Example Costa Rica 2012 earthquake
(data kindly provided by Dr. Ronnie Quintero)
Preliminary steps
Save content of example_costarica folder of the present ISOLA distribution for later comparison
with your own experiments with this example.
Create your example_costarica folder with content identical to example_costarica_start folder of
the present ISOLA distribution.
The content of example_costarica_start represents status of ISOLA after the following actions
(for their explanation, see example example_corinth):
a) Inspecting SAC files from folder data_sac_costarica_sm, using SeisGram software of A.
Lomax (DUNO_Z is problematic)
b) Converting SAC files into ISOLA ascii *unc.dat files (using Import Data from ISOLA main
window, SAC Import). Symbol * stands for station code. Station JTS needed rotation of
horizontal components. See Remark-Rotation below. Copying *unc.dat files into folder data.
c) Preparing folder pzfiles (see Remark-pzfiles below)
d) Creating *raw.dat files in Data Preparation window (Instrumental correction, Origin align,
‘low-cut’ = removing low frequencies f<0.05 Hz due to noise of strong-motion instrument).
These data are saved in folder data_sac_costarica_sm.
e) Copying *raw.dat files to folder invert.
The root of example_costarica folder includes also
event.isl
duration.isl
network_costarica.stn
quinetro_kissling.cru
and *stime.isl files
REMARKS
Remark - pzfiles:
We use 10 identical strong motion accelerographs and work on the flat part of their acceleration
response. The acceleration records are in counts. ISOLA needs response for input velocity. All
pzfiles are the same:
A0
1
… normalization constant
count-->m/sec … conversion constant C … in our case 1 count = 2.340002e-6 m/s^2
2.340002e-006
zeroes
2
0.000000e+000 0.000000e+000
1.000000e+000 0.000000e+000
poles
1
1.000000e+000
0.000000e+000
The pole (1.,0.) and zero (1.,0.) cancel each other; they appear here only for formal reasons
because we assume the instrument response for input acceleration to be constant, independent of
frequency. The second zero (0,0) is included to describe the instrument response for input
velocity.
How to create the pzfile in the Create Pole Zero tool of the ISOLA main window.
Besides other parameters, the meaning of which is obvious, the tool needs the two sensitivities:
Digitizer Sensitivity ... D [count/Volt]
Seismometer Sensitivity ... S [V/(m/s)]
Code uses the product (D.S) [count/(m/s)]
If user does not know D and S, but only the value of the product (D.S) [count/(m/s)],
he formally puts the value of D.S into the D-box and the value 1 into the S-box, or, vice versa
(D.S into S and 1 into D). In both cases the created pzfile will have the value C=1./(D.S)
[(m/s)/count] on its 4th line, as desired.
In the Costarica example we have D.S= 4.27350e5, C=1./(D.S) = 2.340002e-6
For more details, see how_poles_zeros.doc file in the pzfiles folder.
Remark - Rotation
Data of JTS station needed rotation. The recorded EN1 component is positive towards W, EN2
component is positive towards N.
When downloading SAC files, the EN1 component was formally read into 'N component' (in
ISOLA it is understood as X component), and EN2 was read into 'E component' (in ISOLA it is
understood as Y). This is how JTSunc_original.dat was created during conversion from SAC to
ascii.
Rotation from XY to NE was performed with ISOLA tool Rotate data. From XY to NE
anticlockwise by 270 degrees. The rotated file has been called JTSunc_ronnie.dat and copied into
JTSunc.dat.
DEFINING CRUSTAL MODEL and STATIONS
From ISOLA main menu select
Define Crustal Model
and use quintero_kissling.cru file. Continue as in example_corinth.
From ISOLA main menu select
Station Selection
and use network_costarica.stn file; choose all 10 stations.
Work as in example_corinth.
Remember that Event Info must be defined before this step.
INVERSION IN HORIZONTAL PLANE
Purpose - to find centroid; using low frequencies.
From ISOLA main menu select
Seismic Source Definition
Sources on a Line or Plane
The goal is to make a horizontal grid search for the centroid position.
New reference point: Depth 35 km, shift to North 0 km, shift to Eeast 0km
[Choice of the 35 km depth is based on preliminary tests with grid search in vertical direction.]
Strike: 0, Dip 0
[The terms ‘strike’ and ‘dip’ have a different meaning than when dealing with the fault plane.
Choosing strike=dip=0 defines a grid search in horizontal plane.]
