IMPORTING THE ARC EXPORT FILES

The ARC export files with the extension .e00 can be converted to
ARC/INFO vector maps (coverages) and INFO files by running the
import.aml that is included in the database. This will import the
export files and assign standard names (see below). Be sure to
uncompress the of99-504_7a.e00.gz file with the gzip utility before
running the import aml. Run import.aml from the ARC prompt in
the directory containing the export files:

ARC: &run import.aml - run import.aml

Note that the arc coverages and separate INFO files will be given standard names:

of99-504_3.e00 (Landslide Deposit Database) is named al-slid_um
of99-504_4.e00 (Small Landslide Deposit
Database for La Costa
Valley quadrangle only) is named —_Ic-smls_um
of99-504_7a.e00.gz (Quadrangle Index Database) is named al-index_um
20

USGS OF 99-504


Table 6. Line Types Recorded in the LTYPE Field

LINE TYPE DESCRIPTION

contact, certain arcs delineating large landslide polygons
county boundary Alameda County boundary
map boundary quadrangle boundary

e Areas — Landslide units (except those smaller than 500 feet in the La Costa Valley quadrangle)

are recorded as vector polygons with characteristics described in the polygon attribute table (see
Table 7). The primary unit identifier is PTYPE.

Table 7. Content of the Polygon Attribute Table (AL-SLID.PAT)

ITEM NAME WIDTH OUTPUT TYPE N.DEC DESCRIPTION
AREA 4 12 F 3 area of polygon in square meters
PERIMETER 4 12 F 3 length of perimeter in meters
AL-SLID# 4 5 B - unique internal control number
AL-SLID-ID 4 5 B - unique identification number
PTYPE 35 35 Cc - landslide category

¢ Points — Small landslide units in the La Costa Valley quadrangle (maximum dimension 200 to

500 feet) are recorded as point features in ARC/INFO. These features are plotted as arrows
showing the direction of downslope movement and are centered on the location of the slide.
Although the slides range in length from 200 to 500 feet, all arrows are about 200 feet long as
plotted. In both Postscript and PDF plot files, the value for PTTYPE is printed as an annotation at
the head of the landslide arrow for slides attributed as “questionable”.

Table 8. Content of the Point Attribute Table (LC-SMLS.PAT)

ITEM NAME WIDTH OUTPUT TYPE N.DEC DESCRIPTION

AREA 4 12 F 3 area of polygon in square metrs
PERIMETER 4 12 F 3 length of perimeter in meters
PF-SMLS# 4 5 B - unique internal control number
PF-SLID-ID 4 5 B - unique identification number
PTTYPE 35 35 c - landslide category

STRIKE 3 3 I - direction of downslope movement, in

degrees

14
USGS OF 99-504


Counties, California: Calif. Div. Mines and
Geology, Open-file Report 95-15, plate B, map
scale 1:24,000

Nilsen, T.H., 1971, Preliminary
photointerpretation map of landslide and
other surficial deposits of the Mount Diablo
area, Contra Costa and Alameda Counties,
California: U.S. Geol. Survey Misc. Field
Studies map MF-310, scale 1:62,500.

Nilsen, T.H., 1972a, Preliminary
photointerpretation map of landslide and
other surficial deposits of the Altamont and
Carbona 15-minute quadrangles, Alameda
County, California: U.S. Geol. Survey Misc.
Field Studies map MF-321, scale 1:62,500.

Nilsen, T.H., 1972b, Preliminary
photointerpretation map of landslide and
other surficial deposits of the Byron area,
Contra Costa and Alameda Counties,
California: U.S. Geol. Survey Misc. Field
Studies map MF-338, scale 1:62,500.

Nilsen, T.H., 1972c, Preliminary
photointerpretation map of landslide and
other surficial deposits of the Mount Hamilton
quadrangle and parts of the Mount Boardman
and San Jose quadrangles, Alameda and Santa
Clara Counties, California: U.S. Geol. Survey
Misc. Field Studies map MF-339, scale 1:62,500.

Nilsen, T.H., 1973a, Preliminary
photointerpretation map of landslide and
other surficial deposits of the Concord 15-
minute quadrangle and the Oakland West,
Richmond, and part of the San Quentin 7.5-
minute quadrangles, Contra Costa and Alameda
Counties, California: U.S. Geo]. Survey Misc.
Field Studies map MF-493, scale 1:62,500.

Nilsen, T.H., 1973b, Preliminary
photointerpretation map of landslide and

other surficial deposits of the Livermore and.
parts of the Hayward 15-minute quadrangles,
Alameda and Contra Costa Counties,
California: U.S, Geol. Survey Misc. Field
Studies map MF-519, scale 1:62,500.

Nilsen, T.H., Bartow, J.A., Frizzell, V.A., and
Sims, J.D., 1975, Preliminary
photointerpretation maps of landslide and
other surficial deposits of 56 7.5-minute
quadrangles in the southeastern San Francisco
Bay region, Alameda, Contra Costa, ard Santa
Clara Counties, California: U.S. Geolocical
Survey Open-File Report No. 75-277, scale
1:24,000.

Nilsen, T.H., Taylor, F.A., and Brabb, E.F., 1976,
Recent landslides in Alameda County,
California (1940-71)—An estimate of economic
losses and correlations with slope, rairfall, and
ancient landslide deposits: U.S. Geologic
Survey Bulletin 1398, 21 p.

Nilsen, T.H., Wright, R.H., Vlasic, T.C., and
Spangle, W.E., 1979, Relative Slope Stability
and Land-use Planning in the San Francisco Bay
Region, California: U.S. Geologic Survey
Professional Paper 944, 96 p.

Roberts, Sebastian, Barron, Andrew, Bral, E.E.,
and Pike, RJ., 1998, Digital compilation of
“Preliminary map of landslide deposits in
Santa Cruz County, CA, by Cooper-Clark and
Associates, 1975": U.S. Geol. Survey, Open-file
Report 98-792, 22 p.

