The ANSS event ID is ak0208s5foz1 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0208s5foz1/executive.
2020/07/09 16:44:19 62.331 -148.740 19.1 3.6 Alaska
USGS/SLU Moment Tensor Solution
ENS 2020/07/09 16:44:19:0 62.33 -148.74 19.1 3.6 Alaska
Stations used:
AK.CUT AK.DHY AK.DIV AK.EYAK AK.FID AK.GHO AK.GLB AK.GLI
AK.K24K AK.KLU AK.KNK AK.L22K AK.RC01 AK.RND AK.SAW AK.SCM
AK.SLK AK.TRF AT.PMR TA.M22K TA.M24K
Filtering commands used:
cut o DIST/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 5.13e+21 dyne-cm
Mw = 3.74
Z = 51 km
Plane Strike Dip Rake
NP1 246 52 -117
NP2 105 45 -60
Principal Axes:
Axis Value Plunge Azimuth
T 5.13e+21 4 354
N 0.00e+00 21 263
P -5.13e+21 69 94
Moment Tensor: (dyne-cm)
Component Value
Mxx 5.05e+21
Mxy -4.60e+20
Mxz 4.69e+20
Myy -6.09e+20
Myz -1.75e+21
Mzz -4.44e+21
### T ########
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############----------------------------
#########-------------------------------
--######----------------------------------
---####------------------ --------------
----#-------------------- P --------------
----#-------------------- --------------
--####----------------------------------
--######-------------------------------#
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#############------------------#####
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Global CMT Convention Moment Tensor:
R T P
-4.44e+21 4.69e+20 1.75e+21
4.69e+20 5.05e+21 4.60e+20
1.75e+21 4.60e+20 -6.09e+20
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20200709164419/index.html
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STK = 105
DIP = 45
RAKE = -60
MW = 3.74
HS = 51.0
The NDK file is 20200709164419.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:
cut o DIST/3.3 -40 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 255 45 95 2.82 0.1467
WVFGRD96 2.0 70 45 90 2.97 0.1861
WVFGRD96 3.0 265 65 50 3.06 0.1963
WVFGRD96 4.0 260 65 45 3.07 0.2093
WVFGRD96 5.0 70 90 55 3.07 0.2232
WVFGRD96 6.0 75 85 50 3.10 0.2412
WVFGRD96 7.0 235 60 75 3.10 0.2556
WVFGRD96 8.0 80 80 50 3.19 0.2627
WVFGRD96 9.0 75 60 95 3.20 0.2729
WVFGRD96 10.0 240 35 75 3.21 0.2805
WVFGRD96 11.0 240 35 75 3.22 0.2841
WVFGRD96 12.0 240 35 75 3.23 0.2835
WVFGRD96 13.0 240 35 75 3.24 0.2803
WVFGRD96 14.0 65 60 80 3.25 0.2761
WVFGRD96 15.0 65 60 80 3.26 0.2718
WVFGRD96 16.0 65 60 80 3.27 0.2684
WVFGRD96 17.0 65 60 80 3.28 0.2638
WVFGRD96 18.0 290 55 -35 3.29 0.2645
WVFGRD96 19.0 285 55 -40 3.31 0.2739
WVFGRD96 20.0 285 55 -40 3.33 0.2828
WVFGRD96 21.0 285 55 -40 3.35 0.2913
WVFGRD96 22.0 285 55 -40 3.36 0.2990
WVFGRD96 23.0 285 60 -40 3.37 0.3061
WVFGRD96 24.0 285 60 -40 3.39 0.3127
WVFGRD96 25.0 285 60 -40 3.40 0.3172
WVFGRD96 26.0 285 60 -40 3.41 0.3203
WVFGRD96 27.0 120 50 -20 3.42 0.3203
WVFGRD96 28.0 120 50 -20 3.43 0.3212
WVFGRD96 29.0 280 50 -45 3.44 0.3216
WVFGRD96 30.0 115 70 -35 3.45 0.3403
WVFGRD96 31.0 115 70 -40 3.47 0.3557
WVFGRD96 32.0 115 65 -40 3.48 0.3727
WVFGRD96 33.0 115 65 -40 3.49 0.3907
WVFGRD96 34.0 100 55 -60 3.51 0.4094
WVFGRD96 35.0 100 55 -60 3.52 0.4280
WVFGRD96 36.0 105 55 -55 3.53 0.4459
WVFGRD96 37.0 105 55 -55 3.54 0.4596
WVFGRD96 38.0 100 50 -55 3.55 0.4699
WVFGRD96 39.0 100 50 -60 3.57 0.4770
WVFGRD96 40.0 100 50 -60 3.64 0.4864
WVFGRD96 41.0 100 50 -60 3.66 0.4899
WVFGRD96 42.0 100 50 -60 3.67 0.4918
WVFGRD96 43.0 105 50 -60 3.69 0.4963
WVFGRD96 44.0 105 50 -60 3.70 0.5003
WVFGRD96 45.0 95 45 -65 3.71 0.5058
WVFGRD96 46.0 95 45 -65 3.72 0.5092
WVFGRD96 47.0 100 45 -65 3.73 0.5134
WVFGRD96 48.0 100 45 -65 3.73 0.5144
WVFGRD96 49.0 100 45 -60 3.73 0.5166
WVFGRD96 50.0 100 45 -60 3.74 0.5161
WVFGRD96 51.0 105 45 -60 3.74 0.5166
WVFGRD96 52.0 95 40 -65 3.75 0.5151
WVFGRD96 53.0 95 40 -65 3.75 0.5138
WVFGRD96 54.0 95 40 -65 3.75 0.5118
WVFGRD96 55.0 100 40 -60 3.75 0.5086
WVFGRD96 56.0 100 40 -60 3.76 0.5077
WVFGRD96 57.0 100 40 -60 3.76 0.5043
WVFGRD96 58.0 100 40 -60 3.76 0.5013
WVFGRD96 59.0 100 40 -60 3.76 0.4982
The best solution is
WVFGRD96 51.0 105 45 -60 3.74 0.5166
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
cut o DIST/3.3 -40 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 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