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 ak0193yb8dey and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0193yb8dey/executive.
2019/03/27 05:06:35 66.303 -157.283 9.7 3.7 Alaska
USGS/SLU Moment Tensor Solution
ENS 2019/03/27 05:06:35:0 66.30 -157.28 9.7 3.7 Alaska
Stations used:
AK.COLD AK.GCSA TA.B18K TA.C18K TA.D17K TA.D19K TA.D20K
TA.D22K TA.E18K TA.E19K TA.E20K TA.E22K TA.E23K TA.E24K
TA.F14K TA.F15K TA.F17K TA.F18K TA.F19K TA.F20K TA.F21K
TA.F22K TA.G15K TA.G16K TA.G17K TA.G18K TA.G19K TA.G21K
TA.G23K TA.G24K TA.H16K TA.H17K TA.H19K TA.H20K TA.H23K
TA.I17K TA.I20K TA.J18K TA.J19K TA.J20K TA.K20K TA.TOLK
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 0.08 n 3
Best Fitting Double Couple
Mo = 3.16e+21 dyne-cm
Mw = 3.60
Z = 9 km
Plane Strike Dip Rake
NP1 150 70 -25
NP2 249 67 -158
Principal Axes:
Axis Value Plunge Azimuth
T 3.16e+21 2 200
N 0.00e+00 58 294
P -3.16e+21 32 109
Moment Tensor: (dyne-cm)
Component Value
Mxx 2.55e+21
Mxy 1.72e+21
Mxz 3.37e+20
Myy -1.69e+21
Myz -1.38e+21
Mzz -8.59e+20
##############
-#####################
----########################
-----#########################
-------###########################
--------############################
----------##################-------###
-----------#########--------------------
------------####------------------------
-------------#----------------------------
----------#####---------------------------
-------#########--------------------------
-----############---------------- ------
--###############--------------- P -----
-#################-------------- -----
##################--------------------
###################-----------------
####################--------------
###################-----------
#####################-------
### ###############-
T ############
Global CMT Convention Moment Tensor:
R T P
-8.59e+20 3.37e+20 1.38e+21
3.37e+20 2.55e+21 -1.72e+21
1.38e+21 -1.72e+21 -1.69e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190327050635/index.html
|
STK = 150
DIP = 70
RAKE = -25
MW = 3.60
HS = 9.0
The NDK file is 20190327050635.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.
 |
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.
|
|
|
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 0.08 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 340 75 -15 3.23 0.3250
WVFGRD96 2.0 340 75 -15 3.35 0.4290
WVFGRD96 3.0 340 70 -20 3.42 0.4771
WVFGRD96 4.0 335 65 -30 3.47 0.5107
WVFGRD96 5.0 340 70 -20 3.49 0.5344
WVFGRD96 6.0 150 70 -25 3.52 0.5568
WVFGRD96 7.0 150 70 -20 3.54 0.5723
WVFGRD96 8.0 150 70 -25 3.58 0.5814
WVFGRD96 9.0 150 70 -25 3.60 0.5815
WVFGRD96 10.0 150 70 -20 3.61 0.5777
WVFGRD96 11.0 150 70 -20 3.62 0.5726
WVFGRD96 12.0 345 70 20 3.64 0.5716
WVFGRD96 13.0 345 70 15 3.65 0.5694
WVFGRD96 14.0 345 70 15 3.67 0.5656
WVFGRD96 15.0 345 70 15 3.68 0.5602
WVFGRD96 16.0 345 70 15 3.69 0.5533
WVFGRD96 17.0 345 70 15 3.70 0.5453
WVFGRD96 18.0 345 70 15 3.71 0.5369
WVFGRD96 19.0 345 70 15 3.72 0.5282
WVFGRD96 20.0 340 65 10 3.72 0.5199
WVFGRD96 21.0 340 65 10 3.73 0.5118
WVFGRD96 22.0 340 65 10 3.74 0.5031
WVFGRD96 23.0 340 65 5 3.75 0.4942
WVFGRD96 24.0 340 65 5 3.75 0.4847
WVFGRD96 25.0 340 60 5 3.76 0.4751
WVFGRD96 26.0 340 60 10 3.77 0.4652
WVFGRD96 27.0 340 60 10 3.78 0.4556
WVFGRD96 28.0 340 60 10 3.78 0.4457
WVFGRD96 29.0 340 60 5 3.78 0.4358
WVFGRD96 30.0 340 60 5 3.79 0.4258
WVFGRD96 31.0 335 70 -15 3.79 0.4206
WVFGRD96 32.0 335 70 -15 3.79 0.4142
WVFGRD96 33.0 335 70 -15 3.80 0.4076
WVFGRD96 34.0 335 70 -15 3.80 0.4010
WVFGRD96 35.0 335 70 -15 3.81 0.3959
WVFGRD96 36.0 335 70 -15 3.82 0.3919
WVFGRD96 37.0 335 70 -20 3.83 0.3891
WVFGRD96 38.0 330 70 -20 3.85 0.3876
WVFGRD96 39.0 330 70 -20 3.86 0.3882
WVFGRD96 40.0 330 65 -25 3.91 0.3828
WVFGRD96 41.0 330 70 -30 3.92 0.3837
WVFGRD96 42.0 330 70 -30 3.93 0.3836
WVFGRD96 43.0 330 70 -25 3.94 0.3836
WVFGRD96 44.0 330 70 -25 3.95 0.3838
WVFGRD96 45.0 330 70 -25 3.95 0.3833
WVFGRD96 46.0 330 70 -25 3.96 0.3828
WVFGRD96 47.0 330 70 -25 3.97 0.3825
WVFGRD96 48.0 330 70 -25 3.98 0.3816
WVFGRD96 49.0 330 70 -25 3.98 0.3802
WVFGRD96 50.0 330 70 -25 3.99 0.3789
WVFGRD96 51.0 330 70 -25 3.99 0.3774
WVFGRD96 52.0 330 70 -25 4.00 0.3758
WVFGRD96 53.0 330 70 -25 4.00 0.3741
WVFGRD96 54.0 330 70 -25 4.01 0.3722
WVFGRD96 55.0 335 75 -20 4.00 0.3707
WVFGRD96 56.0 335 75 -20 4.01 0.3694
WVFGRD96 57.0 335 75 -20 4.01 0.3677
WVFGRD96 58.0 335 75 -20 4.01 0.3662
WVFGRD96 59.0 335 75 -20 4.02 0.3649
The best solution is
WVFGRD96 9.0 150 70 -25 3.60 0.5815
The mechanism corresponding to the best fit is
|
|
|
The best fit as a function of depth is given in the following figure:
|
|
|
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 0.08 n 3
|
| 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. |
|
| 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