**7. Conclusion**

Our research is an early approach to using Twitter as a social sensor for earthquake observations. It is meaningful that we apply methods by ordinary physical sensors to earthquake detection by social sensors. Furthermore, we present the possibility of earthquake detection without installing numerous physical sensors. The method is effective for earthquake observations in some countries where a few seismic sensors exist. However, it is difficult to detect earthquakes occurring in oceanic areas or less populated areas using methods we introduced in this chapter. Therefore, we must verify that earthquake detection by social sensors is effective when we apply these methods. Furthermore, the applicable scope of the earthquake observation by social sensors can be extended considering a stochastic gradient, more detailed probabilistic models, and so on. Many subjects remain to be explored in future work.
