Earthquake Probabilities in the San Francisco Bay Region, 2002–2031

Working Group on California Earthquake Probabilities, 2002

Chapters 1 & 2

Overview

• History and scope of the Working Group reports (and what is new in this one)

• Uncertainty, and what they mean by it

• The earthquake model, and ‘background events’

• Probability models

• Putting it all together

History of the WGCEP…

• 1988: SAF and HF slip rates & time predictable model, estimated 50% probability of M~7 in 30 years

• 1989: M 6.9 Loma Prieta earthquake

• 1990: Post-Loma Prieta recalculation, added RCF, stress changes, new rupture scenarios, p = 67%

• 1995: SoCal only, included geodetic slip rates, multiple segment ruptures, regional bounds, etc

• 1999: Included CF, SGF, GF, C-GVF, MtDT, 18 segments, 35 scenarios, more sophisticated data/methods as per 1995 report, p = 70%

…and what’s new?

• Improved 1906 stress shadow model

• Probabilities of different magnitude earthquakes included, as are different time intervals

• Incremental improvements to: slip rate estimates (geodetic/geologic), historical eq knowledge (locations/intervals/magnitudes), knowledge of creep, 1906 eq slip, regional strain budget, etc…

2002 Working Group mostly adopts WG99 methods; however there are some improvements:

Area covered by report

Rectangular area between Healdsburg and San Juan Bautista

‘Panhandle’ along San Andreas up to Mendocino, to include 1906-style rupture scenarios

Fault segments considered

17 segments within the Bay Area (+1 near Mendocino)

Mt Diablo Thrust is included, even though its properties are poorly known

A treatise on uncertainty

• Aleatory uncertainties – natural random variability, which is irreducible

• Epistemic uncertainties – owing to our lack of understanding of natural processes, use of incomplete models, measurement error, etc

• The study is devoted to reducing, and quantifying epistemic uncertainty

• A Monte Carlo approach is used

There are essentially two types of model uncertainties:

Monte Carlo treatment of uncertainty

The earthquake model

• Fault segments – their lengths, widths, slip rates and ‘seismogenic scaling factors’ (= % not creeping)

• Rupture sources – 35 combinations of segments that can rupture alone or in groups (+ ‘floaters’)

• Rupture scenarios – possible combinations of rupture sources in a single earthquake cycle

• Fault rupture models – weighted combined probabilities of the various scenarios occurring

• Regional model – all the above must satisfy GPS strain budget

Division of the plate motion budget

Additional provision is made for earthquakes that do not occur on modelled faults

These are modelled with a Gutenberg-Richter distribution based on historic/ instrumental data

Probability models

• Poisson – simple, time invariant (based on mean recurrence)

• Empirical – new for 2002! Modulates Poisson recurr-ences by current seismicity rates (shadow effects)

• Brownian Passage Time – deterministic loading/ stress shadowing + stochastic element

• Time Predictable – using last earthquake rupture time/size and loading rate, how long until next one?

• Estimation of inherent randomness ‘remains a significant scientific challenge’

Strain accumulation and releaseTotal strain budgets are measurable 4 ways

Uncertainty in earthquake strain release is large (variability in models of 1906 event)

However, agreement is good, suggesting that most strain accumulated is released seismically

From rupture area to eq magnitude

• Earthquake scaling relations are used to estimate moment release from the fault segment area(s)

• 3 relations used – Wells & Coppersmith (1994), Ellsworth (1999) and Hanks & Bakun (2002)

• Largest source of uncertainty in the whole process – can get factor of 2 differences in M0 for different scaling relations

Putting it all together 1: recurrences

Putting it all together 2: probabilities