Slip Rate Studies Along the Sierra Madre-Cucamonga Fault System Using Geomorphic and 10Be Cosmogenic Surface Exposure Age Constraints

Sierra Madre-Cucamonga Fault System Large range-bounding reverse fault along northern LA basin 1971 M6.7 San Fernando EQ rupture Large, infrequent EQs (M7+, 4-5 m of slip, several thousand year RI) Rubin et al. (1998) and Tucker and Dolan (2001) Significant hazard to the LA basin and greater metropolitan area Can we better refine slip rate estimates to more accurately characterize the hazard?

Previous Slip Rate Estimates ~2 mm/yr Lindvall et al. (1995) ~1.5-3.0 mm/yr Walls et al. (1997) ~0.6 mm/yr Rubin et al. (2001) ~0.5-1.5 mm/yr Walls et al. (1997) 0.6-0.9 mm/yr Tucker and Dolan (2001) 4.5-5.5 mm/yr Morton and Matti (1987)

Tectonic Geomorphic Analysis and Mapping Fault scarps, fluvial terraces and fan surfaces were identified and mapped using aerial photography and digital topography.

Measuring Uplift of Geomorphic Surfaces Construct topographic profiles across surfaces and fault scarps Total station surveys 5 ft contour maps Measure vertical separation (uplift) of surfaces

Uplift, Shortening, and Dip-Slip Rates

Formation of Uplifted and Abandoned Fluvial Terrace Surfaces

Cosmogenic Nuclide Formation Cosmogenic Surface Exposure Ages Cosmogenic Nuclide Formation Cosmic rays consisting of neutrons, protons and muons strike and penetrate rocks at the earths surface. The cosmic rays interact with Si and O in quartz to produce 10Be and 26Al nuclides.

Cosmogenic Surface Exposure Ages Rocks exposed to cosmic rays contains “exotic” short-lived isotopes. Only rocks near the surface (upper few meters) effected. The older the surface, the higher the concentrations of CRN isotopes. CRN’s produced in quartz grains by cosmic-ray bombardment of Si, O nuclei Production rate variable with altitude, latitude Cosmic-ray flux decreases exponentially with depth below the surface. If a previously exposed surface is buried, nuclide production ceases.

Western Sierra Madre Fault System

Pacoima Wash Terraces Abandoned terraces 1971 rupture Q1-Q6 1971 rupture Three older fault scarps Qt4 surface Moderately preserved Fine-course gravel with discontinuous overbank deposits Agricultural modification

Qt4 Surface Profile Total vertical separation of Qt4 surface across 3 fault scarps = 27 ± 2 m

View of Qt4 Surface

Sampling of Qt4 Surface Processed 5 individual subsurface samples Surveyed site for suitable samples Collected 3 initial samples in depth profile Subsurface depths: PW-1 = 1.09 m PW-2 = 0.70 m PW-3 = 0.33 m

Qt4 Model Surface ages Depth Corrected Ages Sample ID 10Be model age PW-1 33,239 ± 1564 PW-2 31,196 ± 1287 PW-3 31,024 ± 1073 PW-B 69,825 ± 3055 PW-C 64,720 ± 2143

Results - Qt4 Surface Total uplift = 27 ± 2 m Measured from topographic profile 10Be Model surface age = 31,561 ± 729 yr Weighted mean age corrected for depth/latitude/altitude Assumes zero erosion and zero inheritance Uplift rate = 0.9 ± 0.1 mm/yr Horizontal Shortening rate = 0.9 ± 0.3 mm/yr Dip Slip rate = 1.2 ± 0.4 mm/yr Using estimated fault dip of 45 ± 10° Oblique slip rate = ~1.7 mm/yr Using estimated 45 ± 10 ° rake from a 1:1 H:V ratio of left oblique motion during the 1971 San Fernando earthquake

Lopez Canyon Surface

Lopez Canyon Surface Abandoned alluvial surface Limited preservation of remnants (along ridges) in highly dissected surface Two fault scarps Scarps 4 and 5 Cumulative uplift across both scarps = 15.5 ± 07 m

Lopez Canyon Surface Ages Processed 4 surface samples Lat./Alt. Corrected Ages Sample ID 10Be model age LC-1 29,029 ± 884 LC-2 33,443 ± 1009 LC-3 33,181 ± 943 LC-4 24,273 ± 851

Results - Lopez Canyon Surface Total offset = 15.5 ± 0.7 m Measured from topographic profiles 10Be Model surface age = 29,540 ± 458 yr Weighted mean age corrected for latitude/altitude Assumes zero erosion and zero inheritance Uplift rate = 0.5 ± 0.1 mm/yr Horizontal Shortening rate = 0.6 ± 0.2 mm/yr Dip Slip rate = 0.8 ± 0.3 mm/yr Using measured fault dip in exposure of 40 ± 10°

