The ANSS event ID is ak0239nmd2xz and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0239nmd2xz/executive.
2023/07/29 19:32:48 68.620 -148.583 1.3 4 Alaska
USGS/SLU Moment Tensor Solution
ENS 2023/07/29 19:32:48:0 68.62 -148.58 1.3 4.0 Alaska
Stations used:
AK.C27K AK.D25K AK.E19K AK.E24K AK.E25K AK.E27K AK.F20K
AK.F21K AK.FYU AK.G24K AK.H21K AK.H22K AK.H24K AK.I21K
AK.I23K AK.MLY AK.POKR AK.PPD AK.TOLK IM.IL31 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 0.07 n 3
Best Fitting Double Couple
Mo = 1.55e+22 dyne-cm
Mw = 4.06
Z = 13 km
Plane Strike Dip Rake
NP1 215 80 -30
NP2 311 61 -168
Principal Axes:
Axis Value Plunge Azimuth
T 1.55e+22 13 266
N 0.00e+00 59 18
P -1.55e+22 28 169
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.15e+22
Mxy 3.27e+21
Mxz 6.08e+21
Myy 1.42e+22
Myz -4.63e+21
Mzz -2.65e+21
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------------------------####
-----------------------#######
############-----------###########
################-------#############
####################--################
######################--################
####################------##############
####################---------#############
# ###############------------###########
# T #############---------------##########
# ############-----------------#########
##############-------------------#######
#############---------------------######
###########-----------------------####
#########------------------------###
#######----------- ------------#
####------------ P -----------
##------------- ----------
----------------------
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Global CMT Convention Moment Tensor:
R T P
-2.65e+21 6.08e+21 4.63e+21
6.08e+21 -1.15e+22 -3.27e+21
4.63e+21 -3.27e+21 1.42e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20230729193248/index.html
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STK = 215
DIP = 80
RAKE = -30
MW = 4.06
HS = 13.0
The NDK file is 20230729193248.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 0.07 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 40 75 -20 3.67 0.3804
WVFGRD96 2.0 40 75 -20 3.78 0.4810
WVFGRD96 3.0 40 90 -15 3.82 0.5182
WVFGRD96 4.0 220 80 15 3.86 0.5402
WVFGRD96 5.0 220 80 20 3.90 0.5551
WVFGRD96 6.0 220 80 20 3.92 0.5661
WVFGRD96 7.0 220 80 20 3.94 0.5764
WVFGRD96 8.0 220 85 25 3.98 0.5864
WVFGRD96 9.0 40 90 30 4.00 0.5838
WVFGRD96 10.0 215 85 -30 4.02 0.5968
WVFGRD96 11.0 40 90 30 4.03 0.5986
WVFGRD96 12.0 215 80 -30 4.05 0.6120
WVFGRD96 13.0 215 80 -30 4.06 0.6131
WVFGRD96 14.0 215 80 -30 4.07 0.6121
WVFGRD96 15.0 215 80 -30 4.07 0.6079
WVFGRD96 16.0 215 80 -30 4.08 0.6007
WVFGRD96 17.0 215 80 -30 4.09 0.5930
WVFGRD96 18.0 215 80 -30 4.09 0.5845
WVFGRD96 19.0 215 80 -30 4.10 0.5744
WVFGRD96 20.0 215 80 -25 4.11 0.5648
WVFGRD96 21.0 215 80 -30 4.12 0.5552
WVFGRD96 22.0 220 80 -25 4.12 0.5448
WVFGRD96 23.0 220 80 -25 4.13 0.5357
WVFGRD96 24.0 220 85 -30 4.13 0.5268
WVFGRD96 25.0 315 65 10 4.14 0.5250
WVFGRD96 26.0 315 65 10 4.15 0.5287
WVFGRD96 27.0 310 65 10 4.16 0.5312
WVFGRD96 28.0 310 65 10 4.17 0.5318
WVFGRD96 29.0 310 65 5 4.18 0.5317
WVFGRD96 30.0 310 65 5 4.19 0.5310
WVFGRD96 31.0 310 65 5 4.20 0.5279
WVFGRD96 32.0 310 70 5 4.20 0.5246
WVFGRD96 33.0 310 70 5 4.21 0.5199
WVFGRD96 34.0 310 70 5 4.22 0.5149
WVFGRD96 35.0 310 70 5 4.23 0.5090
WVFGRD96 36.0 310 75 5 4.24 0.5042
WVFGRD96 37.0 310 75 5 4.26 0.4984
WVFGRD96 38.0 310 80 10 4.27 0.4928
WVFGRD96 39.0 310 80 5 4.29 0.4883
WVFGRD96 40.0 310 75 15 4.32 0.4852
WVFGRD96 41.0 310 80 15 4.33 0.4870
WVFGRD96 42.0 310 80 15 4.34 0.4890
WVFGRD96 43.0 310 80 15 4.35 0.4895
WVFGRD96 44.0 310 80 15 4.36 0.4886
WVFGRD96 45.0 310 80 10 4.37 0.4866
WVFGRD96 46.0 310 80 10 4.37 0.4842
WVFGRD96 47.0 310 80 10 4.38 0.4806
WVFGRD96 48.0 310 85 10 4.38 0.4766
WVFGRD96 49.0 310 85 10 4.39 0.4718
WVFGRD96 50.0 310 85 10 4.39 0.4663
WVFGRD96 51.0 310 85 10 4.40 0.4601
WVFGRD96 52.0 130 90 -10 4.40 0.4516
WVFGRD96 53.0 310 80 15 4.41 0.4474
WVFGRD96 54.0 310 80 15 4.41 0.4418
WVFGRD96 55.0 310 80 15 4.42 0.4367
WVFGRD96 56.0 310 85 10 4.41 0.4316
WVFGRD96 57.0 310 85 10 4.42 0.4265
WVFGRD96 58.0 310 85 10 4.42 0.4215
WVFGRD96 59.0 130 90 -10 4.42 0.4146
The best solution is
WVFGRD96 13.0 215 80 -30 4.06 0.6131
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 0.07 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