Relevelling Survey over the Epicentral Area of the Earthquake off Izu Peninsula of 1974

Abstract

A locally destructive earthquake occurred on the coast of Izu peninsula (λ=138°48', p=34°34', M=6.9) on May 9th, 1974. The route of bench marks around the epicentral area had been established and surveyed at the level of the third order (closing error 47 mm) by Minami-Izu Town Office in June, 1973. In order to evaluate the coseismic land-deformation, the writers have carried out the resurvey of the first order levelling along the route of bench marks from Shimogamo (B.M. No. 9364) to Iruma Village, in September, 1974. The total length of the route amounts to about 28 km. Comparing the levelling with relevelling results, the relative vertical displacement of bench marks during 14 months are listed in Table I, where B.M. No. 9634 is assumed to be fixed. According to the analysis by means of the "sine-method", we find the coseismic land-tilting having the maximum inclination of 2" to 3" in the direction parallel to the Irozaki Fault which moved actively with a right rateral slip. The vertical displacement previously expected from the fault motion has not been explicitly detected.