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FEBRUARY MEETING NOTICE
***Tuesday, February 12th***
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||"Locating the Raymond Fault at Two Developed Urban Sites in the Highland Park District of Los Angeles, California"
||David L. Perry, C.E.G., Senior Engineering Geologist, MACTEC
Engineering and Consulting, Inc.
||Stevenís Steak House,
5332 Stevens Place, Commerce, California
||6:00 p.m.-Social Hour; 7:00 p.m.-Dinner; 7:45 p.m.-Presentation
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The Raymond fault extends 20 km across the densely populated San Gabriel Valley northeast of Los Angeles. Pioneering geologic studies prior to large-scale urbanization (Eckis 1934, Buwalda 1940) mapped the faultís geomorphologic features (south-facing scarps, pressure and shutter ridges, offset streams, and groundwater and vegetation lineaments). East of Arroyo Seco in Pasadena, the fault is expressed by a distinct topographic break in alluvium and differences in groundwater levels on opposite sides of the fault. However, in the Highland Park District west of Arroyo Seco, the fault is poorly expressed and published geologic maps (Lamar, 1970, Weber et al., 1980, Crook et al., 1987, Dibblee, 1989) are inconsistent with respect to the location of the concealed and inferred traces of the fault.
We studied two sites constructed in the 1930ís and 1940ís in the Highland Park area to determine whether they were transected by traces of the Raymond fault; these sites are located partly within an Alquist-Priolo earthquake fault zone. The first site is located within the floodplain of the Arroyo Seco. Site constraints and safety concerns precluded deep trench excavations through the Holocene alluvium and necessitated drilling continuous core borings with a Sonic drill rig to penetrate through recent alluvium containing cobbles and boulders. Two closely spaced fault traces were identified near the edge of the site by the coincidence of the juxtaposition of dissimilar alluvial subunits and abrupt groundwater level differences on either side of the interpreted fault traces. The fault traces identified at this site are probably the southern boundary of the Raymond fault zone.
The second site is located on a geomorphic surface about 30 feet above the modern Arroyo Seco floodplain. Site constraints (existing structures, buried utilities, locally thick fill) necessitated an investigation approach combining trench excavations, several transects of continuously cored hollowstem auger borings, and seismic refraction methods. Groundwater level data obtained from borings on the site and an adjacent property was key to interpreting fault locations. We identified several fault traces on the property through detailed logging of over 900 lineal feet of trench, core from 25 borings, and interpreting two seismic refraction lines.
One trench exposed a near vertical fault juxtaposing dissimilar alluvial subunits and pedogenic (argillic and/or cambic) soil horizons on either side. This fault coincided with a west-trending zone of abrupt groundwater level differences and abrupt stratigraphic changes identified in boring transects located to the east and west of the trench. The fault location corresponds to a published geologic map
(Dibblee, 1989) and appears to be the main Raymond fault trace. We also observed evidence for faults with minor apparent vertical separation in two trenches located about 100 feet south of the inferred main fault.
Stratigraphic and groundwater differences along a boring transect about 800 feet south of the main fault were interpreted to be a subsidiary fault. Its location and orientation across the site were evaluated with seismic refraction surveys employed in combination with the boring data. The subsidiary fault appears nearly coincident with a southerly trace shown by Dibblee (1989).
The combined approach (trenches, core borings, groundwater data, and geophysical surveys) to these investigations was useful in locating the inferred main and subsidiary traces of the Raymond fault on these sites.
|Brief Speaker Biography:
David L. Perry is a Senior Engineering Geologist at MACTEC Engineering and Consulting, Inc. in Los Angeles. He has 24 years of diverse experience on geologic investigations for design and construction monitoring of infrastructure projects throughout California. A majority of these projects have been applying engineering geology for dams, tunnels, water storage reservoirs, water and sewer pipelines,
highway and subway-rail structures, educational institutions, commercial facilities, sanitary landfills, and land development. Major infrastructure projects include the light rail/subway tunnel for the Red-Line of the Los Angeles Metropolitan Transit Authority and the Emergency Water Storage Reservoir Project (Olivenhain Dam) for the San Diego County Water Authority.
He has performed numerous fault-rupture-hazard studies in southern California and Washington; most recently at a site in the Highland Park district of Los Angeles across suspected traces of the Raymond fault. Mr. Perry received his Bachelors degree in Geological Sciences from the University of California at Santa Barbara and completed post-graduate studies in geology at Cal State Los Angeles.