Co-seismic strain changes of the Wenchuan Mw7.9 earthquake recorded with three four-component borehole strainmeters showed NW-SE and roughly EW extensions,respectively,at two locations in the interior and northern part of Tibetan plateau,and NS shortening at a location south of the epicenter,in agreement with the tectonic stress field of this region.The observed values of as much as 10 -7 are,however,larger than theoretical values obtained with half-space and spherical-earth dislocation theories,implying the existence of other effects,such as local crustal structure and initial stress.