The ANSS event ID is ak0165m3p95m and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0165m3p95m/executive.
2016/05/01 20:38:47 60.114 -152.993 129.5 4.7 Alaska
USGS/SLU Moment Tensor Solution
ENS 2016/05/01 20:38:47:0 60.11 -152.99 129.5 4.7 Alaska
Stations used:
AK.BRLK AK.CAST AK.CNP AK.CUT AK.GHO AK.GLI AK.HOM AK.KNK
AK.KTH AK.PPLA AK.PWL AK.RC01 AK.SAW AT.PMR AT.SVW2 AV.ILSW
II.KDAK TA.K20K TA.L19K TA.M19K TA.M22K TA.N18K TA.N19K
TA.O18K TA.O19K TA.O22K TA.P19K TA.Q19K
Filtering commands used:
cut a -10 a 90
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.06 n 3
Best Fitting Double Couple
Mo = 1.32e+23 dyne-cm
Mw = 4.68
Z = 134 km
Plane Strike Dip Rake
NP1 80 90 -35
NP2 170 55 -180
Principal Axes:
Axis Value Plunge Azimuth
T 1.32e+23 24 131
N 0.00e+00 55 260
P -1.32e+23 24 29
Moment Tensor: (dyne-cm)
Component Value
Mxx -3.69e+22
Mxy -1.01e+23
Mxz -7.45e+22
Myy 3.69e+22
Myz 1.31e+22
Mzz 6.61e+15
##------------
#####-----------------
#######------------- -----
#######-------------- P ------
#########-------------- --------
#########---------------------------
##########----------------------------
###########-----------------------------
###########-----------------------------
############-------------------------#####
############--------------################
############----##########################
#####--------#############################
------------############################
-------------###########################
------------################## #####
------------################# T ####
------------################ ###
-----------###################
------------################
----------############
---------#####
Global CMT Convention Moment Tensor:
R T P
6.61e+15 -7.45e+22 -1.31e+22
-7.45e+22 -3.69e+22 1.01e+23
-1.31e+22 1.01e+23 3.69e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20160501203847/index.html
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STK = 80
DIP = 90
RAKE = -35
MW = 4.68
HS = 134.0
The NDK file is 20160501203847.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 a -10 a 90 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.06 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 5 75 20 3.75 0.2616
WVFGRD96 4.0 5 80 20 3.83 0.2875
WVFGRD96 6.0 5 55 -10 3.89 0.3007
WVFGRD96 8.0 0 50 -15 3.97 0.3264
WVFGRD96 10.0 255 80 5 3.98 0.3511
WVFGRD96 12.0 255 80 5 4.01 0.3800
WVFGRD96 14.0 255 80 0 4.05 0.3945
WVFGRD96 16.0 75 90 5 4.06 0.4055
WVFGRD96 18.0 255 85 -5 4.10 0.4283
WVFGRD96 20.0 75 90 5 4.11 0.4421
WVFGRD96 22.0 75 90 0 4.13 0.4552
WVFGRD96 24.0 75 90 0 4.15 0.4650
WVFGRD96 26.0 75 90 -5 4.17 0.4707
WVFGRD96 28.0 255 90 5 4.18 0.4728
WVFGRD96 30.0 75 90 -5 4.20 0.4717
WVFGRD96 32.0 75 90 -5 4.22 0.4719
WVFGRD96 34.0 75 90 -5 4.25 0.4725
WVFGRD96 36.0 255 90 10 4.27 0.4751
WVFGRD96 38.0 255 90 10 4.30 0.4839
WVFGRD96 40.0 255 85 15 4.36 0.5114
WVFGRD96 42.0 255 85 15 4.38 0.5175
WVFGRD96 44.0 255 85 15 4.40 0.5234
WVFGRD96 46.0 255 85 15 4.41 0.5294
WVFGRD96 48.0 255 90 15 4.43 0.5351
WVFGRD96 50.0 75 90 -15 4.44 0.5430
WVFGRD96 52.0 75 90 -15 4.46 0.5505
WVFGRD96 54.0 75 90 -15 4.47 0.5585
WVFGRD96 56.0 75 85 -20 4.47 0.5668
WVFGRD96 58.0 75 85 -20 4.49 0.5763
WVFGRD96 60.0 75 85 -20 4.50 0.5860
WVFGRD96 62.0 75 85 -20 4.51 0.5964
WVFGRD96 64.0 75 85 -20 4.52 0.6067
WVFGRD96 66.0 75 85 -20 4.53 0.6181
WVFGRD96 68.0 75 85 -20 4.54 0.6297
WVFGRD96 70.0 75 85 -20 4.54 0.6419
WVFGRD96 72.0 75 85 -20 4.55 0.6532
WVFGRD96 74.0 75 85 -20 4.56 0.6651
WVFGRD96 76.0 75 85 -20 4.57 0.6756
WVFGRD96 78.0 75 85 -20 4.57 0.6866
WVFGRD96 80.0 75 85 -20 4.58 0.6967
WVFGRD96 82.0 75 85 -20 4.59 0.7055
WVFGRD96 84.0 75 85 -20 4.59 0.7147
WVFGRD96 86.0 75 85 -20 4.60 0.7225
WVFGRD96 88.0 75 85 -25 4.60 0.7298
WVFGRD96 90.0 75 85 -25 4.60 0.7379
WVFGRD96 92.0 75 85 -25 4.61 0.7459
WVFGRD96 94.0 255 90 30 4.61 0.7440
WVFGRD96 96.0 75 85 -25 4.61 0.7588
WVFGRD96 98.0 255 90 30 4.62 0.7575
WVFGRD96 100.0 255 90 30 4.62 0.7638
WVFGRD96 102.0 255 90 30 4.63 0.7702
WVFGRD96 104.0 255 90 30 4.63 0.7755
WVFGRD96 106.0 75 85 -25 4.63 0.7822
WVFGRD96 108.0 255 90 30 4.64 0.7858
WVFGRD96 110.0 255 90 30 4.64 0.7897
WVFGRD96 112.0 75 90 -35 4.64 0.7929
WVFGRD96 114.0 75 90 -35 4.65 0.7972
WVFGRD96 116.0 80 90 -35 4.65 0.8008
WVFGRD96 118.0 80 90 -35 4.65 0.8027
WVFGRD96 120.0 260 90 35 4.66 0.8069
WVFGRD96 122.0 80 90 -35 4.66 0.8086
WVFGRD96 124.0 80 90 -35 4.66 0.8103
WVFGRD96 126.0 80 90 -35 4.67 0.8124
WVFGRD96 128.0 80 90 -35 4.67 0.8133
WVFGRD96 130.0 260 90 35 4.67 0.8145
WVFGRD96 132.0 80 90 -35 4.68 0.8152
WVFGRD96 134.0 80 90 -35 4.68 0.8152
WVFGRD96 136.0 80 90 -35 4.68 0.8145
WVFGRD96 138.0 75 85 -35 4.68 0.8139
WVFGRD96 140.0 75 85 -35 4.68 0.8131
WVFGRD96 142.0 75 85 -35 4.68 0.8121
WVFGRD96 144.0 75 85 -35 4.69 0.8114
WVFGRD96 146.0 75 85 -35 4.69 0.8094
WVFGRD96 148.0 75 85 -35 4.69 0.8079
The best solution is
WVFGRD96 134.0 80 90 -35 4.68 0.8152
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 a -10 a 90 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.06 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