1

POSSIBLE COSTS ASSOCIATED WITH INVESTIGATING AND

MITIGATING SOME GEOLOGIC HAZARDS IN RURAL PARTS OF

SAN MATEO COUNTY, CALIFORNIA

By

Earl E. Brabb1, Sebastian Roberts1, William R. Cotton2, Alan L. Kropp3,

Robert H. Wright4, and Erik N. Zinn5

With a section on

USING THE USGS WEBSITE TO DETERMINE THE POSSIBLE COST OF

INVESTIGATING LANDSLIDE HAZARDS IN RURAL PARTS OF SAN

MATEO COUNTY, CALIFORNIA

By

Sebastian Roberts1 and Earl E. Brabb1

Digital database by

Sebastian Roberts, Suzanne K. Mills, Jason B. Barnes, and Joanna E. Marsolek

Open-File Report 00-127

2000

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 ‘Possible Costs Associated With Investigating and Mitigating

Some Geologic Hazards in Rural Parts of San Mateo County, California’, has been

approved for release and publication by the Director of the USGS. 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 U.S. Government may be held liable for any damages resulting

from its use.

1) U.S. Geological Survey, Menlo Park, CA

2) Cotton Shires and Associates, Los Gatos, CA

3) Alan Kropp & Associates, Berkeley, CA

4) Mill Valley, CA

5) Zinn Geology, Santa Cruz, CA
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POSSIBLE COSTS ASSOCIATED WITH INVESTIGATING AND

MITIGATING SOME GEOLOGIC HAZARDS IN RURAL AREAS

OF WESTERN SAN MATEO COUNTY, CALIFORNIA

By

Earl Brabb, Sebastian Roberts, William Cotton, Alan Kropp, Robert Wright, and Erik Zinn

INTRODUCTION

This digital publication consists of three parts: 1) A database with ARC/INFO files containing

information about the distribution of landslides, landslide susceptibility and slope; and 2) A Web

site http://kaibab.wr.usgs.gov/geohazweb/intro.htm having interactive landslide hazard maps

and tools for estimating the cost of investigating potential landslide hazards for landowners; and 3)

Plot files for landslide and landslide susceptibility maps, digital orthophoto quadrangles, digital

raster graphic quadrangles, geologic maps, and slope maps. By superimposing these landslide

hazard map layers on property lines either interactively at the website or by using paper plots of the

map layers, an owner can determine which hazards might impact the property and estimate costs

that might be involved in investigating the effects of these hazards. Coastal erosion, earthquake

shaking, liquefaction, and swelling clays are not included, but might be added later when digital

maps are available.

Large parts of rural San Mateo County have been zoned “Resource Management” (RM) by San

Mateo County (1973), meaning that areas with landslides and other geologic hazards are restricted

to one dwelling unit per 40 acres. Plans for a proposed one-dwelling-unit must include a geologic

report indicating that a safe and suitable site is available. Decision-makers in coastal parts of the

County have asked the U.S. Geological Survey to indicate how much the investigation and

mitigation of these geologic hazards might cost. Accordingly, a panel of consulting geologists was

convened to determine these costs, and their deliberations were summarized in a short report sent to

approximately 200 consultants in the San Francisco Bay region. The consultant responses and the

short report form the basis for this publication.

The decision-makers also wanted a way to make this information readily available to the public.

Accordingly, a Web site has been designed to allow a property owner to determine which landslide

hazards might impact her site and the related investigation and mitigation costs. This publication

also provides a way for the public to obtain copies of the landslide hazard maps for the Half Moon

Bay and Montara Mountain Quadrangles and the Pescadero/Butano Creeks drainage areas from the

Web site.

The cost of a geotechnical investigation depends on a large number of complex factors, including:

(1) The thoroughness of the geologic consultant and whether or not the consultant is practicing the

so-called “standard of care”; (2) The extent of the geologic review, and whether or not additional

work is required or recommended by the reviewer; (3) The availability of large-scale, topographic

base maps at the scale needed for the project; (4) Access to the site by heavy equipment (for

example, add another $1000 to $2000 if grading is needed); (5) The amount of liability insurance

needed by some clients; and (6) The wetness of the ground if subsurface investigations, such as

trenches and large diameter borings, are needed. The wetter the ground, the higher the costs.

Because of this complexity, some consultants have stopped offering a fixed price contract and

now provide a client with a worst-case estimate and then charge for time and material. Only a few

consultants provided cost information, so whether or not these costs are representative is not

known. As such, they are intended to be used as general guidelines. Moreover, the costs generally

reflect the conservatism of consultants who fear unanticipated problems -- prospective builders may

be able to negotiate lower investigation and mitigation costs if anticipated problems are minimal.
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COST ASSUMPTIONS

The complexity of factors involved in estimating the cost of investigating and mitigating geologic

hazards in a rural part of western San Mateo County forced us to make certain simplifying

assumptions. Property and developments that differ from those described below may have very

different costs. Assumptions:

A. The development is a single house in a rural part of western San Mateo County.

B. The lot size is one or 40 acres.

C. A geotechnical or geologic report is required.

D. The cost of reviewing the geotechnical and geologic report is not included, but that amount is

typically about $1200 (Jay Mazetta, oral comm. 12/20/99).

THE COST OF INVESTIGATING AND MITIGATING

LANDSLIDE HAZARDS

There are several kinds of landslide hazards in western San Mateo County, each with

different investigation and mitigation requirements. The main landslide hazards and their

associated costs are discussed below.

1. Debris flows - Debris flows are fast-moving shallow landslides that can destroy property

and are a threat to life because they move quickly with great force. During an intense winter storm

in 1982, for example, 25 people were killed and more than $65,000,000 in damages was recorded.

