The ANSS event ID is ak0127cwv72m and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0127cwv72m/executive.
2012/06/08 18:27:36 62.226 -147.875 40.4 4.2 Alaska
USGS/SLU Moment Tensor Solution
ENS 2012/06/08 18:27:36:0 62.23 -147.88 40.4 4.2 Alaska
Stations used:
AK.BAL AK.BMR AK.BWN AK.CCB AK.COLD AK.CRQ AK.CTG AK.DHY
AK.DIV AK.FYU AK.GHO AK.GLM AK.HDA AK.KLU AK.KNK AK.KTH
AK.MCK AK.MDM AK.MLY AK.PAX AK.PPD AK.PPLA AK.RAG AK.RIDG
AK.RND AK.SAW AK.SCM AK.SCRK AK.TGL AK.TRF AK.WRH AT.PMR
CN.DAWY IU.COLA US.EGAK
Filtering commands used:
hp c 0.02 n 3
lp c 0.06 n 3
Best Fitting Double Couple
Mo = 2.69e+22 dyne-cm
Mw = 4.22
Z = 51 km
Plane Strike Dip Rake
NP1 255 60 -40
NP2 8 56 -143
Principal Axes:
Axis Value Plunge Azimuth
T 2.69e+22 2 312
N 0.00e+00 42 44
P -2.69e+22 48 220
Moment Tensor: (dyne-cm)
Component Value
Mxx 5.05e+21
Mxy -1.92e+22
Mxz 1.10e+22
Myy 9.93e+21
Myz 7.72e+21
Mzz -1.50e+22
###########---
################------
###################--------
T ####################--------
# #####################---------
##########################----------
######################-----####-------
###############--------------#########--
###########------------------###########
#########---------------------############
######------------------------############
####-------------------------#############
###--------------------------#############
----------------------------############
------------ ------------#############
----------- P ------------############
---------- -----------############
----------------------############
-------------------###########
-----------------###########
------------##########
------########
Global CMT Convention Moment Tensor:
R T P
-1.50e+22 1.10e+22 -7.72e+21
1.10e+22 5.05e+21 1.92e+22
-7.72e+21 1.92e+22 9.93e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20120608182736/index.html
|
STK = 255
DIP = 60
RAKE = -40
MW = 4.22
HS = 51.0
The NDK file is 20120608182736.ndk The waveform inversion is preferred.
Given the availability of digital waveforms for determination of the moment tensor, this section documents the added processing leading to mLg, if appropriate to the region, and ML by application of the respective IASPEI formulae. As a research study, the linear distance term of the IASPEI formula for ML is adjusted to remove a linear distance trend in residuals to give a regionally defined ML. The defined ML uses horizontal component recordings, but the same procedure is applied to the vertical components since there may be some interest in vertical component ground motions. Residual plots versus distance may indicate interesting features of ground motion scaling in some distance ranges. A residual plot of the regionalized magnitude is given as a function of distance and azimuth, since data sets may transcend different wave propagation provinces.
Left: ML computed using the IASPEI formula for Horizontal components. Center: ML residuals computed using a modified IASPEI formula that accounts for path specific attenuation; the values used for the trimmed mean are indicated. The ML relation used for each figure is given at the bottom of each plot.
Right: Residuals from new relation as a function of distance and azimuth.
Left: ML computed using the IASPEI formula for Vertical components (research). Center: ML residuals computed using a modified IASPEI formula that accounts for path specific attenuation; the values used for the trimmed mean are indicated. The ML relation used for each figure is given at the bottom of each plot.
Right: Residuals from new relation as a function of distance and azimuth.
 |
The focal mechanism was determined using broadband seismic waveforms. The location of the event (star) and the stations used for (red) the waveform inversion are shown in the next figure.
