Left: mLg computed using the IASPEI formula. Center: mLg residuals versus epicentral distance ; the values used for the trimmed mean magnitude estimate are indicated. Right: residuals as a function of distance and azimuth.
The ANSS event ID is ak0195ue13zz and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0195ue13zz/executive.
2019/05/07 21:46:35 61.389 -149.885 38.0 3.7 Alaska
USGS/SLU Moment Tensor Solution
ENS 2019/05/07 21:46:35:0 61.39 -149.88 38.0 3.7 Alaska
Stations used:
AK.BMR AK.CRQ AK.DIV AK.FID AK.GHO AK.GLI AK.KNK AK.MCK
AK.PAX AK.PWL AK.SAW AK.SKN AK.SSN AK.SWD AT.PMR TA.M22K
TA.N25K TA.O22K
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 = 8.61e+21 dyne-cm
Mw = 3.89
Z = 53 km
Plane Strike Dip Rake
NP1 200 80 -20
NP2 294 70 -169
Principal Axes:
Axis Value Plunge Azimuth
T 8.61e+21 7 248
N 0.00e+00 68 354
P -8.61e+21 21 155
Moment Tensor: (dyne-cm)
Component Value
Mxx -5.00e+21
Mxy 5.78e+21
Mxz 2.27e+21
Myy 6.01e+21
Myz -2.12e+21
Mzz -1.01e+21
--------------
-----------------#####
------------------##########
------------------############
-------------------###############
-------------------#################
#########-----------##################
#################---####################
###################---##################
###################--------###############
###################-----------############
##################---------------#########
#################------------------#######
#############--------------------####
T ############-----------------------##
############------------------------
############------------------------
###########-----------------------
########------------ -------
#######------------ P ------
####------------ ---
--------------
Global CMT Convention Moment Tensor:
R T P
-1.01e+21 2.27e+21 2.12e+21
2.27e+21 -5.00e+21 -5.78e+21
2.12e+21 -5.78e+21 6.01e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190507214635/index.html
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STK = 200
DIP = 80
RAKE = -20
MW = 3.89
HS = 53.0
The NDK file is 20190507214635.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: mLg computed using the IASPEI formula. Center: mLg residuals versus epicentral distance ; the values used for the trimmed mean magnitude estimate are indicated.
Right: residuals as a function of distance and azimuth.
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 1.0 295 75 0 2.95 0.2443
WVFGRD96 2.0 295 70 10 3.10 0.3183
WVFGRD96 3.0 295 60 15 3.18 0.3441
WVFGRD96 4.0 295 60 10 3.21 0.3539
WVFGRD96 5.0 290 75 -25 3.24 0.3581
WVFGRD96 6.0 290 75 -25 3.27 0.3626
WVFGRD96 7.0 110 65 -15 3.30 0.3640
WVFGRD96 8.0 110 60 -15 3.35 0.3633
WVFGRD96 9.0 110 65 -15 3.36 0.3572
WVFGRD96 10.0 110 65 -15 3.38 0.3488
WVFGRD96 11.0 25 75 20 3.39 0.3529
WVFGRD96 12.0 25 75 15 3.41 0.3570
WVFGRD96 13.0 25 70 20 3.43 0.3607
WVFGRD96 14.0 25 65 15 3.45 0.3648
WVFGRD96 15.0 25 65 10 3.47 0.3705
WVFGRD96 16.0 25 65 15 3.49 0.3771
WVFGRD96 17.0 25 65 15 3.50 0.3830
WVFGRD96 18.0 25 70 10 3.52 0.3896
WVFGRD96 19.0 25 75 10 3.53 0.4013
WVFGRD96 20.0 25 75 10 3.55 0.4132
WVFGRD96 21.0 25 70 10 3.57 0.4262
WVFGRD96 22.0 25 75 10 3.58 0.4402
WVFGRD96 23.0 25 75 10 3.59 0.4551
WVFGRD96 24.0 25 75 10 3.60 0.4693
WVFGRD96 25.0 200 90 10 3.60 0.4780
WVFGRD96 26.0 25 75 10 3.62 0.5004
WVFGRD96 27.0 200 90 10 3.62 0.5160
WVFGRD96 28.0 200 80 10 3.63 0.5374
WVFGRD96 29.0 200 85 10 3.64 0.5560
WVFGRD96 30.0 200 85 10 3.65 0.5715
WVFGRD96 31.0 200 85 10 3.66 0.5857
WVFGRD96 32.0 205 85 0 3.67 0.5963
WVFGRD96 33.0 205 85 0 3.68 0.6066
WVFGRD96 34.0 200 80 -10 3.67 0.6124
WVFGRD96 35.0 200 80 -15 3.68 0.6212
WVFGRD96 36.0 200 80 -15 3.70 0.6265
WVFGRD96 37.0 200 80 -15 3.71 0.6355
WVFGRD96 38.0 200 80 -10 3.72 0.6396
WVFGRD96 39.0 200 80 -10 3.74 0.6458
WVFGRD96 40.0 200 80 -20 3.78 0.6522
WVFGRD96 41.0 200 80 -20 3.80 0.6561
WVFGRD96 42.0 200 80 -20 3.81 0.6571
WVFGRD96 43.0 200 80 -20 3.82 0.6619
WVFGRD96 44.0 200 80 -20 3.83 0.6626
WVFGRD96 45.0 200 80 -20 3.84 0.6660
WVFGRD96 46.0 200 80 -20 3.85 0.6669
WVFGRD96 47.0 200 80 -20 3.85 0.6690
WVFGRD96 48.0 200 80 -20 3.86 0.6692
WVFGRD96 49.0 200 80 -20 3.87 0.6709
WVFGRD96 50.0 200 80 -20 3.87 0.6702
WVFGRD96 51.0 200 80 -20 3.88 0.6727
WVFGRD96 52.0 200 80 -20 3.89 0.6709
WVFGRD96 53.0 200 80 -20 3.89 0.6728
WVFGRD96 54.0 200 80 -20 3.90 0.6705
WVFGRD96 55.0 200 80 -20 3.90 0.6713
WVFGRD96 56.0 200 80 -20 3.91 0.6689
WVFGRD96 57.0 200 80 -20 3.91 0.6685
WVFGRD96 58.0 200 80 -20 3.91 0.6697
WVFGRD96 59.0 200 80 -20 3.92 0.6663
The best solution is
WVFGRD96 53.0 200 80 -20 3.89 0.6728
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