The ANSS event ID is ak02462bum3w and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak02462bum3w/executive.
2024/05/11 01:53:05 59.828 -152.851 94.6 4.5 Alaska
USGS/SLU Moment Tensor Solution
ENS 2024/05/11 01:53:05:0 59.83 -152.85 94.6 4.5 Alaska
Stations used:
AK.BRLK AK.CAPN AK.CUT AK.DIV AK.FID AK.FIRE AK.L19K
AK.L20K AK.L22K AK.N15K AK.N18K AK.O18K AK.O19K AK.P16K
AK.P17K AK.RC01 AK.SLK AK.SWD AT.TTA AV.STLK II.KDAK
Filtering commands used:
cut o DIST/3.6 -40 o DIST/3.6 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 1.32e+23 dyne-cm
Mw = 4.68
Z = 104 km
Plane Strike Dip Rake
NP1 221 64 134
NP2 335 50 35
Principal Axes:
Axis Value Plunge Azimuth
T 1.32e+23 50 181
N 0.00e+00 39 18
P -1.32e+23 8 281
Moment Tensor: (dyne-cm)
Component Value
Mxx 5.01e+22
Mxy 2.47e+22
Mxz -6.85e+22
Myy -1.25e+23
Myz 1.74e+22
Mzz 7.45e+22
##############
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------------------########------------
--------------###########------------
P ------------##############-----------
- -----------################-----------
-------------##################-----------
------------####################----------
-----------#####################----------
---------#######################--------
--------########################--------
-------########## ###########-------
-----########### T ###########------
----########### ###########-----
-#########################----
########################----
#####################-
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Global CMT Convention Moment Tensor:
R T P
7.45e+22 -6.85e+22 -1.74e+22
-6.85e+22 5.01e+22 -2.47e+22
-1.74e+22 -2.47e+22 -1.25e+23
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20240511015305/index.html
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STK = 335
DIP = 50
RAKE = 35
MW = 4.68
HS = 104.0
The NDK file is 20240511015305.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/3.6 -40 o DIST/3.6 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 20 45 -70 3.90 0.2774
WVFGRD96 4.0 45 90 35 3.89 0.2376
WVFGRD96 6.0 230 75 40 3.96 0.2801
WVFGRD96 8.0 230 75 40 4.05 0.3123
WVFGRD96 10.0 230 75 40 4.09 0.3304
WVFGRD96 12.0 225 75 40 4.12 0.3330
WVFGRD96 14.0 225 75 40 4.14 0.3253
WVFGRD96 16.0 310 40 10 4.15 0.3140
WVFGRD96 18.0 315 40 15 4.18 0.3117
WVFGRD96 20.0 320 35 20 4.20 0.3064
WVFGRD96 22.0 135 50 15 4.24 0.3081
WVFGRD96 24.0 135 50 15 4.26 0.3096
WVFGRD96 26.0 145 60 20 4.29 0.3124
WVFGRD96 28.0 145 55 20 4.31 0.3135
WVFGRD96 30.0 145 60 20 4.33 0.3128
WVFGRD96 32.0 145 65 20 4.34 0.3141
WVFGRD96 34.0 145 65 20 4.36 0.3191
WVFGRD96 36.0 145 65 20 4.38 0.3195
WVFGRD96 38.0 145 70 20 4.41 0.3187
WVFGRD96 40.0 145 55 20 4.48 0.3345
WVFGRD96 42.0 145 50 15 4.51 0.3366
WVFGRD96 44.0 145 50 15 4.54 0.3395
WVFGRD96 46.0 140 65 -20 4.57 0.3403
WVFGRD96 48.0 140 65 -20 4.58 0.3456
WVFGRD96 50.0 140 70 -25 4.60 0.3500
WVFGRD96 52.0 140 70 -25 4.61 0.3535
WVFGRD96 54.0 140 70 -30 4.62 0.3589
WVFGRD96 56.0 140 70 -30 4.63 0.3676
WVFGRD96 58.0 320 55 -15 4.65 0.3778
WVFGRD96 60.0 330 60 20 4.62 0.3934
WVFGRD96 62.0 330 60 25 4.62 0.4122
WVFGRD96 64.0 335 55 30 4.63 0.4320
WVFGRD96 66.0 335 55 30 4.63 0.4500
WVFGRD96 68.0 335 55 30 4.64 0.4669
WVFGRD96 70.0 335 55 30 4.64 0.4832
WVFGRD96 72.0 335 55 30 4.65 0.4977
WVFGRD96 74.0 335 55 30 4.65 0.5135
WVFGRD96 76.0 335 55 35 4.65 0.5260
WVFGRD96 78.0 335 55 35 4.65 0.5392
WVFGRD96 80.0 335 55 35 4.65 0.5506
WVFGRD96 82.0 335 55 35 4.66 0.5602
WVFGRD96 84.0 335 50 35 4.66 0.5708
WVFGRD96 86.0 335 50 35 4.66 0.5797
WVFGRD96 88.0 335 50 35 4.67 0.5873
WVFGRD96 90.0 335 50 35 4.67 0.5945
WVFGRD96 92.0 335 50 35 4.67 0.6001
WVFGRD96 94.0 335 50 35 4.67 0.6041
WVFGRD96 96.0 335 50 35 4.67 0.6074
WVFGRD96 98.0 335 50 35 4.67 0.6108
WVFGRD96 100.0 335 50 35 4.68 0.6144
WVFGRD96 102.0 335 50 35 4.68 0.6164
WVFGRD96 104.0 335 50 35 4.68 0.6171
WVFGRD96 106.0 340 45 40 4.68 0.6168
WVFGRD96 108.0 340 45 40 4.68 0.6160
WVFGRD96 110.0 340 45 40 4.68 0.6162
WVFGRD96 112.0 340 45 40 4.68 0.6160
WVFGRD96 114.0 340 45 40 4.69 0.6144
WVFGRD96 116.0 340 45 40 4.69 0.6121
WVFGRD96 118.0 340 45 40 4.69 0.6112
WVFGRD96 120.0 340 45 40 4.69 0.6098
WVFGRD96 122.0 340 45 45 4.69 0.6070
WVFGRD96 124.0 340 45 45 4.69 0.6056
WVFGRD96 126.0 340 45 45 4.69 0.6041
WVFGRD96 128.0 340 45 45 4.69 0.6020
WVFGRD96 130.0 340 45 45 4.69 0.6001
WVFGRD96 132.0 340 45 45 4.69 0.5983
WVFGRD96 134.0 340 45 45 4.69 0.5951
WVFGRD96 136.0 340 45 45 4.69 0.5932
WVFGRD96 138.0 340 50 45 4.69 0.5901
WVFGRD96 140.0 340 50 45 4.69 0.5870
WVFGRD96 142.0 340 50 45 4.70 0.5857
WVFGRD96 144.0 340 50 45 4.70 0.5831
WVFGRD96 146.0 340 50 45 4.70 0.5811
WVFGRD96 148.0 340 50 45 4.70 0.5776
The best solution is
WVFGRD96 104.0 335 50 35 4.68 0.6171
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/3.6 -40 o DIST/3.6 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 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