The ANSS event ID is ak01881pylyd and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak01881pylyd/executive.
2018/06/24 18:33:00 60.033 -151.434 75.4 4 Alaska
USGS/SLU Moment Tensor Solution
ENS 2018/06/24 18:33:00:0 60.03 -151.43 75.4 4.0 Alaska
Stations used:
AK.BRLK AK.CAPN AK.CNP AK.GHO AK.GLI AK.KLU AK.KNK AK.RC01
AT.PMR TA.K20K TA.M20K TA.N17K TA.N19K TA.N25K TA.O18K
TA.O22K TA.P19K TA.Q19K TA.Q20K
Filtering commands used:
cut o DIST/3.5 -30 o DIST/3.5 +50
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 1.91e+22 dyne-cm
Mw = 4.12
Z = 84 km
Plane Strike Dip Rake
NP1 260 65 45
NP2 147 50 147
Principal Axes:
Axis Value Plunge Azimuth
T 1.91e+22 49 121
N 0.00e+00 40 283
P -1.91e+22 9 20
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.42e+22
Mxy -9.71e+21
Mxz -7.54e+21
Myy 3.87e+21
Myz 7.11e+21
Mzz 1.03e+22
------------
---------------- P ---
##----------------- ------
##----------------------------
####------------------------------
#####-------------------------------
######--------------------------------
#######-------------#############-------
#######-----###########################-
##########################################
######---#################################
###-------################################
#----------################# ###########
-----------################ T ##########
------------############### ##########
-------------#########################
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--------------####################
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Global CMT Convention Moment Tensor:
R T P
1.03e+22 -7.54e+21 -7.11e+21
-7.54e+21 -1.42e+22 9.71e+21
-7.11e+21 9.71e+21 3.87e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20180624183300/index.html
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STK = 260
DIP = 65
RAKE = 45
MW = 4.12
HS = 84.0
The NDK file is 20180624183300.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/3.5 -30 o DIST/3.5 +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 2.0 110 45 -90 3.29 0.2305
WVFGRD96 4.0 150 25 -35 3.35 0.2218
WVFGRD96 6.0 160 30 -25 3.38 0.2574
WVFGRD96 8.0 345 20 -25 3.47 0.2820
WVFGRD96 10.0 85 90 -50 3.52 0.3012
WVFGRD96 12.0 85 90 -50 3.55 0.3140
WVFGRD96 14.0 80 85 -45 3.59 0.3156
WVFGRD96 16.0 80 85 -45 3.61 0.3072
WVFGRD96 18.0 80 80 -45 3.64 0.2908
WVFGRD96 20.0 270 60 55 3.62 0.2771
WVFGRD96 22.0 75 45 40 3.64 0.2716
WVFGRD96 24.0 235 70 -40 3.65 0.2675
WVFGRD96 26.0 75 75 40 3.66 0.2779
WVFGRD96 28.0 70 75 40 3.68 0.2916
WVFGRD96 30.0 70 75 40 3.69 0.3003
WVFGRD96 32.0 80 65 50 3.72 0.3065
WVFGRD96 34.0 80 65 50 3.73 0.3072
WVFGRD96 36.0 245 50 25 3.80 0.3055
WVFGRD96 38.0 235 70 -15 3.80 0.3141
WVFGRD96 40.0 255 40 45 3.92 0.3551
WVFGRD96 42.0 255 40 45 3.95 0.3561
WVFGRD96 44.0 250 45 40 3.96 0.3494
WVFGRD96 46.0 255 45 45 3.98 0.3407
WVFGRD96 48.0 255 50 45 3.99 0.3352
WVFGRD96 50.0 265 55 60 4.00 0.3448
WVFGRD96 52.0 270 55 65 4.01 0.3722
WVFGRD96 54.0 265 60 60 4.03 0.3984
WVFGRD96 56.0 265 60 60 4.04 0.4253
WVFGRD96 58.0 265 60 55 4.06 0.4501
WVFGRD96 60.0 265 60 55 4.07 0.4721
WVFGRD96 62.0 265 60 55 4.08 0.4908
WVFGRD96 64.0 265 60 55 4.08 0.5051
WVFGRD96 66.0 265 60 55 4.09 0.5164
WVFGRD96 68.0 260 65 50 4.10 0.5262
WVFGRD96 70.0 260 65 50 4.10 0.5348
WVFGRD96 72.0 260 65 50 4.10 0.5401
WVFGRD96 74.0 260 65 50 4.11 0.5450
WVFGRD96 76.0 260 65 50 4.11 0.5479
WVFGRD96 78.0 260 65 45 4.12 0.5494
WVFGRD96 80.0 260 65 45 4.12 0.5508
WVFGRD96 82.0 260 65 45 4.12 0.5512
WVFGRD96 84.0 260 65 45 4.12 0.5517
WVFGRD96 86.0 260 65 45 4.13 0.5509
WVFGRD96 88.0 260 65 45 4.13 0.5485
WVFGRD96 90.0 260 65 40 4.14 0.5454
WVFGRD96 92.0 260 65 40 4.14 0.5422
WVFGRD96 94.0 260 65 40 4.14 0.5370
WVFGRD96 96.0 260 65 40 4.14 0.5345
WVFGRD96 98.0 260 65 40 4.14 0.5310
WVFGRD96 100.0 255 65 35 4.15 0.5271
WVFGRD96 102.0 255 65 35 4.15 0.5227
WVFGRD96 104.0 255 65 35 4.15 0.5178
WVFGRD96 106.0 255 65 35 4.15 0.5152
WVFGRD96 108.0 255 65 35 4.16 0.5105
The best solution is
WVFGRD96 84.0 260 65 45 4.12 0.5517
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/3.5 -30 o DIST/3.5 +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