Wentworth, C.M., Graham, S.E., Pike, R.J.,
Beukelman, G.S., Ramsey, D.W., and Farron,
A.D., 1997, Summary distribution of slides and
earth flows in the San Francisco Bay rezion,
CA: USS. Geol. Survey, Open-file Report 97-745
C, 10 p. plus 11 maps.
<http:wrgis.wr.usgs.gov /open-file/of97-

745 /index.html>.

USGS OF 99-504


The report begins at version 1.00. Any revisions will be noted in the revision list and
will result in the recording of a new version number for the report. Small changes will
be indicated by decimal increments and larger changes by integer increments in the
version number. Revisions will be announced and maintained on the Web page for this
report on the Western Region Geologic Information Server (see next section).

OBTAINING THE DATA FILES

The database may be obtained in three ways.

1. The simplest way to obtain the database is to download it over the World Wide
Web from the USGS Western Region Geologic Information Server:

http://wrgis.wr.usgs.gov

From the main page, click on ‘Geologic map databases’ under the heading "Data On-
line’; next click on ‘California’. Scroll down to the listing for this database (Open File
Report 99-504) and click on the Open-File button, which takes you to the page for this
publication. You can also go directly to that final page at:

http://wrgis.wr.usgs.gov /open-file /of99-504

On this page, the several parts of the report in their different file types are separately
available. Set your Web browser to save to a local disk and click on the appropriate
links to download the desired files.

2. To download the files from the Internet via anonymous ftp:

© ftp wrgis.wr.usgs.gov - make ftp connection with USGS computer wrgis.

¢ Name: anonymous - enter "anonymous" as your user name.

¢ Password: [your address] - enter your own email address as password.

° pub/open-file/of99-504 - subdirectory where files from this ‘report are stored

3. To obtain files from the database on magnetic tape, send a blank tape with your
request specifying the desired files and your return address to:

San Francisco Bay Geologic Materials Database
c/o Database Coordinator

U.S. Geological Survey

345 Middlefield Road MS 975

Menlo Park, CA 94025

18
USGS OF 99-504


Home buyers: Areas with relatively low
densities of landslide deposits probably have
good slope stability compared with areas with
high densities of landslide deposits. However,
landslide deposits less than 200 feet long have
not been mapped, and the scale of this map is
such that individual buildings cannot be
precisely located. In fact, areas mapped as

landslide deposits are not necessarily less stable
than adjacent areas. The map, therefore, should
not be used as a substitute for a report by an
engineering geologist or soils engineer, because
detailed site investigations are necessary for
judgments about the slope stability of irdividual
areas. In addition, other types of surfic‘al
deposits may pose construction problems and
require investigation.

REFERENCES

Eckel, E.B., ed., 1958, Landslide and engineering
practice: Highway Research Board Spec. Rept.
29, NAS/NRC 544, Washington, D.C., 232 p.

Flawn, P.T., 1970, Environmental geology;
conservation, landuse planning, and resource
management: New York, Harper & Row, 313 p.

Goldman, H. B., ed., 1969, Geologic and
engineering aspects of San Francisco Bay fill:,
California Div. Mines and Geology Spec. Rept.
97, 130 p.

Leighton, F.B., 1966, Landslides and hillside
development, in Engineering geology in
southern California: Assoc. Eng. Geologists, Los
Angeles Section, Spec. Publ., p. 149-193.

Mitchell, J. K., 1963, Engineering properties and
problems of the San Francisco Bay mud:
California Div. Mines and Geology Spec. Rept.
82, p. 25-32.

10

Nichols, D. R., and Wright, N. A., 1971,
Preliminary map of historic margins of
marshland, San Francisco Bay, Califcrnia: U.S.
Geol. Survey open-file map, scale 1:12° 000.

Sharpe, C.F.S., 1960, Landslides and related
phenomena: Paterson, N.J., Pageant Books, 137

p.

Terzaghi, Karl, 1950, Mechanism of lan‘slides,
in Paige, S.M., chm., Application of geology to
engineering practice (Berkey volume): New
York, Geol. Soc. America, p. 83-123.

Trask, P.D., and Rolston, J.W., 1951, Engineering
geology of San Francisco Bay, California: Geol.
Soc. America Bull., v. 62, no. 9, p. 107S-1110.

Zaruba, Quido, and Mencl, Vojtech, 1969,

Landslides and their control: Amstercam,
Elsevier, 205 p.

USGS OF 99-504


enough to induce landsliding. Relatively stable
areas may be made unstable as a result of
construction activities that involve cutting or
oversteepening of natural slopes. Engineers,
builders, conservationists, and others concerned
with land use must evaluate the potential effects
of all types of development, and maps that show
the nature and distribution of surficial deposits
should provide much of the basic information
they need.

This map, then, shows the cumulative effects of
various processes that have yielded surficial
deposits up to the time the photographs used for
photointerpretation were taken. It does not
indicate directly areas where processes will be
most active, nor does it show the rate at which
they will operate. However, knowledge of the
history of geologic events is a key to
understanding and predicting the evolution of an
area, even where man's activities significantly
change the character of the land. Almost all new
landslides, for example, occur in areas with a
history of landslide activity.

EXPLANATION FOR LANDSLIDE
DEPOSITS SHOWN ON THE MAP

Landslide deposits: debris composed of fresh and
weathered rock fragments, sediment, colluvial
material, and artificial fill, or any combinations
thereof, that has been transported downslope by
falling, sliding, rotational slumping, or flowing.
Landslide deposits smaller than approximately
200 feet in longest dimension are not shown on the
map.

* (polygon symbol) - Landslide deposit
larger than approximately 200 feet in
longest dimension (except in the La
Costa Valley quadrangle, where the
size cutoff is 500 feet); arrows indicate
general direction of downslope
movement; queried where identification
is uncertain.

