Explores the stress effect of the Landers earthquake on a simple model of the San Andreas fault. Uses lnprj, lnsegment, segs2dlc, elfpatch, xss.
Note that answers given in [square braces] are defaults which will be used if no answer is supplied (just hit the carriage return).
The 1992 Landers earthquake in the Mojave Desert of Southern California raised concerns as to whether the southern San Andreas fault, which has not ruptured in historic time, might have been brought closer to failure.
Three groups who worked on this problem arrived at similar answers: That part of the San Andreas fault in the bend north of Riverside and San Bernardino was probably brought closer to failure by 10 to 20 years when the stresses add by Landers were compared to normal loading rates. Other parts of the fault were relaxed. If the nucleation zone for the next Big earthquake on the southern San Andreas fault is in the more loaded region, then it has probably been advanced in time. Unfortunately, we do not yet know where the nucleation zone for that earthquake is likely to be.
This example is a simplified version of the calculation of Harris and Simpson (1992).
The starting point is a model of the Landers and Big Bear ruptures based on geodetic data (Murray and others, 1993). This model was entered using makemodel:
landers.dlc =========== # Murray, Savage, Lisowski, and Gross model for Landers rupture # (GRL, 1992, v.20, p.623-626) # Projected: # iprj,cm,bl,rot,xcen,ycen = # 4 -118.0000 32.0000 0.0000 0.0000 0.0000 ! 1) Dislocation #1 = 1/1/1/0.0 Murray Landers model Camp Rock North segment 3 119.8576 296.7178 126.6881 290.0918 0.000 90.0000 9.0000 -0.7 0. 0. 180.0 ! 2) Dislocation #2 = 1/2/1/0.0 Murray Landers model Camp Rock South segment 3 126.6881 290.0918 130.7291 285.6611 0.000 90.0000 9.0000 -6.6 0. 0. 180.0 ! 3) Dislocation #3 = 1/3/1/0.0 Murray Landers model Emerson segment 3 130.7291 285.6611 135.3941 278.6631 0.000 90.0000 9.0000 -6.1 0. 0. 180.0 ! 4) Dislocation #4 = 1/4/1/0.0 Murray Landers model Homestead North segment 3 135.3941 278.6631 138.5620 271.6084 0.000 90.0000 9.0000 -8. 0. 0. 180.0 ! 5) Dislocation #5 = 1/5/1/0.0 Murray Landers model Homestead South segment 3 138.5620 271.6084 140.6877 266.3643 0.000 90.0000 9.0000 -1.7 0. 0. 180.0 ! 6) Dislocation #6 = 1/6/1/0.0 Murray Landers model Kickapoo segment 3 140.6877 266.3643 143.6539 257.1826 0.000 90.0000 9.0000 -9.2 0. 0. 180.0 ! 7) Dislocation #7 = 1/7/1/0.0 Murray Landers model Johnson Valley North segment 3 143.6539 257.1826 144.7124 247.5127 0.000 90.0000 9.0000 0.1 0. 0. 180.0 ! 8) Dislocation #8 = 1/8/1/0.0 Murray Landers model Johnson Valley South segment 3 144.7124 247.5127 145.2289 242.6602 0.000 90.0000 9.0000 -7. 0. 0. 180.0 ! 9) Dislocation #9 = 1/9/1/0.0 Murray Landers model Eureka Peak segment 3 151.6457 227.2500 148.5672 236.0537 0.000 90.0000 9.0000 -0.8 0. 0. 180.0 ! 10) Dislocation #10 = 1/10/1/0.0 Murray Landers model Big Bear earthquake 3 120.2424 252.2070 104.3675 235.3857 3.000 90.0000 6.0000 1. 0. 0. 0.0
A simple model of a vertical San Andreas fault was made by taking a line file of the fault, projecting it using lnprj, and segmenting it using lnsegment. Finally it was converted to a dislocation file call saf.dlc using segs2dlc. (Because the fault probably dips to the south going around the bend, it would be appropriate to give some of the dislocation surfaces a non-vertical dip. In this case, it could be done most easily in the text editor, perhaps using xss as a guide to identify which patches should be changed. I have not changed dips for this example. Note also that the SAF has more than one strand going through this bend.)
> lnprj
Infile: [saf.lin]
Outfile: [saf.lin] .lin_prj
Outfile: [saf.lin_prj]
Give line type (.OLN/.BOLN/.BLN/.LIN/.GEN/.OPP/.PF/.XYZ/.BLXYZ/QUIT): [.LIN] .lin
Report on progress? (y/n): [n]
Forward or inverse projection? (f/i): [f]
Projection number (99 for help): [0] 4
Give coordinates in decimal degrees or d:m or d:m:s
(e.g. -120.52 or -120:31.2 or -120:31:12)
Central meridian: -118
Base latitude: 32
New azimuth of North-direction: [0.]
Projected coordinates are in kilometers...
