alomax
Hypocenter of an explosion
Hello Anthony,
I am testing NLL for the automatic location of local events. I tried to locate some quarry blasts, which depth should be close to 0 km. Using the Crust1.0 crustal model, most of the quarry blasts are located successfully, but for one of them in particular I get 15 km as depth, which I think is not acceptable and I try to find the reason.
This event was detected on 3 stations, producing P- and S-picks that were manually/visually inspected. The stations have epicentral distances between 14 - 21 km.
I followed the example (sample) that locates the Alaskan events to automatically produce the files necessary to run NLL. These are the files:
For the station coordinates:
GTSRCE GRA1 LATLON 49.690776 11.220436 0.0 0.4995
GTSRCE GRA3 LATLON 49.761076 11.31735 0.0 0.455
GTSRCE GRB4 LATLON 49.467841 11.55948 0.0 0.507
Then the observed travel times:
GRA3 ? HHZ ? P ? 20220120 0822 17.255 GAU 9.00e-02 -1.00e+00 -1.00e+00 -1.00e+00 1
GRA3 ? HHZ ? S ? 20220120 0822 20.175 GAU 6.00e-02 -1.00e+00 -1.00e+00 -1.00e+00 1
GRB4 ? HHZ ? P ? 20220120 0822 18.009 GAU 1.00e-02 -1.00e+00 -1.00e+00 -1.00e+00 1
GRB4 ? HHZ ? S ? 20220120 0822 21.269 GAU 3.00e-02 -1.00e+00 -1.00e+00 -1.00e+00 1
GRA1 ? HHZ ? P ? 20220120 0822 16.999 GAU 3.00e-02 -1.00e+00 -1.00e+00 -1.00e+00 1
GRA1 ? HHZ ? S ? 20220120 0822 19.659 GAU 8.00e-02 -1.00e+00 -1.00e+00 -1.00e+00 1
And finally the control file (here the command Grid2Time is executed once for travel time calculation for P-waves and once for S-waves, by creating a similar file which I don't show here):
CONTROL 0 54321
TRANS LAMBERT Clarke-1880 49.62 11.38 48.62 50.620.0
VGOUT 2022-01-20T08:22:17.000000Z_tiebenn_loc/model_layer
VGTYPE P
VGTYPE S
VGGRID 2 401 102 0.0 0.0 -5.0 1 1 1 SLOW_LEN
LAYER -0.46 2.5 0.0 1.07 0.0 2.11 0.0
LAYER 0.34 6.1 0.0 3.55 0.0 2.74 0.0
LAYER 10.05 6.3 0.0 3.65 0.0 2.78 0.0
LAYER 19.76 6.6 0.0 3.6 0.0 2.86 0.0
LAYER 29.77 8.18 0.0 4.54 0.0 3.37 0.0
LAYER 35.0 8.04 0.0 4.48 0.0 3.38 0.0
LAYER 77.5 8.045 0.0 4.49 0.0 3.38 0.0
LAYER 120.0 8.05 0.0 4.5 0.0 3.36 0.0
GTFILES 2022-01-20T08:22:17.000000Z/model_layer 2022-01-20T08:22:17.000000Z/time_layer P
GTMODE GRID2D ANGLES_YES
INCLUDE 2022-01-20T08:22:17.000000Z/station_coordinates.txt
GT_PLFD 1.0e-3 0
EQFILES 2022-01-20T08:22:17.000000Z/time_layer 2022-01-20T08:22:17.000000Z/obs_synth.obs
EQMODE SRCE_TO_STA
EQSRCE 2022-1-20_08:22:17 LATLON 49.62 11.38 0.0 0.0
EQSTA GRA1 P GAU 0.00 GAU 0.05
EQSTA GRA1 S GAU 0.00 GAU 0.05
EQSTA GRA3 P GAU 0.00 GAU 0.05
EQSTA GRA3 S GAU 0.00 GAU 0.05
EQSTA GRB4 P GAU 0.00 GAU 0.05
EQSTA GRB4 S GAU 0.00 GAU 0.05
EQVPVS 1.68
EQQUAL2ERR 0.05 0.1 0.5 1.0 99999.9
LOCSIG NonLinLoc
LOCCOM 2022-1-20 08:22:17
LOCFILES 2022-01-20T08:22:17.000000Z/obs_ttimes.obs NLLOC_OBS 2022-01-20T08:22:17.000000Z/time_layer 2022-01-20T08:22:17.000000Z/loc_eqdatetime
LOCHYPOUT SAVE_NLLOC_ALL NLL_FORMAT_VER_2
LOCSEARCH OCT 10 10 4 0.01 20000 5000 0 1
LOCGRID 201 201 102 -100.0 -100.0 -5.0 1 1 1 PROB_DENSITY SAVE
LOCMETH EDT_OT_WT 9999.0 6 -1 3 -1 6 -1.0 1
LOCGAU 0.2 0.0
LOCGAU2 0.02 0.05 2.0
LOCQUAL2ERR 0.1 0.5 1.0 2.0 99999.9
LOCANGLES ANGLES_YES 5
LOCPHSTAT 9999.0 -1 9999.0 1.0 1.0 9999.9 -9999.9 9999.9
Do you see anything anomalous in any of the automatically produced files? I must mention that I have not changed the parameters used in the sample/example, so I ignore if there is a more optimal parameter selection.
Best, C.
Hello Cthuulhaa
I tried your location. With P and S readings all looks fine with the configuration and the solution. But the depth is indeed well constrained at ~15km.
I then tried with P readings only - the epicenter is almost the same but the depth is shallower (~6km) and the pdf is smeared vertically, indicating no depth constraint.
I then tried removing each of the S readings in turn. In each case, the maximum likelihood hypocenter is shallow, but the pdf still shows a strong secondary solution at ~15km depth.
I then tried increasing the S pick uncertainty by X10. The The maximum likelihood hypocenter is shallow, and the pdf gives a smearing in depth with a secondary maximum solution at ~15km.
So I am not sure what is happening, but some possible ideas:
- The thin, very low velocity (2.5km/s P) shallow layer over a constant (6.1km/s P) velocity crust is correct, but tends to allow an ambiguous, double solution (shallow near surface and deeper), and small changes or error in the (S?) picks make the maximum likelihood solution jump between the two depths.
- The thin, very low velocity (2.5km/s P) shallow layer over a constant (6.1km/s P) velocity crust is incorrect - are there alternative, smoother models? Is Vp/Vs correct?
- The S picks are surface waves, not S. Or other problem with S picks (they may be delayed).
- ???
Interesting problem...
Best regards, Anthony
PS - I use SeismicityViewer for most of the above analysis, especially for visualization of the pdf.
