Seismic Observatories:
In the past 15 years, a new global, state-of-the-art, digital broadband seismic network has been deployed through the combined efforts of several contries including the Global Seismic Network of the Incorporated Research Institutions for Seismology (IRIS) Consortium in the U.S. and the Geoscope program in France. There are currently over 150 such stations around the world.
Large gaps exist in the global distribution of seismic stations which cannot be filled by installation of instruments on islands; these gaps occur in the Eastern Pacific and the southern oceans. A few of the consequences of this uneven distribution are:
It limits the resolution of three-dimensional tomographic studies of the structure and anisotropy of the Earth's mantle.
It limits the coverage of the core-mantle boundary and lowermost mantle, where vigorous thermal and chemical processes related to mantle convection and interaction between the core and mantle are known to occur. Strong lateral heterogeneity and an anisotropy of structure has recently been documented in this portion of the Earth, but only with poor lateral resolution.
It limits the resolution of inner core anisotropy. The simple model of axisymmetric constant transverse anisotropy, favored since the mid-1980's, is now presumed to be much too simple.
It limits the azimuthal coverage of seismic sources, especially in the southern hemisphere.
and
In some areas, it increases the minimum amount of energy required to detect earthquakes and explosions.
Magnetic Observatory Requirements
Colocation of Magnetic and Seismic Stations
Seismicity in the Monterey Region