THE JUNE-JULY-1993 SEISMO-ACOUSTIC EVENT AT COAXIAL SEGMENT, JUAN-DE-FUCA RIDGE - EVIDENCE FOR A LATERAL DIKE INJECTION

The CoAxial segment (Juan de Fuca Ridge) was the site of an intense sw arm of earthquakes that began at 21:43 GMT on June 26, 1993 (Julian Da y 177). The swarm started near 46 degrees 15'N and migrated northward over the next 40 hours to similar to 46 degrees 36'N, where the majori ty of 676 events occurred during the following 3 weeks of activity. Th e earthquakes propagated NNE at a velocity of 0.3 +/- 0.1 m s(-1) from the southern to northern swarm sites. The activity went undetected by land-based seismic networks along the Oregon and Washington coasts, s uggesting that the earthquakes were all M less than or equal to 4.0. T he character of this earthquake swarm is very similar to dike injectio ns observed at Krafla and Kilauea Volcanoes. The earthquake activity a nd subsequent migration probably represent a lateral dike injection in to faults and fissures comprising the CoAxial segment. The reservoir a cting as the dike's source likely resides beneath, or to the south of, the initial southern swarm of earthquakes. The large magma supply at Axial Volcano cannot be ruled out as the source for the CoAxial dike, even though Axial Volcano exhibited no earthquake activity related to the CoAxial swarm. The T-wave earthquake swarm reported here is the fi rst deep-ocean observation of volcanic seismicity associated with what most likely was a mid-ocean ridge dike injection event.