Abstract
The change of Vp/Vs value before an earthquake corresponds to the change of the elastic constants of a part of the crust. The change of elastic constants distribution in the crust under initial stress field will induce strain changes. If those strain changes can be detected by ordinal geodetic surveys, they will serve as good earthquake precursors as well as Vp/Vs changes.
In order to estimate the values of expected strain changes, a simple plane strain model was adopted for calculation. The results are as follows.
(1) It is very difficult to detect any substantial horizontal strain changes using an ordinal one-wave optical distance meter.
(2) It is well possible to detect the tilting of the free surface as vertical movement by ordinal levelling.
