The ANSS event ID is ak023a0j9eo0 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak023a0j9eo0/executive.
2023/08/06 00:17:51 59.676 -153.278 107.9 4.7 Alaska
USGS/SLU Moment Tensor Solution
ENS 2023/08/06 00:17:51:0 59.68 -153.28 107.9 4.7 Alaska
Stations used:
AK.BRLK AK.BRSE AK.CAPN AK.CNP AK.FIRE AK.GHO AK.HOM
AK.L19K AK.L20K AK.M19K AK.M20K AK.N18K AK.O18K AK.O19K
AK.O20K AK.P17K AK.Q19K AK.RC01 AK.SKN AK.SLK AT.PMR AV.ACH
AV.ANCK AV.AU22 AV.AUCH AV.AUJA AV.AUL AV.AULG AV.AUNO
AV.AUSB AV.AUW AV.AUWS AV.CAHL AV.ILCB AV.ILLG AV.ILNE
AV.ILS AV.ILSW AV.IVE AV.KAB2 AV.KABU AV.KAHG AV.KAKN
AV.KARR AV.KAVE AV.KAWH AV.KBM AV.KEL AV.KJL AV.KVT AV.MGLS
AV.N20K AV.NCT AV.P18K AV.PLK1 AV.PLK2 AV.Q18K AV.Q20K
AV.R17L AV.RDDF AV.RDSO AV.RDT AV.RED AV.REF AV.SPBG
AV.SPCG AV.SPCL AV.SPCN AV.SPCP AV.SPCR AV.SPNN AV.STLK
II.KDAK
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 = 8.41e+22 dyne-cm
Mw = 4.55
Z = 106 km
Plane Strike Dip Rake
NP1 303 71 159
NP2 40 70 20
Principal Axes:
Axis Value Plunge Azimuth
T 8.41e+22 28 261
N 0.00e+00 62 83
P -8.41e+22 1 352
Moment Tensor: (dyne-cm)
Component Value
Mxx -8.08e+22
Mxy 2.20e+22
Mxz -6.55e+21
Myy 6.23e+22
Myz -3.43e+22
Mzz 1.85e+22
--- P --------
------- ------------
---------------------------#
----------------------------##
------------------------------####
########----------------------######
##############----------------########
##################------------##########
######################-------###########
#########################----#############
##########################################
##### ###################---############
##### T #################-------##########
#### ################----------#######
#####################--------------#####
##################-----------------###
###############--------------------#
############----------------------
#######-----------------------
#---------------------------
----------------------
--------------
Global CMT Convention Moment Tensor:
R T P
1.85e+22 -6.55e+21 3.43e+22
-6.55e+21 -8.08e+22 -2.20e+22
3.43e+22 -2.20e+22 6.23e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20230806001751/index.html
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STK = 40
DIP = 70
RAKE = 20
MW = 4.55
HS = 106.0
The NDK file is 20230806001751.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 2.0 310 60 -15 3.53 0.1359
WVFGRD96 4.0 315 65 15 3.63 0.1625
WVFGRD96 6.0 315 65 20 3.70 0.1841
WVFGRD96 8.0 315 60 25 3.79 0.2002
WVFGRD96 10.0 315 55 25 3.84 0.2042
WVFGRD96 12.0 310 60 25 3.85 0.2013