More than 4,000 of these debris flows occurred in San Mateo County during that storm. To

evaluate site vulnerability, a certified engineering geologist or soils engineer should examine steep

slopes where debris flows may be triggered in the future. Debris flows are normally included in any

investigation of deep-seated landslides. If a separate investigation is needed, costs may range from

$1000 to $5000 for this specific investigation. If mitigation measures are required, such as

drainage ditches, diversion structures, or piers, the costs might be in the range of $10,000 to

$100,000.

2. Deep-seated landslides - Deep-seated landslides are estimated to cause $25,000,000 to

$50,000,000 in damages in the San Francisco Bay region in a normal winter, and to exceed

$200,000,000 in very wet winters like 1997-1998. Hundreds of these landslides have been

recognized in San Mateo County. Some of them may never move again, but others, such as the

landslides in La Honda and on Polhemus Road, have gained notoriety by high mitigation costs (at

least $5,000,000 for the Polhemus Road slide). The cost of investigating these landslides might

be on the order of $5,000 to $10,000 if the landslide is small and seems to be inactive, or $10,000

to $100,000 or greater if it is larger and seems to have had recent movement. The cost of

mitigating these landslides is likely to range from $10,000 to $100,000 for small slides and may

range into the millions of dollars for larger slides.

3. Other landslides – Earth falls, rock falls, topples, translational slides and other ground

failures may damage or destroy residences. Most geotechnical studies for deep-seated landslides

or debris flows will generally address these other landslide processes.
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THE COST OF INVESTIGATING AND MITIGATING

EARTHQUAKE HAZARDS

There are several kinds of earthquake hazards in western San Mateo County, each with

different investigation and mitigation requirements. The main earthquake hazards and their

associated costs are discussed below.

A. Fault rupture – (Digital maps not available. See the report by Brabb and Olson, 1986, and

“Fault-rupture Hazard Zones” published by the California Division of Mines and Geology).

Ground rupture along faults during major earthquakes can damage or destroy buildings. The State

Geologist has identified the San Andreas and San Gregorio faults in the area of this report as active

faults requiring special geologic studies. However, most geologists will investigate all linear features

on a site that appear to indicate recent faulting. Special studies usually mean that the consultant will

dig one or more trenches to investigate the fault and to determine the location of the active traces.

Structures for human occupancy should not be put on active fault traces. Investigation costs for

special studies generally range from $10,000 to $15,000. Mitigation, in addition to possible

relocation of structures, might include foundation strengthening and additional support of other

structural members to provide strength during earthquake shaking and ground deformation.

Specially designed utility crossings may also be required. These costs might be on the order of

$10,000 to $50,000.

B. Liquefaction and ground failure – (Digital maps not available. See the reports by Youd

and Perkins, 1987, and Wieczorek and others, 1985) Water-saturated sand and silt as deep as 50

feet from the surface may liquefy during earthquakes. The land next to creeks, lakes and reservoirs

may move laterally causing damage to buildings. Areas with steep slopes and weak soil or bedrock

may fail by landsliding during earthquakes. Geotechnical studies investigating these processes

may cost less than $5,000 to $10,000 or more. Mitigation measures, such as the installation of

concrete columns, removing weak materials, or water pumping are likely to range from $10,000 to

in excess of $100,000. San Mateo County has not previously had requirements for single family

residences to mitigate liquefaction or earthquake-induced ground failure, and few if any houses in

the western part of the county have been built to resist these processes. However, the State of

California is preparing maps of areas susceptible to liquefaction and earthquake-induced

landsliding, and the new California Building Code essentially requires that liquefaction be assessed.

C. Ground shaking –(Digital maps not available. See Perkins, 1987) Shaking generated by

an earthquake may be so violent that buildings and other structures collapse. Ridge tops may also

shatter and crack where earthquake energy is concentrated. In general, post-1955 single family

residences have held up quite well in earthquakes in the San Francisco Bay region. Because new

houses have the benefits of upgraded building codes, San Mateo County does not impose

additional restrictions on single-family houses to deal with earthquake shaking.

THE COST OF INVESTIGATING AND MITIGATING

COASTAL EROSION HAZARDS

The coast at Moss Beach has eroded 165 feet in the past 105 years, according to a Guidebook

published in 1972 by the Friends of the Pleistocene. During 1998, media attention focused on

houses in Pacifica falling into the sea. Coastal erosion is a recurring problem that is difficult and

expensive. Geotechnical and engineering investigations to determine the extent of the hazard and

mitigation measures needed will probably cost $10,000 to $100,000. Mitigation measures are

generally in excess of $100,000 if permission from County and Coastal Commission authorities

can be obtained.
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USING THE USGS WEBSITE

(http://kaibab.wr.usgs.gov/geohazweb/intro.htm)

TO DETERMINE THE POSSIBLE COST OF INVESTIGATING

LANDSLIDES IN RURAL PARTS OF

SAN MATEO COUNTY, CALIFORNIA

By

Sebastian Roberts and Earl E. Brabb

INTRODUCTION

This website contains an interactive link to geological hazard maps of the Pescadero and Butano

Creeks watershed and the Half Moon Bay and Montara Mountain USGS quadrangles. To assist in

planning safe construction, the website also provides a technique to estimate costs required to

investigate potential deep-seated landslide hazards. Users can import and overlay maps showing

deep-seated landslides, deep-seated landslide susceptibility, debris-flow susceptibility, land-use

zoning, shaded relief, and topography.

The website is organized in four principal parts: 1) The main map showing the area of interest; 2)

An index map showing the location of the area selected; 3) Buttons at the top of the main map that

control the positions of the map window and permit the map to be queried and redrawn; and 4) A

legend that explains what is shown on the map, allows changes to what is shown on the map,

provides a link to explanatory text, and provides another link to explain the zoning symbols.

The website was prepared by using Map Objects software developed by Environmental Systems

Research Institute, Redlands, California. This software provides ways to link maps with other

information, such as the cost of investigating landslides and whether or not deep-seated landslides

are located on the property.

