The ANSS event ID is ak0133b7hme1 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0133b7hme1/executive.
2013/03/13 08:05:44 62.556 -151.230 84.4 4.8 Alaska
USGS/SLU Moment Tensor Solution
ENS 2013/03/13 08:05:44:0 62.56 -151.23 84.4 4.8 Alaska
Stations used:
AK.BPAW AK.BWN AK.CAST AK.CCB AK.DHY AK.DOT AK.EYAK AK.HDA
AK.KLU AK.KNK AK.MCK AK.MDM AK.MLY AK.NEA AK.PAX AK.PPD
AK.PPLA AK.RC01 AK.RIDG AK.RND AK.SAW AK.SCM AK.SCRK AK.SWD
AK.TRF AK.WRH AT.PMR AT.SVW2 IU.COLA
Filtering commands used:
hp c 0.02 n 3
lp c 0.07 n 3
Best Fitting Double Couple
Mo = 1.32e+23 dyne-cm
Mw = 4.68
Z = 91 km
Plane Strike Dip Rake
NP1 60 60 55
NP2 294 45 135
Principal Axes:
Axis Value Plunge Azimuth
T 1.32e+23 59 278
N 0.00e+00 30 79
P -1.32e+23 9 174
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.27e+23
Mxy 7.75e+21
Mxz 2.79e+22
Myy 3.33e+22
Myz -5.97e+22
Mzz 9.35e+22
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############ ####################-######
############ T ###################---#####
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------- P ----
Global CMT Convention Moment Tensor:
R T P
9.35e+22 2.79e+22 5.97e+22
2.79e+22 -1.27e+23 -7.75e+21
5.97e+22 -7.75e+21 3.33e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20130313080544/index.html
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STK = 60
DIP = 60
RAKE = 55
MW = 4.68
HS = 91.0
The NDK file is 20130313080544.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 0.07 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 0.5 85 45 -75 3.73 0.2132
WVFGRD96 1.0 85 45 -75 3.77 0.2086
WVFGRD96 2.0 85 50 -80 3.88 0.2457
WVFGRD96 3.0 85 45 -75 3.93 0.2374
WVFGRD96 4.0 95 45 -60 3.93 0.2109
WVFGRD96 5.0 95 45 -55 3.93 0.2003
WVFGRD96 6.0 30 70 -45 3.91 0.2028
WVFGRD96 7.0 25 65 -45 3.93 0.2205
WVFGRD96 8.0 30 70 -50 3.99 0.2286
WVFGRD96 9.0 25 65 -50 4.01 0.2414
WVFGRD96 10.0 30 65 -45 4.01 0.2512
WVFGRD96 11.0 245 65 65 4.04 0.2596
WVFGRD96 12.0 240 65 60 4.04 0.2686
WVFGRD96 13.0 240 65 60 4.05 0.2757
WVFGRD96 14.0 240 65 60 4.06 0.2807
WVFGRD96 15.0 240 65 55 4.07 0.2842
WVFGRD96 16.0 235 65 50 4.08 0.2871
WVFGRD96 17.0 235 65 50 4.09 0.2901
WVFGRD96 18.0 235 65 50 4.10 0.2916
WVFGRD96 19.0 230 70 45 4.11 0.2941
WVFGRD96 20.0 230 70 45 4.12 0.2964
WVFGRD96 21.0 230 70 45 4.13 0.2983
WVFGRD96 22.0 230 70 45 4.14 0.3002
WVFGRD96 23.0 230 70 45 4.15 0.2987
WVFGRD96 24.0 230 70 40 4.16 0.2993
WVFGRD96 25.0 225 70 40 4.17 0.3006
WVFGRD96 26.0 225 70 40 4.17 0.2981
WVFGRD96 27.0 225 70 40 4.18 0.2986
WVFGRD96 28.0 225 70 40 4.19 0.2988
WVFGRD96 29.0 225 65 40 4.20 0.2966
WVFGRD96 30.0 225 65 40 4.21 0.2973
