The ANSS event ID is ak0253v83n8h and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0253v83n8h/executive.
2025/03/25 18:40:54 61.594 -146.385 16.2 3.7 Alaska
USGS/SLU Moment Tensor Solution
ENS 2025/03/25 18:40:54:0 61.59 -146.38 16.2 3.7 Alaska
Stations used:
AK.BAE AK.DIV AK.GHO AK.GLB AK.HIN AK.KLU AK.KNK AK.L22K
AK.M26K AK.MCK AK.PWL AK.RIDG AK.RND AK.SAW AK.SCM AK.SLK
AT.PMR AV.WAZA
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
br c 0.12 0.25 n 4 p 2
Best Fitting Double Couple
Mo = 4.62e+21 dyne-cm
Mw = 3.71
Z = 42 km
Plane Strike Dip Rake
NP1 35 50 -85
NP2 207 40 -96
Principal Axes:
Axis Value Plunge Azimuth
T 4.62e+21 5 121
N 0.00e+00 4 212
P -4.62e+21 84 340
Moment Tensor: (dyne-cm)
Component Value
Mxx 1.20e+21
Mxy -2.03e+21
Mxz -6.71e+20
Myy 3.33e+21
Myz 5.07e+20
Mzz -4.54e+21
##############
#############---------
############--------------##
###########-----------------##
###########-------------------####
##########---------------------#####
##########----------------------######
#########------------------------#######
#########------------------------#######
#########---------- -----------#########
########----------- P -----------#########
########----------- ----------##########
#######------------------------###########
######-----------------------###########
######----------------------############
#####--------------------######### #
#####-----------------########### T
####---------------#############
###------------###############
###-------##################
-#####################
##############
Global CMT Convention Moment Tensor:
R T P
-4.54e+21 -6.71e+20 -5.07e+20
-6.71e+20 1.20e+21 2.03e+21
-5.07e+20 2.03e+21 3.33e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20250325184054/index.html
|
STK = 35
DIP = 50
RAKE = -85
MW = 3.71
HS = 42.0
The NDK file is 20250325184054.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 0.07 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 1.0 195 50 70 3.06 0.3811
WVFGRD96 2.0 205 45 80 3.17 0.4831
WVFGRD96 3.0 35 45 95 3.22 0.4771
WVFGRD96 4.0 15 50 70 3.26 0.4587
WVFGRD96 5.0 225 40 -65 3.29 0.4482
WVFGRD96 6.0 230 80 65 3.32 0.4765
WVFGRD96 7.0 240 80 65 3.31 0.4917
WVFGRD96 8.0 240 80 70 3.37 0.5004
WVFGRD96 9.0 240 80 65 3.36 0.5083
WVFGRD96 10.0 240 80 65 3.35 0.5144
WVFGRD96 11.0 240 80 60 3.35 0.5204
WVFGRD96 12.0 240 80 60 3.35 0.5273
WVFGRD96 13.0 240 80 60 3.35 0.5327
WVFGRD96 14.0 240 85 60 3.35 0.5387
WVFGRD96 15.0 245 85 60 3.37 0.5442
WVFGRD96 16.0 250 85 60 3.38 0.5496
WVFGRD96 17.0 250 85 60 3.39 0.5541
WVFGRD96 18.0 60 85 -60 3.38 0.5582
WVFGRD96 19.0 65 85 -60 3.40 0.5630
WVFGRD96 20.0 60 80 -60 3.40 0.5682
WVFGRD96 21.0 60 80 -60 3.41 0.5725
WVFGRD96 22.0 60 80 -60 3.42 0.5768
WVFGRD96 23.0 60 80 -60 3.43 0.5801
WVFGRD96 24.0 60 75 -60 3.44 0.5848
WVFGRD96 25.0 60 75 -60 3.45 0.5889
WVFGRD96 26.0 60 75 -60 3.45 0.5926
WVFGRD96 27.0 60 70 -60 3.47 0.5979
WVFGRD96 28.0 55 65 -60 3.47 0.6046
WVFGRD96 29.0 55 65 -60 3.48 0.6140
WVFGRD96 30.0 55 60 -60 3.50 0.6268
WVFGRD96 31.0 55 60 -60 3.51 0.6412
WVFGRD96 32.0 50 55 -60 3.52 0.6558
WVFGRD96 33.0 50 55 -65 3.53 0.6701
WVFGRD96 34.0 50 55 -60 3.55 0.6834
WVFGRD96 35.0 50 55 -60 3.56 0.6933
WVFGRD96 36.0 55 55 -60 3.58 0.7007
WVFGRD96 37.0 55 55 -60 3.59 0.7051
WVFGRD96 38.0 55 55 -60 3.60 0.7081
WVFGRD96 39.0 55 55 -60 3.62 0.7080
WVFGRD96 40.0 40 50 -75 3.69 0.7149
WVFGRD96 41.0 35 50 -80 3.70 0.7170
WVFGRD96 42.0 35 50 -85 3.71 0.7174
WVFGRD96 43.0 200 40 -105 3.72 0.7169
WVFGRD96 44.0 30 50 -90 3.72 0.7142
WVFGRD96 45.0 215 40 -80 3.73 0.7110
WVFGRD96 46.0 25 50 -95 3.74 0.7075
WVFGRD96 47.0 220 40 -75 3.74 0.7039
WVFGRD96 48.0 220 40 -75 3.75 0.6990
WVFGRD96 49.0 225 40 -65 3.76 0.6944
WVFGRD96 50.0 225 40 -65 3.76 0.6883
WVFGRD96 51.0 225 40 -65 3.77 0.6824
WVFGRD96 52.0 225 40 -65 3.77 0.6754
WVFGRD96 53.0 235 45 -50 3.78 0.6694
WVFGRD96 54.0 235 45 -50 3.78 0.6639
WVFGRD96 55.0 235 45 -50 3.79 0.6578
WVFGRD96 56.0 235 45 -50 3.79 0.6510
WVFGRD96 57.0 235 45 -45 3.80 0.6439
WVFGRD96 58.0 235 45 -45 3.80 0.6374
WVFGRD96 59.0 235 45 -45 3.81 0.6297
The best solution is
WVFGRD96 42.0 35 50 -85 3.71 0.7174
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 0.07 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