and earthquake faults are an important fact of life in Southern
California. Though using GIS in conjunction with GPS and seismological
data to study earthquake activity is not new, using ArcView GIS
with 3D Analyst extension can give these data a new twist. The author
used publicly available information obtained off the Internet and
published maps to create an interesting, informative project for
a GIS class she took at San Bernardino Valley College in San Bernardino,
more than 200 known faults capable of producing earthquakes of magnitude
6.0 or greater. Some of these faults are hidden beneath the sediments
in valleys, some are visible at the surface. The biggest concentration
of faults is in the valley and mountainous region surrounding the
San Andreas Fault zone and the San Jacinto Fault zone. There are
hundreds of earthquakes every year along these faults though most
are too small to feel.
The biggest concentration of faults is in the valley and mouatainous region surrounding the San Andreas fault zone and the San Jacinto fault zone.
California is such a seismically active area, the United States
Geological Survey (USGS), the California Division of Mines and Geology,
and California Institute of Technology (Caltech) in cooperation
with other agencies, corporate partners, and institutions have developed
the Southern California Earthquake Center (SCEC) and TriNet Seismic
Network. This organization has a number of seismographs strategically
placed around Southern California to monitor earthquake activity.
are linked to a computer system using a combination of GPS technology
and landlines. Seismography records are sent to central computers
within a few seconds or minutes of an event. The seismic information
is provided to the public via the Internet. Visit the ArcUser Jump
Station at the ArcUser Web site for a list of these sites. Data
at these sites, which are updated frequently during the day, are
saved to catalogs that are searchable by various parameters.
the Scope of the Project
to create a project using ArcView GIS and 3D Analyst to study earthquakes
and faults in Southern California and identifying data sources,
the next step was to determine the project's objective and scope.
Once the pertinent data was obtained and converted to a usable form,
maps could be created and analysis performed.
objective was to ascertain whether there is a pattern to the earthquakes
in the inland valleys and mountains of Southern California. The
project hoped to provide answers for the following questions. Are
there earthquakes occurring in areas where no faults are mapped?
If so, do these earthquakes create a pattern suggesting more faulting
not yet mapped? Is there a pattern to the depths of earthquakes?
The study area
needed to be small enough to make the project manageable. The area
chosen stretches from 116.70 degrees to 117.83 degrees west longitude,
and 33.80 degrees to 34.56 degrees north latitude which is an area
approximately 67 miles by 50 miles. The time frame selected was
January 1, 1995, to June 30, 1998. The project would deal with data
on earthquakes of magnitude 2.0 and greater.
3.0a, 3D Analyst, and the beta version of ArcView GIS 3.1 were used
to create the project. USGS maps and information downloaded from
the Caltech and SCEC Internet sites provided the fault data. The
searchable Internet sites of SCEC were used for the basic earthquake
data. The geographic data that comes with ArcView GIS was used to
create the base map.
and Converting Data
The SCEC Data
Center Earthquake and Hypocenter and Phase Database was queried
for data on the study area. Available search parameters available
included start and end dates, minimum and maximum magnitudes, minimum
and maximum depths in kilometers (km), and latitude and longitude
ranges. The results were printed out. A second site, ftp/ca.earthquakes,
a cooperative effort of Caltech and the USGS that is maintained
by Kate Hutton and Lucy Jones, was used for additional data.
These two Internet
sites used different units of measurement to store data. The SCEC
site uses Universal Time for time measurements, has latitude and
longitude expressed as a decimal degree to two decimal places, and
includes depth information not available from the Caltech-USGS site.
site uses local time (either Pacific Standard or Daylight Time)
and has latitude and longitude data expressed in degrees and decimal
minutes. Though this site did not have depth information, it did
have other descriptive information as to location (i.e. 3 miles
NW of San Bernardino) and whether the earthquake was felt by residents.
from the SCEC site were converted to Pacific Standard Time for use
with the Caltech-USGS data. Latitude and longitude measurements
in degrees and decimal minutes from the Caltech-USGS site were converted
and expressed as measurements in decimal degree to three decimal
places. Additional data, whether depth or descriptive data, were
added to the project database.
was used to create an earthquake data table. The tabular data was
saved as a dBASE IV file usable by ArcView GIS. The table was added
to an ArcView GIS project and then used to create an event theme.
