The ANSS event ID is ak0255iimhly and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0255iimhly/executive.
2025/04/30 03:58:52 59.715 -152.729 87.2 4.0 Alaska
USGS/SLU Moment Tensor Solution
ENS 2025/04/30 03:58:52:0 59.72 -152.73 87.2 4.0 Alaska
Stations used:
AK.CAPN AK.GHO AK.N18K AK.O18K AK.O19K AK.P17K AK.PWL
AK.RC01 AK.SLK AK.SSN AK.SWD AT.PMR AV.ACH AV.RED AV.SPCL
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 0.08 n 3
Best Fitting Double Couple
Mo = 2.60e+22 dyne-cm
Mw = 4.21
Z = 120 km
Plane Strike Dip Rake
NP1 310 73 148
NP2 50 60 20
Principal Axes:
Axis Value Plunge Azimuth
T 2.60e+22 34 266
N 0.00e+00 54 104
P -2.60e+22 8 2
Moment Tensor: (dyne-cm)
Component Value
Mxx -2.54e+22
Mxy 1.18e+20
Mxz -4.45e+21
Myy 1.77e+22
Myz -1.22e+22
Mzz 7.70e+21
------ P -----
---------- ---------
----------------------------
------------------------------
######---------------------------#
###########-----------------------##
###############-------------------####
###################---------------######
#####################------------#######
#########################--------#########
###### ##################-----##########
###### T ####################--###########
###### ####################-############
##########################-----#########
########################--------########
#####################------------#####
#################----------------###
###########----------------------#
------------------------------
----------------------------
----------------------
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Global CMT Convention Moment Tensor:
R T P
7.70e+21 -4.45e+21 1.22e+22
-4.45e+21 -2.54e+22 -1.18e+20
1.22e+22 -1.18e+20 1.77e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20250430035852/index.html
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STK = 50
DIP = 60
RAKE = 20
MW = 4.21
HS = 120.0
The NDK file is 20250430035852.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/3.5 -40 o DIST/3.5 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 305 65 -25 3.22 0.0796
WVFGRD96 4.0 310 75 10 3.26 0.0875
WVFGRD96 6.0 310 65 15 3.34 0.0934
WVFGRD96 8.0 315 60 20 3.41 0.0967
WVFGRD96 10.0 35 80 -50 3.49 0.0992
WVFGRD96 12.0 40 85 -50 3.52 0.1021
WVFGRD96 14.0 225 90 50 3.55 0.1042
WVFGRD96 16.0 225 90 45 3.57 0.1072
WVFGRD96 18.0 45 90 -45 3.60 0.1115
WVFGRD96 20.0 45 90 -45 3.63 0.1164
WVFGRD96 22.0 40 90 -45 3.66 0.1219
WVFGRD96 24.0 40 90 -45 3.69 0.1272
WVFGRD96 26.0 225 85 40 3.71 0.1326
WVFGRD96 28.0 40 85 -45 3.74 0.1381
WVFGRD96 30.0 40 85 -45 3.76 0.1427
WVFGRD96 32.0 225 90 40 3.77 0.1456
WVFGRD96 34.0 45 90 -40 3.79 0.1480
WVFGRD96 36.0 225 90 40 3.81 0.1497
WVFGRD96 38.0 45 90 -35 3.84 0.1521
WVFGRD96 40.0 45 85 -50 3.95 0.1567
WVFGRD96 42.0 45 85 -45 3.96 0.1584
WVFGRD96 44.0 45 85 -45 3.98 0.1595
WVFGRD96 46.0 45 80 -40 3.99 0.1602
WVFGRD96 48.0 45 80 -40 4.01 0.1606
WVFGRD96 50.0 45 75 -35 4.03 0.1615
WVFGRD96 52.0 45 75 -35 4.04 0.1637
WVFGRD96 54.0 50 75 -30 4.06 0.1663
WVFGRD96 56.0 50 75 -25 4.07 0.1695
WVFGRD96 58.0 50 70 -25 4.09 0.1733
WVFGRD96 60.0 50 60 -10 4.09 0.1795
WVFGRD96 62.0 50 60 -5 4.10 0.1861
WVFGRD96 64.0 50 60 -5 4.11 0.1925
WVFGRD96 66.0 50 60 -5 4.12 0.1981
WVFGRD96 68.0 50 60 -5 4.13 0.2037
WVFGRD96 70.0 50 55 -5 4.15 0.2098
WVFGRD96 72.0 50 55 -5 4.16 0.2146
WVFGRD96 74.0 50 55 -5 4.16 0.2195
WVFGRD96 76.0 50 55 0 4.16 0.2240
WVFGRD96 78.0 50 55 0 4.17 0.2284
WVFGRD96 80.0 50 55 0 4.18 0.2320
WVFGRD96 82.0 50 55 0 4.18 0.2359
WVFGRD96 84.0 50 55 5 4.18 0.2385
WVFGRD96 86.0 50 55 5 4.18 0.2421
WVFGRD96 88.0 50 55 5 4.19 0.2450
WVFGRD96 90.0 50 55 5 4.19 0.2467
WVFGRD96 92.0 50 55 10 4.19 0.2494
WVFGRD96 94.0 50 55 10 4.20 0.2517
WVFGRD96 96.0 50 55 10 4.20 0.2534
WVFGRD96 98.0 50 55 10 4.20 0.2547
WVFGRD96 100.0 50 55 10 4.21 0.2561
WVFGRD96 102.0 50 55 15 4.21 0.2575
WVFGRD96 104.0 50 55 15 4.21 0.2589
WVFGRD96 106.0 50 55 15 4.21 0.2599
WVFGRD96 108.0 50 55 15 4.22 0.2604
WVFGRD96 110.0 50 60 20 4.20 0.2608
WVFGRD96 112.0 50 60 20 4.20 0.2613
WVFGRD96 114.0 50 60 20 4.20 0.2616
WVFGRD96 116.0 50 60 20 4.21 0.2617
WVFGRD96 118.0 50 60 20 4.21 0.2617
WVFGRD96 120.0 50 60 20 4.21 0.2618
WVFGRD96 122.0 50 60 25 4.21 0.2616
WVFGRD96 124.0 50 60 25 4.21 0.2613
WVFGRD96 126.0 50 60 25 4.21 0.2611
WVFGRD96 128.0 50 60 25 4.22 0.2608
WVFGRD96 130.0 210 90 -75 4.27 0.2612
WVFGRD96 132.0 30 90 75 4.27 0.2612
WVFGRD96 134.0 210 90 -75 4.27 0.2611
WVFGRD96 136.0 30 90 75 4.27 0.2606
WVFGRD96 138.0 30 90 75 4.27 0.2601
WVFGRD96 140.0 210 90 -75 4.26 0.2594
WVFGRD96 142.0 30 90 75 4.26 0.2584
WVFGRD96 144.0 210 90 -75 4.26 0.2569
WVFGRD96 146.0 35 85 75 4.26 0.2557
WVFGRD96 148.0 210 90 -70 4.24 0.2541
The best solution is
WVFGRD96 120.0 50 60 20 4.21 0.2618
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/3.5 -40 o DIST/3.5 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 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