Shane Straw 12/1/11

Final Project

Project Question(s):

How does the 3D geometry of earthquake positions under the Marianas Trench compare to the

3D geometry of earthquakes under the San Andreas Fault? Can the earthquake positions be

correlated with the type of movement associated with these plate boundaries? To answer

these two questions, the plate boundaries and earthquakes associated with the boundaries will

need to be analyzed in Arc Scene. Arc Scene is the ArcGIS tool for visualizing 3D data. Once the

earthquake geometries of both of the plate boundaries have been analyzed, they can be

compared to the known movement of the plates to see if any correlations exist. It should be

noted, however, that if a correlation is found it does not necessarily mean that the movement

of the plates is determining the positions of the earthquakes. Correlation does not equal

causation. It would take more advanced techniques to say for certain whether plate

movements are responsible for earthquake geometry. The answers to the two main questions

will be given in a mostly qualitative form. However, the numerical values for the magnitudes

and depths of the earthquakes will be essential for analyzing their geometries.

Preprocessing and Processing: A ‘How-To’ Methods Description

Before any analysis can be done, however, topography and bathymetry data must be found for

the areas surrounding the two plate boundaries. The 3D capabilities of this ETOPO data are very

helpful for visualizing the two plate boundaries and their associated earthquakes in Arc Scene.

The detailed descriptions of how to find this data and upload it to Arc Scene are listed below.

Bathymetry data for the Mariana Trench

o Go to the NOAA National Geophysical Data Center and click on Bathymetry &

Global Relief

o Create a custom rectangular grid with coordinates at the corners as follows:

Top Latitude: 26° 00’ N

Bottom Latitude: 9° 00’ N Left Longitude: 134° 22’ E Right Longitude: 153° 01’ E

o Use the ETOPO1 1-Minute global relief database with a 1 minute grid cell size

o For the grid format, use the output grid ASCII Raster Format with the ASCII (ARC)

Header

o After the search is complete, save the compressed file to the project folder on

any hard drive

o Extract the data into the folder

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o As stated above, the file is using a .asc file extension: this needs to be converted

to a raster

Use the ASCII to Raster tool to convert the file to a raster

o The coordinate system of the raster file is undefined, so now define it in WGS84

geographical coordinates using decimal degrees

o Now use the Project Raster tool to project the raster file into UTM coordinates

(WGS 1984 UTM Zone 55N)

o Load the file into a new Arc Scene document and set the base heights

Have it float on a custom surface using itself as the file name

o The Marianas Trench should now appear in Arc Scene, but it will appear almost

flat due to the large area it covers

o Go to scene properties and change the vertical exaggeration to 10

This should make the elevation differences more clear (and later the

depths of the earthquakes)

Bathymetry Data for the San Andreas Fault

o Go to the NOAA National Geophysical Data Center and click on Bathymetry &

Global Relief

o Create a custom rectangular grid with coordinates at the corners as follows:

Top Latitude: 42° 15’ N

Bottom Latitude: 29° 40’ N

Left Longitude: 130° 07’ W Right Longitude: 111° 34’ W

o Use the ETOPO1 1-Minute global relief database with a 1 minute grid cell size

o For the grid format, use the output grid ASCII Raster Format with the ASCII (ARC)

Header

o After the search is complete, save the compressed file to the project folder

o Extract the data into the folder

o As stated above, the file is using a .asc file extension: this needs to be converted

to a raster

Use the ASCII to Raster tool to convert the file to a raster

o The coordinate system of the raster file is undefined, so now define it in WGS84

geographical coordinates using decimal degrees

o Now use the Project Raster tool to project the raster file into UTM coordinates

(WGS 1984 UTM Zone 10N)

o Now load the file into a new Arc Scene document and set the base heights

Have it float on a custom surface

o The San Andreas Fault should now appear in Arc Scene

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o Go to the scene properties and change the vertical exaggeration to 10

This should make the elevation differences more clear

The next step in the process is to find earthquake data for the areas surrounding the two plate

boundaries. The detailed process needed to find this data and utilize it in Arc Scene is listed

below.