HOW TO USE THE INTERACTIVE MAPS

A. Map page layout

1. Main map – selected from possible areas of interest.

2. Index map – has no interactive functions. Shows where the selected main map area is

located relative to the rest of San Mateo County and San Francisco.

3. Buttons at the top of the main map – control position of the map window, allow

querying the map, and allow the map to be redrawn at a different scale or with additional

information.

4. Legend and legend buttons at the right side of the map – control changes in the map

content, explain what is on the map, provide a link to text explaining the map, and

provide a link to zoning categories on the map.

B. How to use each part of the map

1. Main map

a. Displays information.

b. Clicking the mouse provides new information depending on top buttons and legend

buttons (see below).

2. Index map

a. Not interactive.

b. Changes to show location of area selected relative to the rest of San Mateo County

and San Francisco.

3. Buttons at top of map

a. Zoom in – Selected by default, or by clicking on the map after the zoom out button

has been used. Enlarges map when mouse arrow is moved to an area on the map and

the mouse is clicked.
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c. Zoom out – Changes the map to a smaller scale to show a larger area.

d. Recenter – Changes position of center of map to a point clicked.

e. Full extent – Clicking on this button causes the map to revert to the original view of

the entire area.

f. Identify zoning – When zoning boundaries have been selected in the legend (see

below), clicking the “identify zoning” button and then clicking a point on the map

will show the zoning designation by a symbol, the zoning boundary with a green line

and the zoned area with hachured yellow lines. For an explanation of the zoning

designation, click the phrase “zoning boundaries” at the lower right corner of the

screen.

g. Identify investigation costs, one or forty acres – These buttons only work when

the landslide susceptibility box has been checked in the legend (see below), and an

area has been selected by clicking on the map. The map will be redrawn with a yellow

square centered at the point clicked. The estimated cost for investigating landslides in

this area will appear opposite a yellow box with red hachures in the lower part of the

explanation.

4. Legend and legend changes - The legend explains what is on the map. The legend

changes are accomplished by clicking in gray boxes or on phrases colored blue:

a. Zoning boundaries –Selected by clicking in the gray box and then clicking

“Legend Changes”. The map will be redrawn with green lines. By clicking

“Identify zoning” and then “Legend Changes”, the map will be redrawn with the

zoning boundaries shown by green lines, the zoned area with yellow hachured lines,

and a zoning designation provided.

b. Zoning categories – By clicking on “Zoning Categories” at the bottom of the

explanation, a link opens a box with an explanation for each code. “PAD”, for

example, is a planned agricultural district.

c. Debris flow susceptibility – When the gray box is checked, and then “Legend

Changes” is clicked, the map will be redrawn to show areas susceptible to debris

flow in pink. Black dots on the map show the origin points of debris flows in 1982.

This map cannot be redrawn at the same time as the map for deep-seated landslide

susceptibility.

d. Deep-seated landslide susceptibility – Clicking in the gray box and then clicking

“Legend Changes” will cause the map to be redrawn with the landslide categories

shown by colors in the legend. To determine the cost for a particular parcel, click on

the one or forty-acre size for the top buttons, and then click on the map in the center

of the parcel to be examined. The map will be redrawn with a yellow square and red

hachures showing the one or forty-acre parcel, and an estimate of the cost of

investigating deep-seated landslides in this parcel in the lower part of the legend.

e. Shaded relief – Selecting this box and then selecting “Legend Changes” will

provide an illusion of mountains and valleys. The impression is particularly helpful

when the debris-flow susceptibility or deep-seated landslide susceptibility box is also

selected.

f. Map explanation - When this phrase near the bottom of the explanation is clicked,

the screen is redrawn to the introduction for the map so that a new area can be

selected or text explaining the map is available.
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EXAMPLE

Let’s assume you have a one-acre parcel near the town of Montara and you want to determine how

much an investigation for landslides would cost. On the first image with the title page, click on the

phrase “Half Moon Bay Map” below the phrase “Geologic Hazard Maps.” On the next map

image, click three times near the town of Montara (the button to zoom or enlarge is automatically

selected). A digital version of the USGS topographic map appears on the screen. Click in the area

closest to your property. The largest-scale version of the topographic map now appears on the

screen. Click on the little shaded box for “Landslide susceptibility.” Now click on the words

“Legend Changes” above the map, and a layer with landslide hazards, from highest (red) to lowest

(green) is displayed on the screen overlying the topographic map. Now click the button with the

area closest to your property size (in this case, one-acre) in the space above the map. A yellow

square of about one-acre is now on the map. If the yellow square depicting your property touches a

highly susceptible landslide area colored red, the investigation cost might exceed $100,000, as

shown in lower part of the legend. If your property is entirely in the green area, with moderate and

low landslide susceptibility, the cost might range from $0 to 10,000, depending on the geologic

hazards that may exist but are not shown on the map. Additional hazards, such earthquakes, floods,

coastal erosion, and swelling clays are not currently a part of this presentation.

If you wish, you can zoom out to get an overview of a larger area, recenter the map image, or

return to the original map by clicking on the words “Full extent”. To recenter, click the recenter

button and then click on the map in the area of interest. You can turn on any of the layers that

appear in the legend with a checkbox to the left of their symbol. Click in the checkbox for those

layers that you would like to have displayed and then click on the "Legend Changes" button to

submit your request. Please note that the debris flow susceptibility, topography, and landslide maps

cannot be displayed at the same time. Activating the shaded relief button with the landslide

susceptibility layer active will give the map a three-dimensional appearance at intermediate scales.

If you want to examine another area, review the explanation for the map, or begin again in the

same area, click on “Map Explanation” in the lower right-hand corner of the map. The computer

will then show the introduction to the web site where you can select the area or text wanted.
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SOURCE MATERIALS FOR THE GEOHAZARDS

DIGITAL DATABASE

All data layers described below are in Universal Transverse Mercator (UTM) projection, zone 10.