* (arrow symbol; La Costa Valley
quadrangle only) - Landslide deposit
between approximately 200 feet and 500
feet in longest dimension; arrow
indicates general direction of downslope
movement, and is positioned over

location of deposit; queried where
identification uncertain.

Complex landslide deposits, which result from
combinations of different types of downslope
movement, are perhaps the most common type of
landslide deposit in the Bay region. In
particular, materials near the head of landslide
deposits typically move in a different manner
than materials at the toe. The landslide
deposits shown on this map have not beer
classified according to either type of movement or
type of material of which the deposit is
composed. The deposits vary in appearance from
clearly discernible, largely unweathered and
uneroded topographic features to indistir ct,
highly weathered and eroded features
recognizable only by their characteristic
topographic configurations. The time of
formation of the mapped landslide deposits
ranges from possibly a few hundred thousand
years ago to 1966. No landslide deposits that
formed since 1966 are shown. The thickness of the
landslide deposits may vary from about 10 feet to
several hundred feet. The larger deposits are
generally thickest; many small deposits may be
very thin and may involve only surficial
materials.

FACTORS AFFECTING MAP
ACCURACY

Date of photography: Modifications of the
landscape that have occurred since the date the
aerial photographs were taken are not shown on
this map. Thus, landslide deposits and large
artificial fills that postdate the photography
are not delineated, although some of the
topographic base maps were photorevised in 1968
and do show the extent of urbanization to that
date.

Urbanization: Surficial geologic features can be
obscured in urbanized areas by (1) modification of
the natural landscape by grading (leveling,
cutting, filling, or terracing), and (2) man-made
structures that cover the natural land surface.
Less than 10 percent of the area included in this
map has been extensively urbanized.

Forest cover: Surficial deposits may be difficult
to recognize in forested areas, so that such areas

USGS OF 99-504


I.

THE SOURCE MAP

The following excerpts from Nilsen (1972c) are reproduced verbatim.

INTRODUCTION

The nine San Francisco Bay region counties lie
within a geologically active, young, and dynamic
part of the central and northern Coast Ranges of
California. Significant movements of the earth's
crust are occurring here at the present time, posing
numerous problems to urbanization, including
some of special concern. Geological processes such
as fault movements, earthquakes, land
subsidence, landsliding, slow downslope
movement of bedrock and surficial materials,
coastal and stream erosion, flooding, and.
sedimentation are all potentially hazardous.
Because of these factors, an understanding of the
operation of physical processes in the Bay region
is desirable for harmonious, efficient, and safe
land-use planning, particularly now, with
greatly expanded pressures for urban growth.

This map presents preliminary information about
one aspect of the physical environment necessary
to sound land-use planning; the nature and
distribution of surficial deposits. Because
surficial deposits are common and well developed
in much of the Bay region, it is useful to know
how and why they have formed, as well as what
properties they possess. When maps like this are
used in combination with other types of
environmental information, such as data on soils,
bedrock geology, slopes, vegetation, climatic
variation, seismic response, and hydrology, it
should be easier to arrive at sound decisions
regarding the physical aspects of land use. The
US. Geological Survey is studying many of these
factors in the Bay region and hopes to provide
the community with much of the required
information as part of its San Francisco Bay
Region Study in cooperation with the
Department of Housing and Urban Development.

The representation of surficial deposits on this
map reflects the way in which a geologist,
working exclusively with aerial photographs,
interpreted the origin of various elements of the

present landscape. The deposits shown here
have not been examined in the field. However,
by viewing overlapping vertical aerial
photographs through a stereoscope, the geologist
sees a three-dimensional relief model of the
ground surface and can study and interpr-t the
origins of landforms with considerable ease. In
fact, for mapping surficial deposits, particularly
in reconnaissance-type studies,
photointerpretation has advantages over both
ground observations and laboratory studies of
surficial materials. Of course, better information
can be provided when all aspects of the study are
integrated. These preliminary photo
interpretation maps are only the first steze in a
detailed study of surficial deposits in tre Bay
region, but they should provide land-use planners
with immediately useful information about the
regional distribution of landslide and other
surficial deposits.

This map indicates the dominant surficial
processes that have probably been operetive by
showing the distribution of different types of
surficial deposits. Processes such as weathering,
erosion, and the slow as well as rapid downslope
movement of earth materials have cons‘antly
reshaped the land surface in the past ard will
continue to in the future, although at varying
rates. The processes are interrelated to varying
degrees. For example, crustal uplift of the Coast
Ranges will lead to increased erosion and
downcutting by streams that in turn generally
results in increased deposition of sediments in
river valleys, lakes, and shoreline areas. Older
flood plains and river deposits may be e~oded,
leaving elevated terrace deposits. In addition,
downcutting, by streams may cause adjacent
slopes to become unstable, thereby increasing the
possibility of slope failures.

Man's activities can alter natural physical
processes in many ways. Simple acts such as
overwatering a lawn or placing a septic tank
drainfield in ground that is marginally stable
may weaken the bedrock and surficial materials

USGS OF 99-504


The specified files bundled in a compressed tar file will be returned to you
on the tape. The acceptable tape types are: 2.3 or 5.0 GB, 8 mm Exabyte tape.

