Location
Location ANSS
The ANSS event ID is aka2026mpakbo and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/aka2026mpakbo/executive.
2026/06/27 01:15:31 60.122 -151.321 58.8 5.1 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2026/06/27 01:15:31.0 60.12 -151.32 58.8 5.1 Alaska
Stations used:
AK.BAE AK.BRLK AK.CAPN AK.CNP AK.CUT AK.FIRE AK.GHO AK.HIN
AK.KNK AK.L22K AK.O18K AK.O19K AK.P23K AK.Q19K AK.RC01
AK.SAW AK.SKN AK.SLK AK.SWD AT.PMR AV.RED AV.SPCL
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 0.12 n 3
Best Fitting Double Couple
Mo = 9.33e+22 dyne-cm
Mw = 4.58
Z = 70 km
Plane Strike Dip Rake
NP1 145 74 -127
NP2 35 40 -25
Principal Axes:
Axis Value Plunge Azimuth
T 9.33e+22 20 262
N 0.00e+00 36 156
P -9.33e+22 47 16
Moment Tensor: (dyne-cm)
Component Value
Mxx -3.83e+22
Mxy 3.45e+20
Mxz -4.91e+22
Myy 7.72e+22
Myz -4.28e+22
Mzz -3.88e+22
--------------
----------------------
##-------------------------#
####------------------------##
######------------ ----------###
########----------- P ----------####
##########---------- -----------####
###########------------------------#####
############-----------------------#####
##############---------------------#######
################-------------------#######
### ###########-----------------########
### T ############----------------########
## #############--------------########
####################-----------#########
####################--------##########
#####################-----##########
######################-###########
##################----########
##############----------####
----------------------
--------------
Global CMT Convention Moment Tensor:
R T P
-3.88e+22 -4.91e+22 4.28e+22
-4.91e+22 -3.83e+22 -3.45e+20
4.28e+22 -3.45e+20 7.72e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20260627011531/index.html
|
Preferred Solution
The preferred solution from an analysis of the surface-wave spectral amplitude radiation pattern, waveform inversion or first motion observations is
STK = 35
DIP = 40
RAKE = -25
MW = 4.58
HS = 70.0
The NDK file is 20260627011531.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 |
USGSW |
USGS/SLU Moment Tensor Solution
ENS 2026/06/27 01:15:31.0 60.12 -151.32 58.8 5.1 Alaska
Stations used:
AK.BAE AK.BRLK AK.CAPN AK.CNP AK.CUT AK.FIRE AK.GHO AK.HIN
AK.KNK AK.L22K AK.O18K AK.O19K AK.P23K AK.Q19K AK.RC01
AK.SAW AK.SKN AK.SLK AK.SWD AT.PMR AV.RED AV.SPCL
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 0.12 n 3
Best Fitting Double Couple
Mo = 9.33e+22 dyne-cm
Mw = 4.58
Z = 70 km
Plane Strike Dip Rake
NP1 145 74 -127
NP2 35 40 -25
Principal Axes:
Axis Value Plunge Azimuth
T 9.33e+22 20 262
N 0.00e+00 36 156
P -9.33e+22 47 16
Moment Tensor: (dyne-cm)
Component Value
Mxx -3.83e+22
Mxy 3.45e+20
Mxz -4.91e+22
Myy 7.72e+22
Myz -4.28e+22
Mzz -3.88e+22
--------------
----------------------
##-------------------------#
####------------------------##
######------------ ----------###
########----------- P ----------####
##########---------- -----------####
###########------------------------#####
############-----------------------#####
##############---------------------#######
