2007/08/06 08:48:40 39.464 -111.218 0.0 3.8 Utah
USGS Felt map for this earthquake
SLU Moment Tensor Solution
2007/08/06 08:48:40 39.464 -111.218 0.0 3.8 Utah
Best Fitting Double Couple
Mo = 6.31e+21 dyne-cm
Mw = 3.80
Z = 6 km
Plane Strike Dip Rake
NP1 176 57 -103
NP2 20 35 -70
Principal Axes:
Axis Value Plunge Azimuth
T 6.31e+21 11 276
N 0.00e+00 11 183
P -6.31e+21 74 50
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.44e+20
Mxy -8.42e+20
Mxz -9.68e+20
Myy 5.72e+21
Myz -2.51e+21
Mzz -5.57e+21
####----------
#######-------------##
#########----------------###
#########------------------###
##########--------------------####
###########--------------------#####
###########----------------------#####
############----------------------######
############---------- ---------######
# #########---------- P ---------#######
# T #########---------- ---------#######
# #########----------------------#######
#############---------------------########
############---------------------#######
#############-------------------########
############------------------########
############----------------########
############-------------#########
##########------------########
###########-------##########
#########---##########
##-----#######
Harvard Convention
Moment Tensor:
R T F
-5.57e+21 -9.68e+20 2.51e+21
-9.68e+20 -1.44e+20 8.42e+20
2.51e+21 8.42e+20 5.72e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20070806084840/index.html
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STK = 20
DIP = 35
RAKE = -70
MW = 3.80
HS = 6
Neither solutions is adequate because of the possibility that this event was a mine collapse. The definite characteristic of this event is the lack of low frequency signal in the waveforms. If there is a collapse which can be represented as a point force, then the source time function is approximately a one-pole high pass filter with corner period related to the tiem of free fall of the roof. If this free fall is asymmetric, then Love waves will be generated. The other way in which to remove the low frequency signal is to have a very shallow vertical dip-slip source. This event requires further study and a better background on the proper source time representation for a collapse.
The focal mechanism was determined using broadband seismic waveforms. The location of the event and the and stations used for the waveform inversion are shown in the next figure.
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The program wvfgrd96 was used with good traces observed at short distance to determine the focal mechanism, depth and seismic moment. This technique requires a high quality signal and well determined velocity model for the Green functions. To the extent that these are the quality data, this type of mechanism should be preferred over the radiation pattern technique which requires the separate step of defining the pressure and tension quadrants and the correct strike.
The observed and predicted traces are filtered using the following gsac commands:
hp c 0.02 n 3 lp c 0.10 n 3The results of this grid search from 0.5 to 19 km depth are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 0.5 320 10 -95 3.86 0.1342
WVFGRD96 1.0 290 15 -10 3.76 0.1497
WVFGRD96 2.0 290 5 -15 3.99 0.1806
WVFGRD96 3.0 310 15 -90 3.97 0.2087
WVFGRD96 4.0 310 20 -90 3.96 0.2121
WVFGRD96 5.0 315 30 -90 3.96 0.2010
WVFGRD96 6.0 310 30 -95 3.95 0.1850
WVFGRD96 7.0 35 50 65 3.91 0.1648
WVFGRD96 8.0 140 55 -85 4.00 0.1436
WVFGRD96 9.0 145 55 -80 3.98 0.1252
WVFGRD96 10.0 150 50 -70 3.97 0.1083
WVFGRD96 11.0 25 55 45 3.92 0.0950
WVFGRD96 12.0 165 40 -50 3.95 0.0842
WVFGRD96 13.0 165 40 -45 3.94 0.0759
WVFGRD96 14.0 170 40 -40 3.94 0.0690
