Location
Location ANSS
The ANSS event ID is aka2026vngzyw and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/aka2026vngzyw/executive.
2026/10/30 19:11:23 61.833 -147.003 19.1 4.4 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2026/10/30 19:11:23.0 61.83 -147.00 19.1 4.4 Alaska
Stations used:
AK.BAE AK.BMR AK.BPAW AK.CCB AK.CUT AK.DHY AK.DIV AK.EYAK
AK.FID AK.GHO AK.GLB AK.GLI AK.HDA AK.HIN AK.J25K AK.KNK
AK.L22K AK.L26K AK.M26K AK.MCK AK.PAX AK.POKR AK.RC01
AK.RND AK.SAW AK.SCM AK.SKN AK.WRH AT.PMR CN.BVCY 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 = 2.34e+22 dyne-cm
Mw = 4.18
Z = 44 km
Plane Strike Dip Rake
NP1 255 70 -35
NP2 358 57 -156
Principal Axes:
Axis Value Plunge Azimuth
T 2.34e+22 8 309
N 0.00e+00 50 49
P -2.34e+22 39 213
Moment Tensor: (dyne-cm)
Component Value
Mxx -9.58e+20
Mxy -1.78e+22
Mxz 1.16e+22
Myy 9.60e+21
Myz 3.68e+21
Mzz -8.64e+21
########------
##############--------
##################----------
##################----------
# T ###################-----------
## ####################-----------
##########################------------
###########################-########----
##################----------############
#############----------------#############
#########--------------------#############
######-----------------------#############
####-------------------------#############
#---------------------------############
----------------------------############
----------- ------------############
---------- P ------------###########
--------- -----------###########
---------------------#########
-------------------#########
--------------########
---------#####
Global CMT Convention Moment Tensor:
R T P
-8.64e+21 1.16e+22 -3.68e+21
1.16e+22 -9.58e+20 1.78e+22
-3.68e+21 1.78e+22 9.60e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20261030191123/index.html
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Preferred Solution
The preferred solution from an analysis of the surface-wave spectral amplitude radiation pattern, waveform inversion or first motion observations is
STK = 255
DIP = 70
RAKE = -35
MW = 4.18
HS = 44.0
The NDK file is 20261030191123.ndk
The waveform inversion is preferred.
Moment Tensor Comparison
The following compares this source inversion to those provided by others. The purpose is to look for major differences and also to note slight differences that might be inherent to the processing procedure. For completeness the USGS/SLU solution is repeated from above.
| SLU |
USGSMWR |
USGS/SLU Moment Tensor Solution
ENS 2026/10/30 19:11:23.0 61.83 -147.00 19.1 4.4 Alaska
Stations used:
AK.BAE AK.BMR AK.BPAW AK.CCB AK.CUT AK.DHY AK.DIV AK.EYAK
AK.FID AK.GHO AK.GLB AK.GLI AK.HDA AK.HIN AK.J25K AK.KNK
AK.L22K AK.L26K AK.M26K AK.MCK AK.PAX AK.POKR AK.RC01
AK.RND AK.SAW AK.SCM AK.SKN AK.WRH AT.PMR CN.BVCY 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 = 2.34e+22 dyne-cm
Mw = 4.18
Z = 44 km
Plane Strike Dip Rake
NP1 255 70 -35
NP2 358 57 -156
Principal Axes:
Axis Value Plunge Azimuth
T 2.34e+22 8 309
N 0.00e+00 50 49
P -2.34e+22 39 213
Moment Tensor: (dyne-cm)
Component Value
Mxx -9.58e+20
Mxy -1.78e+22
Mxz 1.16e+22
Myy 9.60e+21
Myz 3.68e+21
Mzz -8.64e+21
########------
##############--------
##################----------
##################----------
# T ###################-----------
## ####################-----------
##########################------------
###########################-########----
##################----------############
#############----------------#############
#########--------------------#############
######-----------------------#############
####-------------------------#############
#---------------------------############
----------------------------############
----------- ------------############
---------- P ------------###########
--------- -----------###########
---------------------#########
-------------------#########
--------------########
---------#####
Global CMT Convention Moment Tensor:
R T P
-8.64e+21 1.16e+22 -3.68e+21
1.16e+22 -9.58e+20 1.78e+22
-3.68e+21 1.78e+22 9.60e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20261030191123/index.html
|
Regional Moment Tensor (Mwr)
Moment 2.472e+15 N-m
Magnitude 4.20 Mwr
Depth 45.0 km
Percent DC 69%
Half Duration -
Catalog
US Data Source US
Contributor US
Nodal Planes
Plane Strike Dip Rake
NP1 1 54 -151
NP2 253 67 -39
Principal Axes
Axis Value Plunge Azimuth
T 2.649e+15 8 310
N -0.405e+15 45 48
P -2.244e+15 44 212
|
Magnitudes
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.
ML Magnitude
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.
Map showing station locations used for computing the ML's. No distinction is made whether the vertical (Z) or horizontal (H) components were used.
Context
The left panel of the next figure presents the focal mechanism for this earthquake (red) in the context of other nearby events (blue) in the SLU Moment Tensor Catalog. The right panel shows the inferred direction of maximum compressive stress and the type of faulting (green is strike-slip, red is normal, blue is thrust; oblique is shown by a combination of colors). Thus context plot is useful for assessing the appropriateness of the moment tensor of this event.