NUMBER OF LINES READ = 3
NUMBER OF LINES SAVED = 3
STOP:
> lnsegment
** Best to project line before segmenting...
Infile: [saf.lin] saf.lin_prj
Give line type (.OLN/.BOLN/.BLN/.LIN/.GEN/.OPP/.PF/.XYZ/.BLXYZ/QUIT): [.LIN]
Outfile: [saf.lin] saf.lin_10km
Give line type (.OLN/.BOLN/.BLN/.LIN/.GEN/.OPP/.PF/.XYZ/.BLXYZ/QUIT): [.LIN]
Report on progress? (y/n): [n]
Segment length: [0.] 10
NUMBER OF LINES READ = 3
NUMBER OF LINES SAVED = 3
STOP:
> segs2dlc
** Generates a model from segments in a linefile.
** Linefile should be projected and segmented.
Line file: [saf.lin] saf.lin_10km
Give line type (.OLN/.BOLN/.BLN/.LIN/.GEN/.OPP/.PF/.XYZ/.BLXYZ/QUIT): [.LIN]
Name for model file:: [saf.dlc] saf.dlc
Depth to top of dislocations: [0.]
Depth to bot of dislocations: [10.] 15
Height of one dislocation: [15.]
** Number stacked = 1
Approx length of one dislocation: [15.] 10
Want FRIC line also to run BVDS? (y/n): [y] n
Rake angle (0=LL; +/-180=RL; +90=Reverse; -90=Normal): [180.]
linenum, npts = 0 14
linenum, npts = 0 7
linenum, npts = 0 38
** Number of dislocations = 56
STOP
The two dislocation files are now combined using the text editor to yield a file called landers+saf.dlc with patches representing both the rupture and the San Andreas fault. Running elfpatch on this file as input produces a "patch stress" (.pstr) file called landers+saf.pstr. We assume a coefficient of friction of 0.6 for combining shear and normal stress into DeltaCFF values. Note that in this run, the assigned rakes of 180-degrees (RL slip on the SAF) are used to calculate the DeltaCFF.
> elfpatch
Parameter file: [saf.par] landers+saf.par
Dislocation file: [landers+saf.dlc]
Number of dislocations = 66
Displacement table? (None, Read, Write, Zero) (n/r/w/z): [n]
Patch stress (.pstr) outfile: [landers+saf.pstr]
*** Getting slip boundary conditions...
Non-zero slip for dlc # 1
ss,ds,op = -0.700000 0. 0.
Non-zero slip for dlc # 2
ss,ds,op = -6.60000 0. 0.
Non-zero slip for dlc # 3
ss,ds,op = -6.10000 0. 0.
Non-zero slip for dlc # 4
ss,ds,op = -8.00000 0. 0.
Non-zero slip for dlc # 5
ss,ds,op = -1.70000 0. 0.
Non-zero slip for dlc # 6
ss,ds,op = -9.20000 0. 0.
Non-zero slip for dlc # 7
ss,ds,op = 1.00000E-01 0. 0.
Non-zero slip for dlc # 8
ss,ds,op = -7.00000 0. 0.
Non-zero slip for dlc # 9
ss,ds,op = -0.800000 0. 0.
Non-zero slip for dlc # 10
ss,ds,op = 1.00000 0. 0.
Friction coefficient: [0.] .6
Cohesion: [0.]
Include pore effects? (y/n): [n]
If answer to next question is "n" then rakes will be used to calculate deltaCFF...
Add regional stresses? (y/n): [n]
STOP:
The best way to view these results is by using xss. It is also wise to view the model using xss before running elfpatch in order to be sure that it looks as expected. There are a surprising number of ways to come up with incorrect models! Here is a sample run of xss to look at DeltaCFF values. My apologies for the present state of xss - it needs lots of work to make it more user friendly.
>xss
p == >Plot-only, Plot-and-Save, or Save-only? (P/p/Ps/ps/s)
other == Earthquake to view (lp/mh/other)
landers+saf.par == Parameter file
landers+saf.bdlc == Dislocation file
landers+saf.dlc == Dislocation file
landers+saf.see == File with view parameters (or "none")
n == Add regional stresses? (y/n)
n == Scale view heights by value? (y/n)
y == Plot outlines around dislocations? (y/r/n)
/home/simpson/color/12rbplustek.clrs == Color file (h for help)
12 == Colors to use for scale
y == Display color scale? (y/n)
vf == Option (h=help)
landers+saf.pstr == pstr file
vp == Option (h=help)
11 == Give number of contours (0 for file)
-5 == Give MIN level
1 == Give INTerval
== Title
li == Option (h=help)
caout == Line file (h for help with abbrevs)
y == Project lines? (y/n)
q == Option (h=help)
0
Harris, R.A., and Simpson, R.W., 1992, Changes in static stress on southern California faults after the 1992 Landers earthquake: Nature, v. 360, p. 251-254.
Jaume, S.C., and Sykes, L.R., 1992, Change in the state of stress on the southern San Andreas fault resulting from the California earthquake sequence of April to June 1992: Science, v. 258, p. 1325-1328.
Murray, M.H., Savage, J.C., Lisowski, M., and Gross, W.K., 1993, Coseismic displacements: 1992 Landers, California, earthquake: Geophysical Research Letters, v. 20, p. 623-626
Stein, R.S., King, G.C.P., and Lin, Jian, 1992, Change in failure stress on the southern San Andreas fault system caused by the 1992 magnitude=7.4 Landers earthquake: Science, v. 258, p. 1328-1332.