DIKE-INDUCED EARTHQUAKES - THEORETICAL CONSIDERATIONS

Earthquakes of magnitude 1 and greater seem to be ubiquitous features of dike propagation, but their origin is not well understood. We exami ne the elastic stress field surrounding propagating fluid-filled crack s, with an emphasis on assessing the ambient stress required to produc e earthquakes with linear dimensions of similar to 100 m near dikes wi th linear dimensions of a few kilometers. An important feature of the solutions is the dike ''tip cavity,'' a low-pressure region where magm a cannot penetrate and where the: stress field differs most from the c lassical near-tip stress field. Two regions are considered: near the d ike tip but away from the tip cavity and near the tip cavity. The stre ss state most conducive to failure occurs near the tip cavity when the cavity pressure is maintained by influx of host rock pore fluids rath er than by exsolution of magmatic volatiles. Even in this case, howeve r, shear fracture of previously intact rock seems unlikely. Thus most dike-induced seismicity with a frequency content typical of ''tectonic '' earthquakes should be interpreted as resulting from slip along suit ably aligned existing fractures. Production of magnitude 1 earthquakes appears to require either large ambient differential stresses or low ambient confining pressures; in the latter case, the effective normal stress on prospective faults may be low enough far slip to be aseismic . We conclude that the distribution of (recorded) dike-induced seismic ity reflects the distribution of ambient stresses that. are near to fa ilure and does not necessarily reflect the extent of the dike. This re sult is consistent with recent images of the seismicity associated wit h the 1983 dike intrusion at Kilauea.