The theme legend editor was used to classify the earthquakes by
magnitude using graduated symbols. Each size symbol was then given
a different color for easier identification. The location of earthquakes
was double-checked against the descriptive location for accuracy
and any errors were corrected.
the Base Map
comes with a large amount of geographic data that can be used to
construct a base map. The highway and street data (highways and
dtl_st files) were used. Since the data covers the entire United
States, data were limited to the project study area. The select
button was used to select Riverside, San Bernardino, Los Angeles
and Orange Counties. These selections were converted to a shapefile.
Additional editing to the shapefile was performed using the polygon
splitting function. A combination of the select button, conversion
to shapefile, and line splitting functions was used to edit the
There was not
single source for the fault data. A combination of data from several
Internet sites, a California Division of Mines map, and a USGS geologic
map of California were used to determine the locations of faults.
Using the line drawing tool, new a new line theme was created. As
each line was drawn, the name of the fault was added to the table
created. Once drawn, the fault lines were fine-tuned with the vertex
editing function. Highways and county boundaries were used as reference
points for correct line placement. None of the maps completely agreed
with each other, so an average was used where there were discrepancies.
Faults were not included if they were very short and found only
on one map. Some of the minor San Bernardino Mountain faults were
A triangulated irregular network (TIN) created from the earthquake point theme. Faults and earthquake epicenters are visible on the surface. Areas in yellows and reds indicate shallower epicenter depths. Blue areas indicate deeper epicenters.
a Contour, TIN, and Three-Dimensional Scene
Once the map
was completed, a three-dimensional theme and scene could be created.
The earthquake theme was used to create a three-dimensional point
theme with depth as the z value. This was used with the fault theme
to create a three-dimensional scene. Since the depths range from
zero to 20.8 km, the resultant scene looked like a string of beads
hanging down from the surface. A vertical exaggeration of 0.1, set
by adjusting the z factor in the three-dimensional theme properties
dialog, compressed the points into a more intelligible form.
In order to
make a triangular irregular network (TIN) theme, the themes from
the first view were copied into a new second view. A breakline theme,
created using the make new theme choice, was created parallel to
and on each side of an observed pattern of earthquakes perpendicular
to the San Jacinto fault. Using the earthquake point theme and breakline
themes, an inverse distance weighting (IDW) contour theme was created.
The contour theme was used to create a TIN theme. This TIN was added
to the three-dimensional scene and compressed to 0.1 vertical exaggeration.
The addition of this TIN caused the scene to rotate very slowly.
As the three-dimensional scene was spinning, a pattern became apparent.
the Map and Data
of map, TIN and three-dimensional scene revealed a pattern of earthquakes
in the San Bernardino Valley in an area where faults have not been
mapped. There appears to be a pattern of quakes parallel to the
San Jose and Cucamonga faults and perpendicular to the San Jacinto
and San Andreas faults.
One map shows
a barrier near here. There is a pattern of northwest trending quakes
near the San Jacinto and San Andreas faults and another pattern
of earthquakes perpendicular to these faults. At the juncture of
the San Jacinto fault and the barrier earthquake pattern, there
is a large cluster of earthquakes ranging in depth from just below
the surface to 20 km deep.
From this data
there appears to be at least one fault not shown on maps. There
is also a pattern to the depths of earthquakes with the deepest
ones occur in the Banning Pass area where several faults come together
and the shallowest ones located in the mountains.
Go to the ArcUser
Jump Site for the URLs of many excellent Web sites on related topics.
For more information, contact Margaret Gooding at firstname.lastname@example.org.
graduated from California State University at Fullerton with a bachelor's
degree in Geology and a minor in Art. She earned a GIS certificate
at San Bernardino Valley College and completed an internship at
Esri in July of 1998.