The Half Moon Bay study area encompasses the Half Moon Bay 7.5’ quadrangle, and the Montara

Mountain study area encompasses the Montara Mountain 7.5’ quadrangle. On the website these

two quadrangles are treated as one map on the interactive geologic hazards map page, but in the

open file report they are two separate map products. The Pescadero/Butano Creeks Watershed

Study Area spans parts of the following six 7-5’ quadrangles; San Gregorio, Pigeon Point, La

Honda, Franklin Point, Mindego Hill, and Big Basin.

Several of the digital source data layers have already been released in other publications by the

USGS and are available for downloading on the Internet. Those source layers are not included in

this digital database, but a reference is given indicating where the data can be found. Map plots for

the areas of this report are on the publications web site, http://geopubs.wr.usgs.gov.

TOPOGRAPHIC BASE MAPS

 The digital topographic base maps are USGS Digital Raster Graphic (DRG) base maps at a

scale of 1:24,000. These are raster versions of the familiar USGS topographic paper maps. Each

raster cell has dimensions of 2 meters on a side. The DRGs are not included in the database, but

DRGs for the entire San Francisco Bay Area can be downloaded from the USGS website,

http://bard.wr.usgs.gov, and the DRGs for the areas of this report can be plotted from files on the

publication web site, http://geopubs.wr.usgs.gov/. The citation for a DRG of any quadrangle in the

United States is, Digital Raster Graphic Topographic Base Map, U.S. Geological Survey, Reston,

Virginia, 1991.

SHADED RELIEF BASE MAPS

On the geohazard web site, a shaded relief base is always used on the debris flow susceptibility

map, may or may not be used on the deep-seated landslide susceptibility map, or may be displayed

by itself. Shading the hillsides from a sun angle of 315 degrees clockwise from north, at an azimuth

of 45 degrees creates an illusion of three-dimensional topography. The shaded relief is derived

from a 10 meter USGS Digital Elevation Model (DEM). This is a raster data set with cells 10

meters on a side and an elevation value interpolated from the orthophotos described below. A

shaded relief grid can be made from a DEM using the ARC/INFO GRID hillshade command. The shaded relief is combined on the map with the other data layers, such as the pink debris-flow

susceptibility map, by using the ARC/INFO gridcomposite command. The shaded relief maps and

the DEM from which they were derived are not included in the database, but USGS DEMs for the

Bay Area can be downloaded from the USGS website, http://bard.wr.usgs.gov, and the shaded relief

maps for the areas in this report can be plotted from the publications web site,

http://geopubs.wr.usgs.gov. The complete citation for a DEM is, quadrangle name, 10 meter Digital

Elevation Model, U.S. Geological Survey, Reston, Virginia, 1998.

SLOPE MAPS

The slope maps are derived from USGS 10-meter Digital Elevation Models (DEMs) and are

created using the ARC/INFO GRID slope command. For each 10-meter cell, the slope is

calculated from an average of the slope in each of 8 cells surrounding the 10-meter cell. This slope

is expressed as percent rise. The slope grid is included as part of the digital database, and can also

be plotted from the publications web site, http://geopubs.wr.usgs.gov.
9

DIGITAL ORTHOPHOTO QUADRANGLE MAPS

A Digital Orthophoto Quadrangle (DOQ) is a computer-generated image made from aerial

photographs in which the images are digitally rectified to remove distortions caused by relief and

camera angle. Buildings and other objects shown on the quadrangle are positioned to reflect their

true distances from each other.

Four DOQs are needed for each 7.5’ quadrangle. The Pescadero watershed DOQ map includes

parts of 11 separate DOQs spanning 6 quadrangles. The Half Moon Bay quadrangle map includes

4 DOQs in their entirety. The Montara Mountain quadrangle includes 6 DOQs because it is

slightly larger than a standard USGS quadrangle. All the DOQs were derived from airphotos taken

on October 30, 1991. The DOQs are not included in the database, but DOQs can be downloaded

from the USGS website, http://bard.wr.usgs.gov, and for the areas of this report they can be plotted

from the publications web site, http://geopubs.wr.usgs.gov. The complete citation for the DOQs is,

Digital Orthophoto Quadrangles, 1991, US Geological Survey, Reston, VA.

GEOLOGIC MAPS

The geologic map of the Pescadero watershed is from the digital geologic map of the Palo Alto

30’ x 60’ Quadrangle by Brabb and others (1998). The geologic maps of the Half Moon Bay and

Montara Mountain Quadrangles are from the geologic map of San Mateo County by Brabb and

others (1998). These maps show the general distribution of bedrock and surficial deposits that can

influence the distribution of landslide deposits and affect construction costs. The description of the

geologic units represented on the geologic maps can be found on the accompanying unit

description sheet. The geologic maps themselves are not included in the database but they can be

downloaded and plotted from the following web sites http://geopubs.wr.usgs.gov/open-file/of98-

348, for the Pescadero/Butano Creeks watershed, and http://geopubs.wr.usgs.gov/open-file/of98-

137 for the Half Moon Bay and Montara Mountain quadrangles.

DEEP-SEATED LANDSLIDE INVENTORY MAPS

The deep-seated landslide inventories for the Pescadero/Butano watershed and the Half Moon

Bay quadrangle were digitized from the inventory prepared by Brabb and Pampeyan (1972). The

authors identified landslides according to their certainty about the existence of a landslide. Most

landslides were identified as ‘definite’, ‘probable’, and ‘questionable’, but several were

‘unattributed’. In contrast, Pampeyan (1994), whose landslide inventory was used for the Montara

Mountain Quadrangle, mapped young Quaternary landslides (Qyl), old Quaternary landslides

(Qol), questionable young Quaternary landslides (Qyl?), and questionable old Quaternary

landslides (Qol?). All landslide map layers are included as part of the digital database. The

landslide inventories for the areas of this report can de downloaded and plotted from the

publications web site, http://geopubs.wr.usgs.gov.

