Chapter 2

THE KEYS TO THE CITY, AND A QUICK TOUR

2.1 The most important things to know about Coulomb

• Source vs. receiver faults (p. 11)

Sources have slip and impart stress; receivers do not have slip and receive stress

• Coulomb failure hypothesis, Ds_f = Dt_s + m’ Ds_n (p. 35)

Failure is promoted when the Coulomb stress increases. Ds_f is the change in failure stress on the receiver fault caused by slip on the source fault(s), Dt_s is the change in shear stress (positive in the direction of receiver fault slip), Ds_n is change in normal stress (positive when the receiver fault is unclamped), and m’ is effective fault friction coefficient on the receiver fault.

• How faults are defined in the input files (p. 16)

Right-hand rule for start/fin. points, positive dip, rt-lat/reverse or rake/net slip

• Coulomb stress on specified vs. optimally-oriented planes (p. 36)

assumed slip directions vs regional stress; visualizing slip lines, principal axes

• Coulomb Figures (p. 27)

“dead” editable vector .pdf files vs. “live” matlab 3D .fig files that can be spun

•  ‘inp’ vs. ‘mat’ input files (p. 26)

.inp is editable text; .mat is a binary file, but can include fault and quake overlays

• Keep an eye on the command window

For calculation progress, input/output file names, input source moment, errors

• Start simple, and add complexity if needed later

Even if you have a detailed variable-slip source model, start with idealized faults and develop intuition about the problem. Only then add more realistic detail.

• For classroom instruction, use Cartesian coordinates unless you want to use overlays

Its quicker and easier

• How receivers are defined (p. 38)

Specified faults via Aki-Richards, subdivided receivers, CMTs, and nodal planes:

2.2 A quick tour of Coulomb 3.1

Here we just want you to learn the procedure to display and output graphics, and to save the results. We always recommend that you save all your current working documents frequently to prevent lost of files in the event of program crashes.

Menus and their pull-down items turn from light white to black as they become options. Here is the procedure to lunch Coulomb:

1. Launch Matlab on your computer. In the Matlab menu bar top line, click on the icon enter the path of “Coulomb 3.1 folder” in your computer. For example, in the example shown below, the “Coulomb 3.1 folder“ has a path of “/Users/jlin/Desktop/Coulomb 3.1 folder”.

2. In the Matlab Command Window, type “coulomb”. Coulomb will take 30 seconds or so to check the download site and notify you if a newer version is available. Always migrate to the latest version.

Choose Input > Preferences. Colors, line widths, the ‘color map’ (the smoothly varying color gradient maps), and the output file folder can all be changed (below left). For input files with lat/lon information, you can also choose to plot maps with lat/lon rather than km frame tics. For now, leave them in the default setting. Note that ‘Fault’ refers to the source fault, not the active fault file. Replace screen shot; replace ‘Circle diameter’ for Earthquake to ‘Symbol size’ for Earthquake and Volcano.

4. In the pop-up Coulomb 3.1 window, choose Input > Open existing input file (above right). In the pop-up Open input file window, change to sub-directory “input_files”. Choose the “Example-2(LL)_lonlat.inp” from the dialog box, then hit <rtrn>. The program will automatically plot the 2D map view of the Grid menu in Functions.

5. Once you release the mouse button, the calculation is performed and the program displays the result in the Coulomb 3.1 window. It’s a simple grid with a NE-striking fault in the middle of the grid, as shown below. The bottom right corner also shows the version of the Coulomb program you are using, current date/time, name of the input file you are using, type of calculation for the image, and the calculation depth.

6. Now try, Functions > 3D grid, and play with the zoom and swivel tools. If you want to save this image, choose File > Save As. Rename the file in the dialog box, and save it as figure of one of the several types. Do not use “main_menu_window.fig” as the filename.

7. You also have the options to Zoom In, Zoom Out, Pan, and Rotate your image by clicking on the lower line on the Column 3.0 menu bar. You can also perform these operations by choosing items in the Tools pull-down menu.

8. If you click Functions > Displacements, in addition to making an image on your Coulomb 3.1 window, a numerical output file called “Displacement.cou” will appear in the sub-folder you designated in the Preferences menu. Similarly, if you click Functions > Stress, a text output file called “dcff.cou” will be created and can be opened in Excel.

9. To close the Coulomb 3.1 window, click Input > Quit. Do not quit out of Matlab if you want to continue with Coulonb. (If you wanted to quit Matlab program, type “exit” in the Matlab Command Window.)