The ANSS event ID is ak0199ltduib and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0199ltduib/executive.
2019/07/28 10:18:57 63.210 -150.528 123.1 4.1 Alaska
USGS/SLU Moment Tensor Solution
ENS 2019/07/28 10:18:57:0 63.21 -150.53 123.1 4.1 Alaska
Stations used:
AK.BPAW AK.BWN AK.CAST AK.CCB AK.CUT AK.GHO AK.HDA AK.KNK
AK.KTH AK.MCK AK.MLY AK.NEA2 AK.PPLA AK.RND AK.SCM AK.SKN
AK.SLK AK.SSN AK.TRF AK.WRH AT.PMR AV.STLK IM.IL31 IU.COLA
TA.H21K TA.H23K TA.H24K TA.I23K TA.J19K TA.J20K TA.J25K
TA.K20K TA.L19K TA.M19K TA.M22K TA.POKR
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 = 5.19e+22 dyne-cm
Mw = 4.41
Z = 126 km
Plane Strike Dip Rake
NP1 200 70 -70
NP2 333 28 -133
Principal Axes:
Axis Value Plunge Azimuth
T 5.19e+22 22 275
N 0.00e+00 19 13
P -5.19e+22 60 139
Moment Tensor: (dyne-cm)
Component Value
Mxx -7.05e+21
Mxy 2.70e+21
Mxz 1.85e+22
Myy 3.84e+22
Myz -3.30e+22
Mzz -3.13e+22
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#############---######
#################--#########
#################------#######
#################----------#######
#################------------#######
#################---------------######
#################-----------------######
################------------------######
### ###########-------------------######
### T ##########---------------------#####
### #########----------------------#####
###############----------------------#####
#############----------- ---------####
#############----------- P ---------####
############----------- --------####
##########-----------------------###
#########----------------------###
#######---------------------##
######--------------------##
###------------------#
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Global CMT Convention Moment Tensor:
R T P
-3.13e+22 1.85e+22 3.30e+22
1.85e+22 -7.05e+21 -2.70e+21
3.30e+22 -2.70e+21 3.84e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190728101857/index.html
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STK = 200
DIP = 70
RAKE = -70
MW = 4.41
HS = 126.0
The NDK file is 20190728101857.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 2.0 185 45 85 3.50 0.2084
WVFGRD96 4.0 340 75 55 3.49 0.1904
WVFGRD96 6.0 160 80 50 3.54 0.2280
WVFGRD96 8.0 5 70 70 3.66 0.2600
WVFGRD96 10.0 155 80 45 3.67 0.2746
WVFGRD96 12.0 325 80 -45 3.70 0.2837
WVFGRD96 14.0 325 80 -45 3.74 0.2876
WVFGRD96 16.0 65 35 -25 3.78 0.2858
WVFGRD96 18.0 340 75 -50 3.80 0.2820
WVFGRD96 20.0 65 35 -25 3.83 0.2770
WVFGRD96 22.0 65 35 -25 3.86 0.2738
WVFGRD96 24.0 65 35 -25 3.89 0.2682
WVFGRD96 26.0 40 50 -40 3.89 0.2638
WVFGRD96 28.0 40 50 -35 3.91 0.2611
WVFGRD96 30.0 40 55 -35 3.92 0.2579
WVFGRD96 32.0 40 55 -35 3.93 0.2493
WVFGRD96 34.0 40 55 -35 3.94 0.2381
WVFGRD96 36.0 120 50 40 3.96 0.2350
WVFGRD96 38.0 120 55 35 3.99 0.2391
WVFGRD96 40.0 125 50 40 4.07 0.2385
WVFGRD96 42.0 125 55 45 4.09 0.2382
WVFGRD96 44.0 40 50 -35 4.10 0.2406
WVFGRD96 46.0 40 55 -35 4.12 0.2437
WVFGRD96 48.0 40 55 -35 4.14 0.2478
WVFGRD96 50.0 40 55 -30 4.15 0.2525
WVFGRD96 52.0 40 55 -30 4.17 0.2593
WVFGRD96 54.0 45 60 -15 4.17 0.2673
WVFGRD96 56.0 45 60 -15 4.19 0.2781
WVFGRD96 58.0 220 65 -45 4.15 0.2932
WVFGRD96 60.0 220 65 -45 4.17 0.3199
WVFGRD96 62.0 215 60 -50 4.19 0.3442
WVFGRD96 64.0 215 60 -50 4.21 0.3671
WVFGRD96 66.0 215 60 -50 4.22 0.3865
WVFGRD96 68.0 215 60 -50 4.23 0.4051
WVFGRD96 70.0 215 60 -55 4.24 0.4233
WVFGRD96 72.0 215 65 -55 4.25 0.4488
WVFGRD96 74.0 215 65 -55 4.27 0.4860
WVFGRD96 76.0 215 65 -55 4.28 0.5243
WVFGRD96 78.0 215 70 -55 4.30 0.5685
WVFGRD96 80.0 200 70 -65 4.32 0.6118
WVFGRD96 82.0 200 70 -65 4.33 0.6386
WVFGRD96 84.0 200 70 -65 4.33 0.6556
WVFGRD96 86.0 200 70 -65 4.34 0.6698
WVFGRD96 88.0 200 70 -65 4.35 0.6844
WVFGRD96 90.0 200 70 -65 4.35 0.6967
WVFGRD96 92.0 200 70 -65 4.35 0.7091
WVFGRD96 94.0 200 70 -70 4.36 0.7201
WVFGRD96 96.0 200 70 -70 4.36 0.7304
WVFGRD96 98.0 200 70 -70 4.37 0.7418
WVFGRD96 100.0 200 70 -70 4.37 0.7515
WVFGRD96 102.0 200 70 -70 4.38 0.7600
WVFGRD96 104.0 200 70 -70 4.38 0.7679
WVFGRD96 106.0 200 70 -70 4.38 0.7750
WVFGRD96 108.0 200 70 -70 4.39 0.7823
WVFGRD96 110.0 200 70 -70 4.39 0.7869
WVFGRD96 112.0 200 70 -70 4.39 0.7897
WVFGRD96 114.0 200 70 -70 4.40 0.7966
WVFGRD96 116.0 200 70 -70 4.40 0.8001
WVFGRD96 118.0 200 70 -70 4.40 0.8010
WVFGRD96 120.0 200 70 -70 4.40 0.8049
WVFGRD96 122.0 200 70 -70 4.41 0.8063
WVFGRD96 124.0 200 70 -70 4.41 0.8057
WVFGRD96 126.0 200 70 -70 4.41 0.8086
WVFGRD96 128.0 205 70 -70 4.41 0.8071
WVFGRD96 130.0 205 70 -70 4.41 0.8067
WVFGRD96 132.0 205 70 -70 4.42 0.8069
WVFGRD96 134.0 205 70 -70 4.42 0.8032
WVFGRD96 136.0 205 70 -70 4.42 0.8022
WVFGRD96 138.0 205 70 -70 4.42 0.7985
WVFGRD96 140.0 205 70 -70 4.42 0.7954
WVFGRD96 142.0 200 65 -75 4.43 0.7922
WVFGRD96 144.0 200 65 -75 4.43 0.7883
WVFGRD96 146.0 200 65 -75 4.43 0.7863
WVFGRD96 148.0 200 65 -75 4.43 0.7815
The best solution is
WVFGRD96 126.0 200 70 -70 4.41 0.8086
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