The ANSS event ID is ak0234f01qd2 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0234f01qd2/executive.
2023/04/06 14:20:29 63.046 -150.454 100.8 3.6 Alaska
USGS/SLU Moment Tensor Solution
ENS 2023/04/06 14:20:29:0 63.05 -150.45 100.8 3.6 Alaska
Stations used:
AK.BPAW AK.CAST AK.CCB AK.CUT AK.DHY AK.GHO AK.HDA AK.I21K
AK.KNK AK.L20K AK.L22K AK.MCK AK.MLY AK.NEA2 AK.PAX AK.POKR
AK.RC01 AK.RND AK.SAW AK.SKN AK.WAT6 AK.WRH AT.PMR 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 0.08 n 3
br c 0.12 0.25 n 4 p 2
Best Fitting Double Couple
Mo = 7.00e+21 dyne-cm
Mw = 3.83
Z = 114 km
Plane Strike Dip Rake
NP1 290 59 106
NP2 80 35 65
Principal Axes:
Axis Value Plunge Azimuth
T 7.00e+21 71 238
N 0.00e+00 14 101
P -7.00e+21 12 8
Moment Tensor: (dyne-cm)
Component Value
Mxx -6.36e+21
Mxy -5.75e+20
Mxz -2.56e+21
Myy 4.00e+20
Myz -2.01e+21
Mzz 5.96e+21
-------- ---
------------ P -------
--------------- ----------
------------------------------
----------------------------------
------------------------------------
--#################-------------------
#########################---------------
#############################-----------
#################################--------#
###################################------#
############### ###################---##
############### T ####################-###
############## ####################-##
--#################################----#
--##############################------
----#########################-------
------##################----------
------------------------------
----------------------------
----------------------
--------------
Global CMT Convention Moment Tensor:
R T P
5.96e+21 -2.56e+21 2.01e+21
-2.56e+21 -6.36e+21 5.75e+20
2.01e+21 5.75e+20 4.00e+20
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20230406142029/index.html
|
STK = 80
DIP = 35
RAKE = 65
MW = 3.83
HS = 114.0
The NDK file is 20230406142029.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.
 |
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.
|
|
|
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 3 br c 0.12 0.25 n 4 p 2The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 105 50 -90 3.12 0.3185
WVFGRD96 4.0 110 55 -80 3.23 0.2737
WVFGRD96 6.0 310 20 -60 3.25 0.3339
WVFGRD96 8.0 305 20 -65 3.30 0.3628
WVFGRD96 10.0 310 25 -60 3.29 0.3748
WVFGRD96 12.0 320 30 -50 3.28 0.3814
WVFGRD96 14.0 325 35 -40 3.29 0.3862
WVFGRD96 16.0 330 40 -35 3.31 0.3888
WVFGRD96 18.0 330 40 -35 3.32 0.3887
WVFGRD96 20.0 125 65 -60 3.34 0.3860
WVFGRD96 22.0 215 40 30 3.38 0.3822
WVFGRD96 24.0 215 40 30 3.40 0.3835
WVFGRD96 26.0 215 40 30 3.42 0.3816
WVFGRD96 28.0 210 45 25 3.45 0.3760
WVFGRD96 30.0 210 45 25 3.47 0.3667
WVFGRD96 32.0 85 55 65 3.44 0.3569
WVFGRD96 34.0 80 55 60 3.46 0.3502
WVFGRD96 36.0 145 40 -45 3.47 0.3472
WVFGRD96 38.0 145 40 -45 3.49 0.3433
WVFGRD96 40.0 130 35 -60 3.61 0.3454
WVFGRD96 42.0 125 35 -65 3.63 0.3465
WVFGRD96 44.0 125 35 -65 3.65 0.3416
WVFGRD96 46.0 80 50 60 3.67 0.3350
WVFGRD96 48.0 80 45 60 3.68 0.3346
WVFGRD96 50.0 85 55 70 3.68 0.3335
WVFGRD96 52.0 85 55 70 3.70 0.3463
WVFGRD96 54.0 85 50 65 3.71 0.3591
WVFGRD96 56.0 85 50 65 3.72 0.3734
WVFGRD96 58.0 80 50 65 3.73 0.3864
WVFGRD96 60.0 80 50 60 3.74 0.4012
WVFGRD96 62.0 80 50 60 3.75 0.4145
WVFGRD96 64.0 80 50 60 3.75 0.4255
WVFGRD96 66.0 90 40 75 3.77 0.4437
WVFGRD96 68.0 90 40 75 3.77 0.4626
WVFGRD96 70.0 90 40 75 3.78 0.4789
WVFGRD96 72.0 90 40 75 3.78 0.4938
WVFGRD96 74.0 90 40 75 3.78 0.5066
WVFGRD96 76.0 90 40 75 3.78 0.5180
WVFGRD96 78.0 85 40 70 3.79 0.5281
WVFGRD96 80.0 85 40 70 3.79 0.5378
WVFGRD96 82.0 85 40 70 3.79 0.5457
WVFGRD96 84.0 80 40 65 3.79 0.5519
WVFGRD96 86.0 85 40 70 3.79 0.5588
WVFGRD96 88.0 80 40 65 3.79 0.5646
WVFGRD96 90.0 85 35 70 3.80 0.5692
WVFGRD96 92.0 85 35 70 3.80 0.5735
WVFGRD96 94.0 85 35 70 3.80 0.5777
WVFGRD96 96.0 80 35 65 3.81 0.5808
WVFGRD96 98.0 80 35 65 3.81 0.5842
WVFGRD96 100.0 80 35 65 3.81 0.5866
WVFGRD96 102.0 80 35 65 3.81 0.5892
WVFGRD96 104.0 80 35 65 3.81 0.5912
WVFGRD96 106.0 80 35 65 3.82 0.5922
WVFGRD96 108.0 80 35 65 3.82 0.5942
WVFGRD96 110.0 80 35 65 3.82 0.5940
WVFGRD96 112.0 80 35 65 3.82 0.5953
WVFGRD96 114.0 80 35 65 3.83 0.5955
WVFGRD96 116.0 80 35 65 3.83 0.5951
WVFGRD96 118.0 80 35 65 3.83 0.5950
WVFGRD96 120.0 80 35 65 3.83 0.5935
WVFGRD96 122.0 80 35 65 3.84 0.5932
WVFGRD96 124.0 80 35 65 3.84 0.5920
WVFGRD96 126.0 80 35 65 3.84 0.5910
WVFGRD96 128.0 85 35 70 3.84 0.5894
WVFGRD96 130.0 85 35 70 3.85 0.5876
WVFGRD96 132.0 85 35 70 3.85 0.5859
WVFGRD96 134.0 85 35 70 3.85 0.5837
WVFGRD96 136.0 85 35 70 3.85 0.5820
WVFGRD96 138.0 85 35 70 3.86 0.5792
WVFGRD96 140.0 85 35 70 3.86 0.5778
WVFGRD96 142.0 85 35 70 3.86 0.5742
WVFGRD96 144.0 85 35 70 3.86 0.5728
WVFGRD96 146.0 85 35 70 3.87 0.5697
WVFGRD96 148.0 85 35 70 3.87 0.5673
The best solution is
WVFGRD96 114.0 80 35 65 3.83 0.5955
The mechanism corresponding to the best fit is
|
|
|
The best fit as a function of depth is given in the following figure:
|
|
|
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 br c 0.12 0.25 n 4 p 2
|
| 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. |
|
| 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