Due to the limited distribution of continents and ocean islands around the world, the coverage in land-based seismic and other geophysical observatories has many gaps. Because of these gaps, many important scientific issues related to plate tectonics and the deep structure and dynamics of the earth cannot be answered.
Designed in the spirit advocated by the International Ocean Network (ION) program and the U.S. Ocean Seismic Network (OSN) program, MOBB will help fill the regional gap on the ocean side of the Pacific/North American plate boundary.
What questions does MOBB answer?
There are two basic types of scientific problems that require long-term seismic observatories on the ocean floor:
- Global problems that require as broad a distribution of stations as possible, including coverage in the oceans, and
- Regional questions concerning the seismicity, structure, and tectonics of specific areas of the world that are either completely or partially covered by oceans.
For global structural studies, stations must be deployed for at least three years to ensure that enough data is collected from globally-distributed earthquakes. For monitoring future seismic events, permanent stations are needed. These long-term stations should have as broadband a response as possible and they must be located in low-noise areas. They should be located in major gaps in land and island coverage, separated from existing or potential land-based stations by a distance of about 20 degrees.
There are many major regional-scale scientific problems related to deep structure of ridges, subduction zones, ocean basins and hotspots that require the deployment of arrays of broadband ocean bottom seismometers for up to one year. For example, existing broadband stations in northern California necessarily bias coverage towards the eastern side of the North-America/Pacific plate boundary. As a result, the seismic activity of this plate boundary's off-shore fault system is very poorly documented. Due to the uneven distribution of observations, location, mechanism and size of moderate to large events is biased and events smaller than 4 Mb are very imprecisely described, if at all.
The geometry of this active plate boundary thus calls for an expansion of the existing broadband monitoring network to include at least 10 offshore stations. This would not only allow better characterization of the offshore seismicity, but also improve the coverage of onshore seismicity and increase our knowledge of three-dimensional crustal structure.
MOBB is the first such station, and follows on the prototype MOISE experiment conducted in 1997.