The ANSS event ID is ak0235xecrhs and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0235xecrhs/executive.
2023/05/09 03:40:56 63.656 -149.651 117.5 4.4 Alaska
USGS/SLU Moment Tensor Solution
ENS 2023/05/09 03:40:56:0 63.66 -149.65 117.5 4.4 Alaska
Stations used:
AK.BPAW AK.CAST AK.CCB AK.DHY AK.GHO AK.H22K AK.H23K
AK.H24K AK.HDA AK.I23K AK.J20K AK.KNK AK.KTH AK.L19K
AK.L22K AK.M19K AK.MCK AK.MLY AK.NEA2 AK.PAX AK.POKR AK.PWL
AK.RIDG AK.RND AK.SCM AK.SKN AK.WAT6 AK.WRH AT.PMR AV.STLK
IU.COLA
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 0.08 n 3
Best Fitting Double Couple
Mo = 1.66e+22 dyne-cm
Mw = 4.08
Z = 120 km
Plane Strike Dip Rake
NP1 115 85 70
NP2 12 21 166
Principal Axes:
Axis Value Plunge Azimuth
T 1.66e+22 46 4
N 0.00e+00 20 117
P -1.66e+22 37 223
Moment Tensor: (dyne-cm)
Component Value
Mxx 2.11e+21
Mxy -4.67e+21
Mxz 1.41e+22
Myy -4.82e+21
Myz 6.04e+21
Mzz 2.71e+21
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################# T #############---
-################# ##############---
-----###############################----
--------#############################---
------------##########################----
----------------######################----
--------------------##################----
------------------------#############-----
---------------------------#########----
--------------------------------###-----
--------- ----------------------##--
-------- P ---------------------####
------- -------------------#####
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Global CMT Convention Moment Tensor:
R T P
2.71e+21 1.41e+22 -6.04e+21
1.41e+22 2.11e+21 4.67e+21
-6.04e+21 4.67e+21 -4.82e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20230509034056/index.html
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STK = 115
DIP = 85
RAKE = 70
MW = 4.08
HS = 120.0
The NDK file is 20230509034056.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/3.3 -40 o DIST/3.3 +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 50 75 -10 3.03 0.1261
WVFGRD96 4.0 50 90 -30 3.15 0.1520
WVFGRD96 6.0 230 70 15 3.21 0.1736
WVFGRD96 8.0 235 65 25 3.30 0.1948
WVFGRD96 10.0 230 65 15 3.33 0.2064
WVFGRD96 12.0 230 65 15 3.37 0.2161
WVFGRD96 14.0 230 65 20 3.40 0.2224
WVFGRD96 16.0 230 65 20 3.43 0.2255
WVFGRD96 18.0 230 65 20 3.45 0.2270
WVFGRD96 20.0 230 65 20 3.47 0.2258
WVFGRD96 22.0 230 65 20 3.49 0.2223
WVFGRD96 24.0 230 60 20 3.51 0.2166
WVFGRD96 26.0 230 60 15 3.52 0.2108
WVFGRD96 28.0 230 55 15 3.53 0.2059
WVFGRD96 30.0 230 55 15 3.55 0.2016
WVFGRD96 32.0 230 55 15 3.56 0.1977
WVFGRD96 34.0 215 45 15 3.59 0.1954
WVFGRD96 36.0 215 45 15 3.60 0.1966
WVFGRD96 38.0 210 50 15 3.62 0.1978
WVFGRD96 40.0 210 40 15 3.72 0.1995
WVFGRD96 42.0 210 45 15 3.73 0.1961
WVFGRD96 44.0 205 65 25 3.73 0.1971
WVFGRD96 46.0 205 65 25 3.74 0.2005
WVFGRD96 48.0 205 65 25 3.76 0.2052
WVFGRD96 50.0 200 70 20 3.77 0.2099
WVFGRD96 52.0 200 70 20 3.78 0.2145
WVFGRD96 54.0 200 70 20 3.80 0.2197
WVFGRD96 56.0 140 70 -20 3.84 0.2343
WVFGRD96 58.0 325 90 -25 3.83 0.2616
WVFGRD96 60.0 140 50 10 3.88 0.2944
WVFGRD96 62.0 140 55 15 3.89 0.3400
WVFGRD96 64.0 140 60 25 3.90 0.3889
WVFGRD96 66.0 140 65 35 3.91 0.4399
WVFGRD96 68.0 135 70 40 3.93 0.4891
WVFGRD96 70.0 115 80 70 3.96 0.5287
WVFGRD96 72.0 115 80 70 3.97 0.5689
WVFGRD96 74.0 120 80 70 3.98 0.5939
WVFGRD96 76.0 120 80 70 3.99 0.6131
WVFGRD96 78.0 120 80 70 3.99 0.6309
WVFGRD96 80.0 120 80 70 4.00 0.6489
WVFGRD96 82.0 120 80 70 4.01 0.6647
WVFGRD96 84.0 120 80 70 4.01 0.6792
WVFGRD96 86.0 120 80 70 4.02 0.6937
WVFGRD96 88.0 120 80 70 4.03 0.7051
WVFGRD96 90.0 120 80 70 4.03 0.7166
WVFGRD96 92.0 120 80 70 4.04 0.7258
WVFGRD96 94.0 120 80 70 4.04 0.7351
WVFGRD96 96.0 120 80 70 4.04 0.7425
WVFGRD96 98.0 115 85 70 4.05 0.7510
WVFGRD96 100.0 115 85 70 4.05 0.7572
WVFGRD96 102.0 115 85 70 4.05 0.7627
WVFGRD96 104.0 115 85 70 4.06 0.7677
WVFGRD96 106.0 115 85 70 4.06 0.7725
WVFGRD96 108.0 115 85 70 4.06 0.7753
WVFGRD96 110.0 115 85 70 4.07 0.7791
WVFGRD96 112.0 115 85 70 4.07 0.7802
WVFGRD96 114.0 115 85 70 4.07 0.7831
WVFGRD96 116.0 115 85 70 4.08 0.7833
WVFGRD96 118.0 115 85 70 4.08 0.7848
WVFGRD96 120.0 115 85 70 4.08 0.7849
WVFGRD96 122.0 115 85 70 4.08 0.7843
WVFGRD96 124.0 115 85 70 4.08 0.7843
WVFGRD96 126.0 115 85 70 4.09 0.7827
WVFGRD96 128.0 115 85 70 4.09 0.7813
WVFGRD96 130.0 290 90 -65 4.09 0.7733
WVFGRD96 132.0 290 90 -65 4.09 0.7716
WVFGRD96 134.0 290 90 -65 4.10 0.7709
WVFGRD96 136.0 115 85 65 4.10 0.7729
WVFGRD96 138.0 290 90 -65 4.10 0.7664
WVFGRD96 140.0 290 90 -65 4.10 0.7623
WVFGRD96 142.0 115 85 65 4.10 0.7633
WVFGRD96 144.0 115 85 65 4.10 0.7599
WVFGRD96 146.0 115 85 65 4.10 0.7548
WVFGRD96 148.0 115 85 65 4.10 0.7514
The best solution is
WVFGRD96 120.0 115 85 70 4.08 0.7849
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/3.3 -40 o DIST/3.3 +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