The magnitude 7.1 mainshock on April 25, 992, caused substantial changes in the elevation of the Earth's surface in the Cape Mendocino region. Precise measurements of such elevation changes aid in long-term emergency planning and are needed for engineering design and the maintenance of roads, bridges, and waterways. Such measurements also enable researchers to estimate the attitude and slip of the causative fault.
Elevation changes, as well as horizontal displacements of the Earth's surface, are an expected consequence of dip-slip displacement on earthquake faults. the rock surrounding and overlying the fault is forced to stretch and bend to accommodate fault slip. Slip in the case of the April 25 mainshock is thought to have occurred on a gently inclined plane dipping to the northeast at a small angle (see article on preliminary seismological results in this issue).The associated fault-plane solution implies that rock overlying the fault plane (the hanging-wall block west and south of the epicenter) rose and shifted to the northeast. The map on the next page shows the location of the epicenter and approximate extent of uplift and subsidence derived from estimates of the geometry, location. and slip on the buried fault plane.
