Analytical number theory for two integers chosen at random provides a numerical method to estimate π value. An analysis of three datasets of earthquakes including earthquakes occurring in mainland China, South California, and all over the world, thus allowed us to estimate π with less than 3.0% relative deviation. The complexity of earthquakes is expressed by implicit randomness and characteristics of seismicity. If other physical precursors were not taken into account, two or more earthquakes would be correlative with one another, or an earthquake could be partly predictable on the basis of the event sequence itself, with a ratio of about 40% at most.