The ANSS event ID is ak019b0ydkn4 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak019b0ydkn4/executive.
2019/08/28 02:35:00 59.748 -153.824 157.1 4.1 Alaska
USGS/SLU Moment Tensor Solution
ENS 2019/08/28 02:35:00:0 59.75 -153.82 157.1 4.1 Alaska
Stations used:
AK.CAPN AK.CNP AK.HOM AK.SLK AV.STLK II.KDAK TA.N17K
TA.N18K TA.N19K TA.O19K TA.P18K TA.Q19K TA.Q20K
Filtering commands used:
cut o DIST/3.3 -40 o DIST/3.3 +50
rtr
taper w 0.1
hp c 0.05 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 1.78e+22 dyne-cm
Mw = 4.10
Z = 148 km
Plane Strike Dip Rake
NP1 204 80 -102
NP2 75 15 -40
Principal Axes:
Axis Value Plunge Azimuth
T 1.78e+22 34 304
N 0.00e+00 11 206
P -1.78e+22 53 100
Moment Tensor: (dyne-cm)
Component Value
Mxx 3.57e+21
Mxy -4.48e+21
Mxz 6.16e+21
Myy 2.15e+21
Myz -1.53e+22
Mzz -5.72e+21
##############
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####################--------
####################----------
####################--------------
##### ############----------------
###### T ###########------------------
####### ##########--------------------
###################---------------------
####################---------------------#
###################---------- ---------#
##################----------- P ---------#
#################------------ --------##
###############------------------------#
###############-----------------------##
#############-----------------------##
-##########-----------------------##
-#########---------------------###
--#####--------------------###
----##-----------------#####
---###--------########
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Global CMT Convention Moment Tensor:
R T P
-5.72e+21 6.16e+21 1.53e+22
6.16e+21 3.57e+21 4.48e+21
1.53e+22 4.48e+21 2.15e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190828023500/index.html
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STK = 75
DIP = 15
RAKE = -40
MW = 4.10
HS = 148.0
The NDK file is 20190828023500.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: 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 0.05 n 3 lp c 0.10 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 0 80 -15 3.05 0.1550
WVFGRD96 4.0 0 75 -15 3.16 0.1825
WVFGRD96 6.0 0 70 -5 3.23 0.1959
WVFGRD96 8.0 5 60 20 3.34 0.2139
WVFGRD96 10.0 0 80 30 3.37 0.2278
WVFGRD96 12.0 -5 90 35 3.42 0.2340
WVFGRD96 14.0 -5 90 35 3.46 0.2336
WVFGRD96 16.0 165 75 -45 3.51 0.2289
WVFGRD96 18.0 0 50 -10 3.57 0.2205
WVFGRD96 20.0 0 50 -10 3.59 0.2115
WVFGRD96 22.0 0 45 -10 3.63 0.2030
WVFGRD96 24.0 0 50 -10 3.63 0.1926
WVFGRD96 26.0 5 45 -5 3.67 0.1818
WVFGRD96 28.0 5 45 -5 3.67 0.1675
WVFGRD96 30.0 5 40 -5 3.68 0.1524
WVFGRD96 32.0 105 45 45 3.64 0.1382
WVFGRD96 34.0 110 40 45 3.68 0.1372
WVFGRD96 36.0 110 35 45 3.70 0.1348
WVFGRD96 38.0 110 40 45 3.71 0.1321
WVFGRD96 40.0 115 35 50 3.79 0.1212
WVFGRD96 42.0 40 55 35 3.84 0.1208
WVFGRD96 44.0 35 60 25 3.85 0.1212
WVFGRD96 46.0 35 65 30 3.87 0.1194
WVFGRD96 48.0 195 40 45 3.78 0.1168
WVFGRD96 50.0 185 55 25 3.74 0.1155
WVFGRD96 52.0 185 55 25 3.75 0.1153
WVFGRD96 54.0 185 60 25 3.75 0.1156
WVFGRD96 56.0 180 75 25 3.73 0.1167
WVFGRD96 58.0 180 80 25 3.74 0.1185
WVFGRD96 60.0 175 90 30 3.75 0.1207
WVFGRD96 62.0 175 90 30 3.76 0.1234
WVFGRD96 64.0 370 60 -45 3.93 0.1269
WVFGRD96 66.0 15 60 -50 3.93 0.1325
WVFGRD96 68.0 15 60 -50 3.94 0.1381
WVFGRD96 70.0 15 60 -50 3.94 0.1432
WVFGRD96 72.0 15 65 -55 3.93 0.1474
WVFGRD96 74.0 20 65 -50 3.93 0.1503
WVFGRD96 76.0 80 80 -10 3.80 0.1738
WVFGRD96 78.0 80 75 -5 3.84 0.2067
WVFGRD96 80.0 80 70 -5 3.87 0.2464
WVFGRD96 82.0 80 65 0 3.91 0.2906
WVFGRD96 84.0 80 65 0 3.93 0.3331
WVFGRD96 86.0 80 50 0 3.97 0.3690
WVFGRD96 88.0 80 50 0 3.98 0.3872
WVFGRD96 90.0 80 50 -5 3.98 0.3890
WVFGRD96 92.0 80 50 -5 3.99 0.3926
WVFGRD96 94.0 80 50 -5 3.99 0.4007
WVFGRD96 96.0 85 45 25 4.03 0.4132
WVFGRD96 98.0 80 15 -30 4.06 0.4254
WVFGRD96 100.0 85 10 -25 4.06 0.4353
WVFGRD96 102.0 75 10 -40 4.07 0.4417
WVFGRD96 104.0 70 10 -45 4.08 0.4472
WVFGRD96 106.0 60 10 -55 4.08 0.4510
WVFGRD96 108.0 60 10 -55 4.08 0.4526
WVFGRD96 110.0 60 10 -55 4.08 0.4548
WVFGRD96 112.0 10 10 -110 4.10 0.4584
WVFGRD96 114.0 10 10 -105 4.10 0.4619
WVFGRD96 116.0 205 80 -90 4.10 0.4625
WVFGRD96 118.0 10 10 -105 4.10 0.4634
WVFGRD96 120.0 205 80 -90 4.10 0.4660
WVFGRD96 122.0 20 10 -95 4.10 0.4672
WVFGRD96 124.0 35 10 -80 4.10 0.4674
WVFGRD96 126.0 205 80 -90 4.10 0.4690
WVFGRD96 128.0 40 10 -75 4.10 0.4701
WVFGRD96 130.0 40 10 -75 4.10 0.4701
WVFGRD96 132.0 205 80 -90 4.10 0.4711
WVFGRD96 134.0 40 10 -75 4.10 0.4719
WVFGRD96 136.0 40 10 -75 4.10 0.4707
WVFGRD96 138.0 205 80 -95 4.10 0.4716
WVFGRD96 140.0 45 10 -70 4.10 0.4727
WVFGRD96 142.0 75 15 -40 4.10 0.4704
WVFGRD96 144.0 75 15 -40 4.10 0.4724
WVFGRD96 146.0 75 15 -40 4.10 0.4716
WVFGRD96 148.0 75 15 -40 4.10 0.4733
The best solution is
WVFGRD96 148.0 75 15 -40 4.10 0.4733
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 0.05 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