The ANSS event ID is ak018fe5jk85 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak018fe5jk85/executive.
2018/12/01 07:57:22 61.355 -149.991 42.9 5.1 Alaska
USGS/SLU Moment Tensor Solution
ENS 2018/12/01 07:57:22:0 61.35 -149.99 42.9 5.1 Alaska
Stations used:
AK.BRLK AK.CAPN AK.CAST AK.CNP AK.CUT AK.DHY AK.FID AK.GHO
AK.GLI AK.HOM AK.KNK AK.PWL AK.RND AK.SAW AK.SCM AK.SKN
AK.SLK AK.SSN AK.SWD AV.ILSW AV.STLK TA.M19K TA.M20K
TA.M22K TA.O19K TA.O22K TA.P19K
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 = 3.85e+23 dyne-cm
Mw = 4.99
Z = 48 km
Plane Strike Dip Rake
NP1 195 55 -65
NP2 336 42 -121
Principal Axes:
Axis Value Plunge Azimuth
T 3.85e+23 7 267
N 0.00e+00 20 0
P -3.85e+23 69 160
Moment Tensor: (dyne-cm)
Component Value
Mxx -4.46e+22
Mxy 3.34e+22
Mxz 1.21e+23
Myy 3.72e+23
Myz -9.10e+22
Mzz -3.28e+23
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#########---##########
#############---############
############-------###########
#############----------###########
#############-------------##########
#############---------------##########
#############-----------------##########
############-------------------#########
#############--------------------#########
#########---------------------#########
T #########----------------------########
#########--------- ----------########
##########---------- P ----------#######
##########---------- ----------#######
#########-----------------------######
#########----------------------#####
########---------------------#####
######---------------------###
######-------------------###
####-----------------#
#-------------
Global CMT Convention Moment Tensor:
R T P
-3.28e+23 1.21e+23 9.10e+22
1.21e+23 -4.46e+22 -3.34e+22
9.10e+22 -3.34e+22 3.72e+23
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20181201075722/index.html
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STK = 195
DIP = 55
RAKE = -65
MW = 4.99
HS = 48.0
The NDK file is 20181201075722.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 1.0 355 45 85 4.12 0.1527
WVFGRD96 2.0 355 45 85 4.27 0.2036
WVFGRD96 3.0 340 35 60 4.32 0.1933
WVFGRD96 4.0 320 35 30 4.31 0.2061
WVFGRD96 5.0 125 60 -35 4.33 0.2287
WVFGRD96 6.0 130 65 -30 4.35 0.2456
WVFGRD96 7.0 130 65 -30 4.38 0.2601
WVFGRD96 8.0 125 60 -35 4.45 0.2669
WVFGRD96 9.0 125 60 -35 4.46 0.2721
WVFGRD96 10.0 125 60 -35 4.48 0.2761
WVFGRD96 11.0 125 60 -35 4.50 0.2790
WVFGRD96 12.0 125 60 -35 4.51 0.2795
WVFGRD96 13.0 125 60 -35 4.53 0.2782
WVFGRD96 14.0 125 60 -35 4.54 0.2766
WVFGRD96 15.0 125 60 -35 4.55 0.2739
WVFGRD96 16.0 230 65 35 4.56 0.2755
WVFGRD96 17.0 230 65 35 4.58 0.2761
WVFGRD96 18.0 230 65 30 4.59 0.2777
WVFGRD96 19.0 35 80 45 4.59 0.2819
WVFGRD96 20.0 35 80 45 4.60 0.2853
WVFGRD96 21.0 35 80 50 4.62 0.2896
WVFGRD96 22.0 40 80 50 4.64 0.2948
WVFGRD96 23.0 40 80 50 4.65 0.3006
WVFGRD96 24.0 40 80 50 4.67 0.3062
WVFGRD96 25.0 35 85 50 4.67 0.3116
WVFGRD96 26.0 35 85 50 4.69 0.3172
WVFGRD96 27.0 35 90 50 4.70 0.3245
WVFGRD96 28.0 215 90 -50 4.71 0.3325
WVFGRD96 29.0 210 80 -50 4.72 0.3430
WVFGRD96 30.0 210 75 -50 4.73 0.3550
WVFGRD96 31.0 210 75 -50 4.74 0.3702
WVFGRD96 32.0 210 75 -50 4.75 0.3857
WVFGRD96 33.0 210 65 -50 4.76 0.4027
WVFGRD96 34.0 210 65 -50 4.77 0.4191
WVFGRD96 35.0 210 65 -50 4.78 0.4343
WVFGRD96 36.0 210 65 -50 4.79 0.4470
WVFGRD96 37.0 205 60 -55 4.80 0.4587
WVFGRD96 38.0 205 60 -55 4.81 0.4684
WVFGRD96 39.0 205 60 -55 4.82 0.4789
WVFGRD96 40.0 200 60 -60 4.91 0.4791
WVFGRD96 41.0 200 60 -60 4.92 0.4864
WVFGRD96 42.0 200 55 -60 4.93 0.4937
WVFGRD96 43.0 200 55 -60 4.94 0.4995
WVFGRD96 44.0 200 55 -60 4.95 0.5043
WVFGRD96 45.0 200 55 -60 4.96 0.5068
WVFGRD96 46.0 195 55 -65 4.97 0.5105
WVFGRD96 47.0 195 55 -65 4.98 0.5108
WVFGRD96 48.0 195 55 -65 4.99 0.5126
WVFGRD96 49.0 195 55 -65 4.99 0.5117
WVFGRD96 50.0 195 55 -65 5.00 0.5112
WVFGRD96 51.0 195 55 -65 5.00 0.5097
WVFGRD96 52.0 195 55 -65 5.00 0.5071
WVFGRD96 53.0 195 55 -65 5.01 0.5050
WVFGRD96 54.0 195 55 -65 5.01 0.5010
WVFGRD96 55.0 195 50 -65 5.01 0.4988
WVFGRD96 56.0 190 50 -70 5.01 0.4948
WVFGRD96 57.0 195 50 -65 5.01 0.4929
WVFGRD96 58.0 190 50 -70 5.02 0.4887
WVFGRD96 59.0 195 50 -65 5.01 0.4844
The best solution is
WVFGRD96 48.0 195 55 -65 4.99 0.5126
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