THE RELATION BETWEEN EARTHQUAKES, FAULTING, AND SUBMARINE HYDROTHERMAL MINERALIZATION

Although the relationship between submarine hydrothermal activity and earthquakes was recognized over 20 years ago, it has still not been pr ecisely defined. Faulting and permeability control fluid flow in the o ceanic crust and therefore submarine hydrothermal activity at mid-ocea n ridges Microearthquakes associated with submarine hydrothermal activ ity tend to be small in magnitude and occur in swarms Swarms of microe arthquakes associated with hydrothermal vents are thought to be relate d to cracking of the oceanic crust. The depths of the microearthquakes may mark the brittle-ductile transition which denotes the downward li mit of seawater circulation into the oceanic crust. The maximum depth of microearthquakes on the Mid-Atlantic Ridge is 10 km, compared with 3 km on the East Pacific Rise. The brittle layer tends to be thinnest where mid-ocean spreading rates are fastest. Seismic pumping is a mech anism for the transport of hydrothermal fluids within fractures and ma y account for stockwork mineralization within the ocean crust. Submari ne hydrothermal mineralization is episodic on time scales ranging from 1 to 10(5) years and is controlled by volcanic and tectonic processes in a manner not yet completely understood. Emphasis in the study of s ubmarine hydrothermal activity is changing from observation to long-te rm monitoring. The U.S. Navy's Sound Surveillance System (SOSUS) has b een very successful in detecting megaplume events caused by dike injec tion on the Juan de Fuca and Gorda Ridges. The long-term deployment of large arrays of ocean bottom seismometers on the ocean floor will bec ome routine in the future. Such a program will contribute to an unders tanding of the factors controlling the episodicity of submarine hydrot hermal activity and mineralization on the sea floor On the Izu-Bonin A re, hydrothermal mineralization has been observed in a number of volca nic calderas. This region appears to have many advantages for the futu re monitoring and evaluation of microseismicity in hydrothermally acti ve areas.