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 ak019400r8q9 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak019400r8q9/executive.
2019/03/28 08:40:03 66.246 -157.186 5.9 4.1 Alaska
USGS/SLU Moment Tensor Solution
ENS 2019/03/28 08:40:03:0 66.25 -157.19 5.9 4.1 Alaska
Stations used:
AK.ANM AK.BPAW AK.BWN AK.CAST AK.CCB AK.CHUM AK.COLD
AK.GCSA AK.HDA AK.KTH AK.MCK AK.NEA2 AK.PPD AK.PPLA AK.RDOG
AK.RND AK.TNA AK.TRF AK.WRH IU.COLA TA.B18K TA.B21K TA.C17K
TA.C18K TA.C21K TA.C24K TA.D19K TA.D20K TA.D23K TA.E18K
TA.E19K TA.E20K TA.E23K TA.E24K TA.F14K TA.F15K TA.F17K
TA.F19K TA.F20K TA.F21K TA.F24K TA.F25K TA.G15K TA.G16K
TA.G17K TA.G18K TA.G21K TA.G23K TA.G24K TA.H16K TA.H17K
TA.H18K TA.H19K TA.H20K TA.H21K TA.H22K TA.H23K TA.H24K
TA.H25L TA.I20K TA.I23K TA.J16K TA.J17K TA.J18K TA.J19K
TA.J20K TA.K17K TA.K20K TA.L17K TA.L18K TA.L19K TA.L20K
TA.M18K TA.M19K TA.M20K TA.POKR TA.TOLK XV.F1TN XV.F2TN
XV.F3TN XV.F4TN XV.F5MN XV.F6TP XV.F7TV XV.F8KN XV.FAPT
XV.FPAP XV.FTGH
Filtering commands used:
cut o DIST/3.3 -40 o DIST/3.3 +60
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.08 n 3
Best Fitting Double Couple
Mo = 1.84e+22 dyne-cm
Mw = 4.11
Z = 11 km
Plane Strike Dip Rake
NP1 170 85 15
NP2 79 75 175
Principal Axes:
Axis Value Plunge Azimuth
T 1.84e+22 14 35
N 0.00e+00 74 188
P -1.84e+22 7 303
Moment Tensor: (dyne-cm)
Component Value
Mxx 6.03e+21
Mxy 1.65e+22
Mxz 2.34e+21
Myy -6.86e+21
Myz 4.35e+21
Mzz 8.27e+20
---###########
-------###############
-----------############# #
------------############# T ##
------------############# ####
P ------------#####################
- ------------######################
------------------######################
------------------######################
-------------------#####################--
--------------------################------
--------------------############----------
--------------------######----------------
##-------------#####--------------------
####################--------------------
###################-------------------
###################-----------------
##################----------------
#################-------------
################------------
##############--------
##########----
Global CMT Convention Moment Tensor:
R T P
8.27e+20 2.34e+21 -4.35e+21
2.34e+21 6.03e+21 -1.65e+22
-4.35e+21 -1.65e+22 -6.86e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190328084003/index.html
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STK = 170
DIP = 85
RAKE = 15
MW = 4.11
HS = 11.0
The NDK file is 20190328084003.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 +60 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 1.0 170 85 0 3.70 0.3642
WVFGRD96 2.0 170 85 10 3.82 0.4838
WVFGRD96 3.0 170 85 15 3.88 0.5407
WVFGRD96 4.0 170 80 15 3.92 0.5806
WVFGRD96 5.0 170 80 15 3.95 0.6109
WVFGRD96 6.0 170 80 15 3.99 0.6363
WVFGRD96 7.0 170 85 15 4.02 0.6600
WVFGRD96 8.0 170 85 20 4.05 0.6821
WVFGRD96 9.0 170 85 20 4.08 0.6914
WVFGRD96 10.0 170 85 20 4.09 0.6956
WVFGRD96 11.0 170 85 15 4.11 0.6956
WVFGRD96 12.0 170 85 15 4.12 0.6926
WVFGRD96 13.0 165 90 15 4.13 0.6864
WVFGRD96 14.0 165 90 15 4.14 0.6778
WVFGRD96 15.0 165 90 15 4.15 0.6670
WVFGRD96 16.0 165 90 15 4.16 0.6549
WVFGRD96 17.0 165 90 15 4.17 0.6417
WVFGRD96 18.0 345 90 -15 4.18 0.6276
WVFGRD96 19.0 165 90 15 4.19 0.6130
WVFGRD96 20.0 165 90 10 4.19 0.5986
WVFGRD96 21.0 165 90 10 4.20 0.5842
WVFGRD96 22.0 165 90 10 4.21 0.5694
WVFGRD96 23.0 345 85 -10 4.21 0.5558
WVFGRD96 24.0 345 85 -10 4.22 0.5411
WVFGRD96 25.0 345 85 -10 4.22 0.5265
WVFGRD96 26.0 345 85 -10 4.23 0.5120
WVFGRD96 27.0 345 85 -10 4.23 0.4980
WVFGRD96 28.0 165 90 10 4.23 0.4828
WVFGRD96 29.0 345 85 -10 4.24 0.4700
The best solution is
WVFGRD96 11.0 170 85 15 4.11 0.6956
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 +60 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