The ANSS event ID is ak0198ddkciq and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0198ddkciq/executive.
2019/07/01 23:10:34 61.217 -146.884 20.0 3.7 Alaska
USGS/SLU Moment Tensor Solution
ENS 2019/07/01 23:10:34:0 61.22 -146.88 20.0 3.7 Alaska
Stations used:
AK.BARN AK.BRLK AK.CAST AK.CNP AK.CRQ AK.CUT AK.DHY AK.DIV
AK.DOT AK.EYAK AK.FID AK.FIRE AK.GLB AK.GLI AK.HDA AK.HIN
AK.HMT AK.KLU AK.KNK AK.KTH AK.MCAR AK.MCK AK.PAX AK.PPLA
AK.RC01 AK.RIDG AK.SCM AK.SKN AK.SLK AK.SUCK AK.SWD AK.TRF
AK.WAX AK.YAH AT.MENT AT.PMR AV.STLK IU.COLA TA.J25K
TA.J26L TA.K24K TA.K27K TA.L26K TA.L27K TA.M22K TA.M24K
TA.M27K TA.N25K TA.O22K TA.P23K
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 0.07 n 3
Best Fitting Double Couple
Mo = 5.50e+21 dyne-cm
Mw = 3.76
Z = 32 km
Plane Strike Dip Rake
NP1 260 85 -65
NP2 1 25 -168
Principal Axes:
Axis Value Plunge Azimuth
T 5.50e+21 35 329
N 0.00e+00 25 78
P -5.50e+21 44 195
Moment Tensor: (dyne-cm)
Component Value
Mxx 4.75e+19
Mxy -2.32e+21
Mxz 4.87e+21
Myy 8.17e+20
Myz -6.52e+20
Mzz -8.65e+20
###########---
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###### #################----
######## T ##################-----
######### ###################-----
#################################-----
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###################################-----
###############################------#####
###################------------------#####
##########---------------------------#####
###---------------------------------######
-----------------------------------#####
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--------------- ---------------#####
-------------- P --------------#####
------------- -------------#####
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Global CMT Convention Moment Tensor:
R T P
-8.65e+20 4.87e+21 6.52e+20
4.87e+21 4.75e+19 2.32e+21
6.52e+20 2.32e+21 8.17e+20
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190701231034/index.html
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STK = 260
DIP = 85
RAKE = -65
MW = 3.76
HS = 32.0
The NDK file is 20190701231034.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 -30 o DIST/3.3 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.07 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 90 40 90 3.24 0.2429
WVFGRD96 2.0 95 40 95 3.38 0.3343
WVFGRD96 3.0 265 45 90 3.44 0.3260
WVFGRD96 4.0 40 75 15 3.38 0.2869
WVFGRD96 5.0 40 80 15 3.41 0.2869
WVFGRD96 6.0 35 70 -15 3.43 0.2956
WVFGRD96 7.0 260 90 -55 3.41 0.3219
WVFGRD96 8.0 260 90 -60 3.48 0.3467
WVFGRD96 9.0 85 85 65 3.50 0.3819
WVFGRD96 10.0 85 85 65 3.51 0.4127
WVFGRD96 11.0 90 80 65 3.53 0.4414
WVFGRD96 12.0 90 80 65 3.54 0.4680
WVFGRD96 13.0 90 80 65 3.55 0.4923
WVFGRD96 14.0 90 80 65 3.57 0.5151
WVFGRD96 15.0 90 80 65 3.58 0.5362
WVFGRD96 16.0 85 85 60 3.59 0.5557
WVFGRD96 17.0 260 85 -60 3.60 0.5738
WVFGRD96 18.0 85 85 65 3.61 0.5919
WVFGRD96 19.0 260 85 -60 3.63 0.6094
WVFGRD96 20.0 80 90 60 3.64 0.6242
WVFGRD96 21.0 80 90 65 3.65 0.6393
WVFGRD96 22.0 260 85 -65 3.67 0.6552
WVFGRD96 23.0 80 90 65 3.68 0.6682
WVFGRD96 24.0 80 90 65 3.69 0.6810
WVFGRD96 25.0 260 85 -65 3.70 0.6953
WVFGRD96 26.0 80 90 65 3.71 0.7023
WVFGRD96 27.0 260 85 -65 3.72 0.7156
WVFGRD96 28.0 80 90 65 3.73 0.7181
WVFGRD96 29.0 260 85 -65 3.74 0.7292
WVFGRD96 30.0 80 90 65 3.75 0.7273
WVFGRD96 31.0 80 90 65 3.76 0.7301
WVFGRD96 32.0 260 85 -65 3.76 0.7362
WVFGRD96 33.0 260 85 -65 3.77 0.7355
WVFGRD96 34.0 80 90 65 3.77 0.7294
WVFGRD96 35.0 255 85 -65 3.78 0.7307
WVFGRD96 36.0 80 90 65 3.78 0.7241
WVFGRD96 37.0 255 85 -65 3.79 0.7230
WVFGRD96 38.0 80 90 65 3.79 0.7162
WVFGRD96 39.0 260 85 -65 3.79 0.7141
WVFGRD96 40.0 80 90 75 3.93 0.7061
WVFGRD96 41.0 260 90 -75 3.93 0.7020
WVFGRD96 42.0 80 90 75 3.94 0.6972
WVFGRD96 43.0 260 90 -75 3.94 0.6923
WVFGRD96 44.0 80 90 75 3.95 0.6867
WVFGRD96 45.0 85 85 75 3.95 0.6810
WVFGRD96 46.0 85 85 75 3.95 0.6753
WVFGRD96 47.0 260 90 -75 3.96 0.6678
WVFGRD96 48.0 85 85 75 3.96 0.6627
WVFGRD96 49.0 85 85 75 3.97 0.6562
WVFGRD96 50.0 85 85 75 3.97 0.6497
WVFGRD96 51.0 85 85 75 3.97 0.6429
WVFGRD96 52.0 85 85 75 3.98 0.6357
WVFGRD96 53.0 260 90 -75 3.98 0.6256
WVFGRD96 54.0 265 90 -80 3.99 0.6187
WVFGRD96 55.0 85 80 75 3.99 0.6153
WVFGRD96 56.0 170 -10 -5 4.00 0.6050
WVFGRD96 57.0 160 10 -15 4.01 0.5990
WVFGRD96 58.0 160 15 -15 4.02 0.5935
WVFGRD96 59.0 165 15 -10 4.02 0.5878
The best solution is
WVFGRD96 32.0 260 85 -65 3.76 0.7362
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 0.07 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