The ANSS event ID is ak0119a690lk and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0119a690lk/executive.
2011/07/21 06:20:11 60.031 -152.831 92.7 3.7 Alaska
USGS/SLU Moment Tensor Solution
ENS 2011/07/21 06:20:11:0 60.03 -152.83 92.7 3.7 Alaska
Stations used:
AK.CNP AK.FIB AK.GHO AK.HOM AK.SAW AK.SKN AK.SSN AK.SWD
AT.PMR AT.SVW2 II.KDAK
Filtering commands used:
hp c 0.02 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 3.31e+22 dyne-cm
Mw = 4.28
Z = 116 km
Plane Strike Dip Rake
NP1 55 60 40
NP2 302 56 143
Principal Axes:
Axis Value Plunge Azimuth
T 3.31e+22 48 270
N 0.00e+00 42 86
P -3.31e+22 2 178
Moment Tensor: (dyne-cm)
Component Value
Mxx -3.30e+22
Mxy 1.15e+21
Mxz 1.44e+21
Myy 1.46e+22
Myz -1.65e+22
Mzz 1.84e+22
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######### ###################---########
######### T ##############################
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##########################----------####
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------ P -----
Global CMT Convention Moment Tensor:
R T P
1.84e+22 1.44e+21 1.65e+22
1.44e+21 -3.30e+22 -1.15e+21
1.65e+22 -1.15e+21 1.46e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20110721062011/index.html
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STK = 55
DIP = 60
RAKE = 40
MW = 4.28
HS = 116.0
The NDK file is 20110721062011.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 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 50.0 50 70 -10 4.10 0.2975
WVFGRD96 51.0 50 70 -10 4.11 0.2985
WVFGRD96 52.0 225 70 -10 4.15 0.3004
WVFGRD96 53.0 225 70 -10 4.15 0.3013
WVFGRD96 54.0 220 70 -20 4.15 0.3042
WVFGRD96 55.0 220 70 -20 4.16 0.3067
WVFGRD96 56.0 220 70 -20 4.17 0.3084
WVFGRD96 57.0 220 75 -25 4.17 0.3123
WVFGRD96 58.0 220 75 -25 4.17 0.3158
WVFGRD96 59.0 220 75 -25 4.18 0.3175
WVFGRD96 60.0 220 75 -25 4.18 0.3209
WVFGRD96 61.0 220 75 -25 4.19 0.3237
WVFGRD96 62.0 220 75 -25 4.20 0.3258
WVFGRD96 63.0 220 75 -25 4.20 0.3286
WVFGRD96 64.0 220 75 -25 4.20 0.3285
WVFGRD96 65.0 220 80 -25 4.19 0.3321
WVFGRD96 66.0 220 80 -25 4.20 0.3336
WVFGRD96 67.0 220 80 -25 4.20 0.3354
WVFGRD96 68.0 220 80 -25 4.21 0.3374
WVFGRD96 69.0 220 80 -25 4.21 0.3388
WVFGRD96 70.0 220 80 -25 4.21 0.3399
WVFGRD96 71.0 55 80 20 4.22 0.3440
WVFGRD96 72.0 55 75 20 4.21 0.3466
WVFGRD96 73.0 55 75 20 4.22 0.3473
WVFGRD96 74.0 55 75 20 4.22 0.3507
WVFGRD96 75.0 50 70 30 4.19 0.3525
WVFGRD96 76.0 50 70 30 4.19 0.3549
WVFGRD96 77.0 50 70 30 4.20 0.3572
WVFGRD96 78.0 50 70 30 4.20 0.3585
WVFGRD96 79.0 50 70 30 4.20 0.3614
WVFGRD96 80.0 50 70 30 4.20 0.3629
WVFGRD96 81.0 55 70 30 4.23 0.3653
WVFGRD96 82.0 50 70 35 4.21 0.3664
WVFGRD96 83.0 55 65 35 4.22 0.3687
WVFGRD96 84.0 55 65 35 4.22 0.3700
WVFGRD96 85.0 55 65 35 4.22 0.3724
WVFGRD96 86.0 55 65 35 4.23 0.3744
WVFGRD96 87.0 55 65 35 4.23 0.3755
WVFGRD96 88.0 55 65 35 4.23 0.3778
WVFGRD96 89.0 55 65 35 4.23 0.3783
WVFGRD96 90.0 55 65 35 4.24 0.3818
WVFGRD96 91.0 55 65 35 4.24 0.3822
WVFGRD96 92.0 55 65 35 4.24 0.3848
WVFGRD96 93.0 55 65 35 4.24 0.3866
WVFGRD96 94.0 55 65 35 4.25 0.3875
WVFGRD96 95.0 55 65 35 4.25 0.3894
WVFGRD96 96.0 55 65 35 4.25 0.3911
WVFGRD96 97.0 55 65 35 4.25 0.3919
WVFGRD96 98.0 55 60 35 4.24 0.3943
WVFGRD96 99.0 55 60 35 4.24 0.3936
WVFGRD96 100.0 55 60 35 4.24 0.3970
WVFGRD96 101.0 55 60 35 4.24 0.3978
WVFGRD96 102.0 55 60 40 4.25 0.3982
WVFGRD96 103.0 55 60 40 4.25 0.4004
WVFGRD96 104.0 55 60 40 4.25 0.4022
WVFGRD96 105.0 55 60 40 4.25 0.4019
WVFGRD96 106.0 55 60 40 4.26 0.4043
WVFGRD96 107.0 55 60 40 4.26 0.4054
WVFGRD96 108.0 55 60 40 4.26 0.4045
WVFGRD96 109.0 55 60 40 4.26 0.4074
WVFGRD96 110.0 55 60 40 4.27 0.4073
WVFGRD96 111.0 55 60 40 4.27 0.4073
WVFGRD96 112.0 55 60 40 4.27 0.4089
WVFGRD96 113.0 55 60 40 4.27 0.4093
WVFGRD96 114.0 55 60 40 4.27 0.4082
WVFGRD96 115.0 55 60 40 4.28 0.4096
WVFGRD96 116.0 55 60 40 4.28 0.4101
WVFGRD96 117.0 55 60 40 4.28 0.4090
WVFGRD96 118.0 55 60 40 4.28 0.4100
WVFGRD96 119.0 55 60 40 4.28 0.4100
WVFGRD96 120.0 55 60 40 4.28 0.4088
WVFGRD96 121.0 55 60 40 4.29 0.4091
WVFGRD96 122.0 55 60 40 4.29 0.4094
WVFGRD96 123.0 50 60 40 4.28 0.4086
WVFGRD96 124.0 50 60 40 4.28 0.4082
WVFGRD96 125.0 50 60 40 4.28 0.4088
WVFGRD96 126.0 50 60 40 4.28 0.4083
WVFGRD96 127.0 50 60 40 4.28 0.4075
WVFGRD96 128.0 50 60 40 4.28 0.4068
WVFGRD96 129.0 55 55 40 4.28 0.4073
The best solution is
WVFGRD96 116.0 55 60 40 4.28 0.4101
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
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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