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 ak019bj6l40j and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak019bj6l40j/executive.
2019/09/08 04:52:55 60.053 -140.638 9.5 3.5 Alaska
USGS/SLU Moment Tensor Solution
ENS 2019/09/08 04:52:55:0 60.05 -140.64 9.5 3.5 Alaska
Stations used:
AK.BARN AK.BCP AK.BERG AK.BMR AK.CRQ AK.CTG AK.DIV AK.DOT
AK.GLB AK.GRNC AK.HIN AK.HMT AK.ISLE AK.KLU AK.LOGN AK.MCAR
AK.MESA AK.PAX AK.PIN AK.SCM AK.SSP AK.TABL AK.TGL AK.VRDI
AK.WAX AK.YAH AT.MENT AT.SKAG CN.HYT CN.WHY
Filtering commands used:
cut o DIST/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.08 n 3
Best Fitting Double Couple
Mo = 5.13e+21 dyne-cm
Mw = 3.74
Z = 13 km
Plane Strike Dip Rake
NP1 99 69 112
NP2 230 30 45
Principal Axes:
Axis Value Plunge Azimuth
T 5.13e+21 60 41
N 0.00e+00 21 271
P -5.13e+21 21 172
Moment Tensor: (dyne-cm)
Component Value
Mxx -3.63e+21
Mxy 1.23e+21
Mxz 3.41e+21
Myy 4.88e+20
Myz 1.24e+21
Mzz 3.14e+21
--------------
----------------------
-------------############---
---------#####################
---------#########################
-------#############################
-------################ ############
------################## T #############
-----################### #############
#----#####################################
###--#####################################
####--####################################
####-------###############################
##--------------#####################---
##--------------------------------------
##------------------------------------
#-----------------------------------
#---------------------------------
---------------- -----------
--------------- P ----------
------------ -------
--------------
Global CMT Convention Moment Tensor:
R T P
3.14e+21 3.41e+21 -1.24e+21
3.41e+21 -3.63e+21 -1.23e+21
-1.24e+21 -1.23e+21 4.88e+20
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190908045255/index.html
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STK = 230
DIP = 30
RAKE = 45
MW = 3.74
HS = 13.0
The NDK file is 20190908045255.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 +50 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 40 50 -105 3.33 0.2986
WVFGRD96 2.0 45 50 -95 3.46 0.3779
WVFGRD96 3.0 75 65 -55 3.49 0.3173
WVFGRD96 4.0 220 90 -65 3.51 0.3562
WVFGRD96 5.0 80 85 70 3.56 0.4181
WVFGRD96 6.0 260 90 -65 3.57 0.4643
WVFGRD96 7.0 85 80 70 3.58 0.4995
WVFGRD96 8.0 85 80 75 3.66 0.5195
WVFGRD96 9.0 240 20 60 3.69 0.5402
WVFGRD96 10.0 230 25 50 3.70 0.5588
WVFGRD96 11.0 235 25 55 3.71 0.5713
WVFGRD96 12.0 230 30 50 3.72 0.5777
WVFGRD96 13.0 230 30 45 3.74 0.5789
WVFGRD96 14.0 230 30 45 3.74 0.5752
WVFGRD96 15.0 225 35 40 3.75 0.5701
WVFGRD96 16.0 225 35 40 3.76 0.5616
WVFGRD96 17.0 225 35 40 3.76 0.5503
WVFGRD96 18.0 225 40 40 3.77 0.5378
WVFGRD96 19.0 225 40 40 3.78 0.5251
WVFGRD96 20.0 225 40 40 3.78 0.5112
WVFGRD96 21.0 225 40 40 3.79 0.4961
WVFGRD96 22.0 225 40 40 3.79 0.4811
WVFGRD96 23.0 225 40 40 3.80 0.4649
WVFGRD96 24.0 220 40 35 3.80 0.4486
WVFGRD96 25.0 210 70 45 3.79 0.4379
WVFGRD96 26.0 210 70 45 3.79 0.4266
WVFGRD96 27.0 210 70 45 3.80 0.4151
WVFGRD96 28.0 210 70 45 3.80 0.4033
WVFGRD96 29.0 225 55 60 3.80 0.3939
The best solution is
WVFGRD96 13.0 230 30 45 3.74 0.5789
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 +50 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