Earthquake Data for the Marianas Trench

o Go to the USGS Earthquake Hazards Program page and click on “Search for an

Earthquake”

Set the output file type to Spreadsheet Format (comma delimited)

Set the Data Base to USGS/NEIC (PDE) 1973 – 2011 11 28

Do a rectangular area search and enter the same latitude and longitude

coordinates as with the bathymetry data

Under the optional parameters, set the minimum magnitude to 5 and the

maximum magnitude to 10

Submit the search

o Once the search is complete and your data is available, copy all of the

information into a Notepad text document

o Open Excel and import the notepad text document

Make sure you import it as delimited by commas and make sure that all

the columns are marked as general

o Open the Arc Scene document with the bathymetry file of the Marianas Trench

Add the excel document as X,Y data

Make sure that Longitude is in the X field and Latitude is in the Y field

Set the Z coordinates to “depth”

It is very important to note that the earthquake depths are in

Kilometers

o Once the excel coordinates are in Arc Scene, export the layer as a shapefile

o Now project the shapefile into the same coordinate system as the bathymetry

file

o Now that the earthquake points are in Arc Scene, convert them from 2D points

to 3D points by using the tool “Feature To 3D By Attribute”

Make sure that in the height field you use the earthquake depths

o Once the new layer appears in Arc Scene, double-click on it and go to the Base

Heights tab

Under the Elevation from features box, activate the “Use elevation values

in the layer’s features” button and type -1,000 to convert from

kilometers to meters

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o Go to the symbology tab and click on Quantities -> Graduated Symbols

In the value field select Magnitude and make sure the classification is set

to natural breaks (there should be five classes)

This will give each earthquake point a certain spherical radius based on

which class its magnitude falls in (between 5 and 10 on the Richter Scale)

Earthquake Data for the San Andreas Fault

o Go to the USGS Earthquake Hazards Program page and click on “Search for an

Earthquake”

Set the output file type to Spreadsheet Format (comma delimited)

Set the Data Base to USGS/NEIC (PDE) 1973 – 2011 11 28

Do a rectangular area search and enter the same latitude and longitude

coordinates as for the San Andreas Fault bathymetry data

Under the optional parameters, set the minimum magnitude to 5 and the

maximum magnitude to 10

Submit the search

o Once the search is complete and your data is available, copy all of the

information into a Notepad text document

o Open Excel and import the notepad text document

Make sure you import it as delimited by commas and make sure that all

the columns are marked as general

o Open the Arc Scene document with the bathymetry file of the San Andreas Fault

area

Add the excel document as X,Y data

Make sure that Longitude is in the X field and Latitude is in the Y field

Set the Z coordinates to “depth”

o Once the excel coordinates are in Arc Scene, export the layer as a shapefile

o Project the shapefile into the same coordinate system as the bathymetry file

o Now that the earthquake points are in Arc Scene, convert them from 2D points

to 3D points by using the tool “Feature To 3D By Attribute”

Make sure that in the height field you use the earthquake depths

o Once the new layer appears in Arc Scene, double-click on it and go to the Base

Heights tab

Under the Elevation from features box, activate the “Use elevation values

in the layer’s features” button and type -1,000 to convert from

kilometers to meters

o Go to the symbology tab and click on Quantities -> Graduated Symbols

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In the value field select Magnitude and make sure the classification is set

to natural breaks (there should be five classes)

This will give each earthquake point a certain spherical radius based on

which class its magnitude falls in (between 5 and 10 on the Richter Scale)

Now that the earthquake position distribution is visible under the topography data for both the

San Andreas Fault and the Marianas Trench, the images themselves can be made to look more

presentable. Below are the steps for further processing of the images using ArcGIS.

The San Andreas and Mendocino Faults: The steps are in order of how they should be done,

with the first bullet as the first step

Obtain and upload the trace of the San Andreas Fault

o Go to the USGS Earthquake Hazards Program and click on the Hazards tab

o Click on Quaternary Faults in the table of contents

o Next, download the file qfaultsshapefiles.zip, which is located under the GIS

Shapefiles hyperlink

o Once downloaded, extract the data into the project folder

o Open and view the shapefile QuaternaryFaults in Arc Map (it is in the WGS84

geographic coordinate system)

Open its attribute table and select all the listings for “San Andreas

Fault…”

Next, export the selected data into a new shapefile

Project that shapefile into the same coordinate system as the earthquake

and elevation raster in Arc Scene (WGS 1984 UTM Zone 10N)

o Load the newly projected shapefile into the San Andreas Arc Scene document

Have it float on the elevation raster custom surface and add a layer offset

of 500 meters so that it is better visible

Obtain and upload the trace of the Mendocino Fault

o Open the attribute table of the Quaternary Faults in Arc Map

o Select all the listings for “Mendocino fault zone…”

o Export the selected data into a new shapefile and then project it into the WGS

1984 UTM Zone 10N coordinate system

o Load the projected shapefile into the San Andreas Arc Scene document

Have it float on the elevation raster custom surface and add a layer offset

of 500 meters so that it is better visible

Obtain and upload the outline of the state of California (for better referencing of the

fault(s))

