2008/03/27 01:07:14 36.498 -113.641 5.0 3.5 Arizona
USGS Felt map for this earthquake
SLU Moment Tensor Solution
2008/03/27 01:07:14 36.498 -113.641 5.0 3.5 Arizona
Best Fitting Double Couple
Mo = 4.79e+21 dyne-cm
Mw = 3.72
Z = 10 km
Plane Strike Dip Rake
NP1 220 70 -50
NP2 332 44 -150
Principal Axes:
Axis Value Plunge Azimuth
T 4.79e+21 15 282
N 0.00e+00 37 24
P -4.79e+21 49 173
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.87e+21
Mxy -6.58e+20
Mxz 2.61e+21
Myy 4.23e+21
Myz -1.48e+21
Mzz -2.36e+21
--------------
#######---------------
##############-------------#
##################-----#######
##################################
#####################----###########
####################-------###########
###################-----------##########
# #############--------------#########
## T ############---------------##########
## ##########------------------#########
##############--------------------########
#############---------------------########
###########----------------------#######
##########-----------------------#######
########----------- ----------######
######------------ P ----------#####
####------------- ----------####
##-------------------------###
#------------------------###
---------------------#
--------------
Harvard Convention
Moment Tensor:
R T F
-2.36e+21 2.61e+21 1.48e+21
2.61e+21 -1.87e+21 6.58e+20
1.48e+21 6.58e+20 4.23e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080327010714/index.html
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STK = 220
DIP = 70
RAKE = -50
MW = 3.72
HS = 10.0
The waveform inversion is preferred. This is an interesting event in that many of the waveforms show simple pulses rather than a dispersed wavetrain. Although the WUS model was used, these traces would be better fit by the CUS model. The use of the microseism filter in the processing introduced some ringing into the filtered observed/synthetic waveforms used for the fit. The surface-wave solution seems to be different in the position of the steeply dipping nodal plane. The surface-wave solution also admits the waveforms inversion solution as a possibility. There is a good agreement in moment and depth between the two techniques, in spite of the fact that the WUS model may not be adequate for most paths, as seen in the comparison of model and network averaged group velocities.
The following compares this source inversion to others
SLU Moment Tensor Solution
2008/03/27 01:07:14 36.498 -113.641 5.0 3.5 Arizona
Best Fitting Double Couple
Mo = 4.79e+21 dyne-cm
Mw = 3.72
Z = 10 km
Plane Strike Dip Rake
NP1 220 70 -50
NP2 332 44 -150
Principal Axes:
Axis Value Plunge Azimuth
T 4.79e+21 15 282
N 0.00e+00 37 24
P -4.79e+21 49 173
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.87e+21
Mxy -6.58e+20
Mxz 2.61e+21
Myy 4.23e+21
Myz -1.48e+21
Mzz -2.36e+21
--------------
#######---------------
##############-------------#
##################-----#######
##################################
#####################----###########
####################-------###########
###################-----------##########
# #############--------------#########
## T ############---------------##########
## ##########------------------#########
##############--------------------########
