2008/02/22 23:27:46 41.038 -114.866 10.0 4.3 Nevada
USGS Felt map for this earthquake
SLU Moment Tensor Solution
2008/02/22 23:27:46 41.038 -114.866 10.0 4.3 Nevada
Best Fitting Double Couple
Mo = 3.80e+22 dyne-cm
Mw = 4.32
Z = 12 km
Plane Strike Dip Rake
NP1 38 50 -94
NP2 225 40 -85
Principal Axes:
Axis Value Plunge Azimuth
T 3.80e+22 5 131
N 0.00e+00 3 41
P -3.80e+22 84 279
Moment Tensor: (dyne-cm)
Component Value
Mxx 1.65e+22
Mxy -1.86e+22
Mxz -2.86e+21
Myy 2.08e+22
Myz 6.45e+21
Mzz -3.73e+22
##############
######################
##################-------##-
#############----------------#
############-------------------###
###########---------------------####
##########-----------------------#####
#########-------------------------######
########-------------------------#######
########-------- ---------------########
#######--------- P --------------#########
######---------- -------------##########
######-------------------------###########
####-------------------------###########
####------------------------############
###----------------------#############
##--------------------##############
#------------------########### #
--------------############## T
--------###################
######################
##############
Harvard Convention
Moment Tensor:
R T F
-3.73e+22 -2.86e+21 -6.45e+21
-2.86e+21 1.65e+22 1.86e+22
-6.45e+21 1.86e+22 2.08e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080222232746/index.html
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STK = 225
DIP = 40
RAKE = -85
MW = 4.32
HS = 12
The waveform inversion result is preferred. The surface-wave spectral maplitude inversion is compatible with the waveform inversion.
The following compares this source inversion to others
SLU Moment Tensor Solution
2008/02/22 23:27:46 41.038 -114.866 10.0 4.3 Nevada
Best Fitting Double Couple
Mo = 3.80e+22 dyne-cm
Mw = 4.32
Z = 12 km
Plane Strike Dip Rake
NP1 38 50 -94
NP2 225 40 -85
Principal Axes:
Axis Value Plunge Azimuth
T 3.80e+22 5 131
N 0.00e+00 3 41
P -3.80e+22 84 279
Moment Tensor: (dyne-cm)
Component Value
Mxx 1.65e+22
Mxy -1.86e+22
Mxz -2.86e+21
Myy 2.08e+22
Myz 6.45e+21
Mzz -3.73e+22
##############
######################
##################-------##-
#############----------------#
############-------------------###
###########---------------------####
##########-----------------------#####
#########-------------------------######
########-------------------------#######
########-------- ---------------########
#######--------- P --------------#########
######---------- -------------##########
######-------------------------###########
####-------------------------###########
####------------------------############
###----------------------#############
##--------------------##############
#------------------########### #
--------------############## T
--------###################
######################
##############
Harvard Convention
Moment Tensor:
R T F
-3.73e+22 -2.86e+21 -6.45e+21
-2.86e+21 1.65e+22 1.86e+22
-6.45e+21 1.86e+22 2.08e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080222232746/index.html
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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 0.5 190 40 85 3.88 0.3999
WVFGRD96 1.0 165 45 65 3.87 0.2908
WVFGRD96 2.0 355 45 85 4.01 0.3625
WVFGRD96 3.0 125 20 20 4.07 0.3238
WVFGRD96 4.0 115 20 10 4.08 0.4090
WVFGRD96 5.0 115 25 10 4.09 0.4705
WVFGRD96 6.0 270 25 -25 4.11 0.5127
