Seismic Retrofit Needs in Southern California Before the ShakeOut Earthquake, and the Cost of Dithering

Ishmael Legaspi, Sandi Shwe Win, LaMar Anderson, Lucile M. Jones, & Keith A. Porter

Submitted August 30, 2026, SCEC Contribution #15416, 2026 SCEC Annual Meeting Poster #168

Earthquake hazard maps show how strongly the ground will shake with a specified probability. The language of probability tends to hinder rather than to aid decision-making for most people. We used the original ShakeOut scenario shakemap to a different end by joining it with Hazus-based vulnerability functions and a Hazus-based inventory, but one based on the National Structure Inventory [https://www.hec.usace.army.mil/confluence/nsi/technicalreferences/latest/technical-documentation]. We asked what seismic retrofits make sense, how many buildings need it, what will it cost, and how much will it cost if we fail to act?

Our analysis depicts how much loss can be avoided if we retrofit all the most numerous and vulnerable buildings in southern California: older woodframe buildings with unbraced cripple walls; soft-story woodframe apartment buildings; older reinforced masonry and tiltup concrete buildings with weak roof-to-wall connections; and the kinds of nonductile concrete buildings that collapse in earthquake after earthquake: Venezuela, San Fernando, Turkiye, and many others.

Results show that retrofit investment can reduce substantial losses in Southern California, but cost-effectiveness varies strongly by remediation type and location. Under the ShakeOut scenario, the evaluated retrofits avoid an estimated $18.6 billion in property loss, business interruption, living expenses, etc. but require approximately $29.6 billion in total remediation cost when applied across all categories. This indicates that uniform retrofit of all eligible buildings is not the most cost-effective strategy. Instead, manufactured-home tie-downs, unreinforced masonry retrofits, cripple-wall bolt-and-brace retrofits, and soft-story retrofits produce the strongest economic returns, with benefit-cost ratios greater than one. These results support targeted seismic retrofit investment as a practical strategy for reducing earthquake losses before the next major Southern California earthquake.

Citation
Legaspi, I., Win, S., Anderson, L., Jones, L. M., & Porter, K. A. (2026, 08). Seismic Retrofit Needs in Southern California Before the ShakeOut Earthquake, and the Cost of Dithering. Poster Presentation at 2026 SCEC Annual Meeting.

Related Projects & Working Groups
Applied Science Implementation (ASI)