Waveform Inversion using wvfgrd96
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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Location of broadband stations used for waveform inversion
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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 0.08 n 3
br c 0.12 0.25 n 4 p 2
The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 250 80 -15 3.40 0.3095
WVFGRD96 2.0 70 60 -30 3.57 0.4536
WVFGRD96 3.0 75 65 -20 3.60 0.4898
WVFGRD96 4.0 75 65 -20 3.63 0.5130
WVFGRD96 5.0 85 60 20 3.66 0.5317
WVFGRD96 6.0 85 65 25 3.68 0.5531
WVFGRD96 7.0 85 65 25 3.70 0.5667
WVFGRD96 8.0 90 60 30 3.74 0.5742
WVFGRD96 9.0 90 65 25 3.74 0.5808
WVFGRD96 10.0 90 65 25 3.75 0.5854
WVFGRD96 11.0 85 70 20 3.75 0.5887
WVFGRD96 12.0 85 70 20 3.77 0.5921
WVFGRD96 13.0 85 70 20 3.78 0.5959
WVFGRD96 14.0 85 70 20 3.79 0.5994
WVFGRD96 15.0 255 75 -25 3.81 0.6078
WVFGRD96 16.0 255 75 -25 3.82 0.6153
WVFGRD96 17.0 255 75 -25 3.83 0.6221
WVFGRD96 18.0 260 75 -25 3.84 0.6287
WVFGRD96 19.0 260 75 -20 3.85 0.6356
WVFGRD96 20.0 260 75 -20 3.86 0.6423
WVFGRD96 21.0 260 75 -25 3.88 0.6489
WVFGRD96 22.0 260 75 -25 3.89 0.6553
WVFGRD96 23.0 260 75 -25 3.90 0.6615
WVFGRD96 24.0 260 75 -25 3.92 0.6676
WVFGRD96 25.0 260 75 -25 3.93 0.6727
WVFGRD96 26.0 260 75 -25 3.94 0.6796
WVFGRD96 27.0 260 75 -25 3.95 0.6860
WVFGRD96 28.0 260 75 -25 3.96 0.6922
WVFGRD96 29.0 260 75 -25 3.97 0.6968
WVFGRD96 30.0 255 70 -30 3.98 0.7015
WVFGRD96 31.0 255 70 -30 3.99 0.7049
WVFGRD96 32.0 255 70 -30 4.00 0.7069
WVFGRD96 33.0 255 70 -30 4.01 0.7079
WVFGRD96 34.0 260 75 -25 4.02 0.7082
WVFGRD96 35.0 260 75 -25 4.03 0.7085
WVFGRD96 36.0 260 75 -25 4.04 0.7083
WVFGRD96 37.0 260 75 -25 4.06 0.7085
WVFGRD96 38.0 260 75 -25 4.07 0.7095
WVFGRD96 39.0 260 75 -25 4.09 0.7108
WVFGRD96 40.0 255 70 -35 4.14 0.7302
WVFGRD96 41.0 255 70 -35 4.15 0.7336
WVFGRD96 42.0 255 70 -35 4.16 0.7355
WVFGRD96 43.0 255 70 -35 4.17 0.7362
WVFGRD96 44.0 255 70 -35 4.18 0.7363
WVFGRD96 45.0 255 70 -35 4.19 0.7354
WVFGRD96 46.0 255 70 -35 4.20 0.7339
WVFGRD96 47.0 255 70 -35 4.21 0.7322
WVFGRD96 48.0 255 70 -35 4.22 0.7309
WVFGRD96 49.0 255 70 -35 4.22 0.7290
WVFGRD96 50.0 255 70 -35 4.23 0.7263
WVFGRD96 51.0 255 65 -35 4.24 0.7242
WVFGRD96 52.0 255 65 -35 4.24 0.7220
WVFGRD96 53.0 255 65 -35 4.25 0.7195
WVFGRD96 54.0 255 65 -35 4.26 0.7168
WVFGRD96 55.0 255 65 -35 4.26 0.7134
WVFGRD96 56.0 255 70 -30 4.26 0.7107
WVFGRD96 57.0 255 70 -30 4.26 0.7079
WVFGRD96 58.0 255 70 -30 4.27 0.7048
WVFGRD96 59.0 255 70 -30 4.27 0.7018
The best solution is
WVFGRD96 44.0 255 70 -35 4.18 0.7363
The mechanism corresponding to the best fit is
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Figure 1. Waveform inversion focal mechanism
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The best fit as a function of depth is given in the following figure:
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Figure 2. Depth sensitivity for waveform mechanism
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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 0.08 n 3
br c 0.12 0.25 n 4 p 2
|
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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.
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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:
- The origin time and epicentral distance are incorrect
- The velocity model used for the inversion is incorrect
- The velocity model used to define the P-arrival time is not the
same as the velocity model used for the waveform inversion
(assuming that the initial trace alignment is based on the
P arrival time)
Assuming only a mislocation, the time shifts are fit to a functional form:
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.
Velocity Model
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
Last Changed Sat Oct 31 16:47:28 CDT 2026
