2008/02/22 01:50:06 41.023 -114.932 10.0 3.9 Nevada
USGS Felt map for this earthquake
SLU Moment Tensor Solution
2008/02/22 01:50:06 41.023 -114.932 10.0 3.9 Nevada
Best Fitting Double Couple
Mo = 7.76e+21 dyne-cm
Mw = 3.86
Z = 11 km
Plane Strike Dip Rake
NP1 230 55 -60
NP2 5 45 -126
Principal Axes:
Axis Value Plunge Azimuth
T 7.76e+21 6 299
N 0.00e+00 24 32
P -7.76e+21 65 197
Moment Tensor: (dyne-cm)
Component Value
Mxx 5.76e+20
Mxy -3.66e+21
Mxz 3.19e+21
Myy 5.74e+21
Myz 2.27e+20
Mzz -6.32e+21
###########---
#################-----
#####################-------
#######################-------
###################------########
T ###############-----------########
############--------------#########
##############-----------------#########
############-------------------#########
###########---------------------##########
##########----------------------##########
#########-----------------------##########
########------------------------##########
######----------- -----------#########
#####------------ P ----------##########
###------------- ----------#########
##-------------------------#########
#------------------------#########
----------------------########
-------------------#########
--------------########
--------######
Harvard Convention
Moment Tensor:
R T F
-6.32e+21 3.19e+21 -2.27e+20
3.19e+21 5.76e+20 3.66e+21
-2.27e+20 3.66e+21 5.74e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080222015006/index.html
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STK = 230
DIP = 55
RAKE = -60
MW = 3.86
HS = 11
The wveform inversion solution is preferred. The surface-wave solution has a very small data set.
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 0.5 210 45 -90 3.43 0.2226
WVFGRD96 1.0 235 80 -5 3.39 0.1911
WVFGRD96 2.0 30 45 90 3.59 0.2318
WVFGRD96 3.0 85 45 25 3.59 0.2345
WVFGRD96 4.0 95 30 0 3.65 0.2760
WVFGRD96 5.0 75 30 -25 3.67 0.3111
WVFGRD96 6.0 65 35 -40 3.69 0.3469
WVFGRD96 7.0 55 40 -55 3.73 0.3781
WVFGRD96 8.0 220 55 -75 3.82 0.4048
WVFGRD96 9.0 225 55 -70 3.84 0.4294
WVFGRD96 10.0 225 55 -65 3.85 0.4404
WVFGRD96 11.0 230 55 -60 3.86 0.4422
WVFGRD96 12.0 230 55 -60 3.87 0.4371
WVFGRD96 13.0 230 55 -60 3.88 0.4261
WVFGRD96 14.0 235 60 -55 3.88 0.4111
WVFGRD96 15.0 235 60 -50 3.89 0.3947
WVFGRD96 16.0 235 60 -50 3.90 0.3766
WVFGRD96 17.0 235 60 -50 3.90 0.3569
WVFGRD96 18.0 240 65 -45 3.91 0.3367
WVFGRD96 19.0 240 65 -45 3.91 0.3171
WVFGRD96 20.0 240 65 -45 3.92 0.2975
WVFGRD96 21.0 240 65 -45 3.92 0.2789
WVFGRD96 22.0 240 65 -45 3.93 0.2604
WVFGRD96 23.0 240 70 -45 3.93 0.2437
WVFGRD96 24.0 240 70 -45 3.93 0.2285
WVFGRD96 25.0 245 75 -45 3.93 0.2144
WVFGRD96 26.0 335 55 -25 3.93 0.2070
WVFGRD96 27.0 335 55 -25 3.94 0.2041
WVFGRD96 28.0 335 55 -25 3.95 0.2020
WVFGRD96 29.0 335 60 -30 3.95 0.2011
The best solution is
WVFGRD96 11.0 230 55 -60 3.86 0.4422
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= 15.00
DIP= 49.99
RAKE= -115.00
OR
STK= 230.96
DIP= 46.04
RAKE= -63.28
DEPTH = 9.0 km
Mw = 3.99
Best Fit 0.8861 - 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 205 21 -12345 ELK 220 40 -12345 N13A 107 64 -12345 N11A 252 71 -12345 M13A 60 74 -12345 M11A 302 85 -12345 O12A 169 85 -12345 L12A 357 125 -12345 O13A 141 128 -12345 N10A 256 137 -12345 M14A 68 143 -12345 L11A 332 144 -12345 L13A 35 144 -12345 M10A 293 146 -12345 N14A 97 148 -12345 O10A 239 155 -12345 BGU 93 161 -12345 P12A 179 172 -12345 L10A 313 174 -12345 K12A 1 179 -12345 L14A 51 180 -12345 DUG 116 201 -12345 N15A 93 204 -12345 M15A 76 214 -12345 L15A 62 239 -12345 Q11A 195 250 -12345 K10A 321 253 -12345 Q14A 147 266 -12345 NLU 115 270 -12345 J11A 345 276 -12345 P15A 125 277 -12345 AHID 57 372 -12345 DCID1 46 421 -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) N10A 256 137 M14A 68 143 BGU 93 161 L14A 51 180 HVU 64 199 DUG 116 201 N15A 93 204 SPU 80 211 M15A 76 214 K11A 335 215 M09A 283 216 N09A 266 219 Q12A 178 220 K14A 40 223 L15A 62 239 NOQ 99 241 Q13A 161 242 J12A 357 248 Q14A 147 266 NLU 115 270 CTU 97 271 N08A 265 271 J11A 345 276 P15A 125 277 M16A 82 279 J14A 24 281 M08A 280 293 N16A 92 295 R11A 191 302 O16A 106 304 J10A 331 307 L16A 68 312 L08A 296 313 Q15A 135 313 P16A 118 319 R13A 165 326 I11A 346 332 J15A 37 335 K16A 53 342 R14A 151 344 N17A 90 345 WVOR 298 346 K08A 304 351 M07A 278 358 L17A 69 359 M17A 81 361 O17A 103 367 S10A 203 373 S12A 179 379 K17A 59 384 L07A 288 384 H12A 1 392 S13A 166 393 S14A 157 393 RRI2 48 395 P17A 114 396 H13A 8 397 I09A 326 401 M18A 82 410 H11A 348 418 L18A 75 421 SRU 118 432 O19A 97 498
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 0.04 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
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=Sat Oct 24 11:41:27 CDT 2009