Number of points along ‘strike’ (i.e. in the direction from S to N) and along ‘dip’ (i.e. in direction
from W to E): 7
Spacing along strike and dip: 10 km
Reference point along strike: 1, and along dip: 1
[The reference point has its geographic coordinates given at the top of the window. Choosing the
value 1 means that the reference point is in the left bottom corner of the grid.]
Calculate
Three figures appear showing the numbering of the 49 trial sources. It is useful to save some of
the plots (sources.png), for example in folder green\horiz_plane. Save also sourcesinfo.txt and
sources.gmt files from gmtfiles folder.
Exit
From ISOLA main menu select
Green Function Computation
Maximum frequency 0.1 Hz
Time function triangle 10 s
Run
[Avoid answering yes in the Continue Operation menu until the calculation stops.]
Although 49 trial sources are used, the calculation is fast because the depth (35km) is large and
maximum frequency (0.1 Hz) is small.
Finally, when the calculation stops, allow automatic copying of the elementary seismograms into
invert folder.
Wait until the Copy files message appears, confirm OK.
It is useful to save the content of folder green in green\horiz_plane for comparison with the
ISOLA distribution.
From ISOLA main menu select
Inversion
Choose Freq. range common to all stations: 0.03, 0.03, 0.06, 0.06 Hz
Deselect DUNO all 3 components
[See remark below about VR (Variance Reduction) in case of wrong DUNO_Z.]
Choose time search: start 200 dt, step 6 dt, end 800 dt
[Here dt=0.05 s is the time step of all re-sampled waveforms. It is dt = 1./tl = 1/.409.6, where
409.6 is the length of the time window defined in EventInfo.]
Chose Deviatoric Inversion, single subevent
Run
Plot correlation diagram (optimum solution is seen in trial source no. 11)
Exit Inversion window.
Remark – VR
If the inversion is run with DUNO_NS and DUNO_EW (removing only Z), it creates problems
with variance reduction (VR). VR is calculated form used stations [not only used components!],
i.e. including DUNO. Although DUNO_Z is removed from inversion, it causes very small global
VR. GUI lists the VR values of the individual components in folder output, see ***varred_info
file. Global VR calculated from all stations (in case that DUNO_Z is wrong) might even yield a
misleading classification of the global waveform match. The situations in which waveforms in
many components have a good VR, but DUNO_Z is poorly fitted might result in lower global VR
than other situations in which many components are poorly fitted! This will need future
modification of ISOLA.
Open Plot results window from main ISOLA menu
Plot waveforms (see the result in folder output)
Plot moment tensor
Check inv files (or simply see inv1.dat, inv2.dat, inv2c.dat from invert folder)
Save these files and plots in invert\horiz_plane
Optimum position (no. 11) is 30 km northward and 10 km eastward from the reference point.
Point 11: Lat 10.2367144660 Lon -85.3865253851 Depth 35 km [these data can be found in
file sources.gmt, folder gmtfiles].
Optimum mechanism has strike/dip/rake = 300/ 23/ 98 = 112/ 67 / 87 degrees.
This is our centroid-moment solution. [The calculation can be repeated in several horizontal
planes to simulate a complete 3D search.]
Compare with Harvard GCMT
CENTROID LOCATION:
ORIGIN TIME:
14:42:23.7 0.1
LAT: 9.87N 0.01;LON: 85.54W 0.01
DEP: 30.8 0.4;TRIANG HDUR: 15.3
NP1: STRIKE=317;DIP=19;SLIP= 118
From ISOLA main menu select
H-C plot
This will help us to select fault plane among the two nodal planes.
Read parameters from file: hcplot_cr.ini.
[This file was manually prepared before; it contains the coordinates of
hypocenter = those form Event Info, and the coordinates of centroid, see the above result of the
horizontal grid search, optimum point 11; also the strike and dip of the two nodal planes found
above are entered.]
Plot (it is useful to save the plot and numeric values of distances of H from the two planes).
Hypocenter is considerably closer to green plane (strike 300) than to the red plane (strike 112),
i.e. the green plane is the likely fault plane.
For more details, see zahradnik_etal_srl2008.pdf.
INVERSION IN FAULT PLANE
It is possible to start inversion in the same low-frequency range as above to confirm that the
single subevent will dominate and its position will be similar to that one found in the horizontal
plane (folder \invert\fault_plane_0.06hz). This guide skips this test and continues directly with
higher frequencies.
Purpose – to find a possible multiple-point-source solution (MPS); using higher frequencies.
