2006/08/27 03:20:05 66.30N 142.16W 1.0* 4.4ML 280 km NW of Dawson,YT NRCAN 4.5 2006/08/27 03:19:54 66.952 -141.408 2.3 NORTHERN ALASKA AIT (ANSS) 0.580 66.3319 -142.3828 10.0 20060827032003.349 1156648803.35 ELOCATE 2006/08/27 03:20:05.88 66.302 -142.253 10.0 mag 4.5 (AEIC) prelim NEIC
2006/08/27 03:20:05 66.30N 142.25W 10 4.5 Alaska
USGS Felt map for this earthquake
USGS Felt reports page for Alaska
SLU Moment Tensor Solution
2006/08/27 03:20:05 66.30N 142.25W 10 4.5 Alaska
Best Fitting Double Couple
Mo = 3.76e+22 dyne-cm
Mw = 4.35
Z = 20 km
Plane Strike Dip Rake
NP1 319 76 -164
NP2 225 75 -15
Principal Axes:
Axis Value Plunge Azimuth
T 3.76e+22 0 92
N 0.00e+00 69 1
P -3.76e+22 21 182
Moment Tensor: (dyne-cm)
Component Value
Mxx -3.26e+22
Mxy -2.43e+21
Mxz 1.26e+22
Myy 3.75e+22
Myz 6.87e+20
Mzz -4.86e+21
--------------
----------------------
----------------------------
#####----------------------###
##########----------------########
#############-----------############
################-------###############
###################--###################
###################--###################
##################-----###################
#################--------###############
###############------------############# T
#############---------------############
###########-----------------############
##########-------------------###########
#######-----------------------########
#####-------------------------######
###--------------------------#####
#------------ ------------##
------------ P -------------
--------- ----------
--------------
Harvard Convention
Moment Tensor:
R T F
-4.86e+21 1.26e+22 -6.87e+20
1.26e+22 -3.26e+22 2.43e+21
-6.87e+20 2.43e+21 3.75e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20060827031954/index.html
|
The focal mechanism was determined using broadband seismic waveforms. The location of the event and the station distribution are given in Figure 1.
|
|
|
|
STK = 225
DIP = 75
RAKE = -15
MW = 4.35
HS = 20
The waveform inversion is preferred although the depth may not be well controlled. The NEIS location is accepted. The surface-wave mechanism agrees with the waveform inversion but lacks depth control.
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 -35 4.22 0.5311
WVFGRD96 1.0 315 90 -20 4.19 0.5404
WVFGRD96 2.0 135 85 10 4.19 0.5531
WVFGRD96 3.0 135 85 10 4.20 0.5569
WVFGRD96 4.0 140 70 20 4.23 0.5576
WVFGRD96 5.0 315 75 -20 4.23 0.5597
WVFGRD96 6.0 310 75 -15 4.24 0.5624
WVFGRD96 7.0 310 70 -15 4.25 0.5652
WVFGRD96 8.0 310 70 -15 4.25 0.5685
WVFGRD96 9.0 315 75 -15 4.25 0.5714
WVFGRD96 10.0 310 70 -15 4.27 0.5730
WVFGRD96 11.0 50 75 15 4.26 0.5768
WVFGRD96 12.0 230 75 15 4.27 0.5811
WVFGRD96 13.0 230 75 15 4.28 0.5855
WVFGRD96 14.0 225 75 -15 4.29 0.5900
WVFGRD96 15.0 225 75 -15 4.30 0.5953
WVFGRD96 16.0 225 75 -15 4.31 0.5992
WVFGRD96 17.0 225 75 -15 4.32 0.6022
WVFGRD96 18.0 225 75 -15 4.33 0.6040
WVFGRD96 19.0 225 75 -15 4.34 0.6048
WVFGRD96 20.0 225 75 -15 4.35 0.6055
WVFGRD96 21.0 225 75 -15 4.36 0.6053
WVFGRD96 22.0 225 75 -15 4.36 0.6040
WVFGRD96 23.0 225 75 -15 4.37 0.6018
WVFGRD96 24.0 225 75 -15 4.38 0.5988
WVFGRD96 25.0 225 75 -15 4.39 0.5949
WVFGRD96 26.0 225 75 -15 4.40 0.5902
WVFGRD96 27.0 225 75 -15 4.40 0.5848
WVFGRD96 28.0 230 75 -15 4.41 0.5786
WVFGRD96 29.0 230 75 -15 4.41 0.5717
The best solution is
WVFGRD96 20.0 225 75 -15 4.35 0.6055
The mechanism correspond to the best fit is
|
|
|
The best fit as a function of depth is given in the following figure:
|
|
|
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
|
|
|
|
| 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= 227.36
DIP= 80.34
RAKE= -15.22
OR
STK= 319.97
DIP= 75.00
RAKE= -169.99
DEPTH = 2.0 km
Mw = 4.15
Best Fit 0.7956 - P-T axis plot gives solutions with FIT greater than FIT90
 |
The P-wave first motion data for focal mechanism studies are as follow:
Sta Az(deg) Dist(km) First motion DAWY 150 289 eP_X COLD 290 359 iP_D MCK 230 424 eP_X INK 56 441 iP_C BPAW 242 473 iP_+ TRF 233 493 eP_+ KTH 236 509 iP_C CHUM 244 541 eP_X WHY 152 772 eP_X DLBC 143 1111 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.
|
|
|
The velocity model used for the search is a CUS model .
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.
|
|
|
|
| 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. |
|
| 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. |
Sta Az(deg) Dist(km) DAWY 150 283 COLA 241 304 COLD 290 365 MCK 231 426 INK 55 438 BPAW 243 477 TRF 234 496 KTH 237 512 PPLA 236 608 PMR 216 624 EYAK 197 665 RC01 216 689 FIB 218 693 WHY 146 728 PNL 168 754 SWD 211 780 SKAG 152 840 DLBC 139 1078 KDAK 214 1091 SIT 157 1093 OHAK 215 1165 FNBB 121 1281 COWN 80 1415 GLWN 81 1518 MLON 85 1525 RES 41 1928 EDM 119 2149 SRLN 57 2445 ILON 55 2466 FFC 101 2501 WALA 126 2517 FCC 86 2547 HAWA 138 2594 EGMT 121 2772 BMO 136 2820 BOZ 126 2933 HLID 132 3045 ULM 102 3149 BW06 126 3294 ELK 136 3307 EYMN 100 3546
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 velocity model used for the waveform fit is a modified Utah model .
The fits to the waveforms with the given mechanism are show below:
|
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
|
|
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 modified Utah 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.
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
 |
Here we tabulate the reasons for not using certain digital data sets
The following stations did not have a valid response files:
