Identification_Information:
Identification_Information:
Citation:
Citation_Information:
Originator: Ray E. Wells
Originator: Michael J. Rymer
Originator: Carol S. Prentice
Originator: Karen L. Wheeler (database)
Publication_Date: Paper: January 7, 2007
Title: Map showing features and
displacements of the Scenic Drive Landslide, La Honda, California, during the
period March 31, 2005-November 5, 2006
Geospatial_Data_Presentation_Form:
vector digital data
Online_Linkage: http://pubs.usgs.gov/of/2006/1397/
Description:
Abstract: The Scenic Drive landslide in La
Honda, San Mateo County, California began movement during the El Ni–o winter of
1997-98 (Jayko and others, 1998). Recurrent motion occurred during the mild El
Ni–o winter of 2004-05 (Wells and others, 2005) and again during the winter of
2005-06. This report documents the changing geometry and motion of the Scenic
Drive landslide in 2005-2006. The landslide is a complex rotational and
translational earth and weak rock slump with localized slow earthflow behavior
(Varnes, 1978). Because we have digital mapping in successive years, we can
document changes and persistent features that we interpret to reflect
underlying structural control of the landslide. We have also compared the
displacement history to near-real time rainfall history for the continuously
recording La Honda rain gauge LAH for the period October 2004-November 2006.
Purpose: This database and accompanying
graphics files (2 PDFs) depict the structure, topography and history of a
landslide at a 1:400 scale. The report is intended to provide geologic
information for the study of landslide hazards and potential. Changes in
topography over one year of motion are calculated. Estimates of the depth to the
top of the landslide slip surface are determined from well logs, offset drill
holes, and mass balance calculations following the method of Bishop (1999).
Although the scale of this report is large, uncertainties in georeferencing and
absolute elevations provide insufficient detail for site development purposes.
Supplemental_Information:
DIGITAL COMPILATION- Digital map data
released in this report are distributed within an Environmental Systems
Research Institute (ESRI) geodatabase. Data collected during the study of the
landslide (see BASE MAP) and originally cast in State Plane California-III
FIPS-0403 projection, NAD83 were reprojected to UTM, NAD83, zone 10 and are
stored within a single feature dataset (a grouping within a geodatabase of feature
classes with a common spatial reference or logical connection). Other feature
classes in the geodatabase portray features along two lines of cross section
through the landslide. The coordinate system of these feature classes is
technically undefined, but it can be thought of as an un-referenced Cartesian
system. The units represent meters and correspond, along the X axis, to
real-world positions along the line of cross section (from the beginning of the
line at 0 meters to the total length) and, along the Y axis, to the real-world
elevations of features. Each set of feature classes has been organized into a
separate feature dataset by cross section name and year for easier
browsing. The map was exported from the
layout view in ArcMap as an Adobe Illustrator file and added to two Adobe
Illustrator files that show two versions of the map on two map sheets, several
figures, and the two cross sections.
Differences between the maps as they appear in the final map sheet and
as they appear in the either the .mxd or .pmf files represent changes made in
the Adobe Illustrator file to the symbology only and do not reflect any changes
in the actual source data.
BASE MAP- The topographic base map was
created using a Leica differential GPS system with a local base station set up
outside of the moving landslide. We used the differential GPS, a laser
rangefinder, and compass and tape to monitor motion of the Scenic Drive
landslide from April 2005 through November 2006. Part of the landslide was
regraded to improve drainage after motion stopped in the summer of 2005.