At the time the original landslide maps were made, debris flows were not recognized as a landslide

problem. In 1982, more than 4,000 debris flows occurred in San Mateo County, and a new

inventory was made by Wieczorek and others (1988) to show these landslides. In order to

distinguish their map from the one made by Brabb and others, the catchall term “deep-seated

landslide inventory” is used realizing that some shallow landslides are included.

A deep-seated landslide deposit consists of 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, or rotational slumping. Landslide deposits smaller than

approximately 200 feet in longest dimension are not shown on these maps. Complex landslide

deposits, which result from combinations of different types of downslope movement, are probably

the most common type of landslide deposit included on these maps. The deposits vary in
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appearance from clearly discernible, largely unweathered and uneroded topographic features to

indistinct, highly weathered and eroded features recognizable only by their characteristic shape. The

time of formation of the mapped landslides is largely unknown, but may range from as much as a

few hundred thousand years ago to as recent as 1963, the date of the latest photography used for

the interpretation. No landslide deposits that formed since 1963 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.

DEEP-SEATED LANDSLIDE SUSCEPTIBILITY MAPS

The deep-seated landslide susceptibility maps were produced using a procedure developed by

Brabb and others (1972). The maps are constructed using three source maps: a geologic map, a

slope map, and an inventory of earlier landslides. The geologic, slope, and landslide inventory maps

are described above.

To develop the 1972 susceptibility map, Brabb and others superimposed the landslide inventory

map onto a geologic map and arranged the geologic units by increasing percentage of area that had

failed due to landsliding. The units were then given an overall susceptibility ranging from I (low) to

VII (high). The highest susceptibility class (L) was assigned to the landslide deposits themselves,

because they were believed to be more susceptible to future landsliding than the rock units from

which they are derived. The slope map was then superimposed onto the combined geologic map

and landslide inventory map to determine the incidence of landsliding in each geologic unit in each

slope category. The overall susceptibility rating for the geologic unit was adjusted downward where

no landslides had formed on low slopes.

The primary differences between the 1972 map and the susceptibility maps provided here are the

use of new slope maps and the use of polygons around just the landslide deposits. On the 1972

map, areas between the deposits and the scarps were also included as a landslide polygon. The

matrix table used by Brabb and others (1972) for the entire county is the same one used here.

Susceptibility numbers are assigned for each 30-meter cell depending on the unit-slope

combination defined in the table. The deep-seated landslide susceptibility maps are included as part

of the digital database, and are available for downloading and plotting from the publications web site

at: http://geopubs.wr.usgs.gov . Only the three highest hazard zones, including the landslide

deposits (L), are used to determine costs on the web site because these zones are the same ones

used by San Mateo County (1973) to restrict development.

DEBRIS FLOW INVENTORY AND SUSCEPTIBILITY MAPS

Debris flow points of origin recorded by Wieczorek and others (1988) for the January 1982

storm are shown as black dots on these maps. The shaded pink areas indicate places where debris

flows might occur in the future. The shading originates from a statement by Nilsen and others

(1979, p. 36) that most of the landslides in the San Francisco Bay area occur on slopes steeper than

15%. Accordingly, the pink shading shows slopes steeper than 15% derived from the slope map

described above. This shaded area contains 97% of the 900 debris flow origin points in the

Pescadero/Butano drainage area and 98% of the 1597 debris flows mapped in the Half Moon Bay

and Montara Mountains quadrangles mapped after the 1982 storm. Some of the debris flows in

1982 extended beyond the pink shading and into the flat valleys below—future hazard should be

determined by qualified experts on a site-by-site basis. The 1982 debris flow origin points are not

included in the database, but they can be downloaded from the USGS website,

http://geopubs.wr.usgs.gov/open-file/of97-745e.html. The debris flow susceptibility grid is

included as part of the database.
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THE DIGITAL PUBLICATION

INTRODUCTION

This publication consists of a digital map database on a geohazards web site,

http://kaibab.wr.usgs.gov/geohazweb/intro.htm, this text, and 43 digital map images on another web

site, http://geopubs.wr.usgs.gov. The report is stored as several digital files, in ARC export

(uncompressed) format for the database, and Postscript and PDF formats for the map images.

Several of the source data layers for the images have already been released in other publications by

the USGS and are available for downloading on the Internet. These source layers are not included

in this digital database, but rather a reference is given for the web site where the data can be found in

digital format.

The exported ARC coverages and grids lie in UTM zone 10 projection. 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 00-127. 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 spatial database is realized by importing the ARC export files into

ARC/INFO or an equivalent GIS. Other GIS packages, including MapInfo and ARCVIEW, can

also use the ARC export 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.

SPATIAL RESOLUTION

Users 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

compiled from maps at a scale of 1:24,000, 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

combined output will be limited by the lower resolution of this data.

