The ANSS event ID is ak0238ghjqxs and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0238ghjqxs/executive.
2023/07/03 11:19:24 63.120 -150.919 119.1 3.5 Alaska
USGS/SLU Moment Tensor Solution
ENS 2023/07/03 11:19:24:0 63.12 -150.92 119.1 3.5 Alaska
Stations used:
AK.BPAW AK.CAST AK.CCB AK.CUT AK.DHY AK.FIRE AK.GHO AK.H21K
AK.H22K AK.I21K AK.J19K AK.J20K AK.K20K AK.KNK AK.L19K
AK.L20K AK.L22K AK.M19K AK.MCK AK.PAX AK.PPLA AK.RND AK.SAW
AK.SCM AK.SKN AK.WAT6 AK.WRH AT.PMR 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 = 6.10e+21 dyne-cm
Mw = 3.79
Z = 124 km
Plane Strike Dip Rake
NP1 217 50 94
NP2 30 40 85
Principal Axes:
Axis Value Plunge Azimuth
T 6.10e+21 84 156
N 0.00e+00 3 34
P -6.10e+21 5 304
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.79e+21
Mxy 2.76e+21
Mxz -8.80e+20
Myy -4.19e+21
Myz 7.10e+20
Mzz 5.98e+21
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-------------------#######--
----------------############--
-------------###############----
P -----------##################----
- ---------####################-----
------------######################------
-----------#######################------
-----------########################-------
----------########## ############-------
---------########### T ###########--------
--------############ ##########---------
-------#########################--------
-------########################---------
-----#######################----------
----######################----------
---####################-----------
--#################-----------
-##############-------------
#######---------------
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Global CMT Convention Moment Tensor:
R T P
5.98e+21 -8.80e+20 -7.10e+20
-8.80e+20 -1.79e+21 -2.76e+21
-7.10e+20 -2.76e+21 -4.19e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20230703111924/index.html
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STK = 30
DIP = 40
RAKE = 85
MW = 3.79
HS = 124.0
The NDK file is 20230703111924.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 2.0 225 45 -90 2.98 0.1801
WVFGRD96 4.0 265 80 -60 3.01 0.1550
WVFGRD96 6.0 100 80 65 3.02 0.1911
WVFGRD96 8.0 100 75 70 3.12 0.2185
WVFGRD96 10.0 105 65 75 3.16 0.2463
WVFGRD96 12.0 105 60 75 3.19 0.2606
WVFGRD96 14.0 265 35 60 3.22 0.2648
WVFGRD96 16.0 265 35 60 3.24 0.2600
WVFGRD96 18.0 270 35 65 3.25 0.2500
WVFGRD96 20.0 260 40 55 3.28 0.2362
WVFGRD96 22.0 265 40 55 3.30 0.2204
WVFGRD96 24.0 260 45 50 3.31 0.2049
WVFGRD96 26.0 275 70 -35 3.37 0.1974
WVFGRD96 28.0 270 70 -45 3.37 0.1912
WVFGRD96 30.0 270 60 -45 3.38 0.1895
WVFGRD96 32.0 250 45 -60 3.38 0.2023
WVFGRD96 34.0 245 45 -65 3.41 0.2346
WVFGRD96 36.0 240 45 -70 3.43 0.2571
WVFGRD96 38.0 245 45 -70 3.45 0.2687
WVFGRD96 40.0 275 50 -80 3.56 0.2637
WVFGRD96 42.0 275 50 -80 3.60 0.2680
WVFGRD96 44.0 275 50 -80 3.62 0.2641
WVFGRD96 46.0 275 50 -80 3.63 0.2571
WVFGRD96 48.0 280 50 -80 3.64 0.2483
WVFGRD96 50.0 280 50 -80 3.64 0.2405
WVFGRD96 52.0 175 50 65 3.62 0.2349
WVFGRD96 54.0 185 45 65 3.64 0.2600
WVFGRD96 56.0 195 40 70 3.65 0.2816
WVFGRD96 58.0 205 40 80 3.67 0.3070
WVFGRD96 60.0 210 40 85 3.68 0.3312
WVFGRD96 62.0 40 50 95 3.68 0.3526
WVFGRD96 64.0 40 45 85 3.69 0.3705
WVFGRD96 66.0 40 45 85 3.70 0.3869
WVFGRD96 68.0 40 45 85 3.70 0.4012
WVFGRD96 70.0 40 45 85 3.70 0.4146
WVFGRD96 72.0 35 45 80 3.71 0.4268
WVFGRD96 74.0 35 45 85 3.71 0.4386
WVFGRD96 76.0 35 45 85 3.71 0.4501
WVFGRD96 78.0 35 45 85 3.71 0.4611
WVFGRD96 80.0 35 45 80 3.72 0.4710
WVFGRD96 82.0 35 45 80 3.72 0.4794
WVFGRD96 84.0 35 45 85 3.72 0.4871
WVFGRD96 86.0 35 45 85 3.72 0.4950
WVFGRD96 88.0 35 45 85 3.73 0.5018
WVFGRD96 90.0 30 45 80 3.73 0.5074
WVFGRD96 92.0 30 45 80 3.74 0.5129
WVFGRD96 94.0 35 40 85 3.74 0.5194
WVFGRD96 96.0 35 40 85 3.74 0.5260
WVFGRD96 98.0 35 40 85 3.75 0.5314
WVFGRD96 100.0 35 40 85 3.75 0.5362
WVFGRD96 102.0 35 40 85 3.75 0.5396
WVFGRD96 104.0 30 40 85 3.76 0.5434
WVFGRD96 106.0 35 40 85 3.76 0.5465
WVFGRD96 108.0 30 40 85 3.76 0.5494
WVFGRD96 110.0 30 40 85 3.77 0.5513
WVFGRD96 112.0 30 40 85 3.77 0.5546
WVFGRD96 114.0 30 40 85 3.77 0.5576
WVFGRD96 116.0 30 40 85 3.78 0.5576
WVFGRD96 118.0 30 40 85 3.78 0.5602
WVFGRD96 120.0 30 40 85 3.78 0.5608
WVFGRD96 122.0 30 40 85 3.79 0.5616
WVFGRD96 124.0 30 40 85 3.79 0.5631
WVFGRD96 126.0 30 40 85 3.79 0.5630
WVFGRD96 128.0 30 40 85 3.80 0.5629
WVFGRD96 130.0 30 40 85 3.80 0.5628
WVFGRD96 132.0 30 40 80 3.80 0.5623
WVFGRD96 134.0 30 40 80 3.81 0.5608
WVFGRD96 136.0 30 40 80 3.81 0.5609
WVFGRD96 138.0 30 40 80 3.81 0.5590
WVFGRD96 140.0 25 40 75 3.82 0.5587
WVFGRD96 142.0 25 40 75 3.82 0.5559
WVFGRD96 144.0 25 40 75 3.82 0.5571
WVFGRD96 146.0 25 40 75 3.82 0.5539
WVFGRD96 148.0 25 40 75 3.83 0.5543
The best solution is
WVFGRD96 124.0 30 40 85 3.79 0.5631
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