earthquake probabilities in the san francisco bay region, 2002–2031 working group on california...
Post on 18-Dec-2015
221 views
TRANSCRIPT
![Page 1: Earthquake Probabilities in the San Francisco Bay Region, 2002–2031 Working Group on California Earthquake Probabilities, 2002 Chapters 1 & 2](https://reader036.vdocuments.site/reader036/viewer/2022062515/56649d245503460f949fa66a/html5/thumbnails/1.jpg)
Earthquake Probabilities in the San Francisco Bay Region, 2002–2031
Working Group on California Earthquake Probabilities, 2002
Chapters 1 & 2
![Page 2: Earthquake Probabilities in the San Francisco Bay Region, 2002–2031 Working Group on California Earthquake Probabilities, 2002 Chapters 1 & 2](https://reader036.vdocuments.site/reader036/viewer/2022062515/56649d245503460f949fa66a/html5/thumbnails/2.jpg)
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
![Page 3: Earthquake Probabilities in the San Francisco Bay Region, 2002–2031 Working Group on California Earthquake Probabilities, 2002 Chapters 1 & 2](https://reader036.vdocuments.site/reader036/viewer/2022062515/56649d245503460f949fa66a/html5/thumbnails/3.jpg)
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%
![Page 4: Earthquake Probabilities in the San Francisco Bay Region, 2002–2031 Working Group on California Earthquake Probabilities, 2002 Chapters 1 & 2](https://reader036.vdocuments.site/reader036/viewer/2022062515/56649d245503460f949fa66a/html5/thumbnails/4.jpg)
…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:
![Page 5: Earthquake Probabilities in the San Francisco Bay Region, 2002–2031 Working Group on California Earthquake Probabilities, 2002 Chapters 1 & 2](https://reader036.vdocuments.site/reader036/viewer/2022062515/56649d245503460f949fa66a/html5/thumbnails/5.jpg)
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
![Page 6: Earthquake Probabilities in the San Francisco Bay Region, 2002–2031 Working Group on California Earthquake Probabilities, 2002 Chapters 1 & 2](https://reader036.vdocuments.site/reader036/viewer/2022062515/56649d245503460f949fa66a/html5/thumbnails/6.jpg)
Fault segments considered
17 segments within the Bay Area (+1 near Mendocino)
Mt Diablo Thrust is included, even though its properties are poorly known
![Page 7: Earthquake Probabilities in the San Francisco Bay Region, 2002–2031 Working Group on California Earthquake Probabilities, 2002 Chapters 1 & 2](https://reader036.vdocuments.site/reader036/viewer/2022062515/56649d245503460f949fa66a/html5/thumbnails/7.jpg)
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:
![Page 8: Earthquake Probabilities in the San Francisco Bay Region, 2002–2031 Working Group on California Earthquake Probabilities, 2002 Chapters 1 & 2](https://reader036.vdocuments.site/reader036/viewer/2022062515/56649d245503460f949fa66a/html5/thumbnails/8.jpg)
Monte Carlo treatment of uncertainty
![Page 9: Earthquake Probabilities in the San Francisco Bay Region, 2002–2031 Working Group on California Earthquake Probabilities, 2002 Chapters 1 & 2](https://reader036.vdocuments.site/reader036/viewer/2022062515/56649d245503460f949fa66a/html5/thumbnails/9.jpg)
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
![Page 10: Earthquake Probabilities in the San Francisco Bay Region, 2002–2031 Working Group on California Earthquake Probabilities, 2002 Chapters 1 & 2](https://reader036.vdocuments.site/reader036/viewer/2022062515/56649d245503460f949fa66a/html5/thumbnails/10.jpg)
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
![Page 11: Earthquake Probabilities in the San Francisco Bay Region, 2002–2031 Working Group on California Earthquake Probabilities, 2002 Chapters 1 & 2](https://reader036.vdocuments.site/reader036/viewer/2022062515/56649d245503460f949fa66a/html5/thumbnails/11.jpg)
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’
![Page 12: Earthquake Probabilities in the San Francisco Bay Region, 2002–2031 Working Group on California Earthquake Probabilities, 2002 Chapters 1 & 2](https://reader036.vdocuments.site/reader036/viewer/2022062515/56649d245503460f949fa66a/html5/thumbnails/12.jpg)
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
![Page 13: Earthquake Probabilities in the San Francisco Bay Region, 2002–2031 Working Group on California Earthquake Probabilities, 2002 Chapters 1 & 2](https://reader036.vdocuments.site/reader036/viewer/2022062515/56649d245503460f949fa66a/html5/thumbnails/13.jpg)
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
![Page 14: Earthquake Probabilities in the San Francisco Bay Region, 2002–2031 Working Group on California Earthquake Probabilities, 2002 Chapters 1 & 2](https://reader036.vdocuments.site/reader036/viewer/2022062515/56649d245503460f949fa66a/html5/thumbnails/14.jpg)
Putting it all together 1: recurrences
![Page 15: Earthquake Probabilities in the San Francisco Bay Region, 2002–2031 Working Group on California Earthquake Probabilities, 2002 Chapters 1 & 2](https://reader036.vdocuments.site/reader036/viewer/2022062515/56649d245503460f949fa66a/html5/thumbnails/15.jpg)
Putting it all together 2: probabilities