Holocene seismicity of Puget Sound: A Statistical perspective

Presented at the American Geophysical Union 2016 Fall Meeting

Transcript

1. Holocene seismicity of Puget Sound: A Statistical Perspective Richard Styron Earth Analysis, Seattle, WA Global Earthquake Model Foundation, Pavia, Italy Brian Sherrod US Geological Survey, Seattle, WA BIG THANKS TO DATA COLLECTORS!
2. Observations and objectives • Refined dates, offsets, and LiDAR scarp mapping of ~30 paleoearthquakes in Puget Sound region over ~16,000 years (post-glacial tabula rasa) • Improved methods for estimating paleoearthquake magnitude and recurrence intervals • Demographic statistical methods (survival analysis) aid in time-dependent earthquake hazard estimates
3. Premature assertion of conclusions • Regional earthquakes have large measured offsets and short rupture lengths -> Magnitudes between 6.5-7.5 • Earthquakes are clustered in time, and recurrence modes are 20-200 years • Elevated post-earthquake seismic hazard
4. Cascadia • Oblique subduction at 36 mm/yr (Wang et al., 2003) • E-W shortening at subduction zone • N-S shortening in forearc (Puget Lowland) • 3-7 mm/yr (Mazzotti et al., 2003) Juan de Fuca
5. Cascadia • Oblique subduction at 36 mm/yr (Wang et al., 2003) • E-W shortening at subduction zone • N-S shortening in forearc (Puget Lowland) • 3-7 mm/yr (Mazzotti et al., 2003) Juan de Fuca
6. Puget Lowland • Reverse and strike- slip faults throughout forearc low • Population centers (3 M) conveniently astride faults • ~30 earthquakes inferred from many sites S T O E SFZ
7. Paleoearthquake Ages • Radiocarbon data taken from published sources • Remodeled in OxCal with limited constraints to yield PDFs for all ages -14 -10 -6 -2 0 2 Thousand Year (C.E.)
8. Paleoearthquake Ages • Earthquakes are clustered: • 10,000 and 6,000 B.C.E. • Many earthquakes in past 4 k.a. (geologic evidence of absence of older EQs: offset of 16 ka till) • Major cluster at ~1000 C.E. -14 -10 -6 -2 0 2 Thousand Year (C.E.)
9. Paleoearthquake Ages • Earthquakes are clustered: • 10,000 and 6,000 B.C.E. • Many earthquakes in past 4 k.a. (geologic evidence of absence of older EQs: offset of 16 ka till) • Major cluster at ~1000 C.E. -14 -10 -6 -2 0 2 Thousand Year (C.E.)
10. Earthquake age takeaways • Remodeling of published radiocarbon data, minor geological constraints • Earthquakes show clusters • Major cluster at ~1000 C.E.
11. Paleoearthquake magnitude estimation • Fault offsets of 0.5-8 m observed in trenches, scarp profiles, uplifted shorelines • Ruptures 1-30 km mapped in LiDAR, may be much longer (full length of mapped faults) • Both data types can constrain paleoearthquake magnitude • We extend methods of Biasi and Weldon 2006: p(M|D) -> p(M|D,L)
12. Maximum rupture lengths • Full length of mapped/ inferred faults • Faults mostly confined to forearc low S T O E
13. S T O E Minimum rupture lengths • Extent of scarp on LiDAR mapped where possible • Few km surrounding trenches otherwise
14. Calculating p(M|D,L) p(M) p(M|D,L) p(M(L)) p(D|M) ————- p(D) = * * L = [Lmin, Lmax) p(M(L)) = 5.08±0.1 + 1.06 ±0.07 * log10(L) (Wells and Coppersmith, 1994) Bayesian displacement- magnitude scaling D = [Dmin, Dmax) (Biasi and Weldon, 2006) 6.0 7.0 8.0 6.0 7.0 8.0 6.0 7.0 8.0 6.0 7.0 8.0 * * =
15. Results: All earthquakes 6.5-7.5 6.0 6.5 7.0 7.5 8.0 8.5 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Probability p(M | D) 6.0 6.5 7.0 7.5 8.0 8.5 Moment magnitude 2 4 6 8 10 Probability p(M | D,L) n=25
16. Using L decreases M, uncertainty • PNW ruptures have high slip for short length • Accounting for length pushes M estimates lower by M~0.5 • Uncertainty is reduced substantially (esp. long high-M tails) 6.0 6.5 7.0 7.5 8.0 8.5 p(M | D) 6.0 6.5 7.0 7.5 8.0 8.5 p(M | D,L)
