Shear wave splitting above small earthquakes in the Anza seismic gap on the San Jacinto fault, southern California, displays the effective anisotropy to vertical fluid-filled microcracks. The delays between the split shear waves increase significantly over the 3 years of available records at the only station with arrivals from a wide range of azimuths and angles of incidence. These changes can be modeled by increasing the aspect ratio (''bowing'') of stress-oriented microcracks, which is one of the expected elastic effects of an increase of stress on a rock mass containing a distribution of fluid-filled microcracks. This is the first time that temporal variations of shear wave splitting have been observed and suggests that shear wave splitting can be used to monitor the detailed changes in the buildup of stress before an earthquake. ¿ American Geophysical Union 1988