The ANSS event ID is ak0123q2y5io and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0123q2y5io/executive.
2012/03/21 07:42:02 61.309 -150.065 45.4 3.8 Alaska
USGS/SLU Moment Tensor Solution
ENS 2012/03/21 07:42:02:0 61.31 -150.07 45.4 3.8 Alaska
Stations used:
AK.BPAW AK.BRLK AK.GLI AK.RC01 AK.SKN AK.SSN AT.PMR
Filtering commands used:
hp c 0.02 n 3
lp c 0.10 n 3
br c 0.12 0.25 n 4 p 2
Best Fitting Double Couple
Mo = 7.50e+21 dyne-cm
Mw = 3.85
Z = 51 km
Plane Strike Dip Rake
NP1 185 70 -70
NP2 318 28 -133
Principal Axes:
Axis Value Plunge Azimuth
T 7.50e+21 22 260
N 0.00e+00 19 358
P -7.50e+21 60 124
Moment Tensor: (dyne-cm)
Component Value
Mxx -3.84e+20
Mxy 1.98e+21
Mxz 1.34e+21
Myy 4.91e+21
Myz -5.30e+21
Mzz -4.53e+21
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### ##########----------------------####
### T ##########---------- ---------####
## ##########---------- P ---------###
###############---------- ---------###
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Global CMT Convention Moment Tensor:
R T P
-4.53e+21 1.34e+21 5.30e+21
1.34e+21 -3.84e+20 -1.98e+21
5.30e+21 -1.98e+21 4.91e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20120321074202/index.html
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STK = 185
DIP = 70
RAKE = -70
MW = 3.85
HS = 51.0
The NDK file is 20120321074202.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.
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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.
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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'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 0.10 n 3 br c 0.12 0.25 n 4 p 2The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 0.5 215 45 85 3.00 0.1948
WVFGRD96 1.0 175 80 15 2.95 0.1447
WVFGRD96 2.0 5 40 90 3.17 0.2274
WVFGRD96 3.0 170 85 -25 3.20 0.2419
WVFGRD96 4.0 170 85 -30 3.26 0.2897
WVFGRD96 5.0 170 85 -35 3.30 0.3383
WVFGRD96 6.0 170 80 -35 3.33 0.3768
WVFGRD96 7.0 165 70 -40 3.35 0.4107
WVFGRD96 8.0 145 30 35 3.40 0.4308
WVFGRD96 9.0 145 30 35 3.40 0.4490
WVFGRD96 10.0 140 35 25 3.39 0.4604
WVFGRD96 11.0 140 35 25 3.39 0.4705
WVFGRD96 12.0 140 40 20 3.40 0.4793
WVFGRD96 13.0 140 40 20 3.41 0.4888
WVFGRD96 14.0 135 40 15 3.42 0.4969
WVFGRD96 15.0 135 40 15 3.43 0.5040
WVFGRD96 16.0 135 45 15 3.44 0.5100
WVFGRD96 17.0 135 45 10 3.45 0.5163
WVFGRD96 18.0 135 45 10 3.46 0.5209
WVFGRD96 19.0 135 45 10 3.47 0.5255
WVFGRD96 20.0 135 45 10 3.49 0.5289
WVFGRD96 21.0 135 45 10 3.50 0.5294
WVFGRD96 22.0 135 40 10 3.51 0.5308
WVFGRD96 23.0 160 40 -15 3.56 0.5370
WVFGRD96 24.0 160 40 -15 3.57 0.5420
WVFGRD96 25.0 160 40 -15 3.58 0.5461
WVFGRD96 26.0 160 40 -15 3.59 0.5494
WVFGRD96 27.0 165 45 -10 3.60 0.5514
WVFGRD96 28.0 160 40 -15 3.61 0.5545
WVFGRD96 29.0 160 45 -15 3.62 0.5592
WVFGRD96 30.0 30 85 70 3.58 0.5665
WVFGRD96 31.0 30 85 70 3.59 0.5714
