Location
SLU Location
To check the ANSS location or to compare the observed P-wave first motions to the moment tensor solution, P- and S-wave first arrival times were manually read together with the P-wave first motions. The subsequent output of the program elocate is given in the file elocate.txt. The first motion plot is shown below.
Location ANSS
The ANSS event ID is ak0252zva6il and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ak0252zva6il/executive.
2025/03/06 22:42:40 59.814 -152.944 104.3 4.3 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2025/03/06 22:42:40:0 59.81 -152.94 104.3 4.3 Alaska
Stations used:
AK.BRLK AK.CAPN AK.CUT AK.GHO AK.HOM AK.KNK AK.L19K AK.L22K
AK.M20K AK.N18K AK.O18K AK.O19K AK.RC01 AK.SAW AK.SLK
AK.SWD AT.PMR AT.TTA AV.ACH AV.RED AV.STLK II.KDAK
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 0.07 n 3
Best Fitting Double Couple
Mo = 6.84e+22 dyne-cm
Mw = 4.49
Z = 116 km
Plane Strike Dip Rake
NP1 80 88 -85
NP2 190 5 -160
Principal Axes:
Axis Value Plunge Azimuth
T 6.84e+22 43 166
N 0.00e+00 5 260
P -6.84e+22 47 355
Moment Tensor: (dyne-cm)
Component Value
Mxx 2.04e+21
Mxy -5.96e+21
Mxz -6.70e+22
Myy 2.02e+21
Myz 1.16e+22
Mzz -4.06e+21
#-------------
#---------------------
##--------------------------
#-----------------------------
#-------------- ----------------
#--------------- P -----------------
#---------------- ------------------
#---------------------------------------
#---------------------------------------
#-------------------------------------####
#----------------------------#############
#-----------------########################
#------###################################
-#######################################
-#######################################
-#####################################
-################## ##############
-################# T #############
################ ###########
-###########################
-#####################
##############
Global CMT Convention Moment Tensor:
R T P
-4.06e+21 -6.70e+22 -1.16e+22
-6.70e+22 2.04e+21 5.96e+21
-1.16e+22 5.96e+21 2.02e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20250306224240/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 = 10
DIP = -5
RAKE = 20
MW = 4.49
HS = 116.0
The NDK file is 20250306224240.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 |
SLUFM |
USGS/SLU Moment Tensor Solution
ENS 2025/03/06 22:42:40:0 59.81 -152.94 104.3 4.3 Alaska
Stations used:
AK.BRLK AK.CAPN AK.CUT AK.GHO AK.HOM AK.KNK AK.L19K AK.L22K
AK.M20K AK.N18K AK.O18K AK.O19K AK.RC01 AK.SAW AK.SLK
AK.SWD AT.PMR AT.TTA AV.ACH AV.RED AV.STLK II.KDAK
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 0.07 n 3
Best Fitting Double Couple
Mo = 6.84e+22 dyne-cm
Mw = 4.49
Z = 116 km
Plane Strike Dip Rake
NP1 80 88 -85
NP2 190 5 -160
Principal Axes:
Axis Value Plunge Azimuth
T 6.84e+22 43 166
N 0.00e+00 5 260
P -6.84e+22 47 355
Moment Tensor: (dyne-cm)
Component Value
Mxx 2.04e+21
Mxy -5.96e+21
Mxz -6.70e+22
Myy 2.02e+21
Myz 1.16e+22
Mzz -4.06e+21
#-------------
#---------------------
##--------------------------
#-----------------------------
#-------------- ----------------
#--------------- P -----------------
#---------------- ------------------
#---------------------------------------
#---------------------------------------
#-------------------------------------####
#----------------------------#############
#-----------------########################
#------###################################
-#######################################
-#######################################
-#####################################
-################## ##############
-################# T #############
################ ###########
-###########################
-#####################
##############
Global CMT Convention Moment Tensor:
R T P
-4.06e+21 -6.70e+22 -1.16e+22
-6.70e+22 2.04e+21 5.96e+21
-1.16e+22 5.96e+21 2.02e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20250306224240/index.html
|
First motions and takeoff angles from an elocate run.
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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.
