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 ak019esunz4t and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak019esunz4t/executive.
2019/11/18 19:51:28 66.315 -157.229 12.9 3.9 Alaska
USGS/SLU Moment Tensor Solution
ENS 2019/11/18 19:51:28:0 66.32 -157.23 12.9 3.9 Alaska
Stations used:
AK.ANM AK.BPAW AK.BWN AK.COLD AK.H21K AK.H22K AK.I23K
AK.J17K AK.J20K AK.KTH AK.MLY AK.NEA2 AK.RDOG TA.C16K
TA.C19K TA.D20K TA.D22K TA.E18K TA.G19K TA.G23K TA.G24K
TA.I17K 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 = 1.30e+22 dyne-cm
Mw = 4.01
Z = 11 km
Plane Strike Dip Rake
NP1 330 70 -45
NP2 79 48 -153
Principal Axes:
Axis Value Plunge Azimuth
T 1.30e+22 13 29
N 0.00e+00 42 131
P -1.30e+22 45 285
Moment Tensor: (dyne-cm)
Component Value
Mxx 8.98e+21
Mxy 6.89e+21
Mxz 8.00e+20
Myy -3.06e+21
Myz 7.69e+21
Mzz -5.92e+21
##############
-################# #
-------############## T ####
-----------########### #####
--------------####################
-----------------###################
--------------------##################
----------------------##################
-------- ------------#################
--------- P -------------################-
--------- --------------##############--
---------------------------############---
----------------------------#########-----
----------------------------######------
##---------------------------###--------
####-----------------------#----------
########------------########--------
###########################-------
#########################-----
########################----
#####################-
##############
Global CMT Convention Moment Tensor:
R T P
-5.92e+21 8.00e+20 -7.69e+21
8.00e+20 8.98e+21 -6.89e+21
-7.69e+21 -6.89e+21 -3.06e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20191118195128/index.html
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STK = 330
DIP = 70
RAKE = -45
MW = 4.01
HS = 11.0
The NDK file is 20191118195128.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 1.0 335 85 -10 3.59 0.2994
WVFGRD96 2.0 335 75 -25 3.71 0.3383
WVFGRD96 3.0 335 80 -35 3.77 0.3341
WVFGRD96 4.0 175 65 45 3.82 0.3550
WVFGRD96 5.0 270 30 40 3.87 0.3867
WVFGRD96 6.0 325 65 -50 3.88 0.4331
WVFGRD96 7.0 325 65 -50 3.90 0.4693
WVFGRD96 8.0 325 65 -55 3.98 0.5007
WVFGRD96 9.0 325 65 -55 3.99 0.5234
WVFGRD96 10.0 325 65 -50 3.99 0.5341
WVFGRD96 11.0 330 70 -45 4.01 0.5376
WVFGRD96 12.0 330 70 -45 4.02 0.5370
WVFGRD96 13.0 330 70 -40 4.03 0.5327
WVFGRD96 14.0 330 70 -40 4.04 0.5245
WVFGRD96 15.0 330 70 -40 4.05 0.5127
WVFGRD96 16.0 330 70 -40 4.06 0.5001
WVFGRD96 17.0 150 70 -45 4.07 0.4923
WVFGRD96 18.0 150 70 -45 4.08 0.4809
WVFGRD96 19.0 150 70 -45 4.09 0.4682
WVFGRD96 20.0 150 70 -45 4.10 0.4531
WVFGRD96 21.0 150 70 -45 4.11 0.4384
WVFGRD96 22.0 150 70 -45 4.12 0.4215
WVFGRD96 23.0 150 70 -45 4.12 0.4040
WVFGRD96 24.0 150 70 -40 4.13 0.3865
WVFGRD96 25.0 150 65 -40 4.13 0.3699
WVFGRD96 26.0 150 65 -40 4.13 0.3544
WVFGRD96 27.0 150 65 -40 4.13 0.3390
WVFGRD96 28.0 150 65 -40 4.14 0.3241
WVFGRD96 29.0 150 60 -40 4.14 0.3099
The best solution is
WVFGRD96 11.0 330 70 -45 4.01 0.5376
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