TUNE, a proposed fiber-optic observatory offshore the Pacific Northwest,1 provides an ideal opportunity to establish a dense network of submarine seismic and geodetic stations in a region of high seismic risk. To be most effective, NEPTUNE data should be integrated with observations from the proposed Plate Boundary Observatory.2 Deployment of state-of-the-art broadband stations on the ocean floor would also provide better coverage and resolution of seismic sources worldwide, as well as more complete tomographic coverage of the Earth’s interior structure, which is a high priority of the International Ocean Network program. Much like the Program for the Array Seismic Studies of the Continental Lithosphere (PASSCAL) deployments on the continents, the semipermanent or temporary deployment of dense arrays of broadband ocean bottom seismometers can complement the sparse permanent stations for the investigation of regional seismicity on oceanic plate boundaries and in intraplate settings. Plans for the development of a PASSCAL-type facility for broadband deployments on the ocean floor are under way.
Seafloor geodesy is an especially challenging but potentially rewarding area for new research. Absolute gravimeters promise precise vertical positioning on the sea-floor.3 Horizontal positioning relies on acoustic ranging to surface floats precisely positioned by GPS.4 The acoustic link is the largest source of error, due to uncertainties in sound velocity and currents, but despite its lower overall accuracy, such data would still be quite useful for submarine thrust faults with high rates of convergence.
By combining studies of the geological record with seismic and geodetic monitoring, it is possible to forecast which fault systems will produce large earthquakes over long periods of time (decades to centuries). This type of long-range forecasting is essential for seismic hazard analysis, and further work on the problem should receive a high priority. Such research should address the dynamics of rupture propagation over multiple fault segments, the irregularity of the earthquake cycle, and the tendency of earthquakes to cluster in space and time. The diversity of earth-