2006/08/27 02:58:23 66.27N 142.14W 10 4.5 Alaska
USGS Felt map for this earthquake
USGS Felt reports page for Alaska
SLU Moment Tensor Solution
2006/08/27 02:58:23 66.27N 142.14W 10 4.5 Alaska
Best Fitting Double Couple
Mo = 3.51e+22 dyne-cm
Mw = 4.33
Z = 22 km
Plane Strike Dip Rake
NP1 235 80 -15
NP2 328 75 -170
Principal Axes:
Axis Value Plunge Azimuth
T 3.51e+22 3 282
N 0.00e+00 72 22
P -3.51e+22 18 191
Moment Tensor: (dyne-cm)
Component Value
Mxx -2.93e+22
Mxy -1.29e+22
Mxz 1.04e+22
Myy 3.24e+22
Myz -7.37e+19
Mzz -3.10e+21
--------------
----------------------
######----------------------
#########---------------------
#############---------------------
###############---------------######
##################----------##########
##################-----###############
T ###################-##################
##################---##################
##################-------#################
###############-----------################
#############--------------###############
##########-----------------#############
########--------------------############
#####-----------------------##########
##-------------------------#########
---------------------------#######
-------------------------#####
--------- -------------###
------ P -------------
-- ---------
Harvard Convention
Moment Tensor:
R T F
-3.10e+21 1.04e+22 7.37e+19
1.04e+22 -2.93e+22 1.29e+22
7.37e+19 1.29e+22 3.24e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20060827025812/index.html
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STK = 235
DIP = 80
RAKE = -15
MW = 4.33
HS = 22
The waveform-inversion solution is preferred although there is not much faith in the depth control. The surface-wave mechanism agrees but has little depth control. The Preliminary NEIS location is used.
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 0.016 n 3 lp c 0.06 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 80 -30 4.17 0.5057
WVFGRD96 1.0 320 80 -30 4.18 0.5122
WVFGRD96 2.0 140 90 10 4.16 0.5216
WVFGRD96 3.0 150 60 20 4.20 0.5288
WVFGRD96 4.0 150 65 15 4.19 0.5324
WVFGRD96 5.0 150 65 15 4.20 0.5338
WVFGRD96 6.0 155 65 15 4.20 0.5355
WVFGRD96 7.0 235 80 -20 4.20 0.5385
WVFGRD96 8.0 235 80 -20 4.21 0.5438
WVFGRD96 9.0 235 80 -20 4.21 0.5490
WVFGRD96 10.0 235 80 -20 4.22 0.5535
WVFGRD96 11.0 235 80 -20 4.23 0.5576
WVFGRD96 12.0 235 80 -20 4.24 0.5612
WVFGRD96 13.0 235 80 -20 4.25 0.5642
WVFGRD96 14.0 235 80 -20 4.25 0.5667
WVFGRD96 15.0 240 80 -15 4.26 0.5704
WVFGRD96 16.0 240 80 -15 4.27 0.5744
WVFGRD96 17.0 240 80 -15 4.28 0.5776
WVFGRD96 18.0 240 80 -15 4.29 0.5800
WVFGRD96 19.0 240 80 -15 4.30 0.5816
WVFGRD96 20.0 235 75 -15 4.31 0.5825
WVFGRD96 21.0 235 80 -15 4.32 0.5835
WVFGRD96 22.0 235 80 -15 4.33 0.5837
WVFGRD96 23.0 235 80 -15 4.34 0.5831
WVFGRD96 24.0 235 80 -15 4.34 0.5817
WVFGRD96 25.0 235 80 -15 4.35 0.5793
WVFGRD96 26.0 235 80 -15 4.36 0.5761
WVFGRD96 27.0 235 80 -15 4.37 0.5720
WVFGRD96 28.0 230 80 -10 4.38 0.5672
WVFGRD96 29.0 230 80 -10 4.39 0.5619
The best solution is
WVFGRD96 22.0 235 80 -15 4.33 0.5837
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 0.016 n 3 lp c 0.06 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= 344.98
DIP= 64.99
RAKE= -135.00
OR
STK= 232.07
DIP= 50.15
RAKE= -33.41
DEPTH = 3.0 km
Mw = 4.19
Best Fit 0.8332 - 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 DAWY 151 279 eP_+ COLA 242 305 eP_X COLD 290 370 eP_X MCK 232 427 eP_X INK 55 437 iP_D BPAW 244 479 eP_+ TRF 234 497 eP_X KTH 238 513 eP_X WHY 153 754 eP_- DLBC 144 1092 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) DAWY 151 278 COLA 242 307 COLD 290 371 MCK 232 428 INK 55 436 BPAW 244 480 TRF 235 498 KTH 238 514 CHUM 246 548 PPLA 237 611 PMR 216 624 EYAK 197 663 RC01 216 689 WHY 147 723 PNL 168 750 SWD 212 780 DLBC 139 1073 SIT 158 1088 KDAK 215 1091 FNBB 121 1275 CRAG 154 1298 COWN 80 1410 GLWN 81 1513 MLON 85 1521 BMBC 128 1557 RES 41 1927 EDM 119 2144 SRLN 57 2442 ILON 55 2464 FFC 101 2495 WALA 126 2511 FCC 86 2542 HAWA 138 2588 EGMT 121 2766 BMO 136 2814 AHID 129 3233 BW06 126 3288 DUG 133 3435 EYMN 100 3541 JFWS 104 4058
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 0.016 n 3 lp c 0.06 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 figures below show the observed spectral amplitudes (units of cm-sec) at each station and the
theoretical predictions as a function of period for the mechanism given above. The CUS model earth model
was used to define the Green's functions. For each station, the Love and Rayleigh wave spectrail amplitudes are plotted with the same scaling so that one can get a sense fo the effects of the effects of the focal mechanism and depth on the excitation of each.
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Here we tabulate the reasons for not using certain digital data sets
The following stations did not have a valid response files:
