Deformation of the Mitaka Rhombus: Strain buildup following the 1923 Kanto earthquake, central Honshu, Japan

Abstract

Frequently repeated surveys of a local network near Tokyo provide considerable detail on the horizontal strain changes immediately adjacent to the rupture zone of the M = 8.1 Kanto earthquake. Changes in the three independent tensor strain components are determined for 35 epochs during 1916–1980 to a precision of about ±1.5 μstrain. No significant shear or elongational strain changes preceded the earthquake; a change in dilational strain is marginally significant. Postseismic movements are large, accumulating to 40% or more of the coseismic strain drop. They have at least two time scales. A distinct short‐term transient lasting less than a year is explained well by slip or localized deformation that occurs downdip of the coseismic rupture plane and has a time constant of about 3 months. More complex, longer‐period postseismic movements continue for at least another 10 years. Their precise duration, as well as the transition to presumably steadier interseismic deformation, is obscured by notable irregularities in the strain buildup at Mitaka. As a result, neither the interseismic strain rate nor the occurrence time of the next Kanto earthquake can be reliably estimated from these observations.