# Presentation: 907892

## USSD/FERC WorkshopMarch 8, 2007 Philadelphia Evaluation of Earthquake Ground Motions

**User Issues**
**Robin Charlwood, Ph.D., P.E.**
**Robin Charlwood & Associates, PLLC**
**Seattle, WA**

## User Issues – 10 Items for Discussion

- Canvassed a number of Owners and Consultants, mainly in the PNW
- Identified a number of issues of concern
- Not wanting to get in between ongoing specific discussions between Licensees and FERC
- Approach is to raise a number of questions for discussion

- Evaluating EQ Ground Motions             User Issues - Robin Charwlood

- USSD/FERC Workshop     March 8 2007

## User Issues – 10 Items for Discussion

- This is not a new question!
- What fundamental criteria should EQ safety of dams be based on?
- How to apply a deterministic approach in moderate to low seismic activity regions like the Columbia Basin?
- Will FERC accept a probabilistic approach?
- Is so, then what probability level would be reasonable for the MDE or MCE?

- Evaluating EQ Ground Motions             User Issues - Robin Charwlood

- USSD/FERC Workshop     March 8 2007

## User Issues – 10 Items for Discussion

- How best to deal with a Local or Random event which can control in some cases?
- When are local seismological investigations warranted?
- What are the required qualifications for the person doing the seismic hazard analysis?
- Status of the new Chapter 13?
- Changes in MCE/MDE values and potential for repeated upgrades.  How to increase the “half-life” of selected EQs?

- Evaluating EQ Ground Motions             User Issues - Robin Charwlood

- USSD/FERC Workshop     March 8 2007

## 1 This is not a new question..... 

- Idriss 1985 – Evaluating Seismic Risk in Engineering Practice
- USCOLD 1985 – Guidelines for Selecting Seismic Parameters for Dam Projects
- ICOLD Bulletin 72 – Selecting Seismic Parameters for Large Dams
- USCOLD 1999 - Updated Guidelines
- CA DSOD 2002 – Guidelines for the Use of Consequence-Hazard Matrix and Selection of Ground Motions Parameters
- FEMA-ERDC 2004 – Developing Seismic Ground Motions for Embankment Dams
- FEMA 2005 – Federal Guidelines for Dam Safety – EQ Analysis and Design of Dams
- USBR 2007 – Research Needs for the Seismic Safety of Dams

- Evaluating EQ Ground Motions             User Issues - Robin Charwlood

- USSD/FERC Workshop     March 8 2007

## 2 Fundamental criteria for selecting EQs?

- Usually the requirement is to prevent an EQ causing an uncontrolled release of the reservoir water (USSD, FEMA...)
- So shouldn’t criteria be based on such a risk rather than specification of the EQ event or ground motion at the site?
- How do EQ criteria fit within discussions on “Tolerable Risk”, ALARP etc per David Bowles “How Safe is Safe Enough”
- Is using a deterministic MCE equivalent to requiring risk ALARP?  Don’t think so! Maybe it is ALAP!

- Evaluating EQ Ground Motions             User Issues - Robin Charwlood

- USSD/FERC Workshop     March 8 2007

## 3  Applying a deterministic approach in moderate-low seismic activity regions?

- How to deal with a controlling M7.X 1872 type event?  Has a return period of maybe 300 y in a 95% confidence roughly circular area.  Where to put it for an MCE?
- How to deal with various “Potentially Active” faults? (eg Frenchman Hill Fault).  Minimal if any post-glacial activity.
- How does the choice of 84 %ile or mean ground motion in a deterministic approach play into this?
- Don’t  these situation cry out for a probabilistic approach?

- Evaluating EQ Ground Motions             User Issues - Robin Charwlood

- USSD/FERC Workshop     March 8 2007

## 3  Applying a deterministic approach in moderate/low seismic activity regions?

- eg Columbia Basin:
- Bakun 2002 zone of 95% confidence of M7.2 1872 event
- Several major dams inside and just outside the zone!

