The ANSS event ID is ak020406rmbp and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak020406rmbp/executive.
2020/03/27 18:49:57 61.420 -146.560 20.1 3.9 Alaska
USGS/SLU Moment Tensor Solution
ENS 2020/03/27 18:49:57:0 61.42 -146.56 20.1 3.9 Alaska
Stations used:
AK.BMR AK.DHY AK.DIV AK.EYAK AK.GLB AK.GLI AK.HIN AK.KLU
AK.KNK AK.RC01 AK.SCM AK.SWD AT.PMR TA.N25K
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 = 5.69e+21 dyne-cm
Mw = 3.77
Z = 32 km
Plane Strike Dip Rake
NP1 347 62 -139
NP2 235 55 -35
Principal Axes:
Axis Value Plunge Azimuth
T 5.69e+21 4 109
N 0.00e+00 42 16
P -5.69e+21 48 204
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.53e+21
Mxy -2.75e+21
Mxz 2.45e+21
Myy 4.60e+21
Myz 1.55e+21
Mzz -3.07e+21
####----------
##########------------
##############--------------
#################-------------
####################--############
##################----##############
################--------##############
##############-----------###############
############--------------##############
###########-----------------##############
#########-------------------##############
########--------------------##############
#######----------------------#############
#####-----------------------#########
####------------------------######### T
##----------- -----------#########
#----------- P -----------##########
----------- -----------#########
----------------------########
---------------------#######
-----------------#####
------------##
Global CMT Convention Moment Tensor:
R T P
-3.07e+21 2.45e+21 -1.55e+21
2.45e+21 -1.53e+21 2.75e+21
-1.55e+21 2.75e+21 4.60e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20200327184957/index.html
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STK = 235
DIP = 55
RAKE = -35
MW = 3.77
HS = 32.0
The NDK file is 20200327184957.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 2.0 350 60 55 3.26 0.3587
WVFGRD96 4.0 330 90 35 3.31 0.4130
WVFGRD96 6.0 145 80 -30 3.39 0.4549
WVFGRD96 8.0 145 75 -35 3.47 0.4757
WVFGRD96 10.0 145 75 -35 3.51 0.4794
WVFGRD96 12.0 250 65 20 3.52 0.4764
WVFGRD96 14.0 250 65 20 3.56 0.5095
WVFGRD96 16.0 250 60 15 3.60 0.5392
WVFGRD96 18.0 250 60 10 3.63 0.5698
WVFGRD96 20.0 245 55 -10 3.66 0.5987
WVFGRD96 22.0 245 55 -15 3.69 0.6302
WVFGRD96 24.0 245 55 -20 3.70 0.6519
WVFGRD96 26.0 240 55 -25 3.73 0.6722
WVFGRD96 28.0 240 60 -30 3.74 0.6974
WVFGRD96 30.0 235 55 -35 3.76 0.7185
WVFGRD96 32.0 235 55 -35 3.77 0.7254
WVFGRD96 34.0 235 60 -35 3.78 0.7219
WVFGRD96 36.0 235 60 -35 3.79 0.7125
WVFGRD96 38.0 235 60 -35 3.80 0.6998
WVFGRD96 40.0 235 60 -40 3.87 0.6947
WVFGRD96 42.0 235 60 -40 3.90 0.6958
WVFGRD96 44.0 235 60 -40 3.91 0.6877
WVFGRD96 46.0 235 60 -40 3.93 0.6792
WVFGRD96 48.0 235 60 -40 3.94 0.6681
WVFGRD96 50.0 240 65 -35 3.94 0.6591
WVFGRD96 52.0 240 65 -35 3.95 0.6519
WVFGRD96 54.0 240 65 -30 3.95 0.6452
WVFGRD96 56.0 240 65 -30 3.96 0.6401
WVFGRD96 58.0 240 65 -30 3.97 0.6337
WVFGRD96 60.0 240 65 -30 3.97 0.6262
WVFGRD96 62.0 240 65 -30 3.98 0.6219
WVFGRD96 64.0 240 70 -30 3.98 0.6160
WVFGRD96 66.0 240 70 -30 3.99 0.6134
WVFGRD96 68.0 240 70 -25 3.99 0.6112
WVFGRD96 70.0 240 70 -25 4.00 0.6077
WVFGRD96 72.0 240 70 -25 4.00 0.6048
WVFGRD96 74.0 240 75 -20 4.01 0.6012
WVFGRD96 76.0 240 80 -15 4.02 0.5990
WVFGRD96 78.0 240 80 -15 4.02 0.5964
WVFGRD96 80.0 240 85 -10 4.03 0.5939
WVFGRD96 82.0 240 85 -10 4.04 0.5917
WVFGRD96 84.0 240 85 -10 4.04 0.5898
WVFGRD96 86.0 60 90 10 4.05 0.5847
WVFGRD96 88.0 60 90 10 4.05 0.5810
WVFGRD96 90.0 240 85 -10 4.06 0.5808
WVFGRD96 92.0 60 90 5 4.07 0.5748
WVFGRD96 94.0 240 85 -5 4.07 0.5623
WVFGRD96 96.0 60 90 5 4.07 0.5444
WVFGRD96 98.0 60 90 0 4.08 0.5240
The best solution is
WVFGRD96 32.0 235 55 -35 3.77 0.7254
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