OPENING THE DATABASE FILES

Some of the files are packaged as tar files, and the larger files containing
the databases and images have been compressed with gzip. Thus, gzip
is required to uncompress the files, and a tar utility is required to open
the tar files. The necessary utilities are available on-line:

Compressed Gzip Files

Files compressed with gzip (those with a .gz extension) can be
uncompressed with gzip. The gzip utility converts the compressed file
name.gz to its uncompressed equivalent name. The compressed file is
replaced by the uncompressed file. This utility is free of charge over
the Internet at:

http: / /w3.teaser.fr/*jlgailly / gzip

Tar Files (UNIX Tape Archive

To extract the contents of a tar file, first uncompress it with gzip if the
extension is .tar.gz. Once the tar extension is exposed, extract the
contents with a tar utility. This utility is included in most UNIX
systems. Tar utilities for PC and Macintosh can be obtained free of
charge via the Internet from Internet Literacy's Common Internet File
Formats Web Page:

http:/ /www.matisse.net/files /formats.htm

WinZip

This commercial package runs on PCs and can deal with both gzip and
tar files. An evaluation copy of WinZip for Windows 3.1, 95 and NT
can be downloaded from:

http:/ /www.winzip.com/winzip/:

19
USGS OF 99-504


minerals, causing volume changes in the
material, and mixes with fine-grained
unconsolidated materials to produce wet,
unstable slurries;

(5) ground shaking - strong shaking during
earthquakes can jar and loosen bedrock and
surficial materials, thus making them less
stable;

(6) type of vegetation - trees with deep
penetrating roots tend to hold bedrock and
surficial deposits together, thereby
increasing ground stability;

(7) proximity to areas undergoing active erosion -
rapid undercutting and downcutting along
stream courses and shorelines makes slopes
in these areas particularly susceptible to
landsliding.

All the natural factors that promote landsliding
are present in the Bay region. In addition, man
has at times decreased the potential for slope
failures by leveling slopes, building retaining
walls at the base of slopes, planting trees or
seeding forests, as well as practicing soil
conservation. However, other of his activities
have increased the potential for slope failures,
including increasing slope angles for road or
building construction; adding water to marginally
stable slopes by watering lawns, improperly
handling rain-water runoff and choosing poor
sites for septic tank drainfields; adding to the
weight of marginally stable slopes by building
structures as well as by adding fill for
foundations; and removing natural vegetation.
Thus, slope failure, a natural phenomenon that
has occurred throughout the Bay region in the
past, may be aggravated by improper land use.

The landslide deposits shown on the map may or
may not be continuously or intermittently moving
at the present time. The potential for continued
movement varies greatly and depends on many
factors, including the age of the deposits and
their previous histories of activity. Some
deposits may pose no problems for many types of
development, while development on others may
offer serious problems. Most landsliding takes
place in areas where landsliding has occurred
before, and old landslide deposits are commonly
reactivated by either natural or artificial means.
The materials that form landslide deposits may
be so broken up and disturbed that landsliding
may easily recur, especially if slope angles or

moisture contents are changed. Landslide
deposits are characterized by

(1) small isolated ponds, lakes, and other closed
depressions;

(2) abundant natural springs;

(3) abrupt and irregular changes in slope and
drainage pattern;

(4) hummocky irregular surfaces;

(5) smaller landslide deposits that are
commonly younger and form within older and
larger landslide deposits;

(6) steep, arcuate scarps at the upper edge of the
deposit;

(7) irregular soil and vegetation patterns;

(8) disturbed vegetation; and

(9) abundant flat areas that might appear
suitable as construction sites.

In general, fewer of these characteristics will be
noted in the smaller deposits. Detailed g-ound
studies, of course, are required for predicting the
future behavior of landslide deposits under
changing conditions.

SUGGESTIONS FOR MAP USERS
Planning departments and developers: T-e

density of landslide deposits is a crude measure of
the importance of slope failure as an erosional
process and, therefore, a measure of the overall
slope stability of an area. However, this map
cannot be used to determine the probability of
future landsliding, primarily because geclogic
and climatic changes during the past few hundred
thousand years have altered slope stability and
because the map does not provide detailed
information regarding the composition and type
of movement of individual landslide deposits.
Therefore, the map should not be used as a
substitute for detailed site investigations by
engineering geologists and soils engineers; areas
susceptible to landslide activity should l=
carefully studied before any developmert.

Geologists and engineers: This map has keen
prepared to provide a regional context for
interpreting detailed site investigations and
should be used in conjunction with slope maps,
bedrock geology maps, soils maps, and other
available information. It is not intended as a
substitute for site investigations, and its
limitations should be clear. Comments regarding
its usefulness and accuracy would be apreciated.

USGS OF 99-504


ZUSGS

science for a changing world

LANDSLIDES IN ALAMEDA COUNTY,
CALIFORNIA, A DIGITAL DATABASE
EXTRACTED FROM PRELIMINARY
PHOTOINTERPRETATION MAPS OF
SURFICIAL DEPOSITS BY T.H. NILSEN

IN USGS OPEN-FILE REPORT 75-277

by
Sebastian Roberts', Michelle A.
Roberts’, and Eileen M. Brennan’

with a preface by

Richard J. Pike’ and
Sebastian Roberts'

Open-File Report 99-504

Version 1.0

1999

U.S. DEPARTMENT OF THE INTERIOR
U.S. GEOLOGICAL SURVEY

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards
or with the North American Stratigraphic Code. Any use of trade, product, or firm names is for descriptive purposes
only and does not imply endorsement by the U.S. Government.

This database, identified as ‘Landslides in Alameda County, California, a digital database extracted from preliminar,
photointerpretation maps of surficial deposits by T.H. Nilsen in USGS Oper-file Report 75-277, has been approved for
release and publication by the Director of the UdGS. Although this database has been reviewed and is substantially
complete, the USGS reserves the right to revise the data pursuant to further analysis and review. This database is
released on condition that neither the USGS nor the US. Goverment may be held liable for any damages resulting from
its use.

1 Menlo Park, CA



Table 1. General PTYPE and PTTYPE Categories

Descriptions of landslide categories are

Is

Isu

modified from Landslide Map Annctation
- landslide deposit

- landslide deposit, identification
uncertain

SPATIAL RESOLUTION

Uses of this digital map should not violate the
spatial resolution of the data. Although the
digital form of the data removes the physical
constraint imposed by the scale of a paper map,
the detail and accuracy inherent in map scale are
also present in the digital data. Because this
database was extracted from maps at a scale of
1:24,000 and compiled at that same scale, higher-
resolution information is not present.

