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 ak019edv3uuq and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak019edv3uuq/executive.
2019/11/09 13:26:10 67.063 -154.245 4.4 4.7 Alaska
USGS/SLU Moment Tensor Solution
ENS 2019/11/09 13:26:10:0 67.06 -154.24 4.4 4.7 Alaska
Stations used:
AK.BPAW AK.CCB AK.COLD AK.H21K AK.H23K AK.H24K AK.I23K
AK.J19K AK.J20K AK.K20K AK.KTH AK.MLY AK.NEA2 AK.RDOG
AK.TRF AK.WRH IM.IL31 IU.COLA TA.B21K TA.C18K TA.C19K
TA.D19K TA.D20K TA.D22K TA.D23K TA.D25K TA.E18K TA.E19K
TA.E21K TA.E22K TA.E23K TA.E24K TA.E25K TA.F20K TA.F21K
TA.F24K TA.F25K TA.F26K TA.G18K TA.G21K TA.G23K TA.G24K
TA.H17K TA.H18K TA.H19K TA.I20K TA.I21K TA.J18K TA.POKR
TA.TOLK
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 0.10 n 3
Best Fitting Double Couple
Mo = 7.59e+22 dyne-cm
Mw = 4.52
Z = 15 km
Plane Strike Dip Rake
NP1 110 81 150
NP2 205 60 10
Principal Axes:
Axis Value Plunge Azimuth
T 7.59e+22 27 63
N 0.00e+00 59 276
P -7.59e+22 14 161
Moment Tensor: (dyne-cm)
Component Value
Mxx -5.16e+22
Mxy 4.60e+22
Mxz 3.11e+22
Myy 4.02e+22
Myz 2.18e+22
Mzz 1.14e+22
--------------
-----------------#####
-----------------###########
----------------##############
----------------##################
---------------#####################
---------------################ ####
#--------------################# T #####
####----------################## #####
#########-----############################
##########################################
############-----#########################
############----------####################
##########-----------------#############
##########-------------------------#####
#########-----------------------------
########----------------------------
#######---------------------------
#####-------------------------
####-------------- -------
##------------- P ----
-----------
Global CMT Convention Moment Tensor:
R T P
1.14e+22 3.11e+22 -2.18e+22
3.11e+22 -5.16e+22 -4.60e+22
-2.18e+22 -4.60e+22 4.02e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20191109132610/index.html
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STK = 205
DIP = 60
RAKE = 10
MW = 4.52
HS = 15.0
The NDK file is 20191109132610.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 0.10 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 115 80 5 3.97 0.2628
WVFGRD96 2.0 340 50 -85 4.21 0.3403
WVFGRD96 3.0 300 70 35 4.19 0.3473
WVFGRD96 4.0 295 90 45 4.23 0.3721
WVFGRD96 5.0 110 85 -45 4.26 0.4122
WVFGRD96 6.0 110 85 -45 4.29 0.4461
WVFGRD96 7.0 110 85 -40 4.31 0.4744
WVFGRD96 8.0 210 50 15 4.38 0.5022
WVFGRD96 9.0 210 50 15 4.40 0.5336
WVFGRD96 10.0 210 55 15 4.43 0.5590
WVFGRD96 11.0 210 55 15 4.45 0.5780
WVFGRD96 12.0 210 55 15 4.46 0.5917
WVFGRD96 13.0 205 55 10 4.49 0.6021
WVFGRD96 14.0 205 60 10 4.50 0.6089
WVFGRD96 15.0 205 60 10 4.52 0.6118
WVFGRD96 16.0 205 60 10 4.53 0.6105
WVFGRD96 17.0 205 60 10 4.55 0.6058
WVFGRD96 18.0 205 60 10 4.56 0.5978
WVFGRD96 19.0 205 60 10 4.57 0.5868
WVFGRD96 20.0 205 60 10 4.57 0.5731
WVFGRD96 21.0 210 50 15 4.58 0.5581
WVFGRD96 22.0 210 50 15 4.59 0.5441
WVFGRD96 23.0 210 50 15 4.60 0.5296
WVFGRD96 24.0 210 55 20 4.60 0.5158
WVFGRD96 25.0 210 55 20 4.61 0.5029
WVFGRD96 26.0 210 55 20 4.61 0.4898
WVFGRD96 27.0 210 55 20 4.62 0.4764
WVFGRD96 28.0 210 55 20 4.62 0.4626
WVFGRD96 29.0 210 55 20 4.63 0.4487
The best solution is
WVFGRD96 15.0 205 60 10 4.52 0.6118
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 0.10 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