On the relation between solar activity and seismicity

Much attention is recently paid to the role of extraterrestrial factors in terrestrial seismicity, and to the possibility to assess the seismic risk. Seven centuries of records of ancient earthquakes in the Mediterranean region show that the century-scale variations in the number of strong earthquakes closely follow the secular cycle of solar activity. Two well expressed maxima in the global yearly number of earthquakes are seen in the 11-year sunspot cycle - one coinciding with sunspot maximum, and the other on the descending phase of solar activity. A day to day study of the number of earthquakes worldwide reveals that the arrival to the Earth of high speed solar streams is related to significantly greater probability of earthquake occurrence. The possible mechanism includes deposition of solar wind energy into the polar ionosphere where it drives ionospheric convection and auroral electrojets, generating in turn atmospheric gravity waves that interact with neutral winds and deposit their momentum in the neutral atmosphere, increasing the transfer of air masses and disturbing of the pressure balance on tectonic plates. The main sources of high speed solar streams are the solar coronal mass ejections (CMEs) which have a maximum in the sunspot maximum, and the coronal holes with a maximum on the descending phase of solar activity. Both coronal holes and CMEs are monitored by satellite-borne and ground-based instruments, which makes it possible to predict periods of enhanced seismic risk. The geoeffectiveness of solar wind from a coronal hole only depends on the position of the hole relative to the Earth, and for the CMEs an additional factor is their speed. It has been recently found that a useful tool in identifying the population of geoeffective CMEs is the detection of long-wavelength (decameter-hectometer) type II solar radio bursts, as the CMEs associated with them are much faster and wider than average.