WVFGRD96 32.0 30 85 70 3.59 0.5732
WVFGRD96 33.0 205 90 -65 3.60 0.5761
WVFGRD96 34.0 25 90 65 3.60 0.5783
WVFGRD96 35.0 25 90 65 3.61 0.5795
WVFGRD96 36.0 25 90 65 3.62 0.5802
WVFGRD96 37.0 200 85 -65 3.62 0.5834
WVFGRD96 38.0 195 80 -60 3.63 0.5824
WVFGRD96 39.0 195 80 -60 3.63 0.5820
WVFGRD96 40.0 195 80 -70 3.76 0.5716
WVFGRD96 41.0 195 80 -70 3.77 0.5765
WVFGRD96 42.0 195 80 -70 3.78 0.5816
WVFGRD96 43.0 55 15 -45 3.79 0.5858
WVFGRD96 44.0 65 20 -40 3.80 0.5899
WVFGRD96 45.0 185 70 -70 3.81 0.5975
WVFGRD96 46.0 180 65 -70 3.82 0.6014
WVFGRD96 47.0 185 70 -70 3.82 0.6058
WVFGRD96 48.0 180 65 -70 3.83 0.6080
WVFGRD96 49.0 185 70 -70 3.84 0.6108
WVFGRD96 50.0 185 70 -70 3.84 0.6108
WVFGRD96 51.0 185 70 -70 3.85 0.6111
WVFGRD96 52.0 185 70 -70 3.86 0.6099
WVFGRD96 53.0 185 70 -70 3.86 0.6074
WVFGRD96 54.0 180 70 -70 3.88 0.6048
WVFGRD96 55.0 20 15 -65 3.90 0.6029
WVFGRD96 56.0 0 15 -80 3.91 0.6046
WVFGRD96 57.0 -5 15 -85 3.92 0.6029
WVFGRD96 58.0 0 15 -80 3.92 0.6017
WVFGRD96 59.0 10 20 -70 3.92 0.6011
WVFGRD96 60.0 10 20 -70 3.93 0.5986
WVFGRD96 61.0 10 20 -70 3.93 0.5970
WVFGRD96 62.0 10 20 -70 3.94 0.5939
WVFGRD96 63.0 10 20 -70 3.94 0.5895
WVFGRD96 64.0 10 20 -70 3.94 0.5850
WVFGRD96 65.0 10 20 -70 3.95 0.5797
WVFGRD96 66.0 15 20 -65 3.95 0.5727
WVFGRD96 67.0 15 20 -65 3.95 0.5668
WVFGRD96 68.0 75 35 -60 3.93 0.5709
WVFGRD96 69.0 75 35 -60 3.93 0.5663
WVFGRD96 70.0 70 35 -60 3.93 0.5622
WVFGRD96 71.0 70 35 -60 3.93 0.5580
WVFGRD96 72.0 65 35 -65 3.92 0.5532
WVFGRD96 73.0 65 35 -65 3.92 0.5480
WVFGRD96 74.0 65 35 -65 3.93 0.5431
WVFGRD96 75.0 180 90 -70 4.01 0.5376
WVFGRD96 76.0 180 90 -70 4.01 0.5365
WVFGRD96 77.0 5 85 75 4.00 0.5390
WVFGRD96 78.0 180 90 -70 4.02 0.5330
WVFGRD96 79.0 180 90 -70 4.02 0.5308
WVFGRD96 80.0 180 90 -75 4.01 0.5291
WVFGRD96 81.0 180 90 -75 4.02 0.5266
WVFGRD96 82.0 180 90 -75 4.02 0.5243
WVFGRD96 83.0 180 90 -75 4.02 0.5219
WVFGRD96 84.0 0 85 75 4.01 0.5257
WVFGRD96 85.0 180 90 -75 4.02 0.5156
WVFGRD96 86.0 180 90 -75 4.02 0.5129
WVFGRD96 87.0 0 85 80 4.01 0.5182
WVFGRD96 88.0 145 40 -10 4.09 0.5190
WVFGRD96 89.0 145 40 -10 4.09 0.5167
WVFGRD96 90.0 145 45 -10 4.11 0.5145
WVFGRD96 91.0 145 45 -10 4.11 0.5139
WVFGRD96 92.0 145 45 -10 4.11 0.5124
WVFGRD96 93.0 145 45 -10 4.11 0.5106
WVFGRD96 94.0 145 45 -10 4.11 0.5081
WVFGRD96 95.0 145 45 -10 4.11 0.5054
WVFGRD96 96.0 145 45 -10 4.11 0.5037
WVFGRD96 97.0 145 45 -10 4.11 0.5016
WVFGRD96 98.0 145 45 -10 4.11 0.4989
WVFGRD96 99.0 145 45 -10 4.11 0.4958
The best solution is
WVFGRD96 51.0 185 70 -70 3.85 0.6111
The mechanism corresponding 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 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 0.10 n 3 br c 0.12 0.25 n 4 p 2
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| 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. |
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| 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