WVFGRD96 31.0 220 65 35 4.22 0.2959
WVFGRD96 32.0 215 60 30 4.23 0.2984
WVFGRD96 33.0 215 60 30 4.24 0.3012
WVFGRD96 34.0 215 60 30 4.25 0.3020
WVFGRD96 35.0 215 60 30 4.26 0.3044
WVFGRD96 36.0 215 60 30 4.27 0.3056
WVFGRD96 37.0 50 75 30 4.30 0.3077
WVFGRD96 38.0 50 75 25 4.33 0.3104
WVFGRD96 39.0 50 75 25 4.34 0.3133
WVFGRD96 40.0 220 60 40 4.38 0.3225
WVFGRD96 41.0 220 60 40 4.39 0.3250
WVFGRD96 42.0 220 60 40 4.40 0.3283
WVFGRD96 43.0 220 60 40 4.41 0.3324
WVFGRD96 44.0 220 60 40 4.42 0.3360
WVFGRD96 45.0 220 60 40 4.43 0.3400
WVFGRD96 46.0 220 60 40 4.44 0.3433
WVFGRD96 47.0 220 60 40 4.45 0.3465
WVFGRD96 48.0 50 65 35 4.48 0.3532
WVFGRD96 49.0 50 65 35 4.49 0.3598
WVFGRD96 50.0 50 65 35 4.50 0.3678
WVFGRD96 51.0 50 65 35 4.51 0.3754
WVFGRD96 52.0 50 65 35 4.52 0.3838
WVFGRD96 53.0 50 65 35 4.53 0.3923
WVFGRD96 54.0 50 65 35 4.54 0.4009
WVFGRD96 55.0 50 55 45 4.55 0.4117
WVFGRD96 56.0 50 55 45 4.56 0.4270
WVFGRD96 57.0 50 55 45 4.57 0.4427
WVFGRD96 58.0 50 55 45 4.57 0.4581
WVFGRD96 59.0 50 55 45 4.58 0.4731
WVFGRD96 60.0 50 55 45 4.59 0.4881
WVFGRD96 61.0 55 55 50 4.60 0.5023
WVFGRD96 62.0 55 55 50 4.60 0.5174
WVFGRD96 63.0 55 55 50 4.61 0.5321
WVFGRD96 64.0 55 55 50 4.61 0.5452
WVFGRD96 65.0 55 55 50 4.62 0.5586
WVFGRD96 66.0 55 55 50 4.62 0.5709
WVFGRD96 67.0 55 55 50 4.63 0.5841
WVFGRD96 68.0 55 55 50 4.63 0.5951
WVFGRD96 69.0 55 55 50 4.64 0.6060
WVFGRD96 70.0 55 55 50 4.64 0.6168
WVFGRD96 71.0 60 55 55 4.64 0.6263
WVFGRD96 72.0 60 55 55 4.65 0.6362
WVFGRD96 73.0 60 55 55 4.65 0.6450
WVFGRD96 74.0 60 55 55 4.65 0.6534
WVFGRD96 75.0 60 55 55 4.66 0.6607
WVFGRD96 76.0 60 55 55 4.66 0.6682
WVFGRD96 77.0 60 55 55 4.66 0.6738
WVFGRD96 78.0 60 55 55 4.66 0.6799
WVFGRD96 79.0 60 55 55 4.66 0.6845
WVFGRD96 80.0 60 55 55 4.66 0.6889
WVFGRD96 81.0 60 60 55 4.67 0.6942
WVFGRD96 82.0 60 60 55 4.67 0.6982
WVFGRD96 83.0 60 60 55 4.67 0.7022
WVFGRD96 84.0 60 60 55 4.67 0.7053
WVFGRD96 85.0 60 60 55 4.67 0.7080
WVFGRD96 86.0 60 60 55 4.67 0.7102
WVFGRD96 87.0 60 60 55 4.68 0.7118
WVFGRD96 88.0 60 60 55 4.68 0.7132
WVFGRD96 89.0 60 60 55 4.68 0.7144
WVFGRD96 90.0 60 60 55 4.68 0.7141
WVFGRD96 91.0 60 60 55 4.68 0.7147
WVFGRD96 92.0 60 60 55 4.68 0.7142
WVFGRD96 93.0 60 60 55 4.68 0.7139
WVFGRD96 94.0 60 60 55 4.67 0.7125
WVFGRD96 95.0 60 60 55 4.67 0.7119
WVFGRD96 96.0 65 60 60 4.68 0.7103
WVFGRD96 97.0 65 60 60 4.68 0.7092
WVFGRD96 98.0 65 60 60 4.68 0.7076
WVFGRD96 99.0 65 60 60 4.68 0.7065
The best solution is
WVFGRD96 91.0 60 60 55 4.68 0.7147
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 0.07 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