The ANSS event ID is ak0219sja0eh and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0219sja0eh/executive.
2021/08/01 16:47:28 61.606 -146.136 25.8 3.5 Alaska
USGS/SLU Moment Tensor Solution
ENS 2021/08/01 16:47:28:0 61.61 -146.14 25.8 3.5 Alaska
Stations used:
AK.BMR AK.DHY AK.DIV AK.EYAK AK.FID AK.GLB AK.GLI AK.HIN
AK.KLU AK.MCAR AK.RC01 AK.SCM AK.VRDI AT.PMR
Filtering commands used:
cut o DIST/3.3 -30 o DIST/3.3 +40
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.08 n 3
Best Fitting Double Couple
Mo = 2.75e+21 dyne-cm
Mw = 3.56
Z = 42 km
Plane Strike Dip Rake
NP1 225 85 -55
NP2 322 35 -171
Principal Axes:
Axis Value Plunge Azimuth
T 2.75e+21 31 287
N 0.00e+00 35 41
P -2.75e+21 40 167
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.38e+21
Mxy -1.96e+20
Mxz 1.67e+21
Myy 1.77e+21
Myz -1.47e+21
Mzz -3.92e+20
--------------
----------------------
###############-------------
####################---------#
########################----######
####################################
##########################----########
##### #################-------########
##### T ###############----------#######
###### #############-------------#######
####################----------------######
##################------------------######
################--------------------######
##############----------------------####
############------------------------####
#########--------------------------###
#######------------ -----------###
####-------------- P -----------##
#--------------- ----------#
---------------------------#
----------------------
--------------
Global CMT Convention Moment Tensor:
R T P
-3.92e+20 1.67e+21 1.47e+21
1.67e+21 -1.38e+21 1.96e+20
1.47e+21 1.96e+20 1.77e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210801164728/index.html
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STK = 225
DIP = 85
RAKE = -55
MW = 3.56
HS = 42.0
The NDK file is 20210801164728.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 -30 o DIST/3.3 +40 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 55 40 -90 2.77 0.2363
WVFGRD96 2.0 55 40 -90 2.92 0.3259
WVFGRD96 3.0 50 35 -95 2.99 0.3286
WVFGRD96 4.0 310 40 -25 2.97 0.3552
WVFGRD96 5.0 315 40 -15 2.99 0.3831
WVFGRD96 6.0 320 40 0 3.01 0.4074
WVFGRD96 7.0 320 45 0 3.02 0.4231
WVFGRD96 8.0 325 40 10 3.09 0.4326
WVFGRD96 9.0 325 40 15 3.10 0.4368
WVFGRD96 10.0 325 45 15 3.11 0.4389
WVFGRD96 11.0 325 45 20 3.13 0.4394
WVFGRD96 12.0 325 45 20 3.14 0.4377
WVFGRD96 13.0 240 70 40 3.15 0.4348
WVFGRD96 14.0 240 65 35 3.17 0.4430
WVFGRD96 15.0 240 65 35 3.18 0.4506
WVFGRD96 16.0 240 65 35 3.19 0.4569
WVFGRD96 17.0 215 60 -40 3.23 0.4642
WVFGRD96 18.0 215 60 -40 3.24 0.4763
WVFGRD96 19.0 215 60 -40 3.26 0.4879
WVFGRD96 20.0 215 60 -40 3.27 0.4980
WVFGRD96 21.0 220 65 -40 3.28 0.5078
WVFGRD96 22.0 220 65 -40 3.29 0.5205
WVFGRD96 23.0 220 65 -40 3.31 0.5316
WVFGRD96 24.0 50 90 40 3.29 0.5409
WVFGRD96 25.0 50 90 40 3.30 0.5598
WVFGRD96 26.0 225 80 -45 3.32 0.5785
WVFGRD96 27.0 225 80 -45 3.34 0.5969
WVFGRD96 28.0 225 80 -45 3.35 0.6135
WVFGRD96 29.0 225 80 -45 3.36 0.6301
WVFGRD96 30.0 225 80 -45 3.37 0.6450
WVFGRD96 31.0 225 80 -45 3.38 0.6592
WVFGRD96 32.0 225 80 -45 3.39 0.6723
WVFGRD96 33.0 225 80 -45 3.40 0.6829
WVFGRD96 34.0 225 80 -45 3.41 0.6906
WVFGRD96 35.0 225 80 -45 3.42 0.6959
WVFGRD96 36.0 225 80 -45 3.43 0.6996
WVFGRD96 37.0 225 80 -45 3.44 0.7026
WVFGRD96 38.0 225 80 -40 3.44 0.7019
WVFGRD96 39.0 225 80 -40 3.45 0.7040
WVFGRD96 40.0 225 85 -55 3.54 0.7058
WVFGRD96 41.0 225 85 -55 3.55 0.7086
WVFGRD96 42.0 225 85 -55 3.56 0.7090
WVFGRD96 43.0 225 85 -50 3.56 0.7087
WVFGRD96 44.0 225 85 -50 3.57 0.7077
WVFGRD96 45.0 50 90 50 3.57 0.7000
WVFGRD96 46.0 225 85 -50 3.58 0.7047
WVFGRD96 47.0 225 85 -50 3.59 0.7028
WVFGRD96 48.0 225 85 -50 3.59 0.7009
WVFGRD96 49.0 225 85 -50 3.60 0.6971
WVFGRD96 50.0 225 85 -50 3.61 0.6948
WVFGRD96 51.0 225 85 -50 3.61 0.6926
WVFGRD96 52.0 225 85 -50 3.62 0.6888
WVFGRD96 53.0 225 85 -50 3.62 0.6862
WVFGRD96 54.0 225 85 -50 3.63 0.6846
WVFGRD96 55.0 225 85 -50 3.63 0.6816
WVFGRD96 56.0 225 90 -50 3.63 0.6781
WVFGRD96 57.0 225 90 -50 3.63 0.6772
WVFGRD96 58.0 225 90 -50 3.64 0.6746
WVFGRD96 59.0 225 90 -50 3.64 0.6730
The best solution is
WVFGRD96 42.0 225 85 -55 3.56 0.7090
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 -30 o DIST/3.3 +40 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 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