Enlargement of the database to scales larger than
1:24,000 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 combine¢ output
will be limited by the lower resolution cf these
data.

SPECIFICS OF THE LANDSLIDE-DEPOSIT DATABASE

The landslide-deposit spatial database itself
consists of two different data layers each with
the naming convention <pf>-extn. The prefix
abbreviation <pf> is either <al> for Alameda
County or <Ic> for the La Costa Valley
Quadrangle and the abbreviation <extn> is a
data layer extension (suffix). The two data
layers are described in Table 2 below. The map
layer is stored in UTM projection (zone 10) (see

Table 3), and projection files are included to
convert between that and the state plane
projection. Digital tics define a 7.5 minute grid of
latitude and longitude (see 7.5 MINUTE
QUADRANGLE GRID, below). The contents of
the map database are described in terms of the
lines, areas, and points that compose it. Terms in
Table 4 describe the database fields.

Table 2. Data Layers

DATA LAYER NAME

al-slid_um

Ic-smls_um

DESCRIPTION

polygons for landslides, internal quadrangle
boundaries, and Alameda County beundary

point features for small landslides for the La Costa
Valley Quadrangle

12
USGE OF 99-504


II.

DIGITAL COMPILATION

INTRODUCTION

We compiled the landslide-deposit database at
a scale of 1:24,000 in version 7.1.1 of ARC/INFO,
a commercial geographic information system, or
GIS (Environmental Systems Research Institute
[ESRI], Redlands, California), on a UNIX
computer using the menu interface ALACARTE
(versions 1 through 3.1: Fitzgibbon and
Wentworth, 1991; Fitzgibbon, 1991; Wentworth
and Fitzgibbon, 1991).

The digital compilation was derived from
linework inked directly by T.H. Nilsen on 25
USGS Mylar greenline 7.5-minute (1:24,000-scale)
quadrangles that cover Alameda County and
were used to photographically reproduce the
lines on his 1975 and 1976 maps. The linework for
each greenline was scanned (400 dots per inch),
converted from raster to vector form, imported
into ARC/INFO, hand edited to remove all
information save landslides, and combined into a
single digital file. Landslide outlines were
adjusted as needed to align across quadrangle
boundaries. The base map for the landslide data

comprises topography, drainage, and culture from
1:125,000-scale USGS Bay Region Topographic
Sheets (Aitken 1997). A 1:62,500-scale map
image was derived from the digital database.

The boundary of Alameda County (from
Wentworth, 1997) was added to the database to
delineate the county from the surrounding area.
Landslides outside the county boundary are
excluded from this database. The 25 constituent
USGS quadrangles are: Richmond, Briones
Valley, Oakland East, Las Trampas Ridge,
Diablo, Tassajara, Byron Hot Springs, Clifton
Court Forebay (formerly Bethany), San Leandro,
Hayward, Dublin, Livermore, Altamont,
Midway, Newark, Niles, La Costa Valley,
Mendenhall Springs, Cedar Mountain, Lone Tree
Creek, Milpitas, Calaveras Reservoir, Mt. Day,
Eylar Mountain, and Mt. Boardman. Nilsen
mapped no landslides in four additional
quadrangles in western Alameda County—
Oakland West, Hunters Point, Redwood Point,
and Mountain View.

LANDSLIDE-DEPOSIT UNITS

In all quadrangles but one, landslides larger than
200 feet in maximum dimension are represented in
the spatial database by polygons enclosing their
approximate boundaries. In the La Costa Valley
quadrangle, Nilsen denoted landslides 200-500
feet across by a small arrow. In the digital
database, small landslides for that quadrangle
are represented by an arrow; they are plotted on
the landslide-deposit map from the Postscript
and PDF plot files included with the database.
The arrow points in the direction of downslope
movement and is centered on the location of the
deposit. Although smaller landslides in the La
Costa Valley quadrangle range from 200 to 500
feet, all arrow symbols are the same size, about
200 feet long. The primary landslide identifier in

11

the La Costa Valley quadrangle is a character
string in the field PTYPE (polygon type) for the
large landslide data layer and PITYPE (point
type) for the small landslide data layer.

Values in the PTYPE and PTTYPE fields give the
two levels of certainty Nilsen ascribed to his
interpretations. The values for this fielc are
‘Landslide deposit’ and ‘Landslide deposit,
identification uncertain’. Comparatively few
deposits are ‘uncertain’. In both the Postscript
and PDF plot files, large landslide polygons are
color coded accordingly; for the small landslides
in the La Costa Valley quadrangle, the
classification is printed at the head of each
arrow as annotation.

USGS OF 99-504


may be mapped less accurately than grass-
covered areas. Many landslide deposits may be
impossible to recognize on slopes covered with
dense stands of tall trees. Less than 15 percent of
the area included in this map is densely forested.

Quality of photography: The accuracy of the

map varies directly with the clarity and contrast
of the aerial photographs used. Accordingly,
haze, cloud cover, or poor sun angles make
photointerpretation more difficult; also, the
steepness of the topography and the location and
extent of shaded areas-affect the usefulness of
individual photographs. In general, however,
the photographs used to prepare this map are of
excellent quality.

Scale of maps and photography: Landslide and
other surficial deposits less than about 200 feet

long are not shown because they are too small to
be clearly identified on the photographs or
clearly portrayed on the topographic base map.
In addition, no attempt has been made to show
the numerous small areas covered by artificial
fill along highways, railroads and airstrips, in
cemeteries, in populated and farming areas, or
near quarries and mines, even though some are
more than 200 feet in longest dimension.

