Estimating Aftershock Risk for Entry into Earthquake-Damaged Buildings

Jeanne L. Hardebeck, & Alan Poulos

Submitted August 30, 2026, SCEC Contribution #15143, 2026 SCEC Annual Meeting Poster #TBD

Following a major earthquake, entry into damaged buildings may be necessary for response, recovery, and rebuilding activities. However, aftershocks may further damage or collapse these already-damaged structures, posing a risk to the people entering these buildings. Aftershock forecasts can support decision-making around whether and when people can enter damaged buildings or the surrounding areas. Ideally, decision-makers would have access to sequence-specific aftershock hazard models that could be used to quantitatively determine the likelihood of strong ground shaking at a given location. Unfortunately, this type of information is rarely available in real time. Therefore, we present a simple method to estimate the risk of experiencing strong shaking, defined as MMI 7 or greater, from aftershocks during entry into earthquake-damaged buildings. Users of these estimates would need to access only publicly available information such as the magnitude of the mainshock. To calculate the probability that a damaged building location will experience strong shaking from an aftershock, we first compute the expected rate of aftershocks of various magnitudes using the same models as the USGS aftershock forecasts. We next find the expected size of the MMI 7+ shaking area due to an earthquake of a given magnitude, using the Next Generation Attenuation (NGA) ground motion models (GMMs). Then, combining the two, we compute the probability that a location within the MMI 7+ area of the mainshock will also fall within the MMI 7+ area of at least one aftershock. Using this method, we can compute the wait time, as a function of mainshock magnitude and building entry duration, until the probability of MMI 7 or greater shaking from aftershocks reaches a predefined risk threshold. We also develop a relation between aftershock probability and the probability of strong shaking. This allows decision-makers to use the real-time public USGS aftershock forecasts during an ongoing aftershock sequence to determine whether their risk threshold has been met, significantly reducing uncertainty compared to estimates based on mainshock magnitude alone. We demonstrate our method for generic active continental regions, stable continental regions, and subduction zones. Our methodology can similarly be applied to any region where aftershock forecast models and GMMs are available.

Key Words
aftershock, forecast, ground motion, building damage

Citation
Hardebeck, J. L., & Poulos, A. (2026, 08). Estimating Aftershock Risk for Entry into Earthquake-Damaged Buildings. Poster Presentation at 2026 SCEC Annual Meeting.

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