PUBLICATION CONTENTS

The 60 digital files included in the publication are encoded in more than one format. The names

of the files are unique designators based on the report identifier, of00-127, followed by part

numbers (i.e. 1 through 14 below) and an extension indicating file type. Some of the files have been

bundled using the tar (UNIX Tape Archive) utility (.tar extension). All of the 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):

of00-127revs_1.txt ASCII file
12

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

obtain it:

of00-127_2.txt ASCII file (no formatting)

of00-127_2.ps Postscript file

of00-127_2.pdf PDF file

 3. Database for debris flow susceptibility; ARC export grid containing a 10-meter raster grid:

of00-127_3a.e00.gz Arc export grid for Pescadero/Butano watershed

of00-127_3b.e00.gz Arc export grid for Half Moon Bay quadrangle

of00-127_3c.e00.gz Arc export grid for Montara Mountain quadrangle

 4. Database for deep-seated landslide inventory; ARC export coverage containing both lines

and polygons:

 of00-127_4a.e00.gz Arc export coverage for Pescadero/Butano watershed

 of00-127_4b.e00.gz Arc export coverage for Half Moon Bay quadrangle

 of00-127_4c.e00.gz Arc export coverage for Montara Mountain quadrangle

 5. Database for deep-seated landslide susceptibility; ARC export grid containing a 30-meter

raster grid:

 of00-127_5a.tar.gz Arc export grid for Pescadero/Butano watershed

of00-127_5b.tar.gz Arc export grid for Half Moon Bay quadrangle

of00-127_5c.tar.gz Arc export grid for Montara Mountain quadrangle

 6. Database for slope; ARC export grid containing a 10-meter raster grid:

of00-127_6a.tar.gz Arc export grid for Pescadero/Butano watershed

of00-127_6b.tar.gz Arc export grid for Half Moon Bay quadrangle

0f00-127_6c.tar.gz Arc export grid for Montara Mountain quadrangle
13

7-14. Plot Files for the Maps: The map images are provided as both Postscript and PDF

format files. The map layer is listed in the left-hand column. The location and file type are listed in

the top row. Half Moon Bay quadrangle is abbreviated as HMB and Montara Mountain

quadrangle is abbreviated as MM below. File size is rounded up to the closest megabyte.

Pescadero

watershed

Postscript

Pescadero

watershed

 PDF

HMB

quadrangle

 Postscript

HMB

quadrangle

PDF

MM

quadrangle

Postscript

MM

quadrangle

PDF

Debris flow

susceptibility

of00-127

_7a.ps.gz

(32 MB compr

to 4 MB)

of00-127

_7a.pdf

(2 MB)

of00-127

7b.ps.gz

(15 MB

compressed

to 2 MB)

of00-127

_7b.pdf

(1 MB)

of00-127

_7c.ps.gz

(20 MB

compressed

 to 2 MB)

of00-127

_7c.pdf

(2 MB)

Digital ortho–

photo quads

(DOQ)

of00-127

_8a.ps.gz

(231 MB

compressed

to 107 MB)

of00-127

_8a.pdf

(2 MB)

of00-127

_8b.ps.gz

(96 MB

compressed

to 41 MB)

of00-127

_8b.pdf

(1 MB)

of00-127

_8c.ps.gz

(123 MB

compressed

 to 58 MB)

of00-127

_8c.pdf

(2 MB)

Digital raster

graphic

 (DRG)

of00-127

_9a.ps.gz

 (638 MB

compressed

to 22 MB)

of00-127

_9a.pdf

(2 MB)

of00-127

_9b.ps.gz

(285 MB

compressed

to 6 MB)

of00-127

_9b.pdf

(3 MB)

of00-127

_9c.ps.gz

(300 MB

compressed

 to 13 MB)

of00-127

_9c.pdf

(7 MB)

Geology

of00-127

_10a.ps.gz

 (640 MB

compressed

to 25 MB)

of00-127

_10a.pdf

(2 MB)

of00-127

_10b.ps.gz

(286 MB

compressed

 to 7 MB)

of00-127

_10b.pdf

(1 MB)

of00-127

_10c.ps.gz

(303 MB

compressed

to 15 MB)

of00-127

_10c.pdf

(9 MB)

Deep-seated

landslide

inventory

of00-127

_11a.ps.gz

 (523 MB

compressed

 to 15 MB)

of00-127

_11a.pdf

(2 MB)

of00-127

_11b.ps.gz

(286 MB

compressed

to 6 MB)

of00-127

_11b.pdf

(3 MB)

of00-127

_11c.ps.gz

(300 MB

compressed

to 13 MB)

of00-127

_11c.pdf

(8 MB)

Deep-seated

landslide

susceptibility

of00-127

_12a.ps.gz

(521 MB

compressed

to 16 MB)

of00-127

_12a.pdf

(2 MB)

of00-127

_12b.ps.gz

(286 MB

compressed

to 7 MB)

of00-127

_12b.pdf

(4 MB)

of00-127

_12c.ps.gz

(357 MB

compressed

to 14 MB)

of00-127

_12c.pdf

(8 MB)

Slope

of00-127

_13a.ps.gz

(52 MB compr

to 18 MB)

of00-127

_13a.pdf

(1 MB)

of00-127

_13b.ps.gz

(285 MB

compressed

to 8 MB)

of00-127

_13b.pdf

(4 MB)

of00-127

_13c.ps.gz

(300 MB

compressed

to 16 MB)

of00-127

_13c.pdf

(9 MB)

Geologic

unit

description

The unit description sheet is comprehensive and describes the units found on all

three maps. Therefore there are only two image files. The postscript file is named

 of00-127_14.ps.gz (4 MB compressed to 1 MB). The PDF file is named of00-

127_14.pdf (1 MB).
14

REVISIONS

Changes to any parts of the report (the numbered items described above and listed in the revision

list, of00-127revs_1.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.

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. Decimal increments will indicate small

changes and integer increments larger changes 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 digital files may be obtained in three ways.

1. The simplest way to obtain the digital files is to download them over the World Wide Web

from the USGS Western Region Geologic Information Server: http://geopubs.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 00-127) 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://geopubs.wr.usgs.gov/open-file/of00-127. 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 geopubs.wr.usgs.gov - make ftp connection with the USGS computer wrgis.

