NRCAN - no location 4.4 2006/08/26 00:50:04 66.285 -142.290 10.0 NORTHERN ALASKA AIT (ANSS) 66.4118 -142.2593 10.0 20060826005003.060 ELOCATE J-B Tables
2006/08/26 00:50:04 66.28N 142.29W 10 4.4 Alaska
USGS Felt map for this earthquake
USGS Felt reports page for Alaska
SLU Moment Tensor Solution
2006/08/26 00:50:04 66.28N 142.29W 10 4.4 Alaska
Best Fitting Double Couple
Mo = 5.89e+22 dyne-cm
Mw = 4.48
Z = 21 km
Plane Strike Dip Rake
NP1 230 75 -20
NP2 325 71 -164
Principal Axes:
Axis Value Plunge Azimuth
T 5.89e+22 3 278
N 0.00e+00 65 15
P -5.89e+22 25 187
Moment Tensor: (dyne-cm)
Component Value
Mxx -4.67e+22
Mxy -1.42e+22
Mxz 2.26e+22
Myy 5.68e+22
Myz -2.40e+20
Mzz -1.01e+22
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###################-----##############
#####################-##################
###################---#################
T ################-------#################
##############----------################
##############-------------###############
############----------------##############
##########------------------############
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######-----------------------#########
####-------------------------#######
##--------------------------######
------------ ------------###
----------- P ------------##
-------- -----------
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Harvard Convention
Moment Tensor:
R T F
-1.01e+22 2.26e+22 2.40e+20
2.26e+22 -4.67e+22 1.42e+22
2.40e+20 1.42e+22 5.68e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20060826005004/index.html
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The focal mechanism was determined using broadband seismic waveforms. The location of the event and the station distribution are given in Figure 1.
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STK = 230
DIP = 75
RAKE = -20
MW = 4.48
HS = 21
The waveform-inversion is preferred. The surface-wave solution agrees with the mechanism, but indicates a shallower depth. Thus the depth is not weel controlled. Coorespondingly the moment magnitude may be overestimated if the depth is shallower. The preferred location is from ELOCATE
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 0.5 230 90 -25 4.31 0.5023
WVFGRD96 1.0 50 90 30 4.33 0.5106
WVFGRD96 2.0 230 85 -25 4.33 0.5210
WVFGRD96 3.0 230 80 -30 4.35 0.5282
WVFGRD96 4.0 230 80 -25 4.35 0.5319
WVFGRD96 5.0 230 80 -25 4.35 0.5347
WVFGRD96 6.0 235 75 -25 4.36 0.5404
WVFGRD96 7.0 235 75 -25 4.37 0.5473
WVFGRD96 8.0 235 75 -20 4.37 0.5536
WVFGRD96 9.0 235 75 -20 4.38 0.5601
WVFGRD96 10.0 235 75 -25 4.39 0.5653
WVFGRD96 11.0 230 75 -20 4.40 0.5702
WVFGRD96 12.0 230 75 -20 4.40 0.5751
WVFGRD96 13.0 230 75 -20 4.41 0.5793
WVFGRD96 14.0 230 75 -15 4.42 0.5842
WVFGRD96 15.0 230 75 -15 4.43 0.5900
WVFGRD96 16.0 230 75 -15 4.43 0.5953
WVFGRD96 17.0 230 75 -15 4.44 0.5990
WVFGRD96 18.0 230 75 -15 4.45 0.6022
WVFGRD96 19.0 230 75 -15 4.46 0.6040
WVFGRD96 20.0 230 75 -20 4.48 0.6046
WVFGRD96 21.0 230 75 -20 4.48 0.6060
WVFGRD96 22.0 230 75 -20 4.49 0.6057
WVFGRD96 23.0 230 75 -20 4.50 0.6044
WVFGRD96 24.0 230 75 -20 4.51 0.6025
WVFGRD96 25.0 230 75 -20 4.52 0.5993
WVFGRD96 26.0 230 75 -20 4.52 0.5956
WVFGRD96 27.0 230 75 -20 4.53 0.5909
WVFGRD96 28.0 230 75 -20 4.54 0.5852
WVFGRD96 29.0 230 75 -20 4.55 0.5787
The best solution is
WVFGRD96 21.0 230 75 -20 4.48 0.6060
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. |
NODAL PLANES
STK= 228.51
DIP= 76.43
RAKE= -25.77
OR
STK= 324.97
DIP= 65.00
RAKE= -164.99
DEPTH = 3.0 km
Mw = 4.43
Best Fit 0.8100 - 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 150 283 eP_+ COLA 241 301 eP_X COLD 290 364 eP_- MCK 231 424 eP_X INK 55 440 eP_X BPAW 243 475 eP_- TRF 234 494 eP_X KTH 237 510 eP_+ CHUM 245 542 eP_X DIV 198 601 eP_X BMR 192 605 eP_X PPLA 236 606 eP_X PMR 216 622 eP_X WHY 146 728 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.
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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 150 283 COLA 241 301 COLD 290 364 MCK 231 424 INK 55 440 BPAW 243 475 TRF 234 494 KTH 237 510 CHUM 245 542 DIV 198 601 BMR 192 605 PPLA 236 606 PMR 216 622 EYAK 197 663 RC01 216 686 FIB 218 691 WHY 146 728 PNL 168 753 SWD 211 778 SKAG 152 839 DLBC 138 1078 KDAK 214 1088 SIT 157 1092 FNBB 121 1281 COWN 80 1417 GLWN 81 1520 BMBC 127 1563 RES 41 1931 EDM 118 2150 PNT 134 2296 SRLN 57 2447 ILON 55 2469 WALA 126 2518 FCC 86 2549 HAWA 138 2594 MSO 128 2734 EGMT 121 2773 BMO 136 2820 WVOR 140 3029 LAO 118 3038 HLID 132 3045 ULM 102 3150 AHID 128 3239 FRB 60 3291 BW06 126 3294 ELK 136 3307 HWUT 130 3340 DUG 133 3441 EYMN 100 3548 SRU 131 3629
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:
