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To: MR. TOM OSTROM, CHIEF Date: July 15, 2013 OFFICE OF ENGINEERING

File: 07-LA-5, PM 41.55 0000001016 6SSCN Route 5T/405 Separation Bridge No. 53-1548

From: DEPARTMENT OF TRANSPORTATION DIVISION OF ENGINEERING SERVICES Materials Engineering and Testing Services and Geotechnical Services Office of Geotechnical Support – Geotechnical Instrumentation

Subject: Evaluation of Fault Rupture Potential, Route 5T/405 Separation, LA County, CA

SUMMARY

Route 5T/405 Separation, Bridge No. 53-1548, is not crossed by a Caltrans-active fault. The nearest Caltrans-active fault is a strand of the Sierra Madre Fault Zone, San Fernando section (Freeway or [Reservoir] fault) that ruptured in 1971 and is located 250 feet south of the bridge. Regional deformation occurred, however only a few feet of displacement were distributed over 1000 feet or more. No further work is recommended.

INTRODUCTION

This evaluation was prepared as part of the statewide evaluation of fault rupture potential at Caltrans bridges. Caltrans’ policies regarding fault rupture at bridges are described in Memo to Designers (MTD) 20-10. Caltrans requires a fault rupture evaluation if a bridge is located within an Alquist-Priolo (AP) Earthquake Fault Zone (EFZ) or within 1,000 feet of an un-zoned fault that is 15,000 years of age or younger. Route 5T/405 Separation (Separation) is situated within the EFZ established for the San Fernando fault in 1979 in the San Fernando 7-1/2’ Quadrangle; therefore a fault evaluation was required.

An initial estimate of potential offset was based on an analysis developed by Division of Research and Innovation in collaboration with Geotechnical Services, using methods presented in Abrahamson (2008) and Moss and Ross (2011). Both a probabilistic (5% in 50 yr) fault displacement analysis (PFDHA) and a deterministic fault displacement analysis (DFDHA) were performed based on maximum magnitude earthquake, slip rate (for PFDHA), assumed mapping and base map errors, and likelihood of secondary fault traces. If the Freeway fault crosses beneath the Separation, about 2 feet in an east-west direction (probabilistic; estimated deterministic

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MR. TOM OSTROM Route 5T/405 Separation, #53-1548 July 15, 2013 Fault Rupture Potential Page 2 0000001016 6SSCN

displacement is a few inches) would be expected. Mark Yashinsky and Fadel Alameddine reviewed the bridge plans and determined the bridge could not withstand the 2 feet displacement without modification. Therefore additional work, documented herein, was undertaken to better define the fault location.

The existing Route 5T/405 Separation was built in 1974, replacing the bridge with the same name and number that was destroyed in the 1971 M6.6 San Fernando Earthquake. The Separation is a two-span continuous CIP/PS box girder on RC two column bents and RC diaphragm abutments, all supported on spread foundations. The Separation is 355 ft long and 39.5 feet wide.

FAULT RUPTURE EVALUATION

This review consisted of an evaluation of existing data gathered mainly within a short period the 1971 M6.6 San Fernando Earthquake. The magnitude of that event is the same as the MMax value for the fault given by the USGS (USGS, 2008); therefore the damage from that event is similar to what would be expected from the MMax event. Note that the original separation was destroyed as a result of shaking from that event and not ground rupture.

The Separation is near the Sierra Madre Fault Zone (San Fernando fault [Freeway section]) as shown in Figures 1-3. Previous mapping revealed additional faults in this area, but they are not considered active (Yerkes, 1973). Figure 3 shows no faults, either “ancient” or ruptured in 1971, closer than 250 feet to the bridge.

1971 Earthquake

The Separation is discussed in several reports written following this M6.6 earthquake. Prysock and Egan (1971) noted that reverse faulting, north over south on a plane dipping 60N, occurred (primarily in fill) about 250 feet south of the Separation (photos 1 and 2). Smaller scale lateral movement was also observed. Elevation surveys taken pre- and post- earthquake (Yerkes et al, 1974) revealed a regional offset of several feet (Figure 4); so much of the offset that might affect the Separation is distributed over 1000 feet or more.