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o Obtain the shapefile for the outline of all 50 of the U.S. states from a map

database called Mapproj.mbd in the Lab 2 folder

o Add the shapefile to Arc Map and open the attribute table

o Select only California and export it as its own shapefile

o Project the shapefile into the WGS 1984 UTM Zone 10N

o Add the file into the San Andreas Arc Scene document

Do not float it on the elevation raster because the shapefile data will

become corrupted

Add a layer offset of 2,700 meters so that the entire shape of California is

visible above the elevation raster

Add a fault plane to both the San Andreas and Mendocino Faults

o For both shapefiles, go to layer properties and click on the Extrusion tab

o Extrude the feature in the layer and set the expression value to -16093 meters

This will create a 3D vertical wall going straight down about 10 miles,

which, according to the USGS, is the least amount of distance the San

Andreas Fault is known to intrude into the earth

Transfer the image from Arc Scene to Arc GIS

o Take a screen shot of the image and save it as a PNG file in the final project

section of the hard drive

o Open an Arc Map document and insert the PNG into the layout view window

o Take more screen shots of the table of contents in the Arc Scene document in

order to make a legend for the items in Arc Map

o Make sure all the map elements are included in the legend

o Insert a north arrow and angle it so that it faces north with the bathymetry layer

o Insert a title and coordinate system information

Include the amount of exaggeration present in the PNG image

o Label the different continental plates

o The 3D San Andreas Fault (and Mendocino Fault) image should now appear as if

it was made in Arc Map

The Marianas Trench: The steps are in order of how they should be done, with the first bullet as

the first step

Trace the plate boundaries that form the Marianas Trench

o First, load the Marianas Trench bathymetry file into an Arc Map document

o Create a new line shapefile and place the layer on top of the bathymetry layer

Make sure it has the same coordinate system as the bathymetry layer

o Digitize a line tracing the trench in the new shapefile using the editing tool

o Load the new shapefile into the Marianas Trench Arc Scene document

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o Have it float on the Marianas Trench bathymetry surface and add a layer offset

of 900 meters

o The line should conform to the topography but be completely visible above it

Transfer the image from Arc Scene to Arc GIS

o Take a screen shot of the image and save it as a PNG file in the final project

section of the hard drive

o Open an Arc Map document and insert the PNG image into the layout view

window

o Take more screen shots of the table of contents in the Arc Scene document in

order to make a legend for the items in Arc Map

o Trace a line down the axis of the fault plane made visible by the positioning of

the earthquakes

o Make sure all the map elements are included in the legend

o Insert a north arrow and angle it so that it faces north with the bathymetry layer

o Insert a title and coordinate system information

Include the amount of exaggeration present in the PNG image

o Label the different continental plates and the Marianas Trench

o The 3D Marianas Trench image should now appear as if it was made in Arc Map

Analysis and Results:

The first question concerning this project, which is ‘how does the 3D geometry of earthquake

positions under the Marianas Trench compare to the 3D geometry of earthquakes under the

San Andreas Fault?,’ can be answered by visualizing the spatial distribution of the two sets of

earthquakes in Arc Scene.

The Marianas Trench:

The first trend that becomes apparent when viewing the earthquake positions under the

Marianas Trench is that virtually all the earthquakes are located (on the X and Y axes) along an

imaginary fault plane that drops down from the curved trench into the earth. The angle of the

plane can best be described as being just short of 90 degrees straight down, or tilting slightly

towards a westward direction. The easiest way to visualize this is by looking at the “Earthquake

Geometry of the Marianas Trench” image on the second to last page. The earthquakes are

mainly positioned very close to the fault plane and resemble the shape of a half-funnel plunging

into the earth (image on next page). On the Z axis, the deepest earthquakes come to a tip at a

depth of about 612 km beneath the surface of the earth. The only visible trend associated with

the magnitude of the earthquakes (represented by the different sized radii of the earthquake

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spheres) is that most of the deepest earthquakes, from about 570 km to 612 km down, are of

magnitudes 5.7 and higher. Throughout the rest of the funnel shape the magnitudes are fairly

well dispersed. There is a slight cluster, however, of larger earthquakes on the scale of 6.2 and

higher near the southern end of the trench. The image below is a view of the trench looking

from under the ground up to the surface.

