The ANSS event ID is ak0199lpzfmd and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0199lpzfmd/executive.
2019/07/28 04:47:43 59.995 -152.680 103.9 3.8 Alaska
USGS/SLU Moment Tensor Solution
ENS 2019/07/28 04:47:43:0 59.99 -152.68 103.9 3.8 Alaska
Stations used:
AK.CAPN AK.CNP AK.HOM AK.RC01 AK.SKN AK.SLK AK.SSN AK.SWD
AT.PMR AV.ILSW AV.SPU II.KDAK TA.L19K TA.M19K TA.M20K
TA.M22K TA.N18K TA.N19K TA.O18K TA.O22K TA.P18K TA.P19K
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 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 1.26e+22 dyne-cm
Mw = 4.00
Z = 102 km
Plane Strike Dip Rake
NP1 65 65 30
NP2 321 63 152
Principal Axes:
Axis Value Plunge Azimuth
T 1.26e+22 38 284
N 0.00e+00 52 101
P -1.26e+22 1 193
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.15e+22
Mxy -4.50e+21
Mxz 1.72e+21
Myy 6.71e+21
Myz -5.89e+21
Mzz 4.82e+21
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##################------------------
#####################-----------------
#######################---------------##
###### ################-----------####
####### T #################---------######
####### ###################-----########
##############################--##########
#############################--###########
#########################------#########
#####################-----------########
###############----------------#######
-------------------------------#####
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---------------------------#
----- --------------
- P ----------
Global CMT Convention Moment Tensor:
R T P
4.82e+21 1.72e+21 5.89e+21
1.72e+21 -1.15e+22 4.50e+21
5.89e+21 4.50e+21 6.71e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190728044743/index.html
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STK = 65
DIP = 65
RAKE = 30
MW = 4.00
HS = 102.0
The NDK file is 20190728044743.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 0.03 n 3 lp c 0.10 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 300 50 -55 3.12 0.2105
WVFGRD96 4.0 140 90 30 3.12 0.2411
WVFGRD96 6.0 320 90 -30 3.19 0.2615
WVFGRD96 8.0 320 85 -35 3.28 0.2732
WVFGRD96 10.0 325 90 -30 3.32 0.2724
WVFGRD96 12.0 325 90 -30 3.36 0.2648
WVFGRD96 14.0 325 90 -30 3.38 0.2505
WVFGRD96 16.0 325 90 -25 3.40 0.2311
WVFGRD96 18.0 50 70 15 3.42 0.2138
WVFGRD96 20.0 50 70 10 3.44 0.2136
WVFGRD96 22.0 220 60 -30 3.47 0.2194
WVFGRD96 24.0 220 60 -30 3.50 0.2317
WVFGRD96 26.0 225 65 -25 3.53 0.2497
WVFGRD96 28.0 225 65 -20 3.55 0.2694
WVFGRD96 30.0 225 65 -20 3.57 0.2862
WVFGRD96 32.0 225 65 -20 3.59 0.2991
WVFGRD96 34.0 235 85 -15 3.61 0.3067
WVFGRD96 36.0 235 90 -15 3.64 0.3127
WVFGRD96 38.0 55 90 15 3.67 0.3171
WVFGRD96 40.0 60 80 25 3.74 0.3250
WVFGRD96 42.0 240 90 -30 3.80 0.3347
WVFGRD96 44.0 60 85 30 3.82 0.3483
WVFGRD96 46.0 60 85 30 3.84 0.3643
WVFGRD96 48.0 65 80 25 3.87 0.3800
WVFGRD96 50.0 65 80 25 3.89 0.3975
WVFGRD96 52.0 65 75 25 3.90 0.4145
WVFGRD96 54.0 65 70 25 3.90 0.4309
WVFGRD96 56.0 65 70 30 3.92 0.4489
WVFGRD96 58.0 65 70 30 3.93 0.4677
WVFGRD96 60.0 65 70 30 3.94 0.4849
WVFGRD96 62.0 65 70 30 3.95 0.5001
WVFGRD96 64.0 65 70 30 3.96 0.5145
WVFGRD96 66.0 65 70 30 3.96 0.5270
WVFGRD96 68.0 65 70 30 3.97 0.5371
WVFGRD96 70.0 65 70 30 3.97 0.5474
WVFGRD96 72.0 65 70 30 3.98 0.5551
WVFGRD96 74.0 65 70 30 3.98 0.5607
WVFGRD96 76.0 60 70 30 3.97 0.5679
WVFGRD96 78.0 60 70 30 3.97 0.5737
WVFGRD96 80.0 60 70 30 3.98 0.5776
WVFGRD96 82.0 60 70 30 3.98 0.5827
WVFGRD96 84.0 60 70 30 3.98 0.5854
WVFGRD96 86.0 60 70 30 3.98 0.5879
WVFGRD96 88.0 60 70 30 3.99 0.5890
WVFGRD96 90.0 60 70 30 3.99 0.5896
WVFGRD96 92.0 60 70 30 3.99 0.5894
WVFGRD96 94.0 60 70 30 3.99 0.5895
WVFGRD96 96.0 60 70 30 3.99 0.5905
WVFGRD96 98.0 60 70 30 4.00 0.5901
WVFGRD96 100.0 65 65 30 4.00 0.5907
WVFGRD96 102.0 65 65 30 4.00 0.5909
WVFGRD96 104.0 60 65 25 3.99 0.5902
WVFGRD96 106.0 60 65 25 4.00 0.5888
WVFGRD96 108.0 60 65 25 4.00 0.5883
WVFGRD96 110.0 60 65 25 4.00 0.5864
WVFGRD96 112.0 60 65 25 4.00 0.5867
WVFGRD96 114.0 60 65 25 4.01 0.5855
WVFGRD96 116.0 60 65 25 4.01 0.5849
WVFGRD96 118.0 65 60 25 4.01 0.5839
WVFGRD96 120.0 65 60 25 4.01 0.5826
WVFGRD96 122.0 65 60 25 4.02 0.5810
WVFGRD96 124.0 65 60 25 4.02 0.5780
WVFGRD96 126.0 65 60 25 4.02 0.5764
WVFGRD96 128.0 60 65 20 4.02 0.5745
WVFGRD96 130.0 60 65 20 4.02 0.5738
WVFGRD96 132.0 60 65 20 4.03 0.5721
WVFGRD96 134.0 60 65 20 4.03 0.5700
WVFGRD96 136.0 60 65 20 4.03 0.5660
WVFGRD96 138.0 60 65 20 4.03 0.5655
WVFGRD96 140.0 60 65 20 4.03 0.5634
WVFGRD96 142.0 60 65 20 4.04 0.5608
WVFGRD96 144.0 60 65 20 4.04 0.5571
WVFGRD96 146.0 65 60 20 4.04 0.5559
WVFGRD96 148.0 65 60 20 4.04 0.5542
The best solution is
WVFGRD96 102.0 65 65 30 4.00 0.5909
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