The ANSS event ID is ak0238wy2apd and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0238wy2apd/executive.
2023/07/13 06:39:00 63.639 -149.933 141.0 3.7 Alaska
USGS/SLU Moment Tensor Solution
ENS 2023/07/13 06:39:00:0 63.64 -149.93 141.0 3.7 Alaska
Stations used:
AK.BPAW AK.CAST AK.CCB AK.CUT AK.DHY AK.GCSA AK.GHO AK.H23K
AK.J19K AK.J20K AK.K24K AK.KTH AK.L20K AK.L22K AK.MCK
AK.MLY AK.NEA2 AK.PAX AK.POKR AK.PPLA AK.RND AK.SAW AK.SCM
AK.SKN AK.WAT6 AT.PMR AV.SPCP IM.IL31 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 0.10 n 3
Best Fitting Double Couple
Mo = 8.04e+21 dyne-cm
Mw = 3.87
Z = 146 km
Plane Strike Dip Rake
NP1 100 55 -45
NP2 220 55 -135
Principal Axes:
Axis Value Plunge Azimuth
T 8.04e+21 0 160
N 0.00e+00 35 250
P -8.04e+21 55 70
Moment Tensor: (dyne-cm)
Component Value
Mxx 6.77e+21
Mxy -3.46e+21
Mxz -1.35e+21
Myy -1.43e+21
Myz -3.55e+21
Mzz -5.34e+21
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############--------------- ----------
-###########---------------- P -----------
--#########----------------- -----------
----######--------------------------------
------###---------------------------------
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----################################
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############ T
Global CMT Convention Moment Tensor:
R T P
-5.34e+21 -1.35e+21 3.55e+21
-1.35e+21 6.77e+21 3.46e+21
3.55e+21 3.46e+21 -1.43e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20230713063900/index.html
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STK = 100
DIP = 55
RAKE = -45
MW = 3.87
HS = 146.0
The NDK file is 20230713063900.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 0.10 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 255 45 90 2.96 0.2077
WVFGRD96 4.0 40 90 -30 2.92 0.2013
WVFGRD96 6.0 35 80 -30 2.98 0.2309
WVFGRD96 8.0 200 55 -15 3.08 0.2572
WVFGRD96 10.0 200 60 -15 3.12 0.2703
WVFGRD96 12.0 200 60 -15 3.16 0.2725
WVFGRD96 14.0 200 60 -10 3.18 0.2659
WVFGRD96 16.0 200 60 -10 3.21 0.2534
WVFGRD96 18.0 55 70 40 3.24 0.2438
WVFGRD96 20.0 85 80 -50 3.27 0.2346
WVFGRD96 22.0 270 75 -50 3.30 0.2406
WVFGRD96 24.0 270 75 -50 3.33 0.2469
WVFGRD96 26.0 270 75 -50 3.35 0.2506
WVFGRD96 28.0 270 75 -50 3.37 0.2493
WVFGRD96 30.0 275 80 -45 3.38 0.2461
WVFGRD96 32.0 100 90 40 3.39 0.2408
WVFGRD96 34.0 100 85 40 3.41 0.2383
WVFGRD96 36.0 280 90 -35 3.41 0.2347
WVFGRD96 38.0 105 85 35 3.44 0.2341
WVFGRD96 40.0 100 90 45 3.53 0.2316
WVFGRD96 42.0 280 90 -40 3.54 0.2320
WVFGRD96 44.0 105 90 35 3.54 0.2295
WVFGRD96 46.0 285 80 -35 3.56 0.2311
WVFGRD96 48.0 285 80 -30 3.57 0.2331
WVFGRD96 50.0 105 85 30 3.59 0.2340
WVFGRD96 52.0 285 80 -30 3.60 0.2391
WVFGRD96 54.0 105 90 30 3.61 0.2450
WVFGRD96 56.0 105 90 30 3.63 0.2542
WVFGRD96 58.0 105 90 30 3.64 0.2638
WVFGRD96 60.0 105 90 30 3.65 0.2729
WVFGRD96 62.0 105 90 30 3.66 0.2801
WVFGRD96 64.0 285 80 -25 3.66 0.2917
WVFGRD96 66.0 290 80 -25 3.67 0.3037
WVFGRD96 68.0 290 80 -20 3.67 0.3162
WVFGRD96 70.0 110 65 -20 3.67 0.3494
WVFGRD96 72.0 110 65 -20 3.69 0.3895
WVFGRD96 74.0 110 65 -25 3.71 0.4305
WVFGRD96 76.0 105 60 -30 3.73 0.4634
WVFGRD96 78.0 105 60 -30 3.74 0.4869
WVFGRD96 80.0 105 60 -35 3.74 0.4987
WVFGRD96 82.0 105 60 -35 3.75 0.5142
WVFGRD96 84.0 105 60 -35 3.75 0.5313
WVFGRD96 86.0 105 60 -35 3.76 0.5509
WVFGRD96 88.0 105 60 -35 3.77 0.5675
WVFGRD96 90.0 105 60 -35 3.77 0.5764
WVFGRD96 92.0 105 60 -35 3.78 0.5822
WVFGRD96 94.0 105 60 -35 3.78 0.5872
WVFGRD96 96.0 100 55 -40 3.79 0.5935
WVFGRD96 98.0 100 55 -40 3.79 0.5989
WVFGRD96 100.0 100 55 -40 3.80 0.6033
WVFGRD96 102.0 100 55 -40 3.80 0.6066
WVFGRD96 104.0 100 55 -40 3.80 0.6113
WVFGRD96 106.0 100 55 -40 3.81 0.6140
WVFGRD96 108.0 100 55 -40 3.81 0.6164
WVFGRD96 110.0 100 55 -40 3.81 0.6202
WVFGRD96 112.0 100 55 -40 3.82 0.6216
WVFGRD96 114.0 100 55 -40 3.82 0.6232
WVFGRD96 116.0 100 55 -40 3.83 0.6254
WVFGRD96 118.0 100 55 -40 3.83 0.6251
WVFGRD96 120.0 100 55 -40 3.83 0.6274
WVFGRD96 122.0 100 55 -40 3.83 0.6268
WVFGRD96 124.0 100 55 -40 3.84 0.6279
WVFGRD96 126.0 100 55 -40 3.84 0.6283
WVFGRD96 128.0 100 55 -40 3.84 0.6280
WVFGRD96 130.0 100 55 -40 3.85 0.6294
WVFGRD96 132.0 100 55 -40 3.85 0.6288
WVFGRD96 134.0 100 55 -40 3.85 0.6305
WVFGRD96 136.0 100 55 -45 3.86 0.6298
WVFGRD96 138.0 100 55 -45 3.86 0.6310
WVFGRD96 140.0 100 55 -45 3.86 0.6302
WVFGRD96 142.0 100 55 -45 3.86 0.6314
WVFGRD96 144.0 100 55 -45 3.87 0.6316
WVFGRD96 146.0 100 55 -45 3.87 0.6327
WVFGRD96 148.0 100 55 -45 3.87 0.6313
WVFGRD96 150.0 100 55 -45 3.88 0.6316
WVFGRD96 152.0 100 55 -45 3.88 0.6317
WVFGRD96 154.0 100 55 -45 3.88 0.6326
WVFGRD96 156.0 100 55 -45 3.88 0.6306
WVFGRD96 158.0 100 55 -45 3.89 0.6299
The best solution is
WVFGRD96 146.0 100 55 -45 3.87 0.6327
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 0.10 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