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

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

pub/open-file/of00-127 -the subdirectory in which the files from this report are stored

3. To obtain files from the database on magnetic tape, send a blank tape with

a request specifying the desired files and your return address to:

Western San Mateo County Landslide Hazard

c/o Database Coordinator

U.S. Geological Survey

345 Middlefield Road MS 975

Menlo Park, CA 94025

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
15

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/:

IMPORTING THE ARC EXPORT FILES

The ARC export files (.e00 extension) can be converted to ARC/INFO vector maps (coverages)

or GRID raster files and INFO files by using the import command in ARC. Be sure to

uncompress the gzip or tar files before trying to import the files.
16

DIGITAL COMPILATION

INTRODUCTION

The digital database was compiled using version 7.2.1 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 contents of the map databases are described in terms of the lines and/or areas that compose it.

For integer ARC/INFO grids the database records are stored in a value attribute table (.VAT) and

for ARC/INFO coverages the records are stored in polygon attribute tables (.PAT) and arc attribute

tables (.AAT). The terms in Table 1 describe the database fields of these attribute tables.

Table 1. 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 point

Number; N, ASCII floating point number;

I, ASCII integer; C, ASCII character string

NDEC N.DEC, number of decimal places maintained for

floating point numbers

DEBRIS FLOW SUSCEPTIBILITY

The debris flow susceptibility map for each of the three study areas is an ARC/INFO grid that

was derived from the slope grid described in the source materials section. The grid consists of cells

measuring 10 meters on a side. Using the GRID function conditional, the debris flow

susceptibility grid was calculated to give all cells with a slope of 15% or higher a value of 15 and set

all the remaining cells to null. These values are stored as integers in the VALUE database field of

the value attribute table (VAT) of the grid. Note that cells having a value of null are not recorded as

part of the VAT.

Table 2. Content of the value attribute table

ITEM NAME WIDTH OUTPUT TYPE N.DEC DESCRIPTION

VALUE 4 10 B - “15” for slopes >= to 15%

COUNT 4 10 B - Number of cells with this value

LANDSLIDE INVENTORY

The digital compilation of the landslide inventory for the Pescadero watershed and the Half Moon

Bay quadrangle was derived from linework inked directly by the authors on USGS mylar greenline
17

7.5-minute (1:24,000-scale) quadrangle maps. The inked linework for each source map was

scanned (400 dots per inch), converted from raster to vector form, imported into ARC/INFO, and

hand edited and combined into a single digital file. Adjustments were made to fit lines across

quadrangle boundaries. The digital compilation for the Montara Mountain quadrangle was derived

from the geologic map by Pampeyan (1994). Likewise, the original greenline was scanned,

converted to vector form, and hand edited to extract just the landslide information from the geologic

map.

All landslides larger than 200 feet in maximum dimension are represented in the spatial database

by polygons enclosing their approximate boundaries. The primary landslide identifier is a character

string in the field PTYPE (polygon type) in the polygon attribute table for the deep-seated landslide

inventory database.

Because landslides in the Brabb and Pampeyan map (1972), used for the Pescadero watershed

and Half Moon Bay quadrangle, were identified via analysis of aerial photographs without an

opportunity to field check the resulting map, the values in the PTYPE fields reflect the degree of

confidence the author had in his interpretations. The values for this field are; “definite”,

“probable”, “questionable”, and “unattributed”. In both the Postscript and PDF plot files, large

landslide polygons are color coded according to this classification.

Landslides derived from the Pampeyan (1994) geologic map used for the Montara Mountain

quadrangle have a different designation. These landslides have been identified as “young

Quaternary landslide”, “old Quaternary landslide”, “questionable young Quaternary landslide”,

and “questionable old Quaternary landslide”.

The landslide-deposit spatial database for each of the three maps is stored in UTM projection

(zone 10). Digital tics define a 7.5-minute grid of latitude and longitude.

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

described in the arc attribute table (see Table 3). In the landslide inventory map layer they define

the boundaries of the landslide units, topographic scarps, and the quadrangle boundaries. These

distinctions are recorded in the LTYPE database field according to the line types listed in Table 4.

Table 3. Content of the Arc Attribute Table

ITEM NAME WIDTH OUTPUT TYPE N.DEC DESCRIPTION

 FNODE# 4 5 B - Starting node of arc (from node)

 TNODE# 4 5 B - Ending node of arc (to node)

 LPOLY# 4 5 B - Polygon to the left of the arc

 RPOLY# 4 5 B - Polygon to the right of the arc

 LENGTH 4 12 F 3 Length of arc in meters

 PF-SLID# 4 5 B - Unique internal control number

 PF-SLID-ID 4 5 B - Unique identification number

 LTYPE 35 35 C - Line type

Table 4. Line Types Recorded in the LTYPE Field

LINE TYPE DESCRIPTION

Contact, certain Arcs delineating large landslide polygons

Topographic escarpment Landslide scarp at head of a landslide deposit

Map boundary Quadrangle boundary or study area
18

Areas - Landslide areas are recorded as vector polygons with characteristics that are described

in the polygon attribute table (see Table 5). In the landslide inventory map layer they define the

degree of confidence which the author had when identifying the area as a landslide deposit. These

distinctions are recorded in the PTYPE database field according to the polygon types listed in

Tables 6 and 7.

Table 5. Content of the Polygon Attribute Table

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 C - Landslide category

Table 6. Landslide Types Recorded in the PTYPE Field

for Pescadero/Butano watershed and Half Moon Bay quadrangle maps

LANDSLIDE TYPE DESCRIPTION

D Definite landslide deposit

P Probable landslide deposit

? Questionable landslide deposit

UA Unattributed landslide deposit

Table 7. Landslide Types Recorded in the PTYPE Field

for Montara Mountain quadrangle map

LANDSLIDE TYPE DESCRIPTION

Qyl Young Quaternary landslide deposit

Qyl? Questionable young Quaternary landslide deposit

Qol Old Quaternary landslide deposit

Qol? Questionable old Quaternary landslide deposit

LANDSLIDE SUSCEPTIBILITY

The landslide susceptibility map for each of the three study areas is an ARC/INFO grid that was

derived from the slope grid and the geologic map described in the source materials section. The

grid consists of cells measuring 30 meters on a side. Each cell is assigned a relative susceptibility

value between 0 and 7 as described in the source materials section. These values are stored as

integers in the VALUE database field of the value attribute table (VAT) of the grid. Note that cells

that have a value of null are not recorded as part of the VAT.
19

Table 8. Content of the Value Attribute Table

ITEM NAME WIDTH OUTPUT TYPE N.DEC DESCRIPTION

VALUE 4 10 B - Susceptibility value 0 through 7

COUNT 4 10 B - Number of cells with this value

SLOPE

The slope map for each of the three study areas is an ARC/INFO grid that was derived from the