#############---------------------########
###########----------------------#######
##########-----------------------#######
########----------- ----------######
######------------ P ----------#####
####------------- ----------####
##-------------------------###
#------------------------###
---------------------#
--------------
Harvard Convention
Moment Tensor:
R T F
-2.36e+21 2.61e+21 1.48e+21
2.61e+21 -1.87e+21 6.58e+20
1.48e+21 6.58e+20 4.23e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20080327010714/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 0.12 n 3 br c 0.12 0.25 n 4 p 2The results of this grid search from 0.5 to 19 km depth are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 0.5 255 45 -90 3.30 0.1815
WVFGRD96 1.0 45 90 5 3.18 0.1467
WVFGRD96 2.0 45 90 5 3.40 0.2515
WVFGRD96 3.0 230 90 -15 3.47 0.2783
WVFGRD96 4.0 230 90 -35 3.54 0.3024
WVFGRD96 5.0 50 90 50 3.60 0.3472
WVFGRD96 6.0 225 80 -50 3.63 0.3905
WVFGRD96 7.0 220 70 -50 3.65 0.4185
WVFGRD96 8.0 220 70 -55 3.71 0.4341
WVFGRD96 9.0 220 70 -55 3.72 0.4438
WVFGRD96 10.0 220 70 -50 3.72 0.4461
WVFGRD96 11.0 220 70 -45 3.73 0.4450
WVFGRD96 12.0 225 75 -40 3.73 0.4425
WVFGRD96 13.0 225 75 -40 3.74 0.4393
WVFGRD96 14.0 225 75 -40 3.74 0.4341
WVFGRD96 15.0 225 75 -40 3.75 0.4268
WVFGRD96 16.0 225 75 -40 3.76 0.4185
WVFGRD96 17.0 225 75 -35 3.77 0.4095
WVFGRD96 18.0 230 80 -35 3.78 0.4004
WVFGRD96 19.0 230 80 -35 3.79 0.3908
WVFGRD96 20.0 60 75 40 3.79 0.3798
WVFGRD96 21.0 55 75 40 3.81 0.3703
WVFGRD96 22.0 55 75 40 3.82 0.3611
WVFGRD96 23.0 55 75 40 3.82 0.3518
WVFGRD96 24.0 55 75 40 3.83 0.3421
WVFGRD96 25.0 55 75 40 3.84 0.3319
WVFGRD96 26.0 55 75 40 3.84 0.3210
WVFGRD96 27.0 55 75 40 3.85 0.3096
WVFGRD96 28.0 55 75 40 3.85 0.2985
WVFGRD96 29.0 55 75 40 3.86 0.2877
The best solution is
WVFGRD96 10.0 220 70 -50 3.72 0.4461
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 0.12 n 3 br c 0.12 0.25 n 4 p 2
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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= 60.00
DIP= 74.99
RAKE= 54.99
OR
STK= 309.72
DIP= 37.71
RAKE= 154.96
DEPTH = 8.0 km
Mw = 3.85
Best Fit 0.8937 - 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 U13A 253 30 -12345 U14A 102 42 -12345 T13A 338 63 -12345 V13A 203 78 -12345 T14A 38 80 -12345 U12A 265 81 -12345 T12A 285 99 -12345 V14A 153 107 -12345 CCUT 12 119 -12345 U15A 93 121 -12345 S13A 351 122 -12345 T15A 62 126 -12345 W14A 160 151 -12345 V15A 119 152 -12345 U11A 267 156 -12345 W13A 188 157 -12345 S12A 319 163 -12345 T11A 301 163 -12345 W12A 220 173 -12345 V11A 246 177 -12345 R13A 351 189 -12345 W15A 139 192 -12345 T16A 74 198 -12345 LDF 223 206 -12345 R14A 15 207 -12345 X13A 185 212 -12345 R12A 337 220 -12345 R15A 32 225 -12345 S11A 305 226 -12345 U16A 99 229 -12345 WUAZ 117 232 -12345 X14A 163 235 -12345 U10A 269 241 -12345 W16A 129 246 -12345 X15A 150 257 -12345 T17A 77 259 -12345 U17A 87 267 -12345 R11A 321 268 -12345 V17A 110 274 -12345 Q13A 353 275 -12345 Q14A 7 278 -12345 S17A 63 282 -12345 Y14A 168 290 -12345 Y13A 183 298 -12345 Q12A 340 301 -12345 X16A 138 305 -12345 S10A 302 306 -12345 Y15A 157 306 -12345 R10A 311 308 -12345 W17A 120 308 -12345 Q11A 326 315 -12345 P13A 354 330 -12345 T18A 