Potential Uses of OEF at PG&E Norm Abrahamson Mar 16, 2015

OEF for Utilities and Critical Facilities No regulatory criteria or guidance for use of OEF for critical facilities – Utilities moving to risk-informed decision making Need estimate of the risk, but huge uncertainties – PG&E (large portfolio of infrastructure) Not using cost-benefit to justify seismic risk mitigation Instead, have goal of long-term risk reduction over decades

Potential Uses of OEF at PG&E Emergency response following an earthquake Scheduling of high-risk maintenance activities Impacts of long-term changes in hazard that may cause an immediate safety issue

Emergency Response Aftershock probabilities – PG&E’s Emergency Operation Center is looking for additional information on the what to expect in terms of aftershocks beyond the generic USGS statements – Probabilities of M>5 by day for first few weeks – How long to reduce rates of aftershocks by 90% from time of mainshock? Repairs and inspections – pipelines – Main issue is additional ground deformation along pipelines Afterslip for fault crossings Continuing landslides due to aftershocks – How long to get 90% of the final deformation?

Emergency Response Building Tagging (for PG&E facilities) – Using precomputed aftershock risk from generic aftershock models for different damage Want to avoid over-conservative tagging – Have not planned on using OEF to improve the aftershock risk estimates

Emergency Response What to do with a large probability gain? Lake Almanor Dam example – May 24, 2013, M5.7 earthquake under the dam (within a few km) – Hazard increase by factor of ~1000 during the Memorial Day weekend, but still a small risk of failure of the dam – What should a responsible utility do?

Scheduling of High Risk Maintenance Activities If there is short-term increase in hazard, consider postponing high risk maintenance – This was done following the 2003 San Simeon Earthquake at Diablo Canyon Postponed some high-risk maintenance for one week Not based on a formal evaluation A few weeks delay is practical, not months

Impacts of long-term changes in hazard Does the change lead to an immediate safety issue for existing critical facilities? – Change in the 50 yr hazard compared to national hazard map – Significant increase for critical facilities > 20% in GM > 100% in probability for mean hazard at the GM corresponding to the 1E-4 reference hazard level What is the 5-95% uncertainty range on the increase?