 14 reliable observations of the initial motion of P waves gave the same result as 7 times as many observations, used by Ritsema. Nevertheless, as many reliable observations as possible should be used for earthquake mechanism studies.