The ANSS event ID is ak020b5wtrrt and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak020b5wtrrt/executive.
2020/08/30 02:07:33 64.029 -148.804 14.3 3.6 Alaska
USGS/SLU Moment Tensor Solution
ENS 2020/08/30 02:07:33:0 64.03 -148.80 14.3 3.6 Alaska
Stations used:
AK.BWN AK.CCB AK.DIV AK.FYU AK.GHO AK.GLI AK.H21K AK.H23K
AK.H24K AK.HARP AK.I21K AK.I23K AK.I26K AK.J19K AK.J20K
AK.J25K AK.K20K AK.K24K AK.K27K AK.KLU AK.KTH AK.L19K
AK.L26K AK.M19K AK.M20K AK.PPLA AK.PWL AK.RND AK.SCM AK.SSN
AK.WRH AT.MENT AT.PMR AV.STLK IM.IL31 IU.COLA TA.G21K
TA.G23K TA.G24K TA.I20K TA.L27K TA.M22K
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 0.08 n 3
Best Fitting Double Couple
Mo = 2.57e+21 dyne-cm
Mw = 3.54
Z = 13 km
Plane Strike Dip Rake
NP1 271 62 112
NP2 50 35 55
Principal Axes:
Axis Value Plunge Azimuth
T 2.57e+21 66 220
N 0.00e+00 19 80
P -2.57e+21 14 345
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.99e+21
Mxy 8.27e+20
Mxz -1.33e+21
Myy 1.53e+19
Myz -4.62e+20
Mzz 1.98e+21
- ----------
----- P --------------
-------- -----------------
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---------------------------------#
-----------------------------------#
------------------------------------##
------------###############----------###
------###########################----###
---#######################################
-####################################----#
#####################################-----
################ #################------
############### T ################------
############### ##############--------
##############################--------
###########################---------
########################----------
##################------------
-----######-----------------
----------------------
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Global CMT Convention Moment Tensor:
R T P
1.98e+21 -1.33e+21 4.62e+20
-1.33e+21 -1.99e+21 -8.27e+20
4.62e+20 -8.27e+20 1.53e+19
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20200830020733/index.html
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STK = 50
DIP = 35
RAKE = 55
MW = 3.54
HS = 13.0
The NDK file is 20200830020733.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 0.08 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 285 45 90 3.17 0.3421
WVFGRD96 2.0 280 45 85 3.30 0.4216
WVFGRD96 3.0 165 35 -65 3.33 0.3307
WVFGRD96 4.0 30 30 15 3.34 0.4024
WVFGRD96 5.0 30 30 20 3.36 0.4767
WVFGRD96 6.0 35 30 25 3.38 0.5362
WVFGRD96 7.0 35 30 30 3.39 0.5796
WVFGRD96 8.0 40 30 35 3.47 0.6112
WVFGRD96 9.0 45 30 45 3.49 0.6448
WVFGRD96 10.0 45 30 45 3.50 0.6672
WVFGRD96 11.0 50 30 55 3.52 0.6807
WVFGRD96 12.0 50 35 50 3.52 0.6892
WVFGRD96 13.0 50 35 55 3.54 0.6915
WVFGRD96 14.0 50 35 55 3.54 0.6886
WVFGRD96 15.0 50 35 55 3.55 0.6815
WVFGRD96 16.0 55 35 60 3.56 0.6712
WVFGRD96 17.0 55 35 60 3.56 0.6587
WVFGRD96 18.0 55 35 60 3.57 0.6444
WVFGRD96 19.0 55 35 60 3.58 0.6282
WVFGRD96 20.0 55 35 60 3.58 0.6105
WVFGRD96 21.0 55 35 60 3.59 0.5932
WVFGRD96 22.0 55 35 60 3.60 0.5737
WVFGRD96 23.0 55 35 60 3.60 0.5535
WVFGRD96 24.0 250 60 80 3.62 0.5346
WVFGRD96 25.0 15 45 -40 3.60 0.5205
WVFGRD96 26.0 15 50 -45 3.62 0.5055
WVFGRD96 27.0 15 50 -40 3.62 0.4920
WVFGRD96 28.0 15 50 -40 3.62 0.4788
WVFGRD96 29.0 15 50 -40 3.63 0.4660
The best solution is
WVFGRD96 13.0 50 35 55 3.54 0.6915
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 0.08 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