Seismic Study of an Oceanic Ridge Earthquake Swarm in the Gulf of California

Detailed seismic investigation of an unusually intense earthquake swarm which occurred in the northern Gulf of California during March 1969 has provided new information about seismic processes which occur on actively spreading oceanic ridges and has placed some constraints on the elastic wave velocities beneath them. Activity during this swarm was similar to that of a foreshock-mainshock-aftershock sequence, but with a ‘ mainshock ’ composed of over 70 events with magnitudes between 4 and 5.5 occurring in a 6-hr period about a day after swarm activity was initiated. ‘ Aftershocks ’, including many events greater than magnitude 5 , continued for over two weeks. Near-source travel-time data indicate all sources located are within 5-10 km of each other and that hypocentrA are confined to the upper crust. Teleseismic P-delays for rays travelling beneath this ridge may be interpreted in terms of an upper mantle with compressional velocities 5-10 per cent less than normal mantle to a depth of 200 km. Average apparent stresses for all swarm events studied are very similar, show no consistent pattern as a function of time, and are close to values obtained from other ridges. The focal mechanism solution shows a large component of normal faulting. An apparent non-orthogonality of nodal planes common to this mechanism solution and to normal faulting events on other ridges disappears when the indicated low upper mantle velocities beneath the source are taken into account. A survey of recent seismicity (post 1962) in the northern Gulf suggests seismic coupling across about 200 km between adjacent inferred spreading ridge segments. Surface waves from these Gulf Swarm earthquakes have amplitudes from one to two orders of magnitude greater than Northern Baja California events with similar short period body wave excitation.