Abstract
All the silicatube extensometers at Mikawa Crustal Movement Observatory are specially designed to detect strain steps associated with earthquakes with the use of two or three detectors for each silicatube. Various tests indicate that these instruments are sufficiently stable even for shocks as large as felt earthquakes. Since 1972, seven earthquakes have shown apparent strain steps, the minimum of which is as small as 5×10-10. These strain steps are concordant with DAMBARA'S formula (1966), who investigated the relation of magnitude to the radius of region where crustal deformation can be observed. If we assume that the strain step decreses with distance as R-2.5, where R is the epicentral distance, the strain step at the distance defined by DAMBARA is found to be 5×10-6.