- Evaluating EQ Ground Motions             User Issues - Robin Charwlood

- USSD/FERC Workshop     March 8 2007

## 3  Applying a deterministic approach in moderate/low seismic activity regions?

- Should Users use USGS Hazard Results?
- USGS EQ Hazard Program Lat/Long Output:
- The interpolated Probabilistic ground motion values, in %g, at the requested point are:
- 	10%PE in 50 yr   (1/475)   PGA 9.33
- 	2%PE in 50 yr     (1/2475) PGA 20.13
- 	MCE PGA values in range 20 to 40%

- Evaluating EQ Ground Motions             User Issues - Robin Charwlood

- USSD/FERC Workshop     March 8 2007

## 4 Should FERC accept probabilistic methods?

- Should it be used as an option or in parallel with conventional deterministic approaches?
- How would it fit within the legal risk reduction obligations of owners?  Ref to D Bowles paper re risk criteria and ALARP issue.
- There is some concern that changing to a probabilistic approach may dramatically increase MDE/MCE values!
- How different are the approaches by Federal Agencies (FERC, USACE, USBR, USGS)?
- It is sometimes difficult to compare results from other projects in the neighborhood.  Need to understand the basis of each
- Is it ok for the criteria to be different for Feds vs Others?  Perhaps yes...different owners/agencies may have different levels of risk tolerance, etc??

- Evaluating EQ Ground Motions             User Issues - Robin Charwlood

- USSD/FERC Workshop     March 8 2007

## 5 What probability level for the MDE or MCE? 

- USSD and ICOLD have proposed that the MDE is a ground motion with a recurrence rate of between 1/3000 and 1/10,000 years
- The shape of the hazard curve is different in different regions, in CA is rises fast to near “MCE” while in Eastern NA it often rises more slowly.  So a 1:100 or 1:200 year event may be quite close to MCE in CA near a major active fault system while an MCE in a quiet zone may be 1:10,000??
- How does the MDE ground motion probability relate to life risk safety criteria such as those considered by USBR?
- Annual Probability of Failure 10-4/year criteria and justification level and/or 0.001 lives per year for EQ loading plus evaluation of “cost-effectiveness and public trust responsibilities”

- Evaluating EQ Ground Motions             User Issues - Robin Charwlood

- USSD/FERC Workshop     March 8 2007

## 5 What probability level for the MDE or MCE? 

- Shouldn’t  the “reserve strength” or conditional probability of failure given the occurrence of the MDE ground motion play into this?  Refer to Marty McCann’s pres on risk assessment.
- Shouldn’t  criteria vary depending on dam type and consequences?
- There are some issues regarding limits to ground motions that need to be resolved.....

- Evaluating EQ Ground Motions             User Issues - Robin Charwlood

- USSD/FERC Workshop     March 8 2007

## Elements of a Seismic Risk Analysis

- Seismic
- Hazard Analysis

- Seismic Fragility
- Analysis

- Inundation
- Assessment

- Systems
- Analysis

- Consequence
- Assessment

- Risk Quantification
- & Uncertainty
- Assessment

- From USSD 2006 Risk Workshop with permission of Marty McCann

- Evaluating EQ Ground Motions             User Issues - Robin Charwlood

- USSD/FERC Workshop     March 8 2007

## Site Seismic Hazard Example

- Adapted from Marty McCann at USSD 2006 Risk Workshop

- Note increasing values at low probability!!!

- Evaluating EQ Ground Motions             User Issues - Robin Charwlood

- USSD/FERC Workshop     March 8 2007

**(Extracted from Norm Abrahamson – ECEE, 2006)**

- Evaluating EQ Ground Motions             User Issues - Robin Charwlood

- USSD/FERC Workshop     March 8 2007

**(Extracted from Norm Abrahamson – ECEE, 2006)**

- Evaluating EQ Ground Motions             User Issues - Robin Charwlood

- USSD/FERC Workshop     March 8 2007

## Seismic Fragility

- The vulnerability of a dam for seismic events can be quantified in terms of a “fragility curve”
- Quantifies the distribution of ground motions that could cause dam failure.
- The distribution is associated with the variability in soil properties and the variation in ground motions (different time histories with the same PGA).
- The fragility curve defines the conditional probability of failure as a function of the earthquake loading event – **M**, PGA

