Aftershock Modeling in Earthquake Loss Assessment
Summary
Experience indicates that aftershocks often exacerbate financial losses caused by a strong main earthquake. In this blog we explore how significant this increase is, how our team is developing an aftershock model, specifically focusing on its hazard aspect, and how they are integrating it with our modeling platform CatMod.
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Earthquake loss modeling estimates direct losses such as those resulting from structural damage in built environment. It can also focus on estimating indirect losses resulting from business interruption or policy coverage loss. Several factors influence the extent of earthquake loss in a region, and the below illustration lists a few pillars among the important ones. In this blog, we will focus on aftershock modeling.
Importance of Aftershock Modeling
Aftershocks can amplify the damage to already deteriorated structures by the mainshock and can lead to escalated repair and reconstruction expenses. These secondary seismic events can extend the duration of recovery operations and can negatively affect business interruption. Additionally, aftershocks may trigger landslides or other secondary hazards, intensifying the overall impact of earthquake hazard. Consideration of aftershock effects in earthquake loss models will result in more realistic estimation of direct and indirect seismic losses. Besides, the decision maker can envisage the preparedness and response strategies in a more realistic framework to mitigating the adverse effects of future seismic events.
Aftershock Effects: Real-Life Examples
Tangshan Earthquake
Following the 1976 moment magnitude (Mw) 7.8 Tangshan Earthquake in China, the aftershocks, including magnitudes of Mw 7.1 and M 6.9, led to widespread structural failures, collapsing most of the residential buildings in the earthquake affected zone and damaging railways on local bridges.
Hokkaidō Earthquake
Similarly, a Mw 5.0 aftershock after the 2003 Mw 8.0 Hokkaidō Earthquake in Japan ignited a secondary disaster by causing a tank spill that led to a fire.
Canterbury Earthquake
Furthermore, the 2010 Mw 7.1 Canterbury earthquake in New Zealand was followed by a Mw 6.3 aftershock that not only caused significant architectural damage but also resulted in 146 fatalities (Wu et al., 2024).
Kahramanmaraş and Van Earthquake
Major earthquakes in Türkiye have also shown us the negative effects of aftershocks in increasing the casualties and injuries as well as amplifying the property damage. The aftershocks of Mw 6.7 at Nurdağı (Gaziantep) and Mw 6.6 at Defne (Hatay) inflicted a considerable damage and increased the death toll in the February 6, 2023, Kahramanmaraş earthquakes.
The Mw 5.6 aftershock of the 2011 Van Earthquake (Mw 7.1) occurring 2.5 weeks after the main event also led to fatalities and property damage in the earthquake struck area.
CatMod's Approach to Aftershock Modeling
CatMod modeling team is developing the aftershock model of large earthquakes and will integrate it into CatMod’s loss estimations to provide a more comprehensive understanding of earthquake triggered losses. Accurate assessment of earthquake losses, including the aftershock effects, is decisive for pricing, premium rates and fund reserving against claims in insurance. By utilizing well-established hazard models, CatMod’s ability to forecast the annual frequency and geospatial location of aftershocks and their impact on insurance loss will convey enhanced financial and risk mitigation strategies.
The following paragraphs first explain the fundamental theory of aftershock modeling. They are followed by a specific case study to illustrate CatMod’s aftershock modeling capability.
Studies by CatMod Modeling Team for Aftershock Modeling
In recent years, Türkiye has experienced a considerable number of large-magnitude earthquakes, providing good data for aftershock modeling. We investigated the major seismic activities with moment magnitudes greater than 6.0 from 2004 to 2022 for a better understanding of aftershock patterns.
The mainshock and their aftershock data have been compiled from two seismic monitoring agencies: Kandilli Observatory and Earthquake Research Institute (KOERI) and the Disaster and Emergency Management Authority (AFAD). The relevant data on the earthquakes studied are provided in Table 1.
Table 1. Summary of earthquake events studied.
CatMod’s modeling team developed the Omori-Utsu model (1894, 1961) by studying the above listed aftershocks. As an example, Figure 1 shows the first 100-day aftershock sequence of the Mw 6.9 Seferihisar-Samos Earthquake (30 October 2020) and its Omori- Utsu model by CatMod.
Table 2 provides the comparison of CatMod’s aftershock model estimations with the actual observations for the same event.
- The times T1 and T2 in Table 2 represent the time interval in days considered for estimating the number of aftershocks from Omori-Utsu model.
- When T1 is set as zero the time interval is meant to start right after the mainshock.
We have considered a magnitude threshold of Mw 4 in estimating the number of aftershocks. We also present the occurrence probability (Q) of at least one aftershock of Mw 4 in the same table.
Table 2. Comparison of number of estimated and observed aftershocks exceeding a specified magnitude threshold for a specific time interval T1 and T2. The table also presents the occurrence probabilities (Q) of at least one aftershock for the specified magnitude threshold between the specified time intervals. (Data from the 30 October 2020 Seferihisar-Samos Earthquake).
The presented case study indicates that CatMod’s aftershock modeling performs well in estimating the number of aftershocks given that the magnitude threshold is Mw 4. The number of estimations shift to conservative side as the length of time interval widens and shifts to the end of the observation period, which is taken as 100 days in this case.
Conclusion
Aftershock modeling is crucial for realistic earthquake loss estimation. CatMod's model goes beyond traditional assessments by incorporating aftershock scenarios, thereby enhancing financial and risk mitigation strategies. This advanced approach provides (re)insurers with accurate and comprehensive risk evaluations, enabling better preparedness and response.
With CatMod, (re)insurers can offer more precise coverage, optimize their portfolios, and make informed decisions that protect their clients and assets. Contact us to learn more about CatMod's capabilities and how it can transform your risk management strategies. Experience the future of earthquake risk assessment with CatMod.