WVFGRD96 7.0 255 30 -60 4.16 0.5594
WVFGRD96 8.0 245 30 -65 4.25 0.5873
WVFGRD96 9.0 235 35 -80 4.28 0.6331
WVFGRD96 10.0 225 40 -85 4.30 0.6685
WVFGRD96 11.0 225 40 -85 4.31 0.6861
WVFGRD96 12.0 225 40 -85 4.32 0.6875
WVFGRD96 13.0 230 40 -85 4.33 0.6777
WVFGRD96 14.0 230 40 -80 4.33 0.6598
WVFGRD96 15.0 230 40 -80 4.34 0.6361
WVFGRD96 16.0 235 45 -75 4.34 0.6090
WVFGRD96 17.0 235 45 -75 4.35 0.5793
WVFGRD96 18.0 235 45 -75 4.35 0.5486
WVFGRD96 19.0 235 45 -75 4.35 0.5172
WVFGRD96 20.0 325 40 40 4.32 0.4869
WVFGRD96 21.0 325 40 40 4.34 0.4723
WVFGRD96 22.0 325 40 40 4.34 0.4556
WVFGRD96 23.0 325 35 40 4.34 0.4385
WVFGRD96 24.0 320 35 35 4.34 0.4220
WVFGRD96 25.0 320 35 35 4.35 0.4050
WVFGRD96 26.0 320 35 30 4.36 0.3873
WVFGRD96 27.0 315 35 25 4.36 0.3694
WVFGRD96 28.0 315 35 25 4.36 0.3516
WVFGRD96 29.0 35 15 55 4.44 0.3380
The best solution is
WVFGRD96 12.0 225 40 -85 4.32 0.6875
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= 40.00
DIP= 54.99
RAKE= -105.00
OR
STK= 245.04
DIP= 37.70
RAKE= -69.72
DEPTH = 11.0 km
Mw = 4.39
Best Fit 0.8920 - 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 N12A 217 27 -12345 ELK 225 47 -12345 N13A 111 58 -12345 M13A 58 67 -12345 BGU 94 154 -12345 HVU 64 192 -12345 DUG 118 196 -12345 SPU 81 203 -12345 NLU 116 264 -12345 HLID 7 282 -12345 MPU 112 295 -12345 WVOR 297 351 -12345 AHID 57 365 -12345 TMU 121 366 -12345 RRI2 48 388 -12345 CCUT 161 408 -12345 DCID1 46 414 -12345 REDW 51 419 -12345 TPAW 48 421 -12345 SRU 119 427 -12345 SNOW 50 432 -12345 IMW 44 451 -12345 LOHW 49 451 -12345 BMO 335 468 -12345 BW06 65 479 -12345
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) BGU 94 154 HVU 64 192 DUG 118 196 SPU 81 203 NOQ 100 234 CTU 98 264 NLU 116 264 HLID 7 282 MPU 112 295 WVOR 297 351 AHID 57 365 TMU 121 366 RRI2 48 388 CCUT 161 408 DCID1 46 414 REDW 51 419 TPAW 48 421 SRU 119 427 SNOW 50 432 IMW 44 451 LOHW 49 451 BW06 65 479 DLMT 20 514 BOZ 26 574 RWWY 81 643 MSO 6 647 LDF 183 657 GSC 196 660 ISA 209 676 WUAZ 153 684 MVCO 126 694 HUMO 287 695 HAWA 329 704 LTH 330 705 PHWY 85 788 OSI 207 791 COR 302 795 ISCO 97 795 NEW 348 822 EGMT 26 875 SDCO 111 884 GLA 180 887 BAR 190 942 RSSD 65 950 SNCC 207 959 ANMO 130 1000 TUC 158 1035 OGNE 86 1077 DGMT 42 1178 CBKS 96 1313 AMTX 116 1342 ECSD 73 1528 KSU1 92 1568 WMOK 111 1574 AGMN 55 1701 JCT 126 1791 SCIA 80 1805 EYMN 60 2000 MIAR 104 2001 JFWS 76 2042 CCM 92 2049 UALR 102 2091 HKT 120 2110 SLM 89 2116 FVM 91 2120 COWI 66 2141 HDIL 83 2149 BMO 336 2162 PBMO 95 2162 SIUC 91 2231 PVMO 95 2238 MPH 99 2274 OLIL 88 2293 UTMT 95 2311 OXF 100 2346 USIN 89 2353 VBMS 107 2376 WVT 94 2406 BLO 86 2413 PLAL 97 2435 WCI 88 2460 GLMI 70 2488 AAM 76 2587 BRAL 105 2717 TZTN 90 2749 ERPA 76 2889 GOGA 97 2893 MCWV 81 2958 BLA 87 2988 NHSC 95 3183 BINY 75 3213 CNNC 89 3278 PAL 76 3411
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
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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.
Thanks also to the many seismic network operators whose dedication make this effort possible: University of Alaska, University of Washington, Oregon State University, University of Utah, Montana Bureas of Mines, UC Berkely, Caltech, UC San Diego, Saint L ouis University, Universityof Memphis, Lamont Doehrty Earth Observatory, Boston College, the Iris stations and the Transportable Array of EarthScope.
The WUS.REG 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=Sun Feb 24 19:48:21 CST 2008