################-------------------#######
### ###########-----------------########
### T ############----------------########
## #############--------------########
####################-----------#########
####################--------##########
#####################-----##########
######################-###########
##################----########
##############----------####
----------------------
--------------
Global CMT Convention Moment Tensor:
R T P
-3.88e+22 -4.91e+22 4.28e+22
-4.91e+22 -3.83e+22 -3.45e+20
4.28e+22 -3.45e+20 7.72e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20260627011531/index.html
|
W-phase Moment Tensor (Mww)
Moment 9.961e+15 N-m
Magnitude 4.60 Mww
Depth 60.5 km
Percent DC 69%
Half Duration 0.50 s
Catalog US
Data Source US
Contributor US
Nodal Planes
Plane Strike Dip Rake
NP1 167 67 -95
NP2 1 24 -77
Principal Axes
Axis Value Plunge Azimuth
T 9.042e+15 22 261
N 1.636e+15 5 169
P -10.678e+15 68 67
|
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/3.5 -40 o DIST/3.5 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.12 n 3
The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 220 55 85 3.75 0.1659
WVFGRD96 4.0 190 80 60 3.76 0.1840
WVFGRD96 6.0 350 75 -50 3.81 0.2200
WVFGRD96 8.0 190 70 55 3.91 0.2373
WVFGRD96 10.0 190 70 45 3.96 0.2461
WVFGRD96 12.0 210 70 50 3.99 0.2494
WVFGRD96 14.0 210 70 45 4.04 0.2514
WVFGRD96 16.0 210 70 45 4.07 0.2492
WVFGRD96 18.0 210 70 45 4.11 0.2450
WVFGRD96 20.0 250 65 30 4.12 0.2444
WVFGRD96 22.0 245 80 25 4.15 0.2417
WVFGRD96 24.0 55 75 -30 4.16 0.2419
WVFGRD96 26.0 55 70 -30 4.18 0.2434
WVFGRD96 28.0 50 65 -30 4.19 0.2434
WVFGRD96 30.0 50 65 -35 4.21 0.2467
WVFGRD96 32.0 50 60 -30 4.22 0.2615
WVFGRD96 34.0 45 55 -30 4.24 0.2800
WVFGRD96 36.0 45 40 -30 4.27 0.3021
WVFGRD96 38.0 40 35 -40 4.29 0.3203
WVFGRD96 40.0 30 30 -50 4.41 0.3368
WVFGRD96 42.0 35 35 -45 4.42 0.3398
WVFGRD96 44.0 35 35 -45 4.43 0.3425
WVFGRD96 46.0 35 35 -45 4.45 0.3443
WVFGRD96 48.0 35 35 -45 4.46 0.3488
WVFGRD96 50.0 40 40 -40 4.47 0.3543
WVFGRD96 52.0 35 30 -45 4.48 0.3639
WVFGRD96 54.0 35 35 -45 4.50 0.3751
WVFGRD96 56.0 30 35 -35 4.52 0.3846
WVFGRD96 58.0 30 35 -35 4.53 0.3966
WVFGRD96 60.0 30 35 -35 4.54 0.4059
WVFGRD96 62.0 30 35 -35 4.55 0.4127
WVFGRD96 64.0 30 35 -30 4.57 0.4184
WVFGRD96 66.0 35 40 -25 4.57 0.4246
WVFGRD96 68.0 35 40 -25 4.58 0.4274
WVFGRD96 70.0 35 40 -25 4.58 0.4290
WVFGRD96 72.0 35 40 -25 4.59 0.4281
WVFGRD96 74.0 40 40 -20 4.59 0.4268
WVFGRD96 76.0 40 45 -20 4.59 0.4264
WVFGRD96 78.0 40 45 -20 4.60 0.4254
WVFGRD96 80.0 40 45 -20 4.60 0.4227
WVFGRD96 82.0 40 45 -20 4.60 0.4192
WVFGRD96 84.0 40 45 -20 4.60 0.4153
WVFGRD96 86.0 40 45 -20 4.60 0.4115
WVFGRD96 88.0 15 80 70 4.63 0.4139
WVFGRD96 90.0 15 80 70 4.63 0.4146
WVFGRD96 92.0 15 80 70 4.63 0.4147
WVFGRD96 94.0 15 80 75 4.63 0.4134
WVFGRD96 96.0 15 80 75 4.63 0.4102
WVFGRD96 98.0 15 80 75 4.63 0.4077
The best solution is
WVFGRD96 70.0 35 40 -25 4.58 0.4290
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/3.5 -40 o DIST/3.5 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.12 n 3
|
|
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 Fri Jun 26 21:11:08 CDT 2026