Subsequently, motion began again following heavy rains in December 2005, and we
resumed measuring the slide motion. We measured displacement along the
headscarp and along the northwest lateral boundary of up to 13 cm (5 in) on
January 3, 2006. By January 15, displacements of about one meter (3.3 ft) were
visible along the entire slide margin. Between November 6, 2005, and January
16, 2006, our survey monuments had moved as much as 2.5 m. We continued to measure the displacement of
our monument array using differential GPS through the 2005-06 rainy season and
after motion stopped in the summer of 2006. After the landslide stopped moving
in June 2006, we mapped the landslide topography using differential GPS
(measurement uncertainty of 3 cm.) Beneath heavy forest cover, we used a laser
rangefinder (measurement uncertainty 0.5 m) to map critical areas of the toe
and the headscarp that were inaccessible to differential GPS. Even so, there
are impenetrable areas where displacement and topography are poorly resolved.
Position errors in excess of 1 m (3.3 ft) may occur in forested terrain near
the toe of the landslide. Although mapped in metric units, our map is contoured
in feet, and subsurface data are reported in feet, consistent with drillers'
logs and existing work on the landslide. Our measurements are made with respect
to a local reference frame that we established, with a local vertical datum
referenced to the geodetic ellipsoid. We maintained the same datum as the 2005
map (Wells and others, 2005) to more easily quantify relative changes in the
landscape. Our elevations are about 108 feet below elevations referenced to
mean sea level. Some areas beneath trees in the toe area and along parts of the
head scarp have no GPS coverage, and in these areas, the contours do not
reflect the complexity of the ground surface and its deformation.
SPATIAL RESOLUTION- Uses of this digital
geologic map and derivative cross section data should not violate the spatial
resolution of the data. Although the digital form of the data removes the
constraint imposed by the scale of a paper map, the detail and accuracy
inherent in map scale are also present in the digital data. The fact that this
database was edited at a scale of 1:400 means that higher resolution
information is not present in the dataset. Plotting at scales larger than 1:400
will not yield greater real detail, although it may reveal fine-scale
irregularities below the intended resolution of the database. See BASE MAP,
above, for further discussion of uncertainties.
GEOLOGY OF THE LANDSLIDE AREA- This
paragraph is modified slightly from OFR2005-1191 (Wells and others, 2005). The
area of activity in 1998, 2005, and 2006 occurs within an area mapped as a probable
ancient landslide by Brabb and Pampeyan (1972). The landslide area is underlain
by late Miocene and Pliocene Tehana Member of the Purisima Formation, a shallow
marine sequence of interbedded fine sandstone and mudstone that dips gently to
the southwest in the direction of the regional slope (Brabb and others, 1998).
The Purisima Formation is glauconitic at its base, where it unconformably
overlies more steeply dipping Oligocene and Miocene submarine pillow breccia of
the Mindego Basalt (Brabb and others, 1998). These field relations are exposed
just east of the La Honda fire station in a road cut on Highway 84. The head
scarp of the 2005 landslide exposes a 1 m thick, nearly black organic soil
horizon overlying 3-5 m of an orange-brown diamict consisting of weathered
vesicular basalt clasts in a muddy matrix. Locally exposed beneath the diamict
in the head scarp are outcrops of vesicular, weathered pillow basalt and pillow
breccia of the Mindego Basalt. A 2 meter-high exposure of fine sandstone of the
Purisima Formation crops out in a small stream about 70 m southwest of
Recreation Drive. A waterfall over Mindego Basalt occurs about 200 m to the
south off Recreation Drive, where it crosses a small stream. In the landslide mass, a dozen dewatering
wells have been installed since 1998. They encountered landslide debris derived
from both the Purisima Formation and the Mindego Basalt. On Scenic Drive the
wells were drilled through disturbed, mottled, wet, brown to greenish gray fine
sandstone and mudstone of the Purisima Formation containing clay seams and
lenses of cobbles and boulders of basalt. Basaltic debris was more common in
the wells closest to the head scarp. Drilling of the well near Recreation Drive
exposed wet, disturbed, broken, and mottled fine brown and gray sandstone
diamict with horizons containing sparse basalt cobbles to a depth of about 17
feet (R. Wells, field notes). Below that depth, the hole encountered largely
dry, cross-laminated and burrowed, fine-grained, greenish-gray, stiff sandstone
of the Purisima Formation. A trickle of water was noted at 20 feet. The hole
was drilled to refusal at 37 feet (J. de Mouthe, written communication) in
zeolitized basalt pillow breccia probably representing the Mindego Basalt. Well
logs are shown in the cross sections.
