The ANSS event ID is ak0209z121w6 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0209z121w6/executive.
2020/08/04 09:13:26 62.405 -149.512 49.5 3.8 Alaska
USGS/SLU Moment Tensor Solution
ENS 2020/08/04 09:13:26:0 62.40 -149.51 49.5 3.8 Alaska
Stations used:
AK.CAST AK.DIV AK.GHO AK.K24K AK.KNK AK.KTH AK.L22K AK.M20K
AK.MCK AK.PAX AK.PPLA AK.PWL AK.RC01 AK.RND AK.SAW AK.SCM
AK.SKN AK.TRF AT.PMR TA.M22K
Filtering commands used:
cut o DIST/3.3 -40 o DIST/3.3 +30
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 8.32e+21 dyne-cm
Mw = 3.88
Z = 58 km
Plane Strike Dip Rake
NP1 212 76 -111
NP2 90 25 -35
Principal Axes:
Axis Value Plunge Azimuth
T 8.32e+21 28 319
N 0.00e+00 20 218
P -8.32e+21 54 97
Moment Tensor: (dyne-cm)
Component Value
Mxx 3.65e+21
Mxy -2.88e+21
Mxz 3.07e+21
Myy -2.52e+14
Myz -6.18e+21
Mzz -3.65e+21
##############
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#######################-----
#### ##############---------
###### T #############------------
####### ###########---------------
####################------------------
####################--------------------
###################---------------------
###################-----------------------
#################----------- -----------
################------------ P -----------
-##############------------- ----------#
-############--------------------------#
--##########--------------------------##
--#########-------------------------##
---######------------------------###
-----##----------------------#####
-----#------------------######
---#########----############
######################
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Global CMT Convention Moment Tensor:
R T P
-3.65e+21 3.07e+21 6.18e+21
3.07e+21 3.65e+21 2.88e+21
6.18e+21 2.88e+21 -2.52e+14
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20200804091326/index.html
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STK = 90
DIP = 25
RAKE = -35
MW = 3.88
HS = 58.0
The NDK file is 20200804091326.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 +30 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 2.0 45 50 80 3.04 0.1729
WVFGRD96 4.0 0 65 -40 3.09 0.1814
WVFGRD96 6.0 0 70 -40 3.15 0.2146
WVFGRD96 8.0 185 85 -55 3.25 0.2443
WVFGRD96 10.0 15 75 45 3.29 0.2723
WVFGRD96 12.0 15 75 45 3.33 0.2951
WVFGRD96 14.0 15 75 45 3.36 0.3058
WVFGRD96 16.0 15 70 40 3.39 0.3091
WVFGRD96 18.0 15 80 50 3.41 0.3074
WVFGRD96 20.0 110 55 15 3.44 0.3063
WVFGRD96 22.0 110 50 15 3.48 0.3277
WVFGRD96 24.0 110 50 10 3.50 0.3478
WVFGRD96 26.0 105 50 5 3.53 0.3660
WVFGRD96 28.0 105 45 5 3.56 0.3831
WVFGRD96 30.0 110 45 20 3.58 0.3977
WVFGRD96 32.0 110 40 15 3.60 0.4144
WVFGRD96 34.0 110 45 15 3.62 0.4384
WVFGRD96 36.0 100 40 -10 3.63 0.4625
WVFGRD96 38.0 100 40 -10 3.65 0.4903
WVFGRD96 40.0 95 30 -20 3.77 0.5165
WVFGRD96 42.0 95 30 -20 3.79 0.5530
WVFGRD96 44.0 95 30 -25 3.80 0.5880
WVFGRD96 46.0 95 30 -25 3.82 0.6201
WVFGRD96 48.0 90 30 -35 3.83 0.6483
WVFGRD96 50.0 95 30 -30 3.84 0.6695
WVFGRD96 52.0 90 30 -35 3.85 0.6875
WVFGRD96 54.0 95 30 -30 3.86 0.6975
WVFGRD96 56.0 90 25 -35 3.88 0.7065
WVFGRD96 58.0 90 25 -35 3.88 0.7103
WVFGRD96 60.0 90 25 -35 3.89 0.7075
WVFGRD96 62.0 90 25 -35 3.90 0.7059
WVFGRD96 64.0 90 25 -35 3.90 0.7006
WVFGRD96 66.0 95 25 -30 3.91 0.6925
WVFGRD96 68.0 95 25 -30 3.91 0.6842
WVFGRD96 70.0 95 25 -30 3.91 0.6738
WVFGRD96 72.0 95 25 -30 3.92 0.6638
WVFGRD96 74.0 90 25 -40 3.92 0.6532
WVFGRD96 76.0 90 25 -40 3.92 0.6439
WVFGRD96 78.0 90 25 -40 3.92 0.6333
WVFGRD96 80.0 90 25 -40 3.92 0.6240
WVFGRD96 82.0 80 20 -45 3.93 0.6117
WVFGRD96 84.0 75 15 -55 3.95 0.6024
WVFGRD96 86.0 80 15 -55 3.95 0.5932
WVFGRD96 88.0 80 15 -55 3.95 0.5827
WVFGRD96 90.0 75 15 -60 3.95 0.5746
WVFGRD96 92.0 75 15 -60 3.95 0.5650
WVFGRD96 94.0 75 15 -60 3.95 0.5544
WVFGRD96 96.0 65 20 -70 3.93 0.5444
WVFGRD96 98.0 50 20 -85 3.93 0.5362
WVFGRD96 100.0 50 20 -85 3.93 0.5308
WVFGRD96 102.0 225 70 -90 3.93 0.5242
WVFGRD96 104.0 50 20 -85 3.93 0.5156
WVFGRD96 106.0 120 15 -10 3.97 0.5126
WVFGRD96 108.0 50 20 -85 3.93 0.5038
The best solution is
WVFGRD96 58.0 90 25 -35 3.88 0.7103
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 +30 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