From ISOLA main menu select
Seismic Source Definition
Sources on a Line or Plane
The goal is to make grid search in the fault plane passing through the centroid.
New reference point: Depth 35 km, shift to North 30 km, shift to Eeast 10 km
[This choice is dictated by the previous determination of the centroid position.]
Strike: 300, Dip 23; now already the terms ‘strike’ and ‘dip’ have their normal meaning.
[This choice is dictated by previous centroid moment tensor.]
Number of points along strike and dip: 7
Spacing along strike: 10km, and along dip: 5 km
Reference point along strike and dip: 4 (i.e. in the center of the grid).
Calculate
Three figures appear showing the numbering of the 49 trial sources.
Save the map view (e.g. sources.png); the old reference point (from Event Info) is shown by blue
star, the new reference point (center of the fault plane) is yellow.
It is useful to save also sources.gmt from folder gmtfiles where all 49 trial sources appear with
their geographic coordinates (Lon, Lat, Depth).
Exit
From ISOLA main menu select
Green Function Computation
Maximum frequency 0.2 Hz
Time function triangle 10 s
Run
[Avoid answering yes in the Continue Operation menu until the calculation stops.]
Finally, when the calculation stops, copy elementary seismograms into invert folder.
Wait until the Copy files message appears, confirm OK.
It is useful to save the content of folder green in green\fault_plane for comparison with the
ISOLA distribution.
From ISOLA main menu select
Inversion
Choose Freq. range common to all stations: 0.03, 0.04, 0.10, 0.15 Hz
Deselect DUNO all 3 components; see remark below about Variance Reduction in case of wrong
DUNO_Z !!!!!!!
Choose Time search grid: start 200 dt, step 6 dt, end 800 dt [here dt=0.05 s]
Choose Deviatoric Inversion, two subevents
Choose Fixed mechanism: strike/dip/rake = 300./ 23./ 98.
Time function: Triangle 10 s
Run
Use INTEERACTIVE regime (answer y in 'black window' when running inversion).
Otherwise the correlation plot appears only for the last [second] subevent (corr02.ps).
To get the correlation plot also for the first subevent (corr01.ps), plot the correlation during
PAUSE of the inversion after calculation of the first subevent.
Subevents 1 and 2 have their positions no. 25 and 48.
Exit Inversion window and open Plot results window from main ISOLA menu.
Plot waveforms (save using Export Figure option at the top, obtain the plot from folder output
and save it.)
Plot moment tensor; you can choose the 1st or 2nd subevent. The *best.ps file
will be generated (save yourself manually). Plot for second subevent will overwrite the first one.
Check inv files (or simply see inv1.dat, inv2.dat, inv2c.dat from invert folder).
The most interesting result is in inv2.dat showing position of the trial source, its time (in seconds)
and moment (Nm); each line is for the respective subevent. Last number in the line is the
variance reduction. Subevent 2 (position 48) is relatively weak compared to subevent 1 (position
25); the moment ratio sub1/sub2 is around 3; it is relatively large ratio, and thus it may worry
about relevance of subevent 2.
Save results in invert\fault_plane.
From ISOLA main menu select
Time function
The time function will be calculated for 1 source or simultaneously for 2 sources by the NNLS
method described in Appendix of the paper zahradnik_sokos_gji2014.pdf. Linear combination of
24 basic pulses will be calculated (12 for each source). The basic pulse (moment-rate time
function) is the same as used in the Inversion, e.g. triangle of the 10s duration in our example.
The source numbering, frequency range and the station selection remains the same as in the
previous inversion.
Choose Source 1 and 2 in position 25 and 48 as they were obtained during previous inversion.
Their parameters will appear (being transferred from the previous inversion automatically).
Choose optimum time shifts for source 1 and 2. This is the beginning of the respective time
functions of the two sources; it may differ for source 1 and 2. For example, those might be close
(or smaller) than times found in inv2. In this example let us choose the same optimum time shifts
for source 1 and 2 equal 10 s.
Choose Triangle distances 3 s.
Constrain moment: leaving the box un-checked will not put any constraint on the moment.
Duration of triangles for plotting: choose 0.1s.
[This choice helps to see the individual contributions. If you use the true duration, 10 s in our
case, the triangles superimpose and their relative importance is not well seen.]
Calculate and Plot. Check output files, mainly inv111.dat.
Compare with result from Inversion (subevents 1 and 2 were switched at 12.4 and 19s,
respectively). Here we obtain a similar result.
