Potential Uses of OEF at PG&E

Norm AbrahamsonMar 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?