LIMITATIONS AND UNCERTAINTIES- This 2006
map represents a snapshot of an active landslide. Changes in the landslide
since 2005 are shown in the report.
Although individual data points are located within a few centimeters at
the time of measurement, the uncertainties may exceed several meters, given the
continued motion of the landslide, the lack of GPS data beneath trees in
critical areas, and artifacts of the contouring method. Property lines and
cultural features aside from portions of Scenic Drive have not been determined.
Survey elevations are derived directly from the differential GPS and are
related to the geodetic ellipsoid. Relative elevations are likely quite precise
where data coverage is good, but the relation of the GPS frame of reference to
local benchmarks has not been determined.
REFERENCES- Bishop, K.M., 1999,
Determination of translational landslide slip surface depth using balanced
cross sections: Environmental and Engineering Geoscience, v. 5; no. 2, p. 147-156.
Hundemer,
C.R., and Upp, R.R., 2006, Geotechnical investigation, landslide mitigation,
roadway restoration, Scenic Drive landslide, San Mateo County, California: Upp
Geotechnology Report to San Mateo County, 42 pp. and 57 figures.
Jayko,
A.S., Prentice, C.S., Rymer, M., and Wells, R.E., 1998, Scenic Drive landslide
of January-March 1998, La Honda, San Mateo County, California: U. S. Geological
Survey Open File Report OF98-229, 1 oversize sheet.
Upp,
R.R., 1998, Geotechnical investigation, Scenic Drive landslide, San Mateo
County, California: Upp Geotechnology Report to San Mateo County.
Varnes,
David J., 1978, Slope movement and types and processes, in, Landslides:
Analysis and Control: Transportation Research Board, National Academy of
Sciences, Washington D.C., Special Report 176, Chapter 2.
Wells,
Ray E., Rymer, Michael J., Prentice Carol S., and Wheeler, Karen L., 2005, Map
showing features and displacements of the Scenic Drive landslide, La Honda,
California, during the period March 31-May 7, 2005: U.S. Geological Survey Open-File Report 2005-1191,
http://pubs.usgs.gov/of/2005/1191.
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Calendar_Date: 20070107
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West_Bounding_Coordinate: -122.269486
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Keywords:
Theme:
Theme_Keyword_Thesaurus: None
Theme_Keyword: landslide
Theme_Keyword: La Honda
Theme_Keyword: California
Theme_Keyword: Scenic Drive
Theme_Keyword: Recreation Drive
Theme_Keyword: geology
Theme_Keyword: GPS
Theme_Keyword: differential GPS
Theme_Keyword: San Mateo County
Theme_Keyword: late Miocene
Theme_Keyword: Pliocene
Theme_Keyword: Tehana Member
Theme_Keyword: Purisima Formation
Theme_Keyword: Oligocene
Theme_Keyword: Miocene
Theme_Keyword: submarine pillow breccia
Theme_Keyword: Mindego Basalt
Theme_Keyword: cross section
Theme_Keyword: slip surface
Theme_Keyword: dewatering wells
Access_Constraints: None
Use_Constraints: Uses of this digital
geologic map and derivative cross section data should not violate the spatial
resolution of the data. Although the digital form of the data removes the
constraint imposed by the scale of a paper map, the detail and accuracy
inherent in map scale are also present in the digital data. The fact that this
database was edited for a scale of 1:400 means that higher resolution information
is not present in the dataset. Plotting at scales larger than 1:400 will not
yield greater real detail, although it may reveal fine-scale irregularities
below the intended resolution of the database. Similarly, where this database
is used in combination with other data of higher resolution, the resolution of
the combined output will be limited by the lower resolution of these data.
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