Problems in interpretation: Mapping of surficial
deposits by photointerpretation alone presents a

number of difficult problems, some of which can
be resolved only through field checking.
Problems that are especially difficult include:

(1) distinguishing terrace-shaped slump-type
landslide deposits from alluvial terrace
deposits where both are located adjacent to
stream courses;

(2) recognizing bedrock cropping out beneath
surficial deposits, especially where a creek
or stream has cut down through the
overlying surficial deposits to expose
bedrock along the stream bed;

(3) determining boundaries between adjacent
surficial deposits that laterally grade into
or interfinger with one another without
leaving any easily discernible topographic
boundaries, e.g., the downstream gradation
of alluvial terrace deposits into alluvial
deposits;

(4) recognizing landslide deposit boundaries--
whereas the upslope boundary is commonly
defined by an easily recognized scarp, the

toe or downslope boundary is seldom well
defined and is difficult to locate exactly;

(5) recognizing stable masses of bedrock within
landslide deposits, especially where the
bedrock may appear only as a large block
within the surrounding landslide deposit;
and

(6) distinguishing between irregular or
hummocky topography caused eithe~ by
variations in the erosional resistance of
bedrock or by the erosion of landslide
deposits.

CHARACTERISTICS OF SURFICIAL
DEPOSITS RELEVANT TO LAND-USE
PLANNING

General background: The physical properties and
engineering characteristics of the mapped

surficial deposits can be inferred from knowledge
of the geologic processes that formed then. Thus,
with the information provided by this map,
preliminary evaluations of the significance of
the materials and processes with regard to land-
use decisions can be made.

Landslide deposits: Landslides occur wren the
pull of gravity on earth materials overccmes
their frictional resistance to downslope
movement. Slope stability is affected by

(1) type of earth materials - unconsolidated, soft
sediments or surficial deposits will move
downslope easier than consolidated, hard
bedrock;

(2) structural properties of earth materials - the
orientation of the layering of some rocks and
sediments relative to slope directions, as
well as the extent and type of fracturing and
crushing of the materials, will affect
landslide potential;

(3) steepness of slopes - landslides occur more
readily on steeper slopes;

(4) water - landsliding is generally more
frequent in areas of seasonally higt
rainfall, because the addition of water to
earth materials commonly decreases their
resistance to sliding; water decreas>s
internal friction between particles, decreases
cohesive forces that bind clay minerals
together, lubricates surfaces along which
slippage may occur, adds weight to surficial
deposits and bedrock, reacts with some clay

USGE OF 99-504


consultants also is not included here, nor are
landslides mapped more recently by the
California Division of Mines and Geology
(Majmundar, 1991a, b; 1995a, b).

Because landslides in the database were mapped
remotely, from aerial photographs, Nilsen
ascribed two levels of certainty to his
interpretations. Deposits are queried "?" where
identification was uncertain. The 8465 deposits
are not attributed as to type of movement, but
consist primarily of slow-moving slides and earth
flows (Wentworth and others, 1997). Their time
of formation ranges from perhaps a few hundred
thousand years ago to 1966. Deposit thickness
ranges from about three meters to a few hundred
meters; larger deposits are thicker; many small
deposits may be thin and involve surficial
materials only. Debris flows are absent from the
database (The January 3-5, 1982, storm alone
triggered many such failures; Ellen and others,
1997). Locations in Alameda County that might
be susceptible to this type of small landslide are

indicated in a recent terrain-based mathematical
model that predicts source areas for debris flows
(Ellen and others, 1997). Nilsen and others (1976)
offer further background on slope-failure
problems in Alameda County.

This report comprises three separate and distinct
parts. The first reproduces excerpts from T.H.
Nilsen's explanatory text printed on the source
maps. Part II describes compilation of the source
maps as a digital database and the conventions
adopted in encoding the landslide information in
a geographic information system (GIS). The final
section describes details of the digital-map
database, its constituent data files, and their
computer formats. Part III further lists the steps
required to obtain the contents of the landslide-
map database, and the 1:62,500-scale paper plot
derived from it, from USGS. Roberts and others
(1998) is an earlier report in this series of digital
re-releases of mapped USGS and other landslide
inventories for San Francisco Bay Region counties.

REFERENCES

Camerotto, O.G., and Johnson, S.S., 1972, Public
works engineering: San Francisco Bay Region
Study—multi-disciplinary study, first meeting,
Menlo Park, CA, October 11, 1972, unpublished
typescript of presentation, 16 p.

Coe, J.A., Godt, J.W., Brien, Dianne, and Houdre,
Nicolas, 1999, Map showing locations of
damaging landslides in Alameda County,
California, resulting from 1997-98 El Nifio
rainstorms: U.S. Geological Survey,
Miscellaneous Field Studies Map MF-2325-B,
scale 1:125,000.

Ellen, S.D., Mark, R.K., Wieczorek, G.F.,
Wentworth, C.M., Ramsey, D.W., and May,
T.E., 1997, Map showing principal debris-flow
source areas in the San Francisco Bay region,
CA: U.S. Geological Survey, Open-File Report
97-745 E. <http:wrgis.wr.usgs.gov/open-

- file/of97-745/index.html>.

Majmundar, H.H., 1991a, Landslides and related
features, in Landslide hazards in Livermore

Valley and vicinity, Alameda and Contra
Costa Counties, California: Calif. Div. Mines
and Geology, Open-file Report 91-2, plates B1
and B2, map scale 1:24,000.

Majmundar, H.H., 1991b, Landslides and related
features, in Landslide hazards in the Tassajara
and Byron Hot Springs 7.5' quadrangles,
Alameda and Contra Costa Counties,
California: Calif. Div. Mines and Geclogy,
Open-file Report 92-5, plate B, map szale
1:24,000.

Majmundar, H.H., 1996a, Landslides and related
features, in Landslide hazards in the Hayward
quadrangle and parts of the Dublin quadrangle,
Alameda and Contra Costa Counties,
California: Calif. Div. Mines and Geclogy,
Open-file Report 95-14, plate B, map scale
1:24,000.

