The ANSS event ID is ak0234a0aako and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0234a0aako/executive.
2023/04/03 12:27:14 61.359 -150.182 40.9 3.9 Alaska
USGS/SLU Moment Tensor Solution
ENS 2023/04/03 12:27:14:0 61.36 -150.18 40.9 3.9 Alaska
Stations used:
AK.BPAW AK.DIV AK.FIRE AK.GHO AK.KNK AK.PWL AK.RC01 AK.SAW
AK.SCM AK.SKN AK.SLK AK.SWD AT.PMR AV.STLK
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 = 1.40e+22 dyne-cm
Mw = 4.03
Z = 57 km
Plane Strike Dip Rake
NP1 190 81 -93
NP2 30 10 -70
Principal Axes:
Axis Value Plunge Azimuth
T 1.40e+22 36 283
N 0.00e+00 3 190
P -1.40e+22 54 96
Moment Tensor: (dyne-cm)
Component Value
Mxx 4.04e+20
Mxy -1.53e+21
Mxz 2.09e+21
Myy 4.08e+21
Myz -1.30e+22
Mzz -4.49e+21
##########----
#############---------
###############------------#
################--------------
#################----------------#
##################-----------------#
##################-------------------#
###################-------------------##
##### ##########---------------------#
###### T ##########---------------------##
###### ##########---------- --------##
##################----------- P --------##
##################----------- --------##
#################---------------------##
#################---------------------##
###############---------------------##
##############--------------------##
#############-------------------##
###########-----------------##
##########---------------###
#######------------###
-##-------####
Global CMT Convention Moment Tensor:
R T P
-4.49e+21 2.09e+21 1.30e+22
2.09e+21 4.04e+20 1.53e+21
1.30e+22 1.53e+21 4.08e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20230403122714/index.html
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STK = 30
DIP = 10
RAKE = -70
MW = 4.03
HS = 57.0
The NDK file is 20230403122714.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 45 -85 3.19 0.1637
WVFGRD96 2.0 40 45 -85 3.34 0.2337
WVFGRD96 3.0 40 45 -85 3.40 0.2470
WVFGRD96 4.0 55 50 -65 3.41 0.2424
WVFGRD96 5.0 260 65 -15 3.35 0.2474
WVFGRD96 6.0 265 60 0 3.38 0.2589
WVFGRD96 7.0 260 55 -10 3.41 0.2699
WVFGRD96 8.0 260 50 -10 3.46 0.2797
WVFGRD96 9.0 275 50 35 3.50 0.2878
WVFGRD96 10.0 90 45 35 3.51 0.2986
WVFGRD96 11.0 90 45 35 3.52 0.3133
WVFGRD96 12.0 90 45 35 3.54 0.3267
WVFGRD96 13.0 90 45 35 3.55 0.3391
WVFGRD96 14.0 90 45 35 3.56 0.3504
WVFGRD96 15.0 85 45 30 3.57 0.3615
WVFGRD96 16.0 85 45 30 3.58 0.3724
WVFGRD96 17.0 85 45 30 3.59 0.3824
WVFGRD96 18.0 85 45 30 3.60 0.3915
WVFGRD96 19.0 85 45 30 3.62 0.4011
WVFGRD96 20.0 85 40 25 3.62 0.4090
WVFGRD96 21.0 70 35 -15 3.63 0.4199
WVFGRD96 22.0 70 35 -15 3.64 0.4307
WVFGRD96 23.0 70 35 -20 3.65 0.4424
WVFGRD96 24.0 65 35 -25 3.67 0.4522
WVFGRD96 25.0 65 30 -25 3.68 0.4633
WVFGRD96 26.0 65 30 -30 3.69 0.4732
WVFGRD96 27.0 65 30 -30 3.70 0.4823
WVFGRD96 28.0 70 30 -20 3.70 0.4925
WVFGRD96 29.0 65 25 -30 3.72 0.5051
WVFGRD96 30.0 65 25 -30 3.73 0.5179
WVFGRD96 31.0 70 25 -25 3.74 0.5324
WVFGRD96 32.0 70 25 -25 3.75 0.5484
WVFGRD96 33.0 65 20 -30 3.76 0.5648
WVFGRD96 34.0 65 20 -35 3.77 0.5816
WVFGRD96 35.0 60 20 -40 3.78 0.5970
WVFGRD96 36.0 60 20 -40 3.78 0.6109
WVFGRD96 37.0 60 20 -40 3.79 0.6232
WVFGRD96 38.0 60 20 -40 3.80 0.6346
WVFGRD96 39.0 60 20 -45 3.80 0.6433
WVFGRD96 40.0 55 15 -45 3.94 0.6458
WVFGRD96 41.0 50 15 -50 3.94 0.6512
WVFGRD96 42.0 50 15 -50 3.95 0.6546
WVFGRD96 43.0 50 15 -55 3.96 0.6573
WVFGRD96 44.0 50 15 -55 3.96 0.6616
WVFGRD96 45.0 50 15 -55 3.97 0.6644
WVFGRD96 46.0 50 15 -55 3.97 0.6663
WVFGRD96 47.0 50 15 -55 3.98 0.6690
WVFGRD96 48.0 30 10 -75 3.99 0.6696
WVFGRD96 49.0 30 10 -75 3.99 0.6714
WVFGRD96 50.0 30 10 -75 4.00 0.6739
WVFGRD96 51.0 30 10 -70 4.00 0.6751
WVFGRD96 52.0 30 10 -70 4.00 0.6755
WVFGRD96 53.0 30 10 -70 4.01 0.6775
WVFGRD96 54.0 30 10 -70 4.01 0.6785
WVFGRD96 55.0 30 10 -70 4.02 0.6776
WVFGRD96 56.0 30 10 -70 4.02 0.6785
WVFGRD96 57.0 30 10 -70 4.03 0.6788
WVFGRD96 58.0 30 10 -70 4.03 0.6776
WVFGRD96 59.0 30 10 -70 4.03 0.6761
The best solution is
WVFGRD96 57.0 30 10 -70 4.03 0.6788
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