Air photos taken two days after the earthquake show a dusty repaired area of the northbound lanes of 405 about 250 ft south of the Separation ( shown on maps as fault rupture (Photo 3), and the fault may be traced westward towards the Caltrans Maintenance Station where fault rupture was observed in the parking lot. The fault is visible on the air photos east of 405 as a prominent north- dipping scarp and west of 405 as a plane within a rather broad fault zone (Prysock and Egan, 1981). USGS (1971) describes this and other features farther south as landslide-related, however later publications including Yerkes et al (1974) do not reach that conclusion. .

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Caltrans Foundation Investigation Post 1971 Earthquake

A foundation investigation to supplement a 1962 investigation was carried out in 1971 as part of the reconstruction. The 1971 investigation included better coverage of the bridge footprint, extending farther to the north and south. In particular, the boring completed beneath the northernmost Abutment (A3) revealed tension cracks to 1/32nd “ in roughly the center of a 30 ft high section of fill (Caltrans, 1971). It is unlikely that this feature represents faulting and is probably the result of movement at the Separation during the earthquake and/or the resulting bridge failure.

POTENTIAL FOR FAULT RUPTURE

PFDHA is very useful in areas where uncertainties in trace location and base maps exist. Given that we have photos of the historic fault rupture and previous mapping indicating there are not likely to be any traces closer to the bridge, along with pre-earthquake and post-earthquake vertical surveys indicating much of the offset is distributed over a 1000 ft area. I recommend we assume the rupture will occur where it has occurred previously (a basic tenet of paleoseismology) 250 feet south of the bridge, and we assume that a few feet of accompanying deformation may also occur. No surface rupture is expected at this bridge and no design for surface rupture is required.

RECOMMENDATIONS FOR ADDITIONAL INVESTIGATION

No additional work is recommended. If you have any questions, please contact Martha Merriam at (916) 227-7135.

Prepared by: Date: July 15, 2013

Martha Merriam, C.E.G. Engineering Geologist Office of Geotechnical Support Instrumentation Branch

“Caltrans improves mobility across California” MR. TOM OSTROM Route 5T/405 Separation, #53-1548 July 15, 2013 Fault Rupture Potential Page 4 0000001016 6SSCN cc: Geotechnical Support (GS) Shira Rajendra Geotechnical Design South I John Ehsan GS (Instrumentation Branch) Gem-Yeu Ma Research and Innovation Tom Shantz Earthquake Engineering Fadel Alameddine Earthquake Engineering Mark Yashinsky

REFERENCES

Abrahamson, N., 2008, Appendix C, Probabilistic Fault Rupture Hazard Analysis, San Francisco PUC, General Seismic Requirements for the Design on New Facilities and Upgrade of Existing Facilities.

Barrows, A., 1975, Surface effects and related geology of the San Fernando Earthquake in the foothill region between Little Tujunga and Wilson Canyons, in Oakeshott, G., ed., San Fernando, California, Earthquake of 9 February 1971; CGMG Bulletin 196, 462 pp and 5 plates, pages 97- 117: ftp://ftp.consrv.ca.gov/pub/dmg/pubs/bull/B_196/

Barrows, A., and others, 1974, Surface effects map of the San Fernando Earthquake area, in Oakeshott, G., ed., San Fernando, California, Earthquake of 9 February 1971; CGMG Bulletin 196, 462 pp and 5 plates, Plate 3: ftp://ftp.consrv.ca.gov/pub/dmg/pubs/bull/B_196/

CGS, 2002, Guidelines for evaluating hazard of surface fault rupture, CGS Note 49, 4p: http://www.conservation.ca.gov/cgs/information/publications/cgs_notes/note_49/Documen ts/note_49.pdf

CGS, 1979, Special Studies Zone, San Fernando Quad, http://gmw.consrv.ca.gov/shmp/download/quad/SAN_FERNANDO/maps/SANFERN.PDF

Caltrans, 2013, MTD 20-10, Fault Rupture: http://www.dot.ca.gov/hq/esc/techpubs/manual/bridgemanuals/bridge-memo-to- designer/page/Section%2020/20-10.pdf

Caltrans, 2013, MTD 20-10 Implementation Memo: http://www.dot.ca.gov/hq/esc/techpubs/manual/bridgemanuals/bridge-memo-to- designer/page/Section%2020/20-10memo.pdf

Caltrans, 1971a, The San Fernando Earthquake of February 9, 1971, District 7 Draft Report, June 30, 1971.

