2008 National Seismic Hazard Maps

2008 National Seismic Hazard Maps-Fault parameters

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Fault Name	State
Southern Sawatch fault	Colorado

GEOMETRY
Dip (degrees)	50/40/60
Dip direction	E
Sense of slip	normal
Rupture top (km)	0
Rupture bottom (km)	15
Rake (degrees)	-90
Length (km)	45

MODEL VALUES
Probability of activity	1
Minimum magnitude	6.5
Maximum magnitude	6.99
b-value	0.8

Assigned Dip	Fault-Parallel Slip Rate	Width	Annual Rate	a-value	Branch Weight
40	0.10	23.3	8.80e-05	1.095	0.2
50	0.08	19.6	6.19e-05	0.943	0.6
60	0.07	17.3	4.85e-05	0.836	0.2

Comments
The preferred vertical slip rate was calculated from the mean scarp height and mean age of 8-10 m scarps on Bull Lake-equivalent deposits (160-130 ka, Nelson and Shroba, 1998). Results from five trenches demonstrated that at least six episodes of surface rupture have occurred within the past 100-150 k.y., with each rupture producing less than 0.2-0.3 m of vertical displacement. If one assumes 6 ruptures and 0.2-0.3 m of displacement from each, the cross-quotients of displacement divided by age give 0.018 and 0.0080 mm/yr. Additionally, scarps 8-10 m high have been observed on Bull Lake-equivalent (160-130 ka) deposits and multi-event scarps 2 m high are reported on Pinedale-equivalent (15 ka) deposits. These values give vertical slip rates of 0.050-0.077 mm/yr since Bull Lake time, and 0.13 mm/yr since Pinedale time. Dip of fault changed to 50? based on redefined regional default value (Lund, 2006).

Comments

The preferred vertical slip rate was calculated from the mean scarp height and mean age of 8-10 m scarps on Bull Lake-equivalent deposits (160-130 ka, Nelson and Shroba, 1998). Results from five trenches demonstrated that at least six episodes of surface rupture have occurred within the past 100-150 k.y., with each rupture producing less than 0.2-0.3 m of vertical displacement. If one assumes 6 ruptures and 0.2-0.3 m of displacement from each, the cross-quotients of displacement divided by age give 0.018 and 0.0080 mm/yr. Additionally, scarps 8-10 m high have been observed on Bull Lake-equivalent (160-130 ka) deposits and multi-event scarps 2 m high are reported on Pinedale-equivalent (15 ka) deposits. These values give vertical slip rates of 0.050-0.077 mm/yr since Bull Lake time, and 0.13 mm/yr since Pinedale time. Dip of fault changed to 50? based on redefined regional default value (Lund, 2006).

Selected References
Ostenaa, D.A., Losh, S.L., and Nelson, A.R., 1981, Evidence for recurrent late Quaternary faulting, Sawatch fault, upper Arkansas Valley, Colorado, in Junge, W.R., ed., Colorado tectonics, seismicity and earthquake hazards—Proceedings and field trip guide: Colorado Geological Survey Special Publication 19, p. 27-29. Nelson, A.R., and Shroba, R.R, 1998, Soil relative dating of moraine and outwash-terrace sequences in the northern part of the upper Arkansas Valley, central Colorado, U.S.A.: Arctic and Alpine Research, v. 30, p. 349-361. Lund, W.R., ed., 2006, Basin and Range Province Earthquake Working Group seismic-hazard recommendations to the U.S. Geological Survey National Seismic Hazard Mapping Program: Utah Geological Survey Open-File Report 477, 23 p, ugspub.nr.utah.gov/publications/open_file_reports/OFR-477.pdf.

Selected References

Ostenaa, D.A., Losh, S.L., and Nelson, A.R., 1981, Evidence for recurrent late Quaternary faulting, Sawatch fault, upper Arkansas Valley, Colorado, in Junge, W.R., ed., Colorado tectonics, seismicity and earthquake hazards—Proceedings and field trip guide: Colorado Geological Survey Special Publication 19, p. 27-29.
Nelson, A.R., and Shroba, R.R, 1998, Soil relative dating of moraine and outwash-terrace sequences in the northern part of the upper Arkansas Valley, central Colorado, U.S.A.: Arctic and Alpine Research, v. 30, p. 349-361.
Lund, W.R., ed., 2006, Basin and Range Province Earthquake Working Group seismic-hazard recommendations to the U.S. Geological Survey National Seismic Hazard Mapping Program: Utah Geological Survey Open-File Report 477, 23 p, ugspub.nr.utah.gov/publications/open_file_reports/OFR-477.pdf.