Majmundar, H.H., 1996b, Landslides and related
features, in Landslide hazards in the Las
Trampas Ridge quadrangle and parts of the
Diablo quadrangle, Alameda and Cortra Costa

USGS OF 99-504


REFERENCES CITED

Aitken, D.S., 1997, U.S. Geological Survey Bay
Region topographic Sheets 1:125,000 1970: U.S.
Geological Survey, Open-File Report 97-500.

Fitzgibbon, T.T., 1991, ALACARTE installation
and system manual (version 1.0): U.S.
Geological Survey, Open-File Report 91-587 B.

Fitzgibbon, T.T., and Wentworth, C.M., 1991,
ALACARTE user interface - AML code and

demonstration maps (version 1.0): U.S.
Geological Survey, Open-File Report 91-587 A.

Wentworth, C.M., 1997, General Distribution of
Geologic Materials in the San Francisco Bay
Region, California 1: 125,000. A Digital Map
Database: U.S. Geological Survey, Open-File
Report 97-744.

Wentworth, C. M., and Fitzgibbon, T.T., 1991,
ALACARTE version 1 through 3.1: U.S.
Geological Survey, Open-File Report 91-587 C.

II.
THE DIGITAL DATABASE

INTRODUCTION

This report consists of a 1:24,000-scale digital
database, a 1:62,500-scale digital map image
derived from it entitled "Landslides in Alameda
County, California, a digital database extracted
from preliminary photointerpretation maps of
surficial deposits by T.H. Nilsen in USGS Open-
file Report 75-277", and supporting files. The
report is stored as several digital files, for the
spatial data both ARC export (uncompressed)
and ARCVIEW shape formats, and for the map
image both Postscript and PDF formats. The
exported ARC coverages lie in UTM zone 10
projection and the shape versions are coded in
decimal degrees. This pamphlet, which only
describes the content and character of the digital
map database, is included as Postscript, PDF, and
ASCII text files and is also available on paper as
USGS Open-File Report 99-504. A plotted copy of
the 1:62,500-scale map can be ordered from a
USGS Earth Science Information Center or by

15

phone at 1-800-USMAPS. Any or all of the
digital files can be obtained over the Internet or
by magnetic tape copy, as described at the end of
this section.

The full versatility of the 1:24,000-scale spatial
database is realized by importing the AP-C
export files into ARC/INFO or an equivelent GIS.
Other GIS packages, including MapInfo end
ARCVIEW, can use either the ARC export or
shape files. The Postscript map image can be
used for viewing or plotting in computer systems
with sufficient capacity, and the considerably
smaller PDF image files can be viewed or plotted
in full or in part from Adobe ACROBAT software
running on Macintosh, PC, or UNIX platforms.
Please note that the PDF version contains two
plot files (sheets 1 and 2) because of limits to the
dimensions of PDF plots.

USGS OF 99-504


6. Supporting files for ARC/INFO use, bundled as one tar file.
When opened, the tar file yields:

utm2sp.prj and sp2utm.prj: These are projection files to convert between the UTM zone 10
projection of the database and state plane projection.

import.aml: This is an ASCII script written in Arc Macro Language for converting the ARC
export files into usable coverages and INFO files that are assigned standard names (see
IMPORTING THE ARC EXPORT FILES).

0f99-504_6.tar

7. Quadrangle Index Database: The data files representing lines and polygons of the
quadrangle index (ARC export and ARCVIEW shape format). The ARC version
also includes quadrangle names as annotation.

of99-504_7a.e00.gz ARC export coverage containing lines, polygons, and
annotation. Import.aml will name this coverage al-
index_um.

0f99-504_7b.tar.gz Line and polygon ARCVIEW shape files bundled as one
tar file. When opened, the tar file yields:

line files: grdins.dbf, grdins.shp, grdins.shx
polygon files: grdpys.dbf, grdpys.shp,  grdpys.shx

8. Plot Files for the Map: Landslide Deposits in Alameda County, which measures
36 by 53 inches when plotted. Note: The PDF version consists of two sheets
because of limits to the dimensions of PDF plots.

0f99-504_8a.ps.gz Postscript file (12 MB compressed to 3 MB)
of99-504_8b1.pdf PDF file Sheet 1 of 2 (2 MB)

(western half of Alameda County)
0f99-504_8b2.pdf PDF file Sheet 2 of 2 (2 MB)

(eastern half of Alameda County)

REVISIONS

Changes to any parts of the report (the numbered items described above and listed in
the revision list of99-504revs_a.txt) may be made in the future if needed. These could
involve, for example, fixing files that don't work, correcting or adding landslide
details, or adding new file formats or other components. Major revision of the basic
landslide information would result in a new report.

17
USGS OF 99-504


ABSTRACT

All or part of 25 7.5-minute quadrangles identifying 8465
landslides—largely slow-moving slides and earth flows—
in Alameda County, California, have been converted to a
digital-map database, compiled at 1:24,000 scale and plotted
at 1:62,500 scale, that can be acquired from the U.S.
Geological Survey over the Internet or on magnetic tape.

