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 ak0193hwnu43 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0193hwnu43/executive.
2019/03/17 13:13:18 69.528 -144.202 5.0 3.4 Alaska
USGS/SLU Moment Tensor Solution
ENS 2019/03/17 13:13:18:0 69.53 -144.20 5.0 3.4 Alaska
Stations used:
AK.COLD AK.FYU AK.PPD TA.C23K TA.C24K TA.C26K TA.C27K
TA.D23K TA.D28M TA.E23K TA.E25K TA.E27K TA.E28M TA.E29M
TA.F24K TA.F25K TA.F26K TA.F30M TA.G23K TA.G24K TA.G25K
TA.G26K TA.G27K TA.G30M TA.H24K TA.H27K TA.I26K TA.I27K
TA.TOLK
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 0.08 n 3
br c 0.12 0.25 n 4 p 2
Best Fitting Double Couple
Mo = 2.66e+21 dyne-cm
Mw = 3.55
Z = 8 km
Plane Strike Dip Rake
NP1 200 75 35
NP2 100 56 162
Principal Axes:
Axis Value Plunge Azimuth
T 2.66e+21 35 65
N 0.00e+00 52 220
P -2.66e+21 12 326
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.44e+21
Mxy 1.86e+21
Mxz 7.80e+19
Myy 6.79e+20
Myz 1.43e+21
Mzz 7.63e+20
--------------
--------------######
-- P -------------##########
--- -----------#############
------------------################
------------------##################
------------------############ #####
------------------############# T ######
------------------############# ######
#-----------------########################
##---------------#########################
####-------------#########################
######----------##########################
########-------########################-
############--######################----
#############----############---------
###########-------------------------
##########------------------------
########----------------------
#######---------------------
####------------------
--------------
Global CMT Convention Moment Tensor:
R T P
7.63e+20 7.80e+19 -1.43e+21
7.80e+19 -1.44e+21 -1.86e+21
-1.43e+21 -1.86e+21 6.79e+20
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190317131318/index.html
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STK = 200
DIP = 75
RAKE = 35
MW = 3.55
HS = 8.0
The NDK file is 20190317131318.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 -30 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 n 3 br c 0.12 0.25 n 4 p 2The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 195 90 5 3.29 0.5252
WVFGRD96 2.0 15 80 -15 3.38 0.6070
WVFGRD96 3.0 15 80 -15 3.41 0.6200
WVFGRD96 4.0 195 90 30 3.46 0.6326
WVFGRD96 5.0 200 75 35 3.49 0.6531
WVFGRD96 6.0 200 75 30 3.50 0.6724
WVFGRD96 7.0 200 75 30 3.52 0.6825
WVFGRD96 8.0 200 75 35 3.55 0.6872
WVFGRD96 9.0 200 75 35 3.56 0.6863
WVFGRD96 10.0 200 80 35 3.57 0.6830
WVFGRD96 11.0 200 80 30 3.57 0.6790
WVFGRD96 12.0 195 90 25 3.58 0.6740
WVFGRD96 13.0 195 90 25 3.59 0.6698
WVFGRD96 14.0 195 90 25 3.60 0.6651
WVFGRD96 15.0 195 90 25 3.61 0.6590
WVFGRD96 16.0 195 90 25 3.62 0.6521
WVFGRD96 17.0 195 90 25 3.63 0.6445
WVFGRD96 18.0 15 90 -25 3.64 0.6358
WVFGRD96 19.0 15 90 -25 3.65 0.6254
WVFGRD96 20.0 15 90 -25 3.66 0.6144
WVFGRD96 21.0 15 90 -25 3.67 0.6028
WVFGRD96 22.0 15 85 -20 3.67 0.5901
WVFGRD96 23.0 15 85 -20 3.68 0.5771
WVFGRD96 24.0 195 90 25 3.69 0.5614
WVFGRD96 25.0 195 90 20 3.70 0.5464
WVFGRD96 26.0 195 90 20 3.70 0.5315
WVFGRD96 27.0 15 85 -20 3.71 0.5185
WVFGRD96 28.0 195 90 20 3.72 0.4998
WVFGRD96 29.0 15 85 -20 3.73 0.4871
The best solution is
WVFGRD96 8.0 200 75 35 3.55 0.6872
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 -30 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.08 n 3 br c 0.12 0.25 n 4 p 2
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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