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 0.07 n 3
The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 25 85 -5 3.57 0.2399
WVFGRD96 4.0 30 65 15 3.68 0.2751
WVFGRD96 6.0 25 60 -10 3.74 0.3015
WVFGRD96 8.0 30 60 10 3.80 0.3202
WVFGRD96 10.0 30 60 15 3.84 0.3283
WVFGRD96 12.0 30 65 15 3.86 0.3290
WVFGRD96 14.0 30 65 15 3.88 0.3242
WVFGRD96 16.0 25 70 -10 3.90 0.3179
WVFGRD96 18.0 25 70 -10 3.92 0.3136
WVFGRD96 20.0 25 70 -10 3.94 0.3115
WVFGRD96 22.0 25 70 -15 3.96 0.3079
WVFGRD96 24.0 25 70 -15 3.97 0.3032
WVFGRD96 26.0 25 70 -15 3.99 0.2978
WVFGRD96 28.0 25 70 -15 4.00 0.2918
WVFGRD96 30.0 20 65 -20 4.01 0.2859
WVFGRD96 32.0 20 60 -20 4.03 0.2811
WVFGRD96 34.0 20 60 -20 4.04 0.2764
WVFGRD96 36.0 20 60 -20 4.06 0.2713
WVFGRD96 38.0 15 60 -20 4.06 0.2661
WVFGRD96 40.0 10 45 -30 4.16 0.2657
WVFGRD96 42.0 10 45 -30 4.18 0.2641
WVFGRD96 44.0 10 45 -35 4.20 0.2637
WVFGRD96 46.0 10 45 -35 4.22 0.2648
WVFGRD96 48.0 10 45 -30 4.22 0.2669
WVFGRD96 50.0 10 45 -30 4.24 0.2685
WVFGRD96 52.0 10 45 -30 4.25 0.2712
WVFGRD96 54.0 130 85 15 4.27 0.2741
WVFGRD96 56.0 130 85 15 4.29 0.2924
WVFGRD96 58.0 120 75 -15 4.28 0.3113
WVFGRD96 60.0 120 75 -15 4.30 0.3295
WVFGRD96 62.0 120 75 -10 4.30 0.3463
WVFGRD96 64.0 120 75 -10 4.32 0.3602
WVFGRD96 66.0 120 75 -10 4.33 0.3716
WVFGRD96 68.0 120 75 -10 4.34 0.3801
WVFGRD96 70.0 295 15 -60 4.44 0.4001
WVFGRD96 72.0 295 15 -60 4.45 0.4190
WVFGRD96 74.0 295 15 -60 4.45 0.4356
WVFGRD96 76.0 295 15 -60 4.46 0.4507
WVFGRD96 78.0 290 10 -65 4.46 0.4636
WVFGRD96 80.0 285 10 -70 4.46 0.4763
WVFGRD96 82.0 290 10 -65 4.47 0.4866
WVFGRD96 84.0 295 10 -55 4.47 0.4964
WVFGRD96 86.0 295 10 -55 4.47 0.5058
WVFGRD96 88.0 300 10 -50 4.47 0.5136
WVFGRD96 90.0 300 10 -50 4.48 0.5201
WVFGRD96 92.0 300 10 -50 4.48 0.5251
WVFGRD96 94.0 305 10 -45 4.48 0.5285
WVFGRD96 96.0 290 5 -60 4.48 0.5332
WVFGRD96 98.0 290 5 -60 4.48 0.5375
WVFGRD96 100.0 290 5 -60 4.48 0.5403
WVFGRD96 102.0 290 5 -60 4.48 0.5426
WVFGRD96 104.0 290 5 -60 4.48 0.5452
WVFGRD96 106.0 290 5 -60 4.48 0.5462
WVFGRD96 108.0 100 -5 110 4.48 0.5464
WVFGRD96 110.0 30 -5 40 4.48 0.5469
WVFGRD96 112.0 100 -5 110 4.48 0.5468
WVFGRD96 114.0 315 5 -30 4.48 0.5478
WVFGRD96 116.0 10 -5 20 4.49 0.5485
WVFGRD96 118.0 5 -5 15 4.49 0.5472
WVFGRD96 120.0 80 90 -90 4.48 0.5484
WVFGRD96 122.0 320 0 -30 4.48 0.5475
WVFGRD96 124.0 340 0 -10 4.48 0.5465
WVFGRD96 126.0 330 0 -20 4.48 0.5463
WVFGRD96 128.0 60 0 70 4.48 0.5443
WVFGRD96 130.0 340 -5 -10 4.49 0.5431
WVFGRD96 132.0 280 0 -70 4.48 0.5402
WVFGRD96 134.0 335 -5 -15 4.49 0.5407
WVFGRD96 136.0 -20 -5 -10 4.49 0.5379
WVFGRD96 138.0 280 0 -70 4.48 0.5360
WVFGRD96 140.0 -20 -5 -10 4.49 0.5335
WVFGRD96 142.0 -20 -5 -10 4.49 0.5321
WVFGRD96 144.0 85 5 100 4.50 0.5288
WVFGRD96 146.0 300 -5 -50 4.50 0.5278
WVFGRD96 148.0 300 -5 -50 4.50 0.5248
The best solution is
WVFGRD96 116.0 10 -5 20 4.49 0.5485
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 0.07 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.
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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 Thu Mar 6 19:54:24 CST 2025
