**5. Earthquake potential before target earthquakes**

In the previous study as Rundle et al. [24], a circle area around the city was usually used when we want to assess the potential to occur next strong earthquake, and the seismicity in a relatively large region will be selected to build the background database to describe the statistical characteristic. Alternately, here we choose a circle area around the epicenter of four earthquakes, that is, August 3, 2014, Ludian *M*S6.5

earthquake; October 7, 2014, Jinggu *M*S6.6 earthquake; May 21, 2021, Yangbi *M*S6.4 earthquake; and September 5, 2022, Luding *M*S6.8 earthquake, to assess the potential before the occurrence of these four events, and used the seismicity of CSES as the large region to build the background database. We investigate circular regions with radii of 100, 150, and 200 km to test the analysis's resilience to parameter fluctuation. **Figure 5** shows the EPS result (case of radii 200 km) calculated using the nowcasting technique for these four events. For instance, **Figure 5a** shows that since the previous significant event with a magnitude over 6.8 on 2013/4/20, there have been 113 small events within 200 km of the epicenter of the Luding *M*S6.8 earthquake, and the EPS value has reached 81%. The EPS value for the remaining three earthquakes approaches 98, 87, and 88%, respectively, according to **Figure 5b**–**d**. **Table 1** lists the results obtained using various radii. We can observe that the EPS was high but did not reach 100% prior to the occurrence of these four events.

As a result of the big events' ability to segregate smaller events or samples, the amount of larger events will have an impact on how smoothly the EPS curve behaves in each plot. Even yet, the outcome of EPS results before these earthquakes might still

#### **Figure 5.**

*The nowcasting approach was used to compute the Earthquake Potential Score (EPS) up to the occurrence of certain target events in the CSES zone. (a) Assessment for* M ≥ *6.8 before the occurrence of Luding* M*S6.8 on September 20, 2022; (b) assessment for* M ≥ *6.4 before the occurrence of Yangbi* M*S6.4 on May 21, 2021; (c) assessment for* M ≥ *6.6 before the occurrence of Jinggu* M*S6.6 on October 7, 2014; and (d) assessment for* M ≥ *6.5 before the occurrence of Ludian* M*S6.5 on August 3, 2014. The green bars in each plot show the number of small events (*M*<sup>σ</sup>* ≥ *4.0) in the interval of strong earthquakes. The red curve shows the CDF of earthquake intervals varied with the number of small events. The red points on the curve show the current EPS value since the occurrence of the last strong earthquake and the EPS state is shown on the right with a "hot bar." The dashed vertical line in blue indicates the mean count of small events in the intervals according to the statistical analysis for the database.*

*The "Natural Time" Method Used for the Potential Assessment for Strong… DOI: http://dx.doi.org/10.5772/intechopen.110023*


**Table 1.**

*EPS before different target events with different circular regions with radii* R*.*

indicate a rather high likelihood of subsequent large earthquakes. How to comprehend the high EPS with the possibility of the next large earthquake is one issue that has persisted from earlier investigations. Because the EPS is high in this research but does not reach 100%, it is not required to wait for the EPS to reach 100% before the next significant earthquake occurs. When interpreting the EPS using the nowcasting technique, it is useful to keep in mind that the area under investigation is nearing another large earthquake when the EPS is rising high.