Caltrans, 1971b, Reconstruction – Earthquake Damage, memo signed by C. Marek and dated December 3, 1971.

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Caltrans, 1971, Aerial photos (color) taken 2/11/1971: ASC

Dawson, T., and Weldon, R., 2012, UCERF3, Appendix B: Geologic slip-rate data and geologic deformation model (July 9, 2012 draft), http://wgcep.org/sites/wgcep.org/files/AppendixB_GeologicDeformationModel_20120709.pdf

Moss, R., and Ross, Z., 2011, Probabilistic fault displacement hazard analysis for reverse faults: Bull Seism. Soc. Amer., Vol. 101, No. 4, pp. 1542-1553.

Prysock, R., and Egan, J., 1981, Roadway damage during the San Fernando, California Earthquake of Feb.9, 1971: FLWA/CA/LT Report 80-17, 184 pp.

Southern California Earthquake Center (SCEC) Earthquake Catalog: http://www.data.scec.org/eq-catalogs/date_mag_loc.php

US Geological Survey, 2010, http://earthquake.usgs.gov/hazards/qfaults/google.php

US Geological Survey, 2008, National Seismic Hazard Maps – Fault Parameters, http://geohazards.usgs.gov/cfusion/hazfaults_search/hf_search_main.cfm

US Geological Survey, 1971, San Fernando, California, Earthquake, February 9, 1971: Preliminary report published jointly by USGS and NOAA: USGS Professional Paper 733, 254 pp: http://pubs.usgs.gov/pp/0733/report.pdf

Wells, D., and Coppersmith, K., 1994, New empirical relationships among magnitude, rupture length, rupture width, rupture area, and surface displacement: Bull. Seism. Soc. Amer., V. 84, No. 4, pp. 974-1002.

Yerkes, R. and others, 1974, Geologic environment of the Van Norman Reservoirs area, in The Van Norman Reservoirs area, Northern San Fernando Valley, California: USGS Circular 691-A, 35 pp: http://pubs.usgs.gov/circ/1974/0691a/report.pdf

Yerkes, R., and others, 1973, Maps showing faults and ruptures of the Van Norman Reservoirs area, northern San Fernando Valley, California, with a vertical section showing stratigraphy of the surficial deposits at the proposed damsite, USGS Miscellaneous Field Studies Map: 549. Scale 1:4,760: http://pubs.er.usgs.gov/publication/mf549

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Figure 1. Location of Route 5T/405 Separation (within red circle). (Yerkes, 1974)

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Figure 2. 1971 San Fernando Earthquake surface faulting (thick lines) and “ancient faulting” (thin lines) (Yerkes et al, 1974). LD = lower dam; PD = proposed dam.

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Figure 3. 5T/405 Separation (Separation) in relation to the nearest 1971 San Fernando Earthquake fault rupture. Mission Wells, Reservoir, and Freeway faults are the same. Yerkes et al, 1973.

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Figure 4. Changes in elevation after the 1971 Earthquake (Yerkes, 1974). Measurements are meters. See figures 2 and 3 for Separation location.

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Photo 2. Looking easterly across the patched area of Route 5. A break in profile of the median fence may be noted directly in line with the front end of the auto near the photo center. The fault trace is located where dip of marine sedimentary rocks in the foreground changes from vertical to northerly. (Prysock and Egan,1981).

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Photo 3. 5T 405 Separation on 2/11/1971 two days after collapse. North is at top of photo; fault rupture is lighter area on NB 5 in southwest corner of photo trending across the northbound lane of 405 and in line with scarp to east.

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