WVFGRD96 15.0 170 40 -40 3.94 0.0628
WVFGRD96 16.0 170 40 -40 3.95 0.0577
WVFGRD96 17.0 90 25 40 3.94 0.0537
WVFGRD96 18.0 190 60 35 3.92 0.0511
WVFGRD96 19.0 195 60 35 3.93 0.0500
WVFGRD96 20.0 195 60 35 3.94 0.0494
WVFGRD96 21.0 195 60 35 3.95 0.0492
WVFGRD96 22.0 195 60 40 3.96 0.0490
WVFGRD96 23.0 195 60 40 3.97 0.0489
WVFGRD96 24.0 200 55 45 3.97 0.0489
WVFGRD96 25.0 200 55 45 3.98 0.0483
WVFGRD96 26.0 200 60 45 3.99 0.0478
WVFGRD96 27.0 115 30 65 4.04 0.0476
WVFGRD96 28.0 320 65 -75 4.05 0.0476
WVFGRD96 29.0 205 65 45 4.01 0.0480
The best solution is
WVFGRD96 4.0 310 20 -90 3.96 0.2121
The mechanism correspond to the best fit is
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The best fit as a function of depth is given in the following figure:
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The comparison of the observed and predicted waveforms is given in the next figure. The red traces are the observed and the blue are the predicted. Each observed-predicted componnet is plotted to the same scale and peak amplitudes are indicated by the numbers to the left of each trace. The number in black at the rightr of each predicted traces it the time shift required for maximum correlation between the observed and predicted traces. This time shift is required because the synthetics are not computed at exactly the same distance as the observed and because the velocity model used in the predictions may not be perfect. A positive time shift indicates that the prediction is too fast and should be delayed to match the observed trace (shift to the right in this figure). A negative value indicates that the prediction is too slow. The bandpass filter used in the processing and for the display was
hp c 0.02 n 3 lp c 0.10 n 3
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| Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to thewavefroms. Each solution is plotted as a vector at a given value of strike and dip with the angle of the vector representing the rake angle, measured, with respect to the upward vertical (N) in the figure. |
The following figure shows the stations used in the grid search for the best focal mechanism to fit the surface-wave spectral amplitudes of the Love and Rayleigh waves.
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The surface-wave determined focal mechanism is shown here.
NODAL PLANES
STK= 340.50
DIP= 75.52
RAKE= 93.96
OR
STK= 145.01
DIP= 15.00
RAKE= 75.01
DEPTH = 6.0 km
Mw = 3.95
Best Fit 0.7428 - P-T axis plot gives solutions with FIT greater than FIT90
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The P-wave first motion data for focal mechanism studies are as follow:
Sta Az(deg) Dist(km) First motion TMU 177 19 eP_X P16A 293 41 iP_D P17A 88 41 eP_- Q16A 176 61 eP_- MPU 330 71 eP_X SRU 123 72 iP_D O16A 344 86 iP_- P18A 77 86 eP_- NLU 307 91 iP_D P15A 278 92 iP_- Q18A 113 102 iP_- Q15A 243 113 eP_X R17A 159 124 iP_D O18A 49 137 iP_D O15A 311 140 iP_D DUG 301 159 iP_D N16A 353 159 iP_D P14A 276 160 iP_D N17A 11 167 eP_- R15A 214 167 iP_D MVCO 136 345 eP_- AHID 2 367 eP_X ELK 294 371 eP_X BW06 20 392 eP_- REDW 4 434 iP_C WUAZ 182 438 eP_- ISCO 84 483 eP_X
Surface wave analysis was performed using codes from Computer Programs in Seismology, specifically the multiple filter analysis program do_mft and the surface-wave radiation pattern search program srfgrd96.
Digital data were collected, instrument response removed and traces converted
to Z, R an T components. Multiple filter analysis was applied to the Z and T traces to obtain the Rayleigh- and Love-wave spectral amplitudes, respectively.
These were input to the search program which examined all depths between 1 and 25 km
and all possible mechanisms.
