What questions can MOISE 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 5 off-shore stations. This would not only allow better characterization of the off-shore seismicity, but also improve the coverage of on-shore seismicity and increase our knowledge of three dimensional crustal structure.
Before establishing long term remote stations, we must gain experience in how to install broadband instruments so that they will achieve proper deployment without damaging the seismometers. Broadband seismic stations require very precise leveling and are very sensitive to rough treatment, unlike standard short-period OBSs that can just be thrown overboard and minimize the background noise due to imperfect coupling of the instrument with the ground. The question of borehole installation, as opposed to installation on the sea-floor with partial or complete burial in the sediments, is still a fundamental one for which previous experiments have provided only incomplete answers.
Other issues that need to be addressed include:
- How do we provide power for years of operation for these power consuming systems? (With incorporated 24 bit A/D converters and large capacity recording devices, these broadband systems typically consume several Watts) MOISE uses lithium batteries, which can provide sufficient power for an experiment duration of 3 months.
- How do we retrieve the data? In the MOISE experiment, data will be recorded on site and retrieved at the end of the experiment by an ROV.