A survey of 71 earthquake bursts across southern California: Exploring the role of pore fluid pressure fluctuations and aseismic slip as drivers

[ 1] We investigate the cause of seismicity bursts by examining a waveform-relocated catalog for southern California between 1984 and 2002 and systematically identifying 71 isolated sequences of 40 or more earthquakes occurring within a 2-km-radius volume and a 4-week interval. Fifty-seven of the 71 bursts are difficult to interpret as primarily a main shock and its Omori-law-abiding foreshocks and aftershocks because they exhibit a more complicated evolution in space, time, and magnitude; we identify 18 of these sequences as particularly swarm-like. Evidence against a simple cascade of elastic stress triggering includes the presence of an interval of steady seismicity rate, the tendency of the largest event to strike later in the sequence, the large spatial extent of some of the swarms compared to their cumulative moment, and the weak correlation between the number of events in each burst and the magnitude of the largest event in each burst. Shallow sequences and normal faulting mechanism sequences are most likely to be swarm-like. The tendencies of the hypocenters in the swarm-like sequences to occur on vertical planes and expand over time suggest pore fluid pressure fluctuations as the most likely mechanism driving the swarm-like seismicity bursts. However, episodic aseismic slip could also be at least partly responsible and might provide a more compelling explanation for the steady rate of seismicity during swarms, whereas fluid pressure perturbations might be expected to diminish more rapidly with time. Both aftershock-like and swarm-like seismicity bursts are distributed across the entire study region, indicating that they are a general feature of tectonic faulting, rather than limited to a few geological conditions such as volcanic or geothermal areas.