77 343 -12345 V18A 104 345 -12345 Q16A 38 346 -12345 P12A 342 348 -12345 Y16A 145 351 -12345 Z14A 169 354 -12345 Q10A 318 355 -12345 Z15A 159 381 -12345 W18A 112 385 -12345 X18A 122 400 -12345 Y17A 140 402 -12345 Z16A 149 404 -12345 113A 182 414 -12345 W19A 111 414 -12345 DUG 10 417 -12345 114A 170 421 -12345 V19A 101 423 -12345 O12A 347 429 -12345 R19A 62 436 -12345 O11A 337 440 -12345 115A 162 441 -12345 112A 191 448 -12345 X19A 119 457 -12345 116A 157 471 -12345 N13A 354 486 -12345 W20A 107 489 -12345 ELK 344 492 -12345 BGU 6 494 -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) R13A 351 189 W15A 139 192 LDF 223 206 R14A 15 207 X13A 185 212 R12A 337 220 R15A 32 225 S11A 305 226 U16A 99 229 WUAZ 117 232 X14A 163 235 U10A 269 241 W16A 129 246 X15A 150 257 T17A 77 259 U17A 87 267 R11A 321 268 V17A 110 274 Q13A 353 275 R16A 43 275 Q14A 6 278 S17A 63 282 Y14A 168 290 Y13A 183 298 Q15A 21 299 Q12A 340 301 X16A 138 305 S10A 302 306 Y15A 157 306 R10A 311 308 W17A 120 308 GSC 246 315 Q11A 326 315 Y12C 195 315 P13A 354 330 R17A 50 336 U18A 90 338 T18A 77 343 V18A 104 345 Q16A 38 346 P14A 8 347 P12A 342 348 Y16A 145 351 Z14A 170 354 Q10A 318 355 Z13A 180 366 Z15A 159 381 R18A 57 392 GLA 196 397 U19A 92 398 SRU 42 399 X18A 122 400 Y17A 140 402 O13A 356 404 Z16A 149 404 NLU 19 407 113A 182 414 T19A 84 414 W19A 111 414 DUG 10 417 114A 170 421 Q18A 46 423 V19A 100 423 O12A 347 429 O15A 13 432 R19A 62 436 O11A 337 440 115A 162 441 ISA 259 445 112A 191 448 P18A 40 458 U20A 90 459 MVCO 79 465 116A 157 471 V20A 98 472 MWC 239 474 NOQ 16 480 O17A 31 482 N14A 4 485 N13A 354 486 W20A 107 489 Y19A 124 489 ELK 344 492 BGU 6 494 N15A 11 497 R20A 66 502 Z18A 138 502 BAR 214 507 117A 148 510 214A 171 510 109C 219 511 X20A 113 514 O18A 36 525 N10A 333 531 TUC 150 534 Z19A 130 535 P19A 48 537 216A 157 538 118A 141 545 V21A 96 546 N17A 26 550 M14A 2 556 M12A 349 557 M15A 10 561 W21A 104 563 P20A 53 569 O19A 42 577 M11A 342 578 X21A 112 578 217A 153 585 Z20A 128 595 218A 146 601 T22A 83 604 N18A 34 605 Y21A 115 609 M10A 337 612 L14A 3 616 L13A 358 621 L15A 10 621 O20A 48 621 N19A 37 623 W22A 102 632 120A 132 635 L12A 350 638 M09A 330 639 L16A 17 641 X22A 108 641 219A 140 642 Q22A 64 649 318A 148 655 L11A 344 655 L10A 339 666 ANMO 103 672 K14A 3 673 L18A 26 678 O21A 51 679 K13A 357 684 K12A 351 690 121A 128 694 220A 136 694 319A 143 696 L09A 332 704 L19A 29 723 M07A 320 724 K16A 13 725 K11A 344 726 M20A 39 728 AHID 17 729 K17A 18 732 221A 131 734 320A 139 742 K18A 23 748 K10A 339 750 J15A 7 773 K09A 334 773 222A 127 774 BW06 26 779 RRI2 14 786 HLID 355 787 WVOR 328 787 J11A 347 790 ISCO 60 793 REDW 16 798 RWWY 42 801 K08A 330 807 J10A 342 814 DCID1 14 815 M22A 46 818 I12A 352 820 LOHW 17 831 I15A 6 839 K07A 327 839 I11A 347 846 IMW 15 853 J08A 333 866 I10A 343 884 224A 121 894 H12A 354 900 I09A 338 900 H14A 1 902 MNTX 123 929 324A 124 939 H09A 341 968 BMO 343 978 I07A 331 978 325A 123 982 H08A 336 987 G11A 348 1013 425A 126 1022 G09A 342 1035
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 0.12 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.
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 VMODEL 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=Thu Mar 27 10:34:27 CDT 2008