CEUS Mean Earthquake Rates & Observations

1. Figure A9–1 CEUS Mean Earthquake Rates & Observations Steve Harmsen, USGS NRC-USGS Mmax Workshop September 8-9, 2008

2. Basic Approach Figure A9–2 • Compare expected number of background-zone earthquakes in two sub-regions of CEUS with historical data. For expected rates, use 2008 USGS-NSHMP PSHA source model. • Comment on expected rates of M>6 background compared to historical record. • Comment on expected rates of M>7 background sources compared to paleoseismic evidence in those two sub-regions. • These are initial observations. This material is not abstracted from a journal article.

3. What is known about the fault? Figure A9–3 • Wide range of knowledge about faults • Hazardous CEUS faults are Cheraw, Colorado and Meers, Oklahoma (no location uncertainty) and NMSZ (with location uncert.). Earthquake size (Mchar) can be estimated from evidence. • Charleston, SC is a characteristic source zone. Size (Mchar) can be estimated from evidence. • Background sources often at locations with virtually no evidence for estimating fault size. Methodology assumes that long, capable faults are everywhere (close to every location in the CEUS and WUS).

4. Scope of Analysis Figure A9–4 • Background sources only • No faults, fault zones, characteristic-earthquake source zones are included in mean rate calculations, E[N]. • Historical earthquakes used for tabulation of N on the basis of a declustered USGS catalog. Not explicitly considering range of uncertainty of historic earthquake magnitudes. Comment on the range of reported magnitudes.

5. Figure A9–5 CUS Region, Radius 970 km NEUS Region, Radius 700 km Craton Mmax fault length 41 km NEUS region: Radius 700 km CUS region: Radius 970 km Margin Mmax fault Length 90 km

6. Figure A9–6 ANNUAL RATE OF EXCEEDANCE MOMENT MAGNITUDE

7. How Many CEUS Eqs w/M>6 in Last 300 Years? Figure A9–7 • Depends on who you ask. USGS ranked competing estimates, NCEER estimate came out on top of the heap. • Zero earthquakes with m or M>6 in CUS or NEUS circles, according to declustered USGS catalog if you … • Omit NMSZ mainshocks, foreshocks, and aftershocks of 1811-1812 (handled by fault files, not background). • Omit 1843 and 1895 NMSZ earthquakes (all have NCEER M < 5.5, but Johnston and some others give M>6 estimates). • Omit Cape Ann November 1755 earthquake. NCEER M is 5.8 (mb 6+, Ebel) • Omit Charleston, SC mainshock & aftershocks (outside the study area). Also, handled by other files, not background. • Omit St. Lawrence River-Charlevoix (M6+); 1935 Timiskaming (M6.1, Bent; MMI VII); Grand Banks (1929, M7.2) (outside study area). • Omit 1897 Giles County, VA. Inferred M was 5.9. MMI VIII.

8. Figure A9–8 CUS Region, Radius 970 km NEUS Region, Radius 700 km NEUS region: Radius 700 km 300 year completeness CUS region: Radius 970 km 200 year completeness

9. Probability Statement About M>6 Historical Earthquakes Figure A9–9 • Let mu1 = Expected number of earthquakes having M>=6 in random 300 year period in NEUS circle. Mu1=2.85 • Let mu2=Expected number of earthquakes with M>=6 in a random 200 year period in the CEUS circle. Mu2=2.72 • Let mu = mu1 + mu2. • Poisson Pr[0 M6+ | mu=5.57] = 0.0038 • Pr[1 or less | mu=5.57] = 0.025

10. Is the significance test any good? Figure A9–10 • Only if observed N < 2. • Some will claim that omitting Cape Ann, Timiskaming, and Charlevoix is unfair. • Some NMSZ “aftershocks” may be independent sources. 1843 & 1895 M(NCEER)=5.4; other catalogs say M>=6 for these two earthquakes. • Completeness issues will be argued. Large % of these regions was raw frontier for substantial part of the time intervals. • Last 200 or 300 years may not be typical.

11. Even if test is good Figure A9–11 • Do not need to reduce MMax to get a non-significant test result • Could reduce predicted earthquake rates in the M6 to M7.5 range by a factor of 2 or so • 2007 USGS PSHA in California did reduce background-source rates for M>6.5 sources to get better agreement with historical earthquake rate

12. M>7 Paleoearthquakes 6000 yrs Figure A9–12 • 1 earthquake: Vincennes (M7+, Obermeier). 6100 years • Let mu=expected number of M7+ background earthquakes in the last 6000 years in either CUS or NEUS. Omit NMSZ mainshocks, Meers, OK, and South Carolina coastal plain, which are handled by other files. • From a previous slide, mu=12.5. • Pr[1 or less M7+ source | mu=12.5] = 0.000 • No CEUS NPP site applications indicate evidence for Holocene earthquakes with M>=7 near the site. • May want to include 1663 Charlevoix earthquake (M approx 7). • Pr [2 or less | mu=12.5] = 0.0003 • We need 6 or more M7+ paleoearthquakes to produce a non-significant Poisson test when mu=12.5 (5 gives 0.015 p)

13. Summary Figure A9–13 • Initial look at these two CEUS regions shows that expected numbers of earthquakes of M>6 or M>7 seem to exceed observations by a significant amount. • Similar tests should be attempted after resolving some magnitude-uncertainty issues, trying better-researched completeness period estimates, and so on.