I. CEDAR -- an approach to the computer automation of short-period local seismic networks. II. Seismotectonics of the Imperial Valley of southern California

A real-time detection and recording system (CEDAR) is developed as a means of automating the acquisition and processing of data from short- period local networks. This system has been used for the past two years for the analysis of data from 150 stations in Southern California with an annual workload of about 7500 local events. Two minicomputers are used with one dedicated to the real-time detection and digital recording of local earthquakes while the other is used for timing, location, and data archiving based on interactive graphical techniques. The use of this system has substantially reduced the effort required for the routine analysis of local data. The discussion is kept at a general level so as to be useful to those setting up similar systems with somewhat different requirements. In support of the CEDAR system a magnitude scale, M_(CA), is developed that is particularly adapted to the needs of local digital seismic networks. The supporting algorithm is based on median absolute amplitudes of any on-scale portion of the post-S seismic coda. The use of a power law coda shape function in the form a(t) = a_ot^(-q) makes the proposed method directly commensurable with the already widely used and highly successful duration method. The MCA magnitude scale is predicated on the same short-term averages used by the event detection algorithm on the real- time system, permitting a direct stochastic analysis of the spatial magnitude thresholds of a particular configuration of the detection logic. Such an a priori evaluation of detection capability is necessary since detection failure results in considerable extraneous effort. The use of these techniques has permitted the compilation of a local earthquake catalog and attendant phase data base that are substantially more uniform and accurate than what is generally obtained using manual methods. The nature of earthquake swarms in the Imperial Valley is investigated with the goal of placing specific constraints on the physical mechanisms governing their behavior. Within the Imperial Valley most earthquakes occur as swarms concentrated within a narrow, sharply bounded, spindle-shaped zone joining the northern terminus of the Imperial Fault with the southern end of the San Andreas Fault. Although over the past five years the seismicity within this zone, designated the Brawley Seismic Zone, is surprisingly uniform, on time scales of a few weeks activity is highly clustered in both space and time. Seismicity is not confined to a few "hot spots", as might be expected, but rather seems to move around, seldom if ever reactivating the site of a previous swarm. Seven sequences of swarms are analyzed in detail using a master event approach in order to provide some insight into supporting tectonic structures. It is generally observed that swarm sequences comprise discrete bursts of activity, each of which appears to "illuminate" a single planar fracture transverse to the major tectonic elements in the Imperial Valley such as the Imperial Fault and the Brawley Fault. Development of activity during a sequence of swarms generally begins with high clustered activity followed by continuous, progressive involvement of the transverse structures, and progressive but discontinuous development in the form of spatially and temporally isolated clusters along the major fault elements. Observed migration rates range from . 5 km/hr to . 5 km/day. The consistency observed with respect to the pattern of development of independent sequences strongly suggests that a deterministic, physical model can be obtained. One possible model is suggested that relates the swarms on transverse structures with propagating, episodic creep on the major transforms. In this model both the creep rate and the triggering of earthquake swarms is governed by perturbations of pore-pressure in a fluid-infiltrated elastic matrix.