The ANSS event ID is ak0238x1rasm and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0238x1rasm/executive.
2023/07/13 12:59:58 62.987 -150.466 101.5 3.6 Alaska
USGS/SLU Moment Tensor Solution
ENS 2023/07/13 12:59:58:0 62.99 -150.47 101.5 3.6 Alaska
Stations used:
AK.CAST AK.CCB AK.DHY AK.GHO AK.HDA AK.J19K AK.J20K AK.K20K
AK.K24K AK.KNK AK.L20K AK.MCK AK.MLY AK.PAX AK.PPLA AK.RND
AK.SAW AK.SCM AK.SKN AK.WAT6 AK.WRH AT.PMR AV.SPCP AV.STLK
IM.IL31 IU.COLA
Filtering commands used:
cut o DIST/3.5 -40 o DIST/3.5 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 5.89e+21 dyne-cm
Mw = 3.78
Z = 116 km
Plane Strike Dip Rake
NP1 2 84 130
NP2 100 40 10
Principal Axes:
Axis Value Plunge Azimuth
T 5.89e+21 38 307
N 0.00e+00 39 177
P -5.89e+21 28 62
Moment Tensor: (dyne-cm)
Component Value
Mxx 2.98e+20
Mxy -3.67e+21
Mxz 5.97e+20
Myy -1.31e+21
Myz -4.41e+21
Mzz 1.01e+21
########------
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###################---------------
###### ###########----------------
####### T ###########---------- ----
######## ###########---------- P -----
######################---------- -----
-######################-------------------
-######################-------------------
--#####################-------------------
---####################-------------------
----#################-------------------
------###############-----------------##
-------#############---------------###
---------##########------------#####
--------------####-------#########
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Global CMT Convention Moment Tensor:
R T P
1.01e+21 5.97e+20 4.41e+21
5.97e+20 2.98e+20 3.67e+21
4.41e+21 3.67e+21 -1.31e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20230713125958/index.html
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STK = 100
DIP = 40
RAKE = 10
MW = 3.78
HS = 116.0
The NDK file is 20230713125958.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/3.5 -40 o DIST/3.5 +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 2.0 175 50 -70 2.86 0.1871
WVFGRD96 4.0 25 45 0 2.89 0.1946
WVFGRD96 6.0 210 55 25 2.95 0.2301
WVFGRD96 8.0 210 55 25 3.04 0.2522
WVFGRD96 10.0 30 60 20 3.08 0.2639
WVFGRD96 12.0 25 60 15 3.12 0.2690
WVFGRD96 14.0 25 60 15 3.15 0.2690
WVFGRD96 16.0 30 55 15 3.18 0.2631
WVFGRD96 18.0 130 75 40 3.19 0.2548
WVFGRD96 20.0 130 80 40 3.21 0.2541
WVFGRD96 22.0 300 75 30 3.26 0.2628
WVFGRD96 24.0 300 75 25 3.28 0.2719
WVFGRD96 26.0 295 80 25 3.31 0.2793
WVFGRD96 28.0 275 60 5 3.33 0.2881
WVFGRD96 30.0 275 65 0 3.35 0.2937
WVFGRD96 32.0 275 70 5 3.36 0.2954
WVFGRD96 34.0 275 70 5 3.37 0.2967
WVFGRD96 36.0 275 70 5 3.39 0.2922
WVFGRD96 38.0 100 75 20 3.42 0.2858
WVFGRD96 40.0 100 70 25 3.48 0.2896
WVFGRD96 42.0 100 70 25 3.51 0.2950
WVFGRD96 44.0 100 70 25 3.53 0.2962
WVFGRD96 46.0 100 70 25 3.55 0.2975
WVFGRD96 48.0 100 75 25 3.57 0.3023
WVFGRD96 50.0 100 80 30 3.59 0.3107
WVFGRD96 52.0 100 80 30 3.61 0.3191
WVFGRD96 54.0 105 75 40 3.64 0.3287
WVFGRD96 56.0 105 75 40 3.65 0.3376
WVFGRD96 58.0 105 55 20 3.63 0.3500
WVFGRD96 60.0 105 55 20 3.64 0.3640
WVFGRD96 62.0 105 55 20 3.65 0.3775
WVFGRD96 64.0 105 50 20 3.65 0.3933
WVFGRD96 66.0 105 55 20 3.66 0.4074
WVFGRD96 68.0 105 55 20 3.67 0.4204
WVFGRD96 70.0 105 50 20 3.68 0.4325
WVFGRD96 72.0 100 45 15 3.68 0.4455
WVFGRD96 74.0 100 45 15 3.69 0.4570
WVFGRD96 76.0 95 45 10 3.69 0.4691
WVFGRD96 78.0 100 40 15 3.70 0.4800
WVFGRD96 80.0 100 40 15 3.71 0.4893
WVFGRD96 82.0 100 45 15 3.71 0.4992
WVFGRD96 84.0 100 45 15 3.71 0.5101
WVFGRD96 86.0 100 45 15 3.72 0.5198
WVFGRD96 88.0 100 40 15 3.73 0.5294
WVFGRD96 90.0 100 40 15 3.73 0.5392
WVFGRD96 92.0 100 40 15 3.73 0.5489
WVFGRD96 94.0 100 40 15 3.74 0.5583
WVFGRD96 96.0 100 40 15 3.74 0.5659
WVFGRD96 98.0 100 40 15 3.75 0.5726
WVFGRD96 100.0 100 40 15 3.75 0.5791
WVFGRD96 102.0 100 40 15 3.75 0.5843
WVFGRD96 104.0 100 40 10 3.76 0.5888
WVFGRD96 106.0 100 40 10 3.76 0.5933
WVFGRD96 108.0 100 40 10 3.77 0.5969
WVFGRD96 110.0 100 40 10 3.77 0.5989
WVFGRD96 112.0 100 40 10 3.77 0.6001
WVFGRD96 114.0 100 40 10 3.77 0.6011
WVFGRD96 116.0 100 40 10 3.78 0.6021
WVFGRD96 118.0 100 40 10 3.78 0.6013
WVFGRD96 120.0 100 40 10 3.78 0.5989
WVFGRD96 122.0 100 40 10 3.78 0.5966
WVFGRD96 124.0 100 40 10 3.78 0.5938
WVFGRD96 126.0 100 40 10 3.78 0.5916
WVFGRD96 128.0 95 45 10 3.78 0.5893
WVFGRD96 130.0 100 40 15 3.78 0.5868
WVFGRD96 132.0 100 40 15 3.78 0.5833
WVFGRD96 134.0 100 40 15 3.78 0.5796
WVFGRD96 136.0 100 40 15 3.78 0.5761
WVFGRD96 138.0 100 45 15 3.78 0.5734
WVFGRD96 140.0 100 45 15 3.78 0.5708
WVFGRD96 142.0 95 50 5 3.79 0.5671
WVFGRD96 144.0 100 45 15 3.79 0.5631
WVFGRD96 146.0 95 55 5 3.79 0.5612
WVFGRD96 148.0 95 55 5 3.80 0.5592
WVFGRD96 150.0 95 55 5 3.80 0.5569
WVFGRD96 152.0 95 55 5 3.80 0.5532
WVFGRD96 154.0 95 55 5 3.80 0.5517
WVFGRD96 156.0 95 55 5 3.80 0.5498
WVFGRD96 158.0 95 55 5 3.81 0.5467
The best solution is
WVFGRD96 116.0 100 40 10 3.78 0.6021
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/3.5 -40 o DIST/3.5 +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