Work in the Seismology Branch on the problem of earthquake prediction is based primarily on the recording and analysis of seismic (elastic) waves in the Earth generated either by earthquakes themselves or by artificial sources such as quarry blasts or explosions detonated in drill holes. At the present time this work is still very much a research effort focused largely on earthquake occurrence within the San Andreas fault system and related faults in California. Major aspects of this research include 1) studies of the spatial-temporal patterns of earthquake occurrence as a basis for recognizing distinctive sequences of events that may precede moderate-to-large earthquakes, 2) studies of Earth structure as a basis for the accurate location of earthquake hypocenters and understanding the physical properties and state of active fault zones, and 3) studies of the earthquake source mechanisms as a basis for recognizing possible variations in stress-drop, rupture direction, and so on that may precede moderate-to-large earthquakes.
