The ANSS event ID is ak0208dd96es and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0208dd96es/executive.
2020/06/30 22:58:22 61.192 -149.982 45.1 3.6 Alaska
USGS/SLU Moment Tensor Solution
ENS 2020/06/30 22:58:22:0 61.19 -149.98 45.1 3.6 Alaska
Stations used:
AK.DIV AK.FID AK.GHO AK.KLU AK.KNK AK.RC01 AK.SAW AK.SCM
AK.SKN AK.SLK AT.PMR AV.ILSW TA.O22K
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 = 6.76e+21 dyne-cm
Mw = 3.82
Z = 58 km
Plane Strike Dip Rake
NP1 160 55 -120
NP2 25 45 -55
Principal Axes:
Axis Value Plunge Azimuth
T 6.76e+21 5 271
N 0.00e+00 24 179
P -6.76e+21 65 12
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.11e+21
Mxy -3.59e+20
Mxz -2.49e+21
Myy 6.65e+21
Myz -1.16e+21
Mzz -5.54e+21
--------------
###------------------#
#####--------------------###
#####----------------------###
#######----------------------#####
########----------------------######
#########---------- ----------######
##########---------- P ----------#######
##########---------- ----------#######
########----------------------#########
T ########----------------------#########
#########---------------------#########
#############-------------------##########
############------------------##########
#############----------------###########
#############--------------###########
#############------------###########
##############--------############
#############-----############
##############-#############
########------########
--------------
Global CMT Convention Moment Tensor:
R T P
-5.54e+21 -2.49e+21 1.16e+21
-2.49e+21 -1.11e+21 3.59e+20
1.16e+21 3.59e+20 6.65e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20200630225822/index.html
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STK = 25
DIP = 45
RAKE = -55
MW = 3.82
HS = 58.0
The NDK file is 20200630225822.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 1.0 100 65 -25 2.89 0.2324
WVFGRD96 2.0 95 60 -40 3.06 0.2947
WVFGRD96 3.0 100 70 -40 3.10 0.3044
WVFGRD96 4.0 20 75 -30 3.07 0.3280
WVFGRD96 5.0 20 70 -30 3.11 0.3637
WVFGRD96 6.0 25 70 -35 3.13 0.3969
WVFGRD96 7.0 25 65 -35 3.16 0.4258
WVFGRD96 8.0 25 65 -35 3.22 0.4525
WVFGRD96 9.0 25 65 -35 3.24 0.4707
WVFGRD96 10.0 30 70 -35 3.25 0.4842
WVFGRD96 11.0 30 65 -35 3.28 0.4976
WVFGRD96 12.0 30 65 -35 3.29 0.5066
WVFGRD96 13.0 30 65 -35 3.31 0.5127
WVFGRD96 14.0 30 65 -30 3.32 0.5180
WVFGRD96 15.0 35 70 -30 3.33 0.5211
WVFGRD96 16.0 35 70 -30 3.34 0.5252
WVFGRD96 17.0 35 75 -30 3.36 0.5303
WVFGRD96 18.0 35 75 -30 3.37 0.5350
WVFGRD96 19.0 35 75 -25 3.38 0.5419
WVFGRD96 20.0 35 75 -25 3.40 0.5475
WVFGRD96 21.0 35 75 -30 3.41 0.5524
WVFGRD96 22.0 35 75 -30 3.42 0.5594
WVFGRD96 23.0 35 70 -30 3.43 0.5665
WVFGRD96 24.0 35 70 -30 3.44 0.5725
WVFGRD96 25.0 35 70 -30 3.45 0.5786
WVFGRD96 26.0 35 65 -30 3.46 0.5860
WVFGRD96 27.0 35 65 -35 3.48 0.5940
WVFGRD96 28.0 35 60 -35 3.49 0.6010
WVFGRD96 29.0 35 60 -35 3.49 0.6059
WVFGRD96 30.0 35 55 -35 3.50 0.6110
WVFGRD96 31.0 35 55 -35 3.51 0.6157
WVFGRD96 32.0 30 50 -40 3.53 0.6196
WVFGRD96 33.0 35 50 -40 3.53 0.6225
WVFGRD96 34.0 30 45 -40 3.54 0.6230
WVFGRD96 35.0 30 45 -40 3.55 0.6223
WVFGRD96 36.0 30 45 -45 3.56 0.6207
WVFGRD96 37.0 35 45 -40 3.56 0.6191
WVFGRD96 38.0 30 45 -45 3.58 0.6174
WVFGRD96 39.0 30 45 -45 3.59 0.6160
WVFGRD96 40.0 25 40 -45 3.67 0.6109
WVFGRD96 41.0 20 40 -50 3.69 0.6171
WVFGRD96 42.0 20 40 -50 3.70 0.6201
WVFGRD96 43.0 20 40 -55 3.72 0.6231
WVFGRD96 44.0 20 40 -55 3.73 0.6272
WVFGRD96 45.0 20 40 -55 3.74 0.6291
WVFGRD96 46.0 20 40 -55 3.74 0.6285
WVFGRD96 47.0 20 45 -55 3.76 0.6283
WVFGRD96 48.0 15 40 -60 3.77 0.6295
WVFGRD96 49.0 15 40 -60 3.78 0.6302
WVFGRD96 50.0 15 40 -60 3.78 0.6310
WVFGRD96 51.0 15 40 -60 3.79 0.6307
WVFGRD96 52.0 15 40 -60 3.80 0.6320
WVFGRD96 53.0 15 40 -60 3.80 0.6327
WVFGRD96 54.0 15 40 -60 3.81 0.6328
WVFGRD96 55.0 20 45 -60 3.81 0.6323
WVFGRD96 56.0 25 45 -55 3.81 0.6330
WVFGRD96 57.0 25 45 -55 3.82 0.6335
WVFGRD96 58.0 25 45 -55 3.82 0.6358
WVFGRD96 59.0 25 45 -55 3.83 0.6353
The best solution is
WVFGRD96 58.0 25 45 -55 3.82 0.6358
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