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| Pressure-tension axis trends. Since the surface-wave spectra search does not distinguish between P and T axes and since there is a 180 ambiguity in strike, all possible P and T axes are plotted. First motion data and waveforms will be used to select the preferred mechanism. The purpose of this plot is to provide an idea of the possible range of solutions. The P and T-axes for all mechanisms with goodness of fit greater than 0.9 FITMAX (above) are plotted here. |
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| Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to the Love and Rayleigh wave radiation patterns. Each solution is plotted as a vector at a given value of strike and dip with the angle of the vector representing the rake angle, measured, with respect to the upward vertical (N) in the figure. Because of the symmetry of the spectral amplitude rediation patterns, only strikes from 0-180 degrees are sampled. |
The distribution of broadband stations with azimuth and distance is
Sta Az(deg) Dist(km) P17A 88 41 Q16A 176 61 O16A 344 86 P18A 77 86 P15A 278 92 Q18A 113 102 Q15A 243 113 R17A 159 124 O18A 49 137 O15A 311 140 N16A 353 159 P14A 276 160 R18A 136 166 N17A 11 167 R15A 214 167 Q19A 108 178 Q14A 254 185 S17A 170 206 N18A 38 213 R19A 127 214 S18A 151 224 N14A 313 228 O13A 288 248 H13A 337 620 H12A 333 640 G15A 351 642 G14A 345 668 G13A 339 673 H11A 327 704 H10A 323 730 F13A 341 747 119A 166 763 118A 171 765 117A 177 766 116A 183 767 E15A 352 782 I08A 311 786 318A 172 897
Since the analysis of the surface-wave radiation patterns uses only spectral amplitudes and because the surfave-wave radiation patterns have a 180 degree symmetry, each surface-wave solution consists of four possible focal mechanisms corresponding to the interchange of the P- and T-axes and a roation of the mechanism by 180 degrees. To select one mechanism, P-wave first motion can be used. This was not possible in this case because all the P-wave first motions were emergent ( a feature of the P-wave wave takeoff angle, the station location and the mechanism). The other way to select among the mechanisms is to compute forward synthetics and compare the observed and predicted waveforms.
The fits to the waveforms with the given mechanism are show below:
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This figure shows the fit to the three components of motion (Z - vertical, R-radial and T - transverse). For each station and component, the observed traces is shown in red and the model predicted trace in blue. The traces represent filtered ground velocity in units of meters/sec (the peak value is printed adjacent to each trace; each pair of traces to plotted to the same scale to emphasize the difference in levels). Both synthetic and observed traces have been filtered using the SAC commands:
hp c 0.02 n 3 lp c 0.10 n 3 br c 0.12 0.25 n 4 p 2
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Should the national backbone of the USGS Advanced National Seismic System (ANSS) be implemented with an interstation separation of 300 km, it is very likely that an earthquake such as this would have been recorded at distances on the order of 100-200 km. This means that the closest station would have information on source depth and mechanism that was lacking here.
Dr. Harley Benz, USGS, provided the USGS USNSN digital data. The digital data used in this study were provided by Natural Resources Canada through their AUTODRM site http://www.seismo.nrcan.gc.ca/nwfa/autodrm/autodrm_req_e.php, and IRIS using their BUD interface
The WUS used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows:
MODEL.01
Model after 8 iterations
ISOTROPIC
KGS
FLAT EARTH
1-D
CONSTANT VELOCITY
LINE08
LINE09
LINE10
LINE11
H(KM) VP(KM/S) VS(KM/S) RHO(GM/CC) QP QS ETAP ETAS FREFP FREFS
1.9000 3.4065 2.0089 2.2150 0.302E-02 0.679E-02 0.00 0.00 1.00 1.00
6.1000 5.5445 3.2953 2.6089 0.349E-02 0.784E-02 0.00 0.00 1.00 1.00
13.0000 6.2708 3.7396 2.7812 0.212E-02 0.476E-02 0.00 0.00 1.00 1.00
19.0000 6.4075 3.7680 2.8223 0.111E-02 0.249E-02 0.00 0.00 1.00 1.00
0.0000 7.9000 4.6200 3.2760 0.164E-10 0.370E-10 0.00 0.00 1.00 1.00
Here we tabulate the reasons for not using certain digital data sets
The following stations did not have a valid response files:
DATE=Tue Aug 7 21:51:48 CDT 2007