The ANSS event ID is ak024asoq1gu and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak024asoq1gu/executive.
2024/08/22 02:40:21 61.253 -147.035 18.9 3.8 Alaska
USGS/SLU Moment Tensor Solution
ENS 2024/08/22 02:40:21:0 61.25 -147.04 18.9 3.8 Alaska
Stations used:
AK.BAE AK.DHY AK.DIV AK.EYAK AK.FID AK.GHO AK.GLI AK.HIN
AK.KLU AK.KNK AK.PWL AK.RC01 AK.SAW AK.SCM
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 = 6.10e+21 dyne-cm
Mw = 3.79
Z = 43 km
Plane Strike Dip Rake
NP1 225 70 -70
NP2 358 28 -133
Principal Axes:
Axis Value Plunge Azimuth
T 6.10e+21 22 300
N 0.00e+00 19 38
P -6.10e+21 60 164
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.18e+20
Mxy -1.84e+21
Mxz 3.61e+21
Myy 3.80e+21
Myz -2.60e+21
Mzz -3.68e+21
##########----
#################-----
#######################-----
#########################--###
########################-----#####
## #################---------#####
### T ###############-----------######
#### ############---------------######
#################------------------#####
#################-------------------######
###############---------------------######
#############-----------------------######
############------------------------######
##########-------------------------#####
########------------- ----------######
######-------------- P ----------#####
#####-------------- ---------#####
###--------------------------#####
--------------------------####
-----------------------#####
------------------####
-----------###
Global CMT Convention Moment Tensor:
R T P
-3.68e+21 3.61e+21 2.60e+21
3.61e+21 -1.18e+20 1.84e+21
2.60e+21 1.84e+21 3.80e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20240822024021/index.html
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STK = 225
DIP = 70
RAKE = -70
MW = 3.79
HS = 43.0
The NDK file is 20240822024021.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 1.0 20 50 75 2.92 0.2085
WVFGRD96 2.0 210 45 90 3.10 0.3036
WVFGRD96 3.0 25 50 85 3.17 0.3097
WVFGRD96 4.0 335 50 -35 3.14 0.3272
WVFGRD96 5.0 335 60 -40 3.17 0.3664
WVFGRD96 6.0 335 60 -35 3.20 0.3940
WVFGRD96 7.0 335 60 -30 3.22 0.4131
WVFGRD96 8.0 335 60 -40 3.29 0.4168
WVFGRD96 9.0 335 60 -35 3.30 0.4228
WVFGRD96 10.0 265 55 35 3.33 0.4345
WVFGRD96 11.0 265 50 35 3.36 0.4461
WVFGRD96 12.0 265 50 35 3.38 0.4550
WVFGRD96 13.0 265 50 35 3.40 0.4615
WVFGRD96 14.0 265 50 35 3.41 0.4652
WVFGRD96 15.0 265 50 30 3.43 0.4676
WVFGRD96 16.0 245 65 -30 3.42 0.4675
WVFGRD96 17.0 245 65 -30 3.44 0.4805
WVFGRD96 18.0 245 65 -35 3.45 0.4928
WVFGRD96 19.0 245 65 -35 3.47 0.5052
WVFGRD96 20.0 245 65 -35 3.48 0.5176
WVFGRD96 21.0 245 65 -35 3.50 0.5283
WVFGRD96 22.0 245 70 -35 3.50 0.5371
WVFGRD96 23.0 245 70 -35 3.52 0.5501
WVFGRD96 24.0 240 65 -40 3.53 0.5623
WVFGRD96 25.0 240 65 -40 3.54 0.5725
WVFGRD96 26.0 240 65 -40 3.55 0.5804
WVFGRD96 27.0 240 75 -50 3.56 0.5926
WVFGRD96 28.0 235 75 -55 3.58 0.6154
WVFGRD96 29.0 235 75 -55 3.59 0.6379
WVFGRD96 30.0 230 75 -60 3.60 0.6567
WVFGRD96 31.0 225 70 -65 3.62 0.6735
WVFGRD96 32.0 225 70 -65 3.63 0.6874
WVFGRD96 33.0 225 70 -65 3.64 0.6989
WVFGRD96 34.0 225 70 -65 3.64 0.7043
WVFGRD96 35.0 225 70 -65 3.65 0.7062
WVFGRD96 36.0 225 70 -65 3.65 0.7091
WVFGRD96 37.0 225 70 -65 3.66 0.7087
WVFGRD96 38.0 225 70 -65 3.66 0.7077
WVFGRD96 39.0 225 70 -65 3.67 0.7076
WVFGRD96 40.0 220 70 -75 3.78 0.7160
WVFGRD96 41.0 220 70 -75 3.78 0.7203
WVFGRD96 42.0 225 70 -70 3.79 0.7230
WVFGRD96 43.0 225 70 -70 3.79 0.7240
WVFGRD96 44.0 225 70 -70 3.80 0.7220
WVFGRD96 45.0 225 70 -70 3.80 0.7187
WVFGRD96 46.0 225 70 -70 3.81 0.7142
WVFGRD96 47.0 225 70 -70 3.81 0.7096
WVFGRD96 48.0 220 70 -70 3.81 0.7042
WVFGRD96 49.0 220 70 -70 3.82 0.6981
WVFGRD96 50.0 225 70 -65 3.82 0.6920
WVFGRD96 51.0 225 70 -65 3.82 0.6851
WVFGRD96 52.0 225 75 -65 3.82 0.6783
WVFGRD96 53.0 225 75 -65 3.82 0.6727
WVFGRD96 54.0 225 75 -65 3.82 0.6658
WVFGRD96 55.0 225 75 -65 3.83 0.6599
WVFGRD96 56.0 225 70 -65 3.83 0.6523
WVFGRD96 57.0 225 70 -65 3.83 0.6457
WVFGRD96 58.0 225 70 -65 3.83 0.6399
WVFGRD96 59.0 225 70 -65 3.83 0.6323
The best solution is
WVFGRD96 43.0 225 70 -70 3.79 0.7240
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