The ANSS event ID is ak018fj96qbd and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak018fj96qbd/executive.
2018/12/04 16:02:28 61.394 -150.076 38.3 4.5 Alaska
USGS/SLU Moment Tensor Solution
ENS 2018/12/04 16:02:28:0 61.39 -150.08 38.3 4.5 Alaska
Stations used:
AK.CNP AK.CUT AK.FID AK.GHO AK.GLI AK.KNK AK.PWL AK.RC01
AK.SAW AK.SCM AK.SKN AK.SSN AT.PMR AV.ILSW AV.STLK TA.M22K
Filtering commands used:
cut o DIST/3.3 -30 o DIST/3.3 +70
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 6.84e+22 dyne-cm
Mw = 4.49
Z = 47 km
Plane Strike Dip Rake
NP1 185 60 -60
NP2 316 41 -131
Principal Axes:
Axis Value Plunge Azimuth
T 6.84e+22 10 254
N 0.00e+00 26 349
P -6.84e+22 62 144
Moment Tensor: (dyne-cm)
Component Value
Mxx -4.75e+21
Mxy 2.47e+22
Mxz 1.96e+22
Myy 5.60e+22
Myz -2.80e+22
Mzz -5.13e+22
-------#######
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---######--#################
###########-----##############
############---------#############
#############------------###########
#############---------------##########
#############-----------------##########
#############-------------------########
##############--------------------########
#############----------------------#######
#############-----------------------######
# #########-----------------------######
T #########----------- ---------#####
#########----------- P ----------####
############---------- ----------###
###########-----------------------##
##########----------------------##
#########---------------------
#########-------------------
#######---------------
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Global CMT Convention Moment Tensor:
R T P
-5.13e+22 1.96e+22 2.80e+22
1.96e+22 -4.75e+21 -2.47e+22
2.80e+22 -2.47e+22 5.60e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20181204160228/index.html
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STK = 185
DIP = 60
RAKE = -60
MW = 4.49
HS = 47.0
The NDK file is 20181204160228.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 -30 o DIST/3.3 +70 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 1.0 335 40 75 3.61 0.1839
WVFGRD96 2.0 330 45 70 3.77 0.2549
WVFGRD96 3.0 315 40 45 3.82 0.2686
WVFGRD96 4.0 300 50 10 3.81 0.2827
WVFGRD96 5.0 295 50 -10 3.84 0.3003
WVFGRD96 6.0 295 50 -10 3.86 0.3180
WVFGRD96 7.0 115 65 -25 3.90 0.3337
WVFGRD96 8.0 110 60 -35 3.97 0.3433
WVFGRD96 9.0 110 55 -30 3.98 0.3550
WVFGRD96 10.0 110 55 -30 4.01 0.3657
WVFGRD96 11.0 110 55 -30 4.02 0.3735
WVFGRD96 12.0 25 90 40 4.02 0.3768
WVFGRD96 13.0 25 90 40 4.03 0.3857
WVFGRD96 14.0 30 80 40 4.05 0.3932
WVFGRD96 15.0 30 80 40 4.07 0.4006
WVFGRD96 16.0 30 80 40 4.09 0.4075
WVFGRD96 17.0 30 80 40 4.10 0.4136
WVFGRD96 18.0 25 90 40 4.11 0.4204
WVFGRD96 19.0 25 90 40 4.12 0.4271
WVFGRD96 20.0 25 90 40 4.13 0.4338
WVFGRD96 21.0 25 90 45 4.15 0.4412
WVFGRD96 22.0 25 90 45 4.16 0.4497
WVFGRD96 23.0 25 90 45 4.17 0.4582
WVFGRD96 24.0 25 90 45 4.19 0.4689
WVFGRD96 25.0 25 90 45 4.20 0.4818
WVFGRD96 26.0 205 85 -45 4.21 0.4933
WVFGRD96 27.0 205 85 -45 4.22 0.5085
WVFGRD96 28.0 200 80 -45 4.23 0.5202
WVFGRD96 29.0 200 80 -50 4.24 0.5367
WVFGRD96 30.0 200 80 -50 4.26 0.5525
WVFGRD96 31.0 200 75 -45 4.26 0.5711
WVFGRD96 32.0 195 70 -50 4.28 0.5887
WVFGRD96 33.0 195 70 -50 4.28 0.6065
WVFGRD96 34.0 195 70 -50 4.29 0.6191
WVFGRD96 35.0 195 70 -50 4.30 0.6285
WVFGRD96 36.0 195 65 -45 4.31 0.6385
WVFGRD96 37.0 195 65 -45 4.32 0.6449
WVFGRD96 38.0 195 65 -45 4.33 0.6522
WVFGRD96 39.0 195 65 -45 4.34 0.6572
WVFGRD96 40.0 190 65 -55 4.42 0.6448
WVFGRD96 41.0 190 65 -55 4.43 0.6535
WVFGRD96 42.0 185 60 -60 4.44 0.6604
WVFGRD96 43.0 185 60 -60 4.45 0.6662
WVFGRD96 44.0 185 60 -60 4.46 0.6720
WVFGRD96 45.0 185 60 -60 4.47 0.6750
WVFGRD96 46.0 185 60 -60 4.48 0.6766
WVFGRD96 47.0 185 60 -60 4.49 0.6776
WVFGRD96 48.0 185 60 -60 4.49 0.6759
WVFGRD96 49.0 185 60 -60 4.50 0.6748
WVFGRD96 50.0 185 60 -60 4.50 0.6706
WVFGRD96 51.0 185 60 -60 4.50 0.6674
WVFGRD96 52.0 185 60 -60 4.51 0.6637
WVFGRD96 53.0 185 60 -60 4.51 0.6590
WVFGRD96 54.0 185 60 -60 4.51 0.6543
WVFGRD96 55.0 185 60 -60 4.52 0.6482
WVFGRD96 56.0 185 60 -60 4.52 0.6412
WVFGRD96 57.0 180 60 -65 4.53 0.6360
WVFGRD96 58.0 180 60 -65 4.53 0.6280
WVFGRD96 59.0 180 60 -65 4.53 0.6209
The best solution is
WVFGRD96 47.0 185 60 -60 4.49 0.6776
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 -30 o DIST/3.3 +70 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