Contribution of aftershocks to seismic hazard assessment

Contribution of aftershocks to seismic hazard assessment

Probabilistic seismic hazard assessment (PSHA) aims at providing probabilities of occurrence of ground-motions at the locations under study. These probabilities of occurrence are needed to define ground-motion of reference for seismic building design. PSHA aims at combining the knowledge on earthquake generation (seismology, tectonics, geodesy) with the knowledge on ground-motions produced by earthquakes, to build hazard models for the regions under study.
We have a long experience in seismic hazard assessment in Ecuador, a country exposed to seismic risk. We have built hazard models based on information from earthquake catalogs and active fault tectonics (e.g. Beauval et al. 2018). The models built up to now are all long-term models assuming a Poisson occurrence for earthquakes. Aftershocks are identified and retrieved from earthquake catalogs before earthquake recurrence is modeled. The contribution of aftershocks is ignored.
The aim of the present internship is to evaluate the contribution of aftershocks to the hazard assessment by comparing Aftershock PSHA (Wiemer 2000, Perruzza et al. 2016) with respect to the usual long-term PSHA. The main aftershock sequences in the instrumental earthquake catalog will be identified (e.g. from the 1979 MW 8.1 subduction event), and for each sequence the increase in the hazard level with respect to the long-term hazard will be evaluated. An APSHA model will be established for every significant aftershock sequence in the earthquake catalog, going back to the beginning of the XIXth century (Beauval et al. 2013).
This study will enable to understand if the long-term hazard levels, used in establishing country building codes, underestimate the hazard shortly after the occurrence of a significant event, taking advantage of existing hazard models and earthquake catalogs in Ecuador. This work will contribute to the current discussion on how aftershocks should be best integrated into hazard assessment (Marzocchi and Taroni 2014, Llenos and Michael 2020).

References

• Beauval C., J. Marinière, H. Yepes, L. Audin, JM. Nocquet, A. Alvarado, S. Baize, J. Aguilar, JC Singaucho, and H. Jomard (2018). A New Seismic Hazard Model for Ecuador, BSSA, 108 (3A) : 1443-1464.
• Beauval C., H. Yepes, P. Palacios, M. Segovia, A. Alvarado, Y. Font, J. Aguilar, L. Troncoso, and S. Vaca (2013). An earthquake catalog for seismic hazard assessment in Ecuador, Bull. Seism. Soc. Am., v. 103, p. 773-786, doi:10.1785/0120120270.
• Llenos A.L. and A.J. Michael (2020). Regionally Optimized Background Earthquake Rates from ETAS (ROBERE) for Probabilistic Seismic Hazard Assessment, Bulletin of the Seismological Society of America, Volume 110 Number 3, 1172-1190.
• Marzocchi W. and M. Taroni (2014). Some thoughts on declustering in probabilistic Seismic-Hazard Analysis, Bulletin of the Seismological Society of America, Vol. 104, No. 4, pp. 1838–1845.
• Peruzza L., Gee R., Pace B., Roberts G., Scotti O., Visini F., Benedetti L., and Pagani M. (2016). PSHA after a strong earthquake : hints for the recovery, Annals of Geophysics, 59, 1–9.
• Wiemer S. (2000). Introducing probabilistic aftershock hazard mapping, Geophysical Research Letter, 27, 3405-3408.