The ANSS event ID is ak0184nb2fzw and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0184nb2fzw/executive.
2018/04/11 16:44:11 60.579 -147.613 6.6 4 Alaska
USGS/SLU Moment Tensor Solution
ENS 2018/04/11 16:44:11:0 60.58 -147.61 6.6 4.0 Alaska
Stations used:
AK.CRQ AK.DIV AK.EYAK AK.FID AK.GHO AK.GLI AK.HIN AK.KLU
AK.KNK AK.RC01 AK.RND AK.SAW AK.SWD AK.TRF AT.PMR TA.L26K
TA.M26K TA.N25K TA.O22K
Filtering commands used:
cut o DIST/3.3 -30 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 = 1.17e+22 dyne-cm
Mw = 3.98
Z = 27 km
Plane Strike Dip Rake
NP1 205 65 -75
NP2 353 29 -119
Principal Axes:
Axis Value Plunge Azimuth
T 1.17e+22 19 284
N 0.00e+00 14 19
P -1.17e+22 67 142
Moment Tensor: (dyne-cm)
Component Value
Mxx -5.58e+20
Mxy -1.56e+21
Mxz 4.25e+21
Myy 9.25e+21
Myz -6.07e+21
Mzz -8.69e+21
#########-----
################-#####
##################----######
#################-------######
#################-----------######
#################-------------######
################----------------######
## ###########------------------######
## T ##########-------------------######
### #########---------------------######
##############----------------------######
##############----------------------######
#############---------- ----------######
############---------- P ----------#####
###########----------- ---------######
##########-----------------------#####
#########----------------------#####
#######----------------------#####
#####---------------------####
#####-------------------####
##-----------------###
------------##
Global CMT Convention Moment Tensor:
R T P
-8.69e+21 4.25e+21 6.07e+21
4.25e+21 -5.58e+20 1.56e+21
6.07e+21 1.56e+21 9.25e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20180411164411/index.html
|
STK = 205
DIP = 65
RAKE = -75
MW = 3.98
HS = 27.0
The NDK file is 20180411164411.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.
 |
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.
|
|
|
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 +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 1.0 185 45 85 3.37 0.2649
WVFGRD96 2.0 185 45 85 3.53 0.3768
WVFGRD96 3.0 175 40 75 3.58 0.3316
WVFGRD96 4.0 135 50 20 3.55 0.2915
WVFGRD96 5.0 120 50 -15 3.58 0.3120
WVFGRD96 6.0 120 55 -20 3.61 0.3419
WVFGRD96 7.0 130 60 -30 3.62 0.3685
WVFGRD96 8.0 330 30 25 3.68 0.3910
WVFGRD96 9.0 325 35 20 3.70 0.4145
WVFGRD96 10.0 335 35 30 3.72 0.4345
WVFGRD96 11.0 335 35 30 3.74 0.4490
WVFGRD96 12.0 330 35 30 3.75 0.4591
WVFGRD96 13.0 40 55 -45 3.78 0.4695
WVFGRD96 14.0 40 55 -45 3.80 0.4937
WVFGRD96 15.0 205 65 -70 3.80 0.5188
WVFGRD96 16.0 205 65 -70 3.82 0.5462
WVFGRD96 17.0 205 65 -70 3.84 0.5706
WVFGRD96 18.0 200 65 -75 3.85 0.5921
WVFGRD96 19.0 200 65 -80 3.87 0.6120
WVFGRD96 20.0 210 65 -65 3.89 0.6303
WVFGRD96 21.0 210 65 -70 3.91 0.6451
WVFGRD96 22.0 205 65 -70 3.92 0.6590
WVFGRD96 23.0 205 65 -70 3.94 0.6703
WVFGRD96 24.0 205 65 -75 3.95 0.6794
WVFGRD96 25.0 205 65 -75 3.96 0.6859
WVFGRD96 26.0 205 65 -75 3.97 0.6900
WVFGRD96 27.0 205 65 -75 3.98 0.6911
WVFGRD96 28.0 205 65 -80 3.99 0.6886
WVFGRD96 29.0 200 65 -80 4.00 0.6821
WVFGRD96 30.0 200 65 -85 4.01 0.6714
WVFGRD96 31.0 5 25 -105 4.01 0.6563
WVFGRD96 32.0 200 65 -85 4.02 0.6358
WVFGRD96 33.0 195 65 -90 4.02 0.6129
WVFGRD96 34.0 10 25 -95 4.02 0.5866
WVFGRD96 35.0 10 25 -95 4.02 0.5582
WVFGRD96 36.0 195 65 -90 4.02 0.5310
WVFGRD96 37.0 190 65 -90 4.02 0.5049
WVFGRD96 38.0 190 65 -90 4.02 0.4835
WVFGRD96 39.0 190 65 -90 4.02 0.4654
WVFGRD96 40.0 195 70 -85 4.12 0.4476
WVFGRD96 41.0 25 45 -65 4.14 0.4411
WVFGRD96 42.0 25 45 -65 4.15 0.4395
WVFGRD96 43.0 25 45 -65 4.16 0.4329
WVFGRD96 44.0 25 45 -65 4.16 0.4229
WVFGRD96 45.0 25 45 -65 4.17 0.4120
WVFGRD96 46.0 30 45 -60 4.17 0.4001
WVFGRD96 47.0 30 45 -60 4.18 0.3884
WVFGRD96 48.0 60 50 25 4.13 0.3826
WVFGRD96 49.0 60 50 25 4.14 0.3833
The best solution is
WVFGRD96 27.0 205 65 -75 3.98 0.6911
The mechanism corresponding to the best fit is
|
|
|
The best fit as a function of depth is given in the following figure:
|
|
|
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 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3
|
| 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. |
|
| 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