17. Magnitude estimation takeaways • New methods incorporating rupture length into magnitude estimation • All earthquakes between M 6.4 and 7.5 • Using length reduces magnitude and uncertainty for Puget Lowland events
18. Earthquake Recurrence: methods 600 700 800 900 1000 1100 1200 1300 1400 Calendar Years B.P. −0.01 0.00 0.01 0.02 0.03 0.04 0.05 0.06 probability 0 100 200 300 400 500 600 Recurrence Interval (years) 0 200 400 600 800 1000 1200 1400 count PDF CDF
19. Recurrence results: SFZ 0 2000 4000 6000 8000 10000 12000 14000 16000 Earthquake time, calendar years BP 0.000 0.001 0.002 0.003 0.004 0.005 Seattle Fault EQ times (8 EQs) 0 1000 2000 3000 4000 5000 6000 7000 8000 Earthquake recurrence interval (years) 0.0000 0.0005 0.0010 0.0015 0.0020 0.0025 0.0030 mean = 1690 yrs median = 752 yrs mode = 236 yrs Seattle Fault recurrence PDF 0 100 200 300 400 (detail) total PDF total CDF 0.0 0.2 0.3 0.5 0.7 0.8 1.0 cumulative probability
20. Recurrence results: All Puget EQs 0 2000 4000 6000 8000 10000 12000 14000 16000 Earthquake time, calendar years BP 0.000 0.001 0.002 0.003 0.004 0.005 Puget Lowland EQ times (31 EQs) 0 1000 2000 3000 4000 5000 6000 Earthquake recurrence interval (years) 0.000 0.001 0.002 0.003 0.004 0.005 0.006 mean = 409 yrs median = 166 yrs mode = 19 yrs Puget Lowland recurrence PDF 0 100 200 300 400 (detail) total PDF total CDF 0.0 0.2 0.3 0.5 0.7 0.8 1.0 cumulative probability
21. Recurrence interval takeaways • Regional fault recurrence PDF shows very short mode (earthquake clusters), long tail • Very short modal recurrence may result from earthquake triggering
22. Survival Analysis • Statistics of timing of events • sociology, epidemiology, engineering applications • Hazard rates (instantaneous probability of occurrence) λ(t) = pdf(t) / 1 - cdf(t) • Expected time until next event, probability in time interval, etc.
23. Earthquake hazard (rate) 0 1000 2000 3000 4000 5000 6000 0.000 0.001 0.002 0.003 0.004 0.005 0.006 instantaneous earthquake probability 0 200 600 1000 0.0002 0.0004 0.0006 0.0008 0.0010 0.0012 0.0014 0.0016 0 1000 2000 3000 4000 5000 6000 years since last earthquake 0.000 0.001 0.002 0.003 0.004 0.005 0.006 0 100 300 500 0.000 0.001 0.002 0.003 0.004 0.005 0.006 Seattle Fault Zone Puget Lowland
24. Earthquake hazard (rate) 2000 3000 4000 5000 6000 7000 0.000 0.001 0.002 0.003 0.004 0.005 0.006 1200 1600 2000 0.0002 0.0004 0.0006 0.0008 0.0010 0.0012 0.0014 0.0016 2000 3000 4000 5000 6000 7000 calendar year C.E. 0.000 0.001 0.002 0.003 0.004 0.005 0.006 1800 2000 calendar year C.E. 0.000 0.001 0.002 0.003 0.004 0.005 0.006 instantaneous earthquake probability Seattle Fault Zone Puget Lowland
25. Survival analysis: EQ expectations • 2.5% chance of M 6.5+ earthquake on Seattle Fault Zone in next 50 years, given 750 years since last event • 12% chance of M 6.5+ earthquake in Puget Lowland in next 50 years, given 312 years since last event 10 20 30 40 50 0.005 0.010 0.015 0.020 0.025 probability EQ Probabilities in next T years 0 10 20 30 40 50 0.00 0.02 0.04 0.06 0.08 0.10 0.12 probability SFZ Puget Lowland
26. Survival analysis: takeaways • Significant time-dependent earthquake hazards on SFZ, Puget Lowland faults • Earthquake hazard is highest in decades following an earthquake • Previously-damaged infrastructure may be very risky • Mitigation plans need to account for repeated events
27. Conclusions • Refined ages and magnitude estimates for 30 earthquakes • Magnitudes between 6.5-7.5 for all events • Short modal recurrence (20-200 years), with longer (1000s of years) tails • Elevated seismic hazard following earthquake
28. Questions?