Chapter 5
EARTHQUAKE,
FAULT, AND COASTLINE OVERLAYS
5.1 Overlays (coastlines, active fault traces, seismicity)
1. It
is often enormously valuable to plot your map with lat/lon tic marks and
overlay geographic or seismic catalog information. This is easy to do as long
as the input file (.inp or .mat) has lat/lon information which is the last item
in the input file, such as
To plot with lat/lon tic marks, Input > Preferences > Coordinates > lon.
& lat. and then launch the input
file. (If you do not have this information, you can calculate it using the
Coulomb tools: Functions > Tools > Convert lat/lon to Cartesian, which opens a calculator window that will create
your input file for you.)
Use of the
National Geophysical Data Center Coastline Extractor. Go to http://www.ngdc.noaa.gov/mgg_coastline/ and input the boundary coordinates, or Ôuse Java Map
for lat/lonÕ, zooming in as needed. We recommend the Medium Resolution
(1:70,000) Coastline database. Select the Matlab Format, choose no compression,
and preview it with a GMT plot. Hit the ÔSUBMITÕ button. Save the resulting
Ô.datÕ file in the Coulomb Ôcoastline_dataÕ folder, and rename it if you like.
5.2 How to use
overlays in 2D and 3D plots
1. Launch
ÔExample-SFBayArea.inpÕ. Now, Overlay > Coastlines.
Open the Ôcoastline dataÕ folder in the Coulomb30 folder, and select
Ôcalifornia_coastline_di_neg.datÕ. The ÔdiÕ refers to intermediate resolution,
there is also a high resolution ÔdhÕ file in the folder. Answer the pop-up question, ÔnoÕ. Some
datasets treat western longitudes as negative; others do not. The coastline
will appear on your plot. Now try, Overlay > Active faults. (You could
load folds or roads or anything you like; the formats are explained in a pop-up
window). Choose ÔCalifornia faults_longlat_datÕ. It's a large file but shortly
will plot on your map. The Coastline
and active fault data format windows
are shown below:
2.
Now choose Overlay > Earthquakes, and choose theÔ(NCSN) readableÕ format in the Earthquake catalog
formats menu below left. Then select
the ÔreadableÕ format of the actual data file in the data folder. It is only
important that the format chosen matches the format of the dataset you are
opening. Fourteen formats are
supported by Coulomb from global data, the U.S., Japan, and Turkey, and we plan
to add more.
An
Earthquake_screening window (shown
above right) will appear that lets you filter the time, location, depth, and
magnitude of earthquakes you would like to plot. It if is a large catalog, this
can take time. For the CMT ÔndkÕ format, go to
http://www.globalcmt.org/cmtfiles.html.
3. For this exercise, just
click ÔOKÕ to plot all 324 earthquakes that occurred in the grid are defined in
the input file, and you should see the image below. The default colors appear
below, but the colors of all overlays are controlled in the Preferences panel.
You
can remove any overlay by Overlay
> Clear overlay from memory, if
you want to use a different dataset instead, or you can toggle on/off any
overlay by unchecking it. Note that coastlines are continuous lines; by editing
them in Illustrator, you can fill the land or sea area. Faults are line
segments.
4. Save an input file with all of the overlays by Input > Save Input + Overlays as .mat. Name it uniquely. This will create a compact binary file
that you can use in the future—a great time savings—although it
cannot be epened in an editor.
5.3 Adding source or receiver faults by tracing active
faults (or folds)
1. It
is very fast and easy to build source or receiver faults by tracing the active
faults on the map. Starting from where you left off in the previous exercise, Overlay > Trace and put faults into input. Instructions pop up to remind you that
you use your left mouse button for the Start end of the fault and your right mouse
button for the Finish
end. Remember that the
right-hand rule means that for non-vertical sources, the Start-Finish order
determines which direction it dips: the fault dips to the right when you are
looking toward the Finish from the Start (see the all-important Aki-Richards convention diagram in Chapter
4).
The
Element input panel appears (see
below) with the Start-Finish coordinates you moused on the map. Now, add the
slip and top/bottom depth and hit ÔOKÕ. Remember that if the fault is non-vertical and blind, it will
be offset from the trace as a function of its dip and upper depth (refer to the ÔMap PositioningÕ figure in Chapter 4 to see
this). The new fault appears in red (below); when you hit OK, it turns
green to indicate it is now the trace of a source or receiver. If you save the
input file, the new source or receivers will be included. A corresponds to Start, and B corresponds to Finish.
5.4 Making 3D source and seismicity
figures that you can spin and roll
Matlab lets you create very compact
ÔfigureÕ files that can be manipulated in 3D to see, for example, if your
coseismic source fault is properly located with respect to its aftershocks.
- Input > Preferences > Cartesian. Open an input file,
add the coastline and earthquake catalog overlays. Now, Functions > Grid > 3D view (see below left).
- Switch to the Matlab command window, and type the
plug-in, Ôearthquake_plot3Õ. You will asked on screen if you would like
the earthquake magnitude scaled by circle size and depth scaled by color
(choose Y); circles or filled disks (choose N).
- You can use the
tool to blow up the area of
interest, but the file will still contain the full data. Now use the 
to spin and roll the view. A portion is shown below, at
right. With this tool selected, a right mouse-click lets you to reset to a
regular map, side, or depth view. The source slip is scaled by its color. - If
you are using a PC,
you can save this file and re-open it later. File > Save as > Ôyourname.fig.Õ Its an
amazingly small file.
- If
you simply double-click on this figure, it opens in Matlab (not Coulomb)
and can again be maninpulated. Matlab has yet to implement this feature on
MacÕs.
3D view in Grid
using Ôearthquake_plot3Õ plugin
