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

2. 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?

3. Previous Slip Rate Estimates
~2 mm/yr
Lindvall et al. (1995)
~ mm/yr
Walls et al. (1997)
~0.6 mm/yr
Rubin et al. (2001)
~ mm/yr
Walls et al. (1997)
mm/yr
Tucker and Dolan (2001)
mm/yr
Morton and Matti (1987)

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

5. 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

6. Uplift, Shortening, and Dip-Slip Rates

7. Formation of Uplifted and Abandoned Fluvial Terrace Surfaces

8. 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.

9. 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.

10. Western Sierra Madre Fault System

11. 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

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

13. View of Qt4 Surface

14. 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

15. 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

16. 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

17. Lopez Canyon Surface

18. 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

19. 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

20. 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°

21. Wilson Canyon Fan Surface

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

23. Southwest View of Wilson Canyon Fan Surface Remnant

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

25. 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

26. 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

27. 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°

28. 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

29. Cucamonga Fault Zone

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

31. Oblique Aerial Photograph of Day Canyon Fan Surface

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

33. 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

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

35. 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)

36. 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 ~ mm/yr is significantly greater than our estimate of ~1.9 mm/yr using cosmogenic ages of fan surface

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

38. 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

39. 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?