Aftershock Matters

Nicole S. Gage, David J. Wald, & Kristin D. Marano

Submitted August 10, 2018, SCEC Contribution #8385, 2018 SCEC Annual Meeting Poster #048

Aftershocks can be inconsequential, or they can cause more extensive damage and more human casualties than their associated mainshocks. Most studies of aftershocks focus on characterizing statistical properties such as their likelihood, frequency, size, and spatial distribution. However, an understanding of the impacts of aftershocks is important for operational aftershock forecasting, response protocols, and loss modeling efforts. The definition of “aftershock” has always been and remains open for interpretation. We acknowledge the ambiguous nature of aftershocks and as such, we consider only a practical definition. The events immediately following an earthquake of societal significance are of importance to scientists, responders, infrastructure managers and financers, among others. The goal of this study is to improve loss calculations for future earthquake sequences. In addition to the emotional toll often experienced by inhabitants, a typical result of an earthquake mainshock includes the reduction of human exposure to hazards during ensuing aftershocks (evacuation, sheltering, casualties), and changes to both the value and vulnerability of the building stock. To this end, we study the scope of effects from aftershocks by creating an inventory of losses for 20 California mainshock/aftershock sequences. Overall, few significant historic aftershock sequences caused extensive damage or fatalities. Though many of the 20 earthquake sequences studied generated aftershocks that caused additional damage, only the aftershocks of the 1952 Kern County and the 1987 Whittier Narrows earthquakes produced additional documented fatalities. Damage and possible fatalities resulting from aftershocks of the 1933 Long Beach and the 1906 Great San Francisco earthquakes are difficult to discern, though some commentary implies that they were important. Contemporary accounts of two of the mainshock/aftershock sequences studied (1940 Imperial Valley and 1992 Cape Mendocino) suggest that aftershocks may have produced greater damage than the mainshocks. We envision that analysis of the California aftershock data collected will support the development of strategies that can be applied operationally during an aftershock sequence to more accurately project losses. We expect that any such strategies will be regionally dependent, reflecting variable aftershock statistics and building vulnerabilities.

Key Words
Aftershocks, Hazards, California

Gage, N. S., Wald, D. J., & Marano, K. D. (2018, 08). Aftershock Matters. Poster Presentation at 2018 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Forecasting and Predictability (EFP)