PREFACE

Public agencies in the 1960’s became increasingly
aware of their responsibility to take geologic
hazards into consideration when planning future
regional development. One reason for this
growing awareness was a series of court decisions
that increased the liability of public agencies in
cases where property was damaged by natural
hazards—and the agency had been involved in
approving the affected site for development.
Another was the realization that many geologic
hazards can be anticipated and thus can be used
to make informed development decisions
(Camerotto and Johnson, 1972). As a result of this
shift in attitude the San Francisco Bay Region
Environmental and Resources Planning Study
(SFBRS) was formed in 1970 with support from
the U.S. Geological Survey (USGS) and the
Department of Housing and Urban Development's
Office of Policy Development and Research. The
study sought to provide, in forms understandable
and usable by non-specialists, the earth-science
information needed to solve problems related to
growth and development in the nine-county,

7 A400-square-mile San Francisco Bay Region
(Nilsen and others, 1979).

Among the scores of publications resulting from
the SFBRS effort was a 1:24,000-scale
photointerpretive inventory of (predominantly)
old and ancient landslides measuring over 200
feet across and other surficial geologic deposits in
the southeastern part of the Bay Region (Nilsen
and others, 1975). The present report is a
1:24,000-scale digital database that reproduces
the landslide content (n = 8465) of that inventory
for Alameda County. The area included by the
database was mapped by Nilsen only.

Several series of vertical aerial photogrephs
taken for USGS were used by Nilsen to prepare
the original maps; series BUU taken in 1958,
1959, and 1966 at the scale of 1:20,000, ser‘es GS-
Yf taken in 1953 at the scale of 1:23,600, and
series GS-JL taken in 1949 at the scale of 1:23,600.
In addition larger scale maps at the scale of
1:80,000 from series GS-VCMI were used cver the
entire county.

The 1:24,000-scale landslide-inventory maps
that cover Alameda County had been drafted
prior to 1975 and released in the SFBRS
publications series as six paper maps at 1:62,500-
scale (Nilsen, 1971, 1972a-c, 1973a, b; see also,
Nilsen and others, 1976). These coarser-scale
maps do not contain the fine detail of the 1:24,000
source maps—all of which is, however, preserved
in the digital database of this report. A
generalized broad-scale compilation of landslide
deposits in Alameda and nine other Bay Pegion
counties was also published as a paper map by
Nilsen and others (1979) and in digital-map
format by Wentworth and others (1997).

Although the information in this database has
not been updated or amended beyond that in the
original publication, we are releasing it in
digital form because it is the only systematic
landslide-inventory that covers Alameda County
in its entirety. Users are cautioned that these 35-
to 40-year-old data have shortcomings in
addition to caveats cited in the original map text
(see Part I, below). Information on the hazard is
incomplete; no slope failures since 1966 are
shown. The many landslides from the severe
winters of 1982, 1983, and 1998 (Coe and others,
1999) are absent. Detailed mapping by frivate

USGS OF 99-504


Cover illustration: Reactivated 40-hectare (100-acre) earth flow
on Mission Peak, in southern Alameda County, which
threatened several homes in the town of Fremont in the spring
of 1998, when it moved downslope at the rate of a meter per day.
The Mission Peak slide is part of a much larger landslide
complex initially mapped by Nilsen (1973b) and included in the
digital database released in this report. Photo 1998 by J.D. Rogers

USGS OF 99-504


Table 3. Map Projection

Projection UTM (universal transverse mercator)

Units meters

Zone 10

Datum NAD27

spheroid Clarke 1866

Table 4. Field Definition Terms
ITEM NAME NAME of the DATABASE FIELD (item)

WIDTH maximum number of digits or characters stored

OUTPUT output width

TYPE B - binary integer, F - binary floating poirt number,
N - ASCII floating point number, I - ASCII
integer, C - ASCII character string

NDEC number of decimal places maintained for floating
point numbers

e Lines — Database lines (arcs) are recorded as strings of vectors with characteristics that are

described in the arc attribute table (see Table 5). In the al-slid_um map layer they define the
boundaries of the landslide units, the quadrangle boundaries, and the Alameda County Boundary.
These distinctions are recorded in the LTYPE database field according to the line types listed in

Table 6.

Table 5. Content of the Arc Attribute Table (AL-SLID.AAT)

ITEM NAME WIDTH OUTPUT

FNODE#
TNODE#
LPOLY#
RPOLY#
LENGTH
PF-SLID#
PF-SLID-ID
LTYPE

a

8

auan

a

35

TYPE
B
B
B
B
F
B
B
c
13

N.DEC DESCRIPTION

- starting node of arc (from noe)
- ending node of arc (to node)

- polygon to the left of the arc

- polygon to the right of the erc
3 length of arc in meters

- unique internal control number
- unique identification numbe~

- line type

USGS OF 99-504


DATABASE CONTENTS

The eight sets of digital files comprising the database are encoded in more than one
format. Names of the files are unique designators based on the report identifier, of99-
504, followed by part numbers (i.e. 1 through 8 below) and an extension indicating file
type. Some files have been bundled using the tar (UNIX Tape Archive) utility (.tar
extension). All larger files have been compressed with the gzip utility, indicated by
the .gz extension. The files and their identities are as follows:

1. Revision List: A list of the parts of the report and at what version number of the
report each was last revised (if at all) followed by a chronological list that describes
any revisions (see REVISIONS, below).

of99-504revs_la.txt ASCII file

2. Open File Text: The open-file pamphlet (this text), which describes the database
and. how to obtain it.

o0f99-504_2a.txt ASCII file
of99-504_2b.ps Postscript file
0f99-504_2c.pdf PDF file

3. Database for Landslide Deposits: ARC export coverage containing both lines and
polygons. Import aml will name this coverages al-slid_um.

of99-504_3.e00.gz Arc export coverage
4. Database for Small Landslide Deposits: ARC export coverage for La Costa Valley
quadrangle containing points. Import aml will name this coverage Ic-smls_um.
o0f99-504_4.e00.gz Arc export coverage

5. Line and polygon ARCVIEW shape files bundled as one tar file. When opened,
the tar file yields:

-line files al-arc.dbf al-arc.shp and al-arc.shx
-polygon files al-poly.dbf al-poly.shp and al-poly.shx
-point files Ic-smls.dbf lc-smls.shp and Ic-smls.shx

Note: Point files exist only for the La Costa Valley quadrangle.

of99-504_5.tar.gz ARCVIEW shape files

16
USGE OF 99-504