The San Andreas Fault:

While analyzing the positioning of earthquakes under the San Andreas (plus the Mendocino)

fault, it became obvious that the overall geometry was nothing like the earthquake geometry

associated with the Marianas Trench. The earthquakes are positioned around a fault plane that

extends vertically down into the earth, not off at an angle. The earthquakes are also not

positioned quite as close to the fault plane as under the Marianas Trench. They tend to vary in

XY distance to the east and to the west of the fault plane. The only real trend that is visible in

the XY field is that some earthquakes tend to cluster to the north of the Mendocino fault, with

very few located to the south. When it comes to the Z axis geometry of the San Andreas Fault,

the earthquakes all occurred at much more shallow depths than in the Marianas Trench (refer

to the image on the next page). The deepest earthquake associated with the fault is at a depth

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of about 33 kilometers. There is no obvious trend in the positioning of earthquakes with

regards to their magnitude; they seem to be fairly well dispersed on all three X, Y and Z planes.

This view of the San Andreas Fault area is looking from under the ground up to the surface at an

angle.

The second question concerning this project (Can the earthquake positions be correlated with

the type of movement associated with these plate boundaries?) can be answered by first

researching the type of plate boundaries represented by the Marianas Trench and the San

Andreas Fault. By comparing this research with the visual knowledge gained from analyzing the

Arc Scene images, it can then be deduced whether or not earthquake positioning is correlated

with plate movement.

The Marianas Trench:

While analyzing the overall geometry of the positioning of earthquakes under the Marianas

Trench, it was very obvious that the earthquakes were correlated with each other based on

some outside force. After researching how the Marianas Trench was formed, it became clear

that this force was most likely the movement of tectonic plates. The Marianas Trench is the

product of the denser Pacific Plate being forced under the less-dense Mariana Plate in what is

known as ocean-ocean subduction. The ‘imaginary plane’ that the earthquakes tend to occur

around is the surface between the subducting Pacific plate and the ‘floating’ Mariana Plate.

Therefore, the answer to the second question is yes, the earthquakes associated with the

Marianas Trench are correlated with the movement of the Pacific Plate under the Mariana

Plate.

The San Andreas Fault:

Based on online resources, the type of fault movement associated with the San Andreas Fault is

strike-slip, or more precisely “right-lateral strike slip (USGS).” This type of motion is mainly

horizontal, with the Pacific Plate on the west side of the fault moving slowly north and rubbing

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against the North American Plate on the eastern side of the fault. The fault plane associated

with the boundary of these plates, which can be seen in the image “Earthquake Geometry of

the Sand Andreas Fault Area” (located two pages down), extends vertically down into the earth.

By analyzing the overall geometry of the earthquakes in the area of the San Andreas Fault, a

clear correlation can be seen between plate movement and earthquake depth. While the

earthquakes do not trend right along the plate boundary, they do, however, exhibit the same

shallow depths. The deepest earthquake, which occurred at a depth of about 33 km, does not

go too much further down than the shallowest limit of the plate boundary (about 10 km). Based

on these observations, it can be said that the earthquake positions around the San Andreas

Fault are correlated with the strike-slip movement of the plates.

Conclusions:

While the Marianas Trench and the San Andreas Fault are both formed by tectonic plate

boundaries, the movement of these plates at their boundaries is quite different. The San

Andreas Fault exhibits a “right-lateral strike-slip” motion while the Marianas Trench is being

formed by the downward motion of a plate in an ocean to ocean subduction zone. Based on all

the evidence and correlations gathered in this study, the differences in earthquake position

geometry associated with these two plate boundaries is most likely caused by the different

movements of the tectonic plates.

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References

Schulz, Sandra S. and Wallace, Robert E. “The San Andreas Fault.” United States Geological

Survey (USGS). 06/24/1997. U.S. Department of the Interior. Nov. 28, 2011.

< http://pubs.usgs.gov/gip/earthq3/safaultgip.html>.

“The Mariana Trench – Oceanography.” The Mariana Trench. 04/04/2003. Nov. 28, 2011.

< http://www.marianatrench.com/mariana_trench-oceanography.htm>.

“Mariana Trench.” United States Geological Survey (USGS). 10/21/2009. U.S. Department of the

Interior. Nov. 28, 2011.

< http://earthquake.usgs.gov/earthquakes/world/guam/mariana_trench.php>.