Play with the parameters [opt. time shift and triangle distance] to find out if the inversion is stable
enough. For example, use only 1 source, opt. time shift 10s, triangle distance 1 or 2 s. Check the
variability of the time function plot.
The results appear in folder timefunc. If you want to plot synthetics you have to copy *syn.dat
files (where * denotes the station code) from timefunc into invert folder. Then you can use the
standard Plot waveform tool.
Some other options:
Jackknifing [repeatedly inverting same waveforms while deselecting each time a different
component, or complete 3-component station]. This might help to see stability of the result,
particularly useful if there was an indication of a multiple event.
Caution: Polarity tool cannot be used with the crustal model quintero_kissling.cru.
It contains the deepest discontinuity at 104 km, but ISOLA expects the deepest discontinuity to
be Moho. Manual adjustment of the crustal.dat file in the plarity folder is possible according the
instruction in example_corinth.
Some experiments recommended for advanced users.
Repeating inversion in horizontal plane with various choices of the triangle duration T. Watch
VR and moment while varying duration T, e.g. T=5, 10, 15, 20 s. As a rule, moment is always
increasing, but VR has a local maximum thus indicating the preferable T.
Playing with inversion in fault plane, one, two or three subevents (free deviatoric mechanism,
various options of the fixed mechanism, various frequency bands, selecting-deselecting stations).
Watch mainly the variablity of the subevent positions, time, moment, DC% and VR to find out a
stable solution. Adding more subevents has no reason if their moments are low, if VR does not
increase considerably, or if the total moment reached a value known from GCMT. File inv2.dat
gives moments of the individual subevent (column 3), while file inv2c.dat gives the cumulative
moment (column 3); inv2c.dat provides also the cumulative moment magnitude (column 4).
Repeating all modeling with a different crustal model, if available for the studied region.
In case of large uncertainty of the solution (large scatter of nodal planes in the Uncertainty tool,
e.g. when using only very few stations) it is useful to design a fictitious station network and
observe how adding more stations the uncertainty decreases. It is always useful to play with
uncertainty by changing the source position (e.g. depth) and frequency range. The uncertainty can
be calculated without inversion. For fictitious stations, see example_corinth.
Try inversion with DUNO_NS and DUNO_EW (remove only Z). It creates problems with
variance reduction (VR). VR is calculated form used stations [not only used components!], i.e.
including DUNO. Although DUNO_Z is removed from inversion, it causes very small global
VR. Good examples - inversions with JTS incorrect rotations. See invert\problems_rotation.
Simultaneously inverting for position, time and moment of the pairs of subevents on the fault
plane. This is not yet implemented in GUI. Work manually in folder timefunc, see
runtimfun_pairs.bat. See also paper zahradnik_sokos_gji2014.
Calculating uncertainty maps (zahradnik_custodio_BSSA2012); not yet in GUI, but fortran codes
are available.
Weak events, recorded at near distances have P onset close to origin time. This often needs a trick
– artificial shift of waveforms “to the right”, e.g. by 30 seconds. To this goal, construct *raw.dat
[in Data preparation window] with origin time artificially decreased by 30 seconds. Make the
temporal grid search in the inversion centered at time increased by 30 s with respect to origin
time (the common centering of the time search is at the origin time). It of course needs SAC data
available at least 30 s before origin time.
Playing with low-cut (pre-filtering) in Data preparation. It removes very low noisy frequencies
when preparing *raw.dat. It is useful because some problems of the noisy low frequencies might
be not removable during inversion! [The waveforms are aligned and cut at origin time, the cut
will produce an artificial impulse if the record is not pre-filtered enough.]
If the correlation plot has almost parallel strips with beachballs of opposite
compression/dilatation sectors (mainly for weak events inverted up to relatively high
frequencies) the moment tensor solution might disagree with polarities. Choose the solution with
lower correlation from the neighboring space-time strip using interactive mode of the inversion
run. To find exact time of the solution from the ‘opposite correlation strip’, use numeric values
from corr01.dat; column 1 = trial source position, column 2 = correlation.
Play with using polarities form more stations than those used in waveform inversion, play in
polarity folder with source depth and crustal model.
Try to compare two examples in which you always invert in the same frequency range, e.g. up to
0.15Hz, but Green functions were calculated either up to 0.2 or 0.5 Hz. The results should not
differ at all, but they might differ 'a bit'. Explanation is in 'artificial attenuation' used in the
Discrete Wavenumber method. This is an unpleasant feature for which we have not yet found a
good solution.