Digital Elevation Model (DEM) described in the source materials section. The grid consists of

cells measuring 10 meter on a side. Using the GRID function slope, a continuous slope is

calculated using a 3 by 3 cell (30 meter by 30 meter) neighborhood to arrive at an average slope

expressed as percent rise. This operation results in a grid with continuous floating-point numbers,

and thus a value attribute table (VAT) is not created for the grid.

CONCLUSIONS

An interactive website for landslide hazards has been provided for rural areas in western San

Mateo County. People who wish to determine if their property is impacted by these hazards can

use the website to locate their property, overlay deep-seated landslide hazard maps and determine

the cost to investigate them. Information is provided for investigating and mitigating several other

kinds of geologic hazards, but the data are not interactive. The maps will be also useful for other

purposes, such as determining where sediment from landslides might adversely affect fish in the

various creeks, or where roads might be closed after heavy rainfall.

ACKNOWLEDGEMENTS

We are grateful to Russell Graymer, David Howell, William Lettis, Bruce Clark, John O’Rourke,

Betsy Mathieson, Reid Fisher, Roberta Smith, Gerald Weber, Earl Hart, and John Wade who kindly

provided excellent comments used to revise various drafts. Suzanne Mills kindly made corrections

to the text, figures and database, and made plots of all the maps. Scott Graham helped to revise the

Web text.

REFERENCES

Brabb, E.E., and Pampeyan, E.H., 1972, Preliminary Map of Landslides in San Mateo County,

California: U.S. Geological Survey Miscellaneous Field Studies Map MF-344, scale 1:62,500.

Brabb, E.E., Pampeyan, E.H., Bonilla, M.G., 1972, Landslide Susceptibility in San Mateo

County, California, U.S. Geological Survey Miscellaneous Field Studies Map MF-360, scale

1:62,500.

Brabb, E.E., and Olson, J.A., 1985, Map showing faults and earthquake epicenters in San Mateo

County, California: U.S. Geological Survey Miscellaneous Investigations Map I-1257-F, scale

1:62,500.

Brabb, E.E., Graymer, R.W., and Jones, D.L., 1998, Geology of the onshore part of San Mateo

County, California: a digital database: U.S. Geological Survey Open-File Report 98-137.

Brabb, E.E., Graymer, R.W., and Jones, D.L., 1999, Digital geology of the Palo Alto 30X60

Minute Quadrangle, California: U.S. Geological Survey Miscellaneous Field studies Map MF2332.

Ellen, S.D., Mark, R.K., Wieczorek, G.F., Wentworth, C.M., Ramsey, D.W., and May, T.E.,

1997, San Francisco Bay Region Landslide Folio, U.S. Geological Survey Open-File Report 97-

745 scale, 1:125,000.

Fitzgibbon, T.T., 1991, ALACARTE installation and system manual (version 1.9): U.S.

Geological Survey Open-File Report 91-587-B.
20

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.

Friends of the Pleistocene, 1972, Progress report on the USGS Quaternary studies in the San

Francisco Bay area: National Technical Information Service, Springfield, VA, Publication PB206826.

Nilsen, T.H., and others, 1979, Relative slope stability and land-use planning in the San

Francisco Bay region, California: U.S. Geological Survey Professional Paper 944, 96p.

Perkins, J.B., 1987, Maps showing cumulative damage potential from earthquake ground

shaking, San Mateo County, California: U.S. Geological Survey Miscellaneous Investigations Map

I-1257-I, scale 1:62,500.

 Pampeyan, E.H., 1994 Geologic Map of the Montara Mountain and San Mateo 7-1/2'

Quadrangles, San Mateo County, California: U.S. Geological Survey Miscellaneous Investigations

Map I-2390, scale 1:24,000.

San Mateo County, 1973, Ordinances and local laws related to the Resource Management

Zoning District: Redwood City, CA, 44p.

Wentworth, C.M., and Fitzgibbon, T.T., 1991, ALACARTE user manual (version 1.0): U.S.

Geological Survey Open-File Report 91-587-C.

Wieczorek, G.F., Wilson, R.C., and Harp, E.L., 1985, Map showing slope stability during

earthquakes in San Mateo County, California: U.S. Geological Survey Miscellaneous Geologic

Investigations Map I-1257-E, scale 1:62,500.

Wieczorek, G.F., and others, 1988, Debris flows and other landslides in San Mateo, Santa Cruz,

Contra Costa, Alameda, Napa, Solano, Sonoma, Lake, and Yolo counties, and factors influencing

debris-flow distribution in Ellen, S.D., and Wieczorek, G.F. eds., Landslides, floods, and marine

effects of the storm of January 3-5, 1982, in the San Francisco Bay region, California: U.S.

Geological Survey Professional Paper 1434, p. 133-162.

Youd, T.L., and Perkins, J.B., 1987, Map showing liquefaction susceptibility of San Mateo

County, California: U.S. Geological Survey Miscellaneous Investigations Map I-1257-G, scale

1:62,500.

(Maps in stock can be obtained from the Earth Sciences Information Center of the US

Geological Survey, 345 Middlefield Road, Menlo Park, CA, 8am to 4pm. If the maps are out of

stock, they may be examined at the USGS Library at the same address.)