The ANSS event ID is ak018frkncdg and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak018frkncdg/executive.
2018/12/09 19:00:32 61.420 -149.837 41.2 4.7 Alaska
USGS/SLU Moment Tensor Solution
ENS 2018/12/09 19:00:32:0 61.42 -149.84 41.2 4.7 Alaska
Stations used:
AK.BRLK AK.CNP AK.DHY AK.FID AK.FIRE AK.GLI AK.KNK AK.PWL
AK.RC01 AK.RND AK.SAW AK.SCM AK.SKN AK.SLK AK.SSN AK.SWD
AT.PMR AV.ILSW AV.STLK TA.M22K
Filtering commands used:
cut o DIST/3.3 -40 o DIST/3.3 +40
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.08 n 3
Best Fitting Double Couple
Mo = 1.11e+23 dyne-cm
Mw = 4.63
Z = 48 km
Plane Strike Dip Rake
NP1 165 60 -85
NP2 335 30 -99
Principal Axes:
Axis Value Plunge Azimuth
T 1.11e+23 15 251
N 0.00e+00 4 342
P -1.11e+23 74 88
Moment Tensor: (dyne-cm)
Component Value
Mxx 1.06e+22
Mxy 3.12e+22
Mxz -9.63e+21
Myy 8.51e+22
Myz -5.46e+22
Mzz -9.57e+22
-#############
####--------##########
#######------------#########
#######---------------########
#########-----------------########
#########--------------------#######
##########---------------------#######
###########----------------------#######
###########-----------------------######
#############----------- ---------######
#############----------- P ---------######
#############----------- ---------######
##############----------------------######
## ########----------------------#####
## T #########---------------------#####
# ##########--------------------####
##############------------------####
##############-----------------###
#############---------------##
##############------------##
#############--------#
###########---
Global CMT Convention Moment Tensor:
R T P
-9.57e+22 -9.63e+21 5.46e+22
-9.63e+21 1.06e+22 -3.12e+22
5.46e+22 -3.12e+22 8.51e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20181209190032/index.html
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STK = 165
DIP = 60
RAKE = -85
MW = 4.63
HS = 48.0
The NDK file is 20181209190032.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 +40 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 345 45 90 3.81 0.1753
WVFGRD96 2.0 350 40 90 3.95 0.2282
WVFGRD96 3.0 165 50 80 4.00 0.2077
WVFGRD96 4.0 315 80 65 4.01 0.2119
WVFGRD96 5.0 310 85 70 4.03 0.2465
WVFGRD96 6.0 305 85 70 4.04 0.2784
WVFGRD96 7.0 305 85 70 4.05 0.3036
WVFGRD96 8.0 310 75 85 4.14 0.3239
WVFGRD96 9.0 310 75 85 4.15 0.3473
WVFGRD96 10.0 140 15 100 4.16 0.3677
WVFGRD96 11.0 145 20 105 4.17 0.3859
WVFGRD96 12.0 310 70 85 4.19 0.4019
WVFGRD96 13.0 310 70 90 4.20 0.4158
WVFGRD96 14.0 140 20 100 4.21 0.4274
WVFGRD96 15.0 135 20 95 4.22 0.4363
WVFGRD96 16.0 125 20 80 4.23 0.4438
WVFGRD96 17.0 120 20 75 4.24 0.4492
WVFGRD96 18.0 100 20 55 4.24 0.4535
WVFGRD96 19.0 85 25 35 4.25 0.4572
WVFGRD96 20.0 85 25 35 4.26 0.4604
WVFGRD96 21.0 90 20 40 4.28 0.4629
WVFGRD96 22.0 90 20 40 4.29 0.4633
WVFGRD96 23.0 90 20 40 4.30 0.4631
WVFGRD96 24.0 90 20 40 4.31 0.4613
WVFGRD96 25.0 95 15 45 4.32 0.4596
WVFGRD96 26.0 320 80 85 4.33 0.4583
WVFGRD96 27.0 105 10 55 4.33 0.4548
WVFGRD96 28.0 100 10 50 4.34 0.4513
WVFGRD96 29.0 80 15 25 4.34 0.4465
WVFGRD96 30.0 75 15 20 4.35 0.4420
WVFGRD96 31.0 35 50 -45 4.36 0.4438
WVFGRD96 32.0 350 25 -85 4.37 0.4679
WVFGRD96 33.0 165 65 -90 4.38 0.4931
WVFGRD96 34.0 340 30 -95 4.40 0.5203
WVFGRD96 35.0 165 60 -90 4.41 0.5439
WVFGRD96 36.0 340 30 -95 4.42 0.5647
WVFGRD96 37.0 165 60 -85 4.43 0.5831
WVFGRD96 38.0 165 60 -85 4.44 0.5980
WVFGRD96 39.0 165 55 -85 4.46 0.6097
WVFGRD96 40.0 165 60 -85 4.56 0.6194
WVFGRD96 41.0 165 60 -85 4.57 0.6299
WVFGRD96 42.0 165 60 -85 4.58 0.6388
WVFGRD96 43.0 165 60 -85 4.59 0.6454
WVFGRD96 44.0 165 60 -85 4.60 0.6509
WVFGRD96 45.0 165 60 -85 4.61 0.6547
WVFGRD96 46.0 165 60 -85 4.61 0.6570
WVFGRD96 47.0 165 60 -85 4.62 0.6582
WVFGRD96 48.0 165 60 -85 4.63 0.6587
WVFGRD96 49.0 165 60 -85 4.63 0.6570
WVFGRD96 50.0 165 60 -85 4.64 0.6557
WVFGRD96 51.0 170 60 -80 4.64 0.6522
WVFGRD96 52.0 165 60 -85 4.64 0.6488
WVFGRD96 53.0 165 60 -80 4.65 0.6444
WVFGRD96 54.0 165 60 -80 4.65 0.6399
WVFGRD96 55.0 165 60 -80 4.65 0.6342
WVFGRD96 56.0 165 60 -80 4.66 0.6289
WVFGRD96 57.0 165 60 -80 4.66 0.6225
WVFGRD96 58.0 165 60 -80 4.66 0.6157
WVFGRD96 59.0 165 60 -80 4.66 0.6093
The best solution is
WVFGRD96 48.0 165 60 -85 4.63 0.6587
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 +40 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 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