Earthquake sequences and seismicity of the Watsonville Region of Central California

The Watsonville area designates here the region ofCentral California between the latitude 36°40' and 37°05' N and the longitudes121°25' and 121°50' W. This area can be considered as a regionalseismic unit. The seismic activity during the years 1960-1966 gives abackground level of 2 or 3 earthquake of magnitude 2 < il/ ^ 3, per month,with periods of increased activity associated with the occurrence of largershocks. The presence and nature of sequences of earthquakes in the area isinvestigated for the period 1952-1966. Three sequences in 1963 are studiedin more detail. Tentatively the sequences of the area are groupedin three general types