Left: mLg computed using the IASPEI formula. Center: mLg residuals versus epicentral distance ; the values used for the trimmed mean magnitude estimate are indicated. Right: residuals as a function of distance and azimuth.
The ANSS event ID is ak021bt4ffvw and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak021bt4ffvw/executive.
2021/09/14 05:56:36 64.437 -146.737 3.9 4.9 Alaska
USGS/SLU Moment Tensor Solution
ENS 2021/09/14 05:56:36:0 64.44 -146.74 3.9 4.9 Alaska
Stations used:
AK.BPAW AK.CAST AK.CCB AK.COLD AK.CUT AK.DHY AK.DOT AK.E24K
AK.F21K AK.FIRE AK.G23K AK.G24K AK.G27K AK.GHO AK.GLB
AK.GLI AK.H21K AK.H22K AK.H23K AK.H24K AK.HARP AK.I21K
AK.I23K AK.I26K AK.J19K AK.J20K AK.J25K AK.J26L AK.K20K
AK.K24K AK.K27K AK.KNK AK.KTH AK.L22K AK.L26K AK.M20K
AK.M26K AK.M27K AK.MCAR AK.MCK AK.MLY AK.NEA2 AK.PPD
AK.PPLA AK.RC01 AK.RIDG AK.RND AK.SAW AK.SCRK AK.SKN AK.SSN
AK.TRF AK.VRDI AK.WRH AT.MENT AT.PMR AV.STLK CN.DAWY
IU.COLA
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 = 1.11e+23 dyne-cm
Mw = 4.63
Z = 10 km
Plane Strike Dip Rake
NP1 148 71 159
NP2 245 70 20
Principal Axes:
Axis Value Plunge Azimuth
T 1.11e+23 28 106
N 0.00e+00 62 288
P -1.11e+23 1 197
Moment Tensor: (dyne-cm)
Component Value
Mxx -9.51e+22
Mxy -5.36e+22
Mxz -1.13e+22
Myy 7.07e+22
Myz 4.46e+22
Mzz 2.44e+22
--------------
----------------------
###-------------------------
####--------------------------
#######---------------------------
########----------------------------
##########------------------##########
###########-----------##################
############------######################
##############-###########################
############---###########################
##########------##########################
#######----------################# #####
####-------------################ T ####
###----------------############## ####
-------------------###################
--------------------################
--------------------##############
--------------------##########
----------------------######
--- ----------------
P ------------
Global CMT Convention Moment Tensor:
R T P
2.44e+22 -1.13e+22 -4.46e+22
-1.13e+22 -9.51e+22 5.36e+22
-4.46e+22 5.36e+22 7.07e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210914055636/index.html
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STK = 245
DIP = 70
RAKE = 20
MW = 4.63
HS = 10.0
The NDK file is 20210914055636.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: mLg computed using the IASPEI formula. Center: mLg residuals versus epicentral distance ; the values used for the trimmed mean magnitude estimate are indicated.
Right: residuals as a function of distance and azimuth.
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 1.0 240 90 5 4.23 0.3313
WVFGRD96 2.0 60 90 -10 4.35 0.4324
WVFGRD96 3.0 245 70 20 4.44 0.4705
WVFGRD96 4.0 245 70 25 4.48 0.5129
WVFGRD96 5.0 245 70 25 4.51 0.5479
WVFGRD96 6.0 245 70 25 4.53 0.5749
WVFGRD96 7.0 245 70 20 4.55 0.5965
WVFGRD96 8.0 245 70 25 4.60 0.6164
WVFGRD96 9.0 245 70 20 4.62 0.6239
WVFGRD96 10.0 245 70 20 4.63 0.6266
WVFGRD96 11.0 245 70 20 4.64 0.6250
WVFGRD96 12.0 245 70 20 4.65 0.6197
WVFGRD96 13.0 240 75 15 4.66 0.6122
WVFGRD96 14.0 240 75 15 4.67 0.6032
WVFGRD96 15.0 240 75 15 4.68 0.5917
WVFGRD96 16.0 240 75 15 4.69 0.5785
WVFGRD96 17.0 240 75 15 4.70 0.5642
WVFGRD96 18.0 240 75 15 4.70 0.5489
WVFGRD96 19.0 240 75 15 4.71 0.5332
WVFGRD96 20.0 240 75 15 4.71 0.5176
WVFGRD96 21.0 240 70 15 4.72 0.5024
WVFGRD96 22.0 240 70 15 4.72 0.4879
WVFGRD96 23.0 240 70 15 4.73 0.4738
WVFGRD96 24.0 240 70 15 4.73 0.4607
WVFGRD96 25.0 65 75 15 4.76 0.4527
WVFGRD96 26.0 65 75 15 4.77 0.4463
WVFGRD96 27.0 65 75 15 4.78 0.4403
WVFGRD96 28.0 65 75 15 4.78 0.4347
WVFGRD96 29.0 65 75 15 4.79 0.4294
The best solution is
WVFGRD96 10.0 245 70 20 4.63 0.6266
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