Wilson Canyon Fan Surface

Wilson Canyon Fan Surface Abandoned fan surface Uplifted from modern valley floor Remnants of surface preserved along ridge lines

Southwest View of Wilson Canyon Fan Surface Remnant

Topographic Profile Uplift across fault scarp is 63.5 ± 5 m Hospital fault ‘Wilson Canyon’ fault

Wilson Canyon Fan Surface Ages Processed 6 surface samples Lat./Alt. Corrected Ages Sample ID 10Be model age PW-13 52,071 ± 1435 PW-14 61,489 ± 1534 PW-15 49,265 ± 1705 PW-16 72,255 ± 1955 PW-17 41,688 ± 1082 PW-18 64,584 ± 2122

Wilson Canyon Fan Surface Ages Lat./Alt. Corrected Ages Sample ID 10Be model age PW-14 61489 ± 1534 PW-16 72255 ± 1955 PW-18 64584 ± 2122 W* Mean 65,245 ± 1049

Results - Wilson Canyon Fan Total Uplift = 63.5 ± 5 m 10Be Model surface age = 65,345 ± 1049 yr Weighted mean age corrected for latitude/altitude Assumes zero erosion and zero inheritance. Uplift rate = 1.0 ± 0.1 mm/yr Horizontal Shortening rate = 1.2 ± 0.7 mm/yr Dip Slip rate = 1.5 ± 0.9 mm/yr Using estimated fault dip of 40 ± 20°

Cumulative Horiz. Shortening and Dip Slip Rates Across Zone 1.2 ± 0.7 mm/yr 0.6 ± 0.2 mm/yr + 0.9 ± 0.3 mm/yr 2.6 ± 0.8 mm/yr Cumulative Dip Slip Rate = 3.5 ± 1.1 mm/yr

Cucamonga Fault Zone

Day Canyon Fan Study Site Modified from Matti and Morton (1987)

Oblique Aerial Photograph of Day Canyon Fan Surface

Topographic Profile Analysis Three profiles across strand C One profile across strand A and B All profiles were constructed from total station surveys

Topographic Profile Analysis Uplift across scarps A and B (Qyf1a)= 20 ± 0.5 m Total uplift of Qyf1a surface across A, B, and C = 34 ± 0.7 m

Sample Ages of Qyf1a Surface (West) Weighted mean model surface age = 33,395 ± 332 years Excluding samples that plot outside the yellow box

Results - Day Canyon Fan Total uplift = 34 ± 0.7 m (across 3 scarps) 10Be Model surface age = 33,395 ± 332 yr Weighted mean age corrected for depth/latitude/altitude Assumes zero erosion and zero inheritance Uplift rate = 1.1 ± 0.1 mm/yr Horizontal Shortening rate = 1.6 ± 0.3 mm/yr Dip Slip rate = 1.9 ± 0.35 mm/yr Using measured fault dip of 32.5 ± 5° from Matti et al. (1982)

Lower Rate than Morton and Matti, (1987) Geomorphic and soil chronologic study 36 m of uplift of surface Qyf1a across 3 strands Surface age of ~13 ka estimated using soil comparisons with radiometrically dated soil at Cajon Pass Dip slip rate of ~4.5 - 5.5 mm/yr is significantly greater than our estimate of ~1.9 mm/yr using cosmogenic ages of fan surface

Dip Slip Rate Estimates ~2 mm/yr Lindvall et al. (1995) ~1.5-3.0 mm/yr Walls et al. (1997) ~0.6 mm/yr Rubin et al. (2001) ~0.5-1.5 mm/yr Walls et al. (1997) 0.6-0.9 mm/yr Tucker and Dolan (2001) 4.5-5.5 mm/yr Morton and Matti (1987) 3.5 ± 1.1 mm/yr This Study 1.9 ± 0.35 mm/yr This Study

Escape Tectonics vs Crustal Thickening Walls et al. (1998) Argus et al. (1999) > 50% of N-S contraction is accommodated by E-W extension N-S contraction rates 7-9 mm/yr E-W extension rates ~6 mm/yr Does not consider viscoelastic effects of the SAF and SJF N-S contraction is almost entirely accommodated by crustal thickening N-S contraction rates 5.8 ± 1.9 mm/yr E-W extension 0-2 mm/yr Removes viscoelastic effects of the SAF and SJF

Conclusions Dip-slip rate of Western Sierra Madre fault zone across multiple strands = 3.5 ± 1.1 mm/yr Dip-slip rate of Cucamonga fault across multiple strands = 1.9 ± 0.35 mm/yr Rates for the west and east ends of this fault system still appear to be greater than the Central and Eastern SMF (large, infrequent EQs) Is there a slip gradient along fault system or has the entire width of deformation not been captured for the Central and Eastern SMF?