Earthquake Nowcasting: Retrospective Testing in Sarpol-e Zahab, Iran

Sarpol-e Zahab, in the west of Iran, is subjected to a high earthquake risk. Located in the north part of the Zagros seismic belt and is surrounded by several active faults that show some M7+ earthquake records.

Nowcasting refers to the process of determining the uncertain state of the seismicity at the present time by indirect means for the seismically active regions, where the goal is to estimate the current state of risk, the current state of the fault system, and its current level of progress through the earthquake cycle. The local catalog of earthquakes is used, using “small” earthquakes to determine the level of hazard from “large” earthquakes in the region. To evaluate the statistical distribution of the inter-event counts of small events that occur between large events, the natural time concept was used rather than clock time (Origin Time of Earthquake). This method does not involve any model other than the idea of an earthquake cycle. Rather, a specific radius and a specific large earthquake magnitude of interest are defined, ensuring that we have enough data to span at least ~10 or larger earthquake cycles in the region. We then compute the earthquake potential score (EPS) which is defined as the cumulative probability distribution P (n<n(t)) for the current count n(t) for the small earthquakes in the region. The EPS was calculated as the total number of small earthquakes after the last large occurrence in the studied area. EPS is therefore the current level of hazard and assigns a number between 0% and 100% to every region so defined, thus providing a unique measure. Physically, the EPS corresponds to an estimate of the level of progress through the earthquake cycle in the defined region at the current time.

We have determined the EPS values to consider events of Mw≥4.4 within two different radiuses (250 and 350 km) around Sarpol-e Zahab. The EPS values for Sarpol-e Zahab at 250 km corresponding to Mw≥5.5 and six events were found to be almost 0.86 and 0.97, respectively, while at 350 km these values are equal to 0.73 and 0.50 for Mw≥5.5 and six events, respectively.