- From USSD 2006 Risk Workshop with permission of Marty McCann

- Evaluating EQ Ground Motions             User Issues - Robin Charwlood

- USSD/FERC Workshop     March 8 2007

## Seismic Fragility

- PGA (g)

- 1.0
- 0.5
-    0

- Dam Section i

- P(f|PGA), Conditional Prob of Failure

- Â = median capacity

- Median = best estimate (50/50 chance of failure) of the ground motion
- that will cause failure or a specified performance state (level of damage).

- Â exp(β)

- From USSD 2006 Risk Workshop with permission of Marty McCann

- Evaluating EQ Ground Motions             User Issues - Robin Charwlood

- USSD/FERC Workshop     March 8 2007

## Seismic Systems Model

- Systems model – Event tree (possibly fault trees as well)
- Event Tree – possible top events
- Initiating events: Earthquakes (M) & PGAs
- Variation in reservoir levels
- Estimated deformations or damage to structures
- Post-seismic stability
- Either- Seepage/piping through damaged embankment
- Or - Flow through damaged concrete structures
- Evacuation of the reservoir

- From USSD 2006 Risk Workshop with permission of Marty McCann

- Evaluating EQ Ground Motions             User Issues - Robin Charwlood

- USSD/FERC Workshop     March 8 2007

## Conclusions re application of probabilistic method?

- Need to tighten range of variation in ground motion estimates to control “tail” of hazard curves?
- Need to recognize that occurrence of MDE GM does not mean “failure” – use “systems model”?
- Need research to quantify fragility, ie “reserve strength”?
- All these problems are also present in deterministic methods but are not addressed explicitly!!

- Evaluating EQ Ground Motions             User Issues - Robin Charwlood

- USSD/FERC Workshop     March 8 2007

## 6 How to deal with a Local or Random event? 

- Put it into an overall Cornell-McGuire style Probabilistic Seismic Hazard Analysis?
- Deal with it separately in a local area/radius, in which case, at what probability?
- How to relate this it USGS hazard maps?

- Evaluating EQ Ground Motions             User Issues - Robin Charwlood

- USSD/FERC Workshop     March 8 2007

## 7 When are local seismological investigations warranted? 

- When is trenching etc required
- Micro seismic networks?
- How far out should the investigation reach?  100 km or?

- Evaluating EQ Ground Motions             User Issues - Robin Charwlood

- USSD/FERC Workshop     March 8 2007

## 8 Required qualifications for the person doing the seismic hazard analysis? 

- Can a knowledgeable civil engineer do it satisfactorily?
- Is a “seismology company” required and if so at what cost?
- How much money is on the table?
- What skills are required?  Shouldn’t one cover the whole risk analysis spectrum?
- 	(Tectonics/geology/GM/response/fragility/PFMs/Consequences)

- Evaluating EQ Ground Motions             User Issues - Robin Charwlood

- USSD/FERC Workshop     March 8 2007

## 9 Status of the new Chapter 13? 

- When will update be ready?
- What will be the review and implementation process?

- Evaluating EQ Ground Motions             User Issues - Robin Charwlood

- USSD/FERC Workshop     March 8 2007

## 10 Ongoing changes in MCE/MDE values and potential for repeated upgrades?

- The sciences of seismology and EQ response and potential failure will continue to evolve, so changes are inevitable?
- Changing societal values and requirements?
- Epistemic (science/knowledge) and aleatory (natural/random) uncertainty and all that jazz..
- Status of NGA project for new attenuation relations
- Should agencies/owners build in provision for future changes?  If so, how?  HV 2004 Panel

- Evaluating EQ Ground Motions             User Issues - Robin Charwlood

- USSD/FERC Workshop     March 8 2007