WVFGRD96 14.0 310 60 25 3.88 0.1936
WVFGRD96 16.0 310 50 30 3.91 0.1825
WVFGRD96 18.0 315 45 35 3.94 0.1707
WVFGRD96 20.0 220 70 50 3.93 0.1570
WVFGRD96 22.0 220 70 45 3.95 0.1590
WVFGRD96 24.0 220 75 35 3.96 0.1599
WVFGRD96 26.0 220 80 25 3.97 0.1633
WVFGRD96 28.0 220 80 20 3.99 0.1679
WVFGRD96 30.0 220 80 15 4.00 0.1729
WVFGRD96 32.0 220 85 10 4.02 0.1784
WVFGRD96 34.0 40 90 -5 4.04 0.1829
WVFGRD96 36.0 40 90 0 4.07 0.1900
WVFGRD96 38.0 220 90 0 4.11 0.2008
WVFGRD96 40.0 35 80 5 4.15 0.2198
WVFGRD96 42.0 35 75 5 4.19 0.2326
WVFGRD96 44.0 215 80 -15 4.23 0.2518
WVFGRD96 46.0 215 85 -15 4.26 0.2808
WVFGRD96 48.0 35 80 20 4.28 0.3140
WVFGRD96 50.0 35 75 20 4.31 0.3553
WVFGRD96 52.0 40 75 15 4.35 0.3964
WVFGRD96 54.0 40 70 15 4.37 0.4301
WVFGRD96 56.0 40 70 10 4.39 0.4537
WVFGRD96 58.0 40 70 10 4.40 0.4696
WVFGRD96 60.0 40 70 10 4.42 0.4847
WVFGRD96 62.0 40 70 10 4.43 0.4968
WVFGRD96 64.0 40 70 10 4.44 0.5089
WVFGRD96 66.0 40 70 10 4.45 0.5180
WVFGRD96 68.0 40 70 10 4.46 0.5281
WVFGRD96 70.0 40 70 10 4.46 0.5371
WVFGRD96 72.0 40 70 10 4.47 0.5456
WVFGRD96 74.0 40 70 10 4.48 0.5523
WVFGRD96 76.0 40 70 10 4.48 0.5583
WVFGRD96 78.0 40 70 10 4.49 0.5633
WVFGRD96 80.0 40 70 15 4.49 0.5684
WVFGRD96 82.0 40 70 15 4.50 0.5734
WVFGRD96 84.0 40 70 15 4.50 0.5775
WVFGRD96 86.0 40 70 15 4.51 0.5815
WVFGRD96 88.0 40 70 15 4.51 0.5845
WVFGRD96 90.0 40 70 15 4.52 0.5876
WVFGRD96 92.0 40 70 15 4.52 0.5900
WVFGRD96 94.0 40 70 15 4.53 0.5921
WVFGRD96 96.0 40 70 15 4.53 0.5940
WVFGRD96 98.0 40 70 15 4.54 0.5955
WVFGRD96 100.0 40 70 15 4.54 0.5968
WVFGRD96 102.0 40 70 20 4.54 0.5982
WVFGRD96 104.0 40 70 20 4.54 0.5988
WVFGRD96 106.0 40 70 20 4.55 0.5996
WVFGRD96 108.0 40 70 20 4.55 0.5985
WVFGRD96 110.0 40 70 20 4.55 0.5990
WVFGRD96 112.0 40 70 20 4.56 0.5988
WVFGRD96 114.0 40 70 20 4.56 0.5986
WVFGRD96 116.0 40 70 20 4.57 0.5970
WVFGRD96 118.0 40 70 20 4.57 0.5964
WVFGRD96 120.0 40 70 20 4.57 0.5960
WVFGRD96 122.0 40 70 20 4.58 0.5946
WVFGRD96 124.0 40 70 20 4.58 0.5923
WVFGRD96 126.0 40 70 20 4.58 0.5925
WVFGRD96 128.0 40 70 20 4.59 0.5911
WVFGRD96 130.0 40 70 20 4.59 0.5890
WVFGRD96 132.0 40 70 20 4.59 0.5875
WVFGRD96 134.0 40 70 20 4.60 0.5862
WVFGRD96 136.0 40 70 20 4.60 0.5835
WVFGRD96 138.0 40 70 20 4.60 0.5821
WVFGRD96 140.0 40 70 20 4.60 0.5802
WVFGRD96 142.0 40 70 20 4.61 0.5772
WVFGRD96 144.0 40 70 20 4.61 0.5758
WVFGRD96 146.0 40 70 20 4.61 0.5730
WVFGRD96 148.0 40 70 20 4.62 0.5713
The best solution is
WVFGRD96 106.0 40 70 20 4.55 0.5996
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