Possible correlation between exogenous parameters and seismicity

Exogenous parameter is basically referred to the external activities that may have been the important factors in modulating the atmosphere, ionosphere and the earth's surface. Due to its significant impacts, there is possibility to link solar activities and seismicity. Associated investigations have been done by previous researchers in order to explore the solar - terrestrial connection; nevertheless, the physical mechanism is still controversial. To comprehend the investigation of this coupling mechanism, we propose another exogenous source to be analyzed which is cosmic ray. As solar activity, cosmic ray also has minimum and maximum phases or called as cosmic ray cycle, but it is anti-correlation between phases of sunspot and cosmic ray cycles. In this brief report, we examine the trend of shallow earthquake occurrence as the caused effect during recent 4 complete solar cycles (SC 20-23) in order to study its possible link to sun spot number (SSN). The earthquakes were categorized into very shallow earthquakes with epicenter depth less than 35 km and deeper earthquakes with epicenter depth between 35-70 km. For very shallow earthquakes, the analysis shows two interesting features. First, its occurrence rate shows a steady increase during the 40 years period of 1964-2005, with average increase rate about 150/year. Second, a distinct increase of the occurrence rate occurs during each solar minimum of SC 21-23. Neither of these features is found in the earthquakes with deeper epicenters, suggesting that the solar influence on seismicity, if exists, is likely to exist only in the case of very shallow earthquakes whose epicenter is in the crust region.