Left: mLg computed using the IASPEI formula. Center: mLg residuals versus epicentral distance ; the values used for the trimmed mean magnitude estimate are indicated. Right: residuals as a function of distance and azimuth.
The ANSS event ID is ak018fo7boo4 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak018fo7boo4/executive.
2018/12/07 15:22:57 69.594 -144.947 6.9 4.2 Alaska
USGS/SLU Moment Tensor Solution
ENS 2018/12/07 15:22:57:0 69.59 -144.95 6.9 4.2 Alaska
Stations used:
AK.FYU TA.C24K TA.C26K TA.C27K TA.D23K TA.D25K TA.D27M
TA.D28M TA.E24K TA.E27K TA.E28M TA.E29M TA.F24K TA.F25K
TA.F26K TA.G23K TA.G24K TA.G27K TA.H24K TA.TOLK
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 = 2.51e+22 dyne-cm
Mw = 4.20
Z = 10 km
Plane Strike Dip Rake
NP1 271 85 -170
NP2 180 80 -5
Principal Axes:
Axis Value Plunge Azimuth
T 2.51e+22 4 45
N 0.00e+00 79 297
P -2.51e+22 11 136
Moment Tensor: (dyne-cm)
Component Value
Mxx -4.31e+15
Mxy 2.46e+22
Mxz 4.35e+21
Myy 7.49e+20
Myz -2.06e+21
Mzz -7.49e+20
-------#######
----------############
-------------##############
-------------############### T
---------------############### #
----------------####################
-----------------#####################
------------------######################
------------------######################
------------------########################
------############------------############
##################------------------------
##################------------------------
##################----------------------
##################----------------------
#################---------------------
################--------------------
###############------------- ---
#############------------- P -
#############------------
##########------------
#######-------
Global CMT Convention Moment Tensor:
R T P
-7.49e+20 4.35e+21 2.06e+21
4.35e+21 -4.31e+15 -2.46e+22
2.06e+21 -2.46e+22 7.49e+20
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20181207152257/index.html
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STK = 180
DIP = 80
RAKE = -5
MW = 4.20
HS = 10.0
The NDK file is 20181207152257.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: mLg computed using the IASPEI formula. Center: mLg residuals versus epicentral distance ; the values used for the trimmed mean magnitude estimate are indicated.
Right: residuals as a function of distance and azimuth.
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 5 90 -5 3.81 0.3224
WVFGRD96 2.0 5 75 25 3.96 0.4289
WVFGRD96 3.0 180 80 -5 3.98 0.4687
WVFGRD96 4.0 180 80 -10 4.03 0.5014
WVFGRD96 5.0 180 80 -5 4.06 0.5268
WVFGRD96 6.0 180 80 -10 4.10 0.5491
WVFGRD96 7.0 180 80 -5 4.13 0.5675
WVFGRD96 8.0 180 80 -10 4.16 0.5838
WVFGRD96 9.0 180 80 -5 4.18 0.5889
WVFGRD96 10.0 180 80 -5 4.20 0.5900
WVFGRD96 11.0 180 80 -5 4.22 0.5892
WVFGRD96 12.0 180 80 -5 4.23 0.5858
WVFGRD96 13.0 180 80 -5 4.25 0.5794
WVFGRD96 14.0 180 85 -5 4.26 0.5708
WVFGRD96 15.0 0 85 5 4.27 0.5605
WVFGRD96 16.0 0 85 5 4.28 0.5526
WVFGRD96 17.0 0 85 5 4.29 0.5441
WVFGRD96 18.0 0 85 5 4.29 0.5345
WVFGRD96 19.0 0 85 5 4.30 0.5255
WVFGRD96 20.0 0 85 5 4.31 0.5182
WVFGRD96 21.0 0 85 10 4.32 0.5111
WVFGRD96 22.0 0 85 5 4.32 0.5073
WVFGRD96 23.0 0 85 10 4.33 0.5033
WVFGRD96 24.0 0 85 5 4.34 0.5013
WVFGRD96 25.0 0 85 5 4.34 0.5005
WVFGRD96 26.0 0 85 5 4.35 0.5002
WVFGRD96 27.0 0 85 10 4.36 0.5004
WVFGRD96 28.0 0 85 10 4.36 0.4991
WVFGRD96 29.0 0 85 10 4.37 0.4998
WVFGRD96 30.0 0 85 10 4.38 0.4980
WVFGRD96 31.0 0 85 10 4.38 0.4945
WVFGRD96 32.0 0 85 10 4.39 0.4920
WVFGRD96 33.0 0 85 10 4.40 0.4861
WVFGRD96 34.0 180 85 -10 4.41 0.4795
WVFGRD96 35.0 180 85 -10 4.42 0.4724
WVFGRD96 36.0 0 85 10 4.42 0.4673
WVFGRD96 37.0 0 85 10 4.43 0.4603
WVFGRD96 38.0 180 85 -10 4.45 0.4556
WVFGRD96 39.0 0 85 10 4.46 0.4540
WVFGRD96 40.0 5 80 15 4.50 0.4544
WVFGRD96 41.0 5 80 15 4.51 0.4560
WVFGRD96 42.0 5 80 15 4.52 0.4547
WVFGRD96 43.0 5 80 15 4.53 0.4544
WVFGRD96 44.0 5 85 15 4.54 0.4524
WVFGRD96 45.0 5 85 15 4.55 0.4518
WVFGRD96 46.0 5 85 15 4.56 0.4499
WVFGRD96 47.0 5 85 15 4.57 0.4487
WVFGRD96 48.0 5 85 15 4.58 0.4475
WVFGRD96 49.0 5 80 20 4.58 0.4461
WVFGRD96 50.0 5 80 20 4.58 0.4458
WVFGRD96 51.0 5 80 25 4.59 0.4436
WVFGRD96 52.0 5 80 25 4.60 0.4447
WVFGRD96 53.0 5 80 25 4.60 0.4433
WVFGRD96 54.0 5 80 25 4.61 0.4432
WVFGRD96 55.0 5 80 25 4.61 0.4422
WVFGRD96 56.0 5 80 25 4.62 0.4410
WVFGRD96 57.0 5 80 25 4.62 0.4405
WVFGRD96 58.0 5 80 25 4.63 0.4401
WVFGRD96 59.0 5 80 25 4.63 0.4384
The best solution is
WVFGRD96 10.0 180 80 -5 4.20 0.5900
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