|
|
|
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's 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 0.06 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 0.5 230 45 95 3.41 0.2288
WVFGRD96 1.0 230 45 90 3.45 0.2389
WVFGRD96 2.0 50 45 95 3.56 0.3002
WVFGRD96 3.0 225 45 85 3.63 0.3090
WVFGRD96 4.0 0 75 15 3.58 0.2970
WVFGRD96 5.0 0 80 15 3.60 0.2871
WVFGRD96 6.0 90 80 -10 3.62 0.2893
WVFGRD96 7.0 275 70 30 3.66 0.3032
WVFGRD96 8.0 275 70 35 3.71 0.3184
WVFGRD96 9.0 275 70 35 3.72 0.3263
WVFGRD96 10.0 275 70 35 3.73 0.3304
WVFGRD96 11.0 275 70 35 3.74 0.3325
WVFGRD96 12.0 275 70 35 3.75 0.3333
WVFGRD96 13.0 280 65 35 3.76 0.3344
WVFGRD96 14.0 5 90 60 3.77 0.3392
WVFGRD96 15.0 105 60 50 3.75 0.3489
WVFGRD96 16.0 80 75 -45 3.75 0.3603
WVFGRD96 17.0 80 75 -45 3.76 0.3751
WVFGRD96 18.0 80 75 -45 3.77 0.3882
WVFGRD96 19.0 80 75 -45 3.78 0.4001
WVFGRD96 20.0 80 70 -45 3.79 0.4109
WVFGRD96 21.0 80 70 -45 3.81 0.4228
WVFGRD96 22.0 80 70 -40 3.83 0.4333
WVFGRD96 23.0 80 70 -40 3.84 0.4433
WVFGRD96 24.0 80 70 -40 3.85 0.4520
WVFGRD96 25.0 260 45 -30 3.88 0.4638
WVFGRD96 26.0 260 50 -30 3.89 0.4748
WVFGRD96 27.0 260 50 -30 3.90 0.4858
WVFGRD96 28.0 260 50 -30 3.92 0.4957
WVFGRD96 29.0 260 50 -30 3.93 0.5043
WVFGRD96 30.0 260 50 -30 3.94 0.5120
WVFGRD96 31.0 260 55 -30 3.95 0.5197
WVFGRD96 32.0 260 55 -25 3.97 0.5278
WVFGRD96 33.0 260 60 -30 3.97 0.5384
WVFGRD96 34.0 260 60 -30 3.98 0.5485
WVFGRD96 35.0 260 60 -30 3.99 0.5578
WVFGRD96 36.0 260 60 -30 4.00 0.5661
WVFGRD96 37.0 260 60 -30 4.02 0.5729
WVFGRD96 38.0 260 60 -30 4.03 0.5781
WVFGRD96 39.0 260 60 -30 4.04 0.5797
WVFGRD96 40.0 255 55 -35 4.14 0.6033
WVFGRD96 41.0 255 55 -35 4.15 0.6136
WVFGRD96 42.0 255 60 -35 4.15 0.6225
WVFGRD96 43.0 255 60 -40 4.16 0.6310
WVFGRD96 44.0 255 60 -40 4.16 0.6382
WVFGRD96 45.0 255 60 -40 4.17 0.6438
WVFGRD96 46.0 255 60 -40 4.18 0.6488
WVFGRD96 47.0 255 60 -40 4.19 0.6521
WVFGRD96 48.0 255 60 -40 4.20 0.6545
WVFGRD96 49.0 255 60 -40 4.20 0.6554
WVFGRD96 50.0 255 60 -40 4.21 0.6557
WVFGRD96 51.0 255 60 -40 4.22 0.6559
WVFGRD96 52.0 255 60 -40 4.22 0.6545
WVFGRD96 53.0 255 60 -40 4.23 0.6525
WVFGRD96 54.0 255 60 -35 4.24 0.6493
WVFGRD96 55.0 255 65 -35 4.24 0.6457
WVFGRD96 56.0 255 65 -35 4.24 0.6424
WVFGRD96 57.0 255 65 -35 4.25 0.6389
WVFGRD96 58.0 255 65 -35 4.25 0.6341
WVFGRD96 59.0 255 65 -35 4.26 0.6287
WVFGRD96 60.0 255 65 -35 4.26 0.6227
WVFGRD96 61.0 255 65 -35 4.26 0.6161
WVFGRD96 62.0 255 65 -35 4.26 0.6082
WVFGRD96 63.0 255 65 -35 4.27 0.6014
WVFGRD96 64.0 255 65 -35 4.27 0.5930
WVFGRD96 65.0 255 65 -35 4.27 0.5848
WVFGRD96 66.0 260 70 -30 4.26 0.5777
WVFGRD96 67.0 260 70 -30 4.27 0.5704
WVFGRD96 68.0 260 70 -30 4.27 0.5629
WVFGRD96 69.0 260 70 -30 4.27 0.5552
The best solution is
WVFGRD96 51.0 255 60 -40 4.22 0.6559
The mechanism corresponding 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 component is plotted to the same scale and peak amplitudes are indicated by the numbers to the left of each trace. A pair of numbers is given in black at the right of each predicted traces. The upper number 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, the velocity model used in the predictions may not be perfect and the epicentral parameters may be be off. 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 lower number gives the percentage of variance reduction to characterize the individual goodness of fit (100% indicates a perfect fit).
The bandpass filter used in the processing and for the display was
hp c 0.02 n 3 lp c 0.06 n 3
|
| Figure 3. Waveform comparison for selected depth. Red: observed; Blue - predicted. The time shift with respect to the model prediction is indicated. The percent of fit is also indicated. The time scale is relative to the first trace sample. |
|
| Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to the waveforms. 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. |
A check on the assumed source location is possible by looking at the time shifts between the observed and predicted traces. The time shifts for waveform matching arise for several reasons:
Time_shift = A + B cos Azimuth + C Sin Azimuth
The time shifts for this inversion lead to the next figure:
The derived shift in origin time and epicentral coordinates are given at the bottom of the figure.
The WUS.model used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows (The format is in the model96 format of Computer Programs in Seismology).
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