SURFACE DEFORMATION IN PART OF THE SAN JACINTO VALLEY, SOUTHERN CALIFORNIA.

Ground fissuring and subsidence occur in a deep alluvium-filled graben between the Casa Loma and San Jacinto faults, San Jacinto Valley, southern California. These processes are attributed chiefly to compaction due to withdrawal of ground water. Subsidence due to compaction is estimated to be a maximum of 3. 5 centimeters per year; tectonic subsidence of the graben is estimated to be from 0. 3 to 0. 6 cm per year. Ground fissuring occurs primarily on the west side of a closed depression in the graben. The area of ground fissures has expanded from 1 square kilometer in 1953 to 12 km**2 in 1974 and could grow even more. Individual fissures have attained lengths of 850 meters.