2008 National Seismic Hazard Maps

2008 National Seismic Hazard Maps-Fault parameters

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Fault Name	State
Chemult graben fault system (east)	Oregon

GEOMETRY
Dip (degrees)	50/60/40
Dip direction	W
Sense of slip	normal
Rupture top (km)	0
Rupture bottom (km)	15
Rake (degrees)	-90
Length (km)	62

MODEL VALUES
Probability of activity	1
Minimum magnitude	6.5
Maximum magnitude	7.16
b-value	0.8

Assigned Dip	Fault-Parallel Slip Rate	Width	Annual Rate	a-value	Branch Weight
40	0.08	23.3	5.66e-05	0.914	0.2
50	0.07	19.6	3.98e-05	0.762	0.6
60	0.06	17.3	3.12e-05	0.655	0.2

Comments
The fault system forms the eastern side of the Chemult graben. Two slip rate estimates are available for the graben, 0.05 mm/yr from 300 m of offset in 4.5-7 Ma volcanic rocks, and 0.4 mm/yr from 5- to 6-m-high scarps on latest Pleistocene (15 ka) deposits (Geomatrix Consultants, Inc., 1995). Geomatrix Consultants, Inc. (1995) notes geomorphic expressions indicating that the eastern side of the graben probably has slower slip rates than the western side. The Quaternary fault and fold database for Oregon (http://gldims.cr.usgs.gov/webapps/cfusion/Sites/qfault/qf_web_disp.cfm?qfault_or=1757&ims_cf_cd=cf&disp_cd=C) explains why 0.4 mm/yr probably applies only to the western side of the graben. Thus, in 2002, 0.052 mm/yr was used for the eastern side. Dip of fault changed to 50? based on redefined regional default value (Lund, 2006).

Comments

The fault system forms the eastern side of the Chemult graben. Two slip rate estimates are available for the graben, 0.05 mm/yr from 300 m of offset in 4.5-7 Ma volcanic rocks, and 0.4 mm/yr from 5- to 6-m-high scarps on latest Pleistocene (15 ka) deposits (Geomatrix Consultants, Inc., 1995). Geomatrix Consultants, Inc. (1995) notes geomorphic expressions indicating that the eastern side of the graben probably has slower slip rates than the western side. The Quaternary fault and fold database for Oregon (http://gldims.cr.usgs.gov/webapps/cfusion/Sites/qfault/qf_web_disp.cfm?qfault_or=1757&ims_cf_cd=cf&disp_cd=C) explains why 0.4 mm/yr probably applies only to the western side of the graben. Thus, in 2002, 0.052 mm/yr was used for the eastern side. Dip of fault changed to 50? based on redefined regional default value (Lund, 2006).

Selected References
Geomatrix Consultants, Inc., 1995, Seismic design mapping, State of Oregon: Technical report to Oregon Department of Transportation, Salem, Oregon, under Contract 11688, January 1995, unpaginated, 5 pls., scale 1:1,250,000. 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

Geomatrix Consultants, Inc., 1995, Seismic design mapping, State of Oregon: Technical report to Oregon Department of Transportation, Salem, Oregon, under Contract 11688, January 1995, unpaginated, 5 pls., scale 1:1,250,000.
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.