Colocation of Magnetic and Seismic Observatories:
There are two important areas in which colocation of these two types of observatories would be beneficial: mantle imaging and natural hazards. In practice, only very few such joint observatories exist. The two colocated magnetic/seismic observatories of the Berkeley Digital Seismic Network at SAO and PKD are among the only such stations in the world.
Mantle imaging:
One of the goals of geomagnetic studies is the imaging of the electrical conductivity in the mantle. So far, the data comes from poorly distributed geomagnetic observatories, which omit the 70% of the Earth's surface covered by oceans.
For historical reasons, very few sites have in place both a long period or broad band seismometer and and a long period magnetometer (when ideally all stations should be equipped with both). As a result, the comparison of the mantle models determined from magnetic data to seismic tomography models is very difficult because of scale differences. Geomagnetic sounding averages locally the deep structure over about 1000 km and the tomographic images that average globally with a similar characteristic dimension. Hence, if there is no colocation, the comparison is almost impossible; this is regretable because both electromagnetic parameters and seismic parameters provide complementary information about the deep structure (thermodynamics, composition) of the Earth.
Natural hazards:
There have been a variety of studies using electric and magnetic (EM) data that showed that in active tectonic zones (volcanic and seismic) there are EM signals associated with volcanic eruptions and seismic activity. It seems that the signal is carried by motion of water in the porous basement; these signals are very difficult to detect or recognize inland because of the complexity of the hydrous systems. Furthermore, there must be enough water to drive enough electric currents to generate the coseismic (and possibly, even a pre-seismic) signal.
At sea, we are in a very good conductor and the porous medium is completely saturated. Thus, relative motion between the rock matrix (which is electrically resistive) and the water may generate a significant signal.
Generally inland, we have several stations so we may compare the records, clean the natural EM signal of exospheric origin and the anthropomorphic noise, to emphasize seismogenic-EM signals. At sea, it is not yet possible to deploy so many instruments. Hence we emphasize another approach which is to try to correlate the LF seismic signal to the magnetic and electric signal.
Seafloor Seismic Observatories
Magnetic Observatory Requirements
Seismicity in the Monterey Region