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 ak01958kp09o and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak01958kp09o/executive.
2019/04/24 02:40:02 66.304 -157.167 5.4 3.4 Alaska
USGS/SLU Moment Tensor Solution
ENS 2019/04/24 02:40:02:0 66.30 -157.17 5.4 3.4 Alaska
Stations used:
AK.BPAW AK.CHUM AK.COLD AK.GCSA AK.NEA2 AK.RDOG TA.C18K
TA.D19K TA.D20K TA.D22K TA.E18K TA.E19K TA.E20K TA.E21K
TA.E22K TA.E23K TA.E24K TA.F17K TA.F18K TA.F19K TA.F20K
TA.F21K TA.F22K TA.F24K TA.G16K TA.G17K TA.G18K TA.G19K
TA.G21K TA.G22K TA.G23K TA.H16K TA.H17K TA.H18K TA.H19K
TA.H20K TA.H21K TA.H22K TA.H23K TA.H24K TA.I20K TA.I21K
TA.I23K TA.J18K TA.J19K TA.J20K TA.TOLK XV.F7TV XV.F8KN
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 = 2.32e+21 dyne-cm
Mw = 3.51
Z = 12 km
Plane Strike Dip Rake
NP1 155 85 -45
NP2 250 45 -173
Principal Axes:
Axis Value Plunge Azimuth
T 2.32e+21 26 211
N 0.00e+00 45 330
P -2.32e+21 34 102
Moment Tensor: (dyne-cm)
Component Value
Mxx 1.30e+21
Mxy 1.16e+21
Mxz -5.53e+20
Myy -1.02e+21
Myz -1.52e+21
Mzz -2.85e+20
##############
---###################
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--------######################
----------##########-----#########
------------#--------------------###
-----------###-----------------------#
---------#######------------------------
-------##########-----------------------
------############------------------------
-----##############-----------------------
---#################------------- ------
---##################------------ P ------
-####################----------- -----
-#####################------------------
######################----------------
######################--------------
####### ############------------
##### T #############---------
#### ##############-------
###################---
##############
Global CMT Convention Moment Tensor:
R T P
-2.85e+20 -5.53e+20 1.52e+21
-5.53e+20 1.30e+21 -1.16e+21
1.52e+21 -1.16e+21 -1.02e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190424024002/index.html
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STK = 155
DIP = 85
RAKE = -45
MW = 3.51
HS = 12.0
The NDK file is 20190424024002.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 1.0 310 45 -85 3.14 0.2700
WVFGRD96 2.0 305 45 -90 3.28 0.3568
WVFGRD96 3.0 335 55 -40 3.27 0.3296
WVFGRD96 4.0 345 80 55 3.32 0.3660
WVFGRD96 5.0 345 80 55 3.34 0.4125
WVFGRD96 6.0 345 80 50 3.36 0.4481
WVFGRD96 7.0 350 75 50 3.38 0.4743
WVFGRD96 8.0 160 90 -50 3.44 0.4856
WVFGRD96 9.0 160 90 -50 3.46 0.5017
WVFGRD96 10.0 160 90 -45 3.48 0.5102
WVFGRD96 11.0 155 85 -45 3.49 0.5149
WVFGRD96 12.0 155 85 -45 3.51 0.5155
WVFGRD96 13.0 155 85 -45 3.52 0.5122
WVFGRD96 14.0 155 85 -40 3.54 0.5062
WVFGRD96 15.0 155 85 -40 3.55 0.4982
WVFGRD96 16.0 340 90 40 3.56 0.4848
WVFGRD96 17.0 340 90 40 3.57 0.4718
WVFGRD96 18.0 155 85 -40 3.58 0.4615
WVFGRD96 19.0 155 80 -40 3.59 0.4474
WVFGRD96 20.0 155 80 -40 3.60 0.4330
WVFGRD96 21.0 155 80 -40 3.61 0.4183
WVFGRD96 22.0 155 80 -40 3.61 0.4039
WVFGRD96 23.0 155 80 -40 3.62 0.3893
WVFGRD96 24.0 155 80 -45 3.63 0.3748
WVFGRD96 25.0 160 80 -40 3.63 0.3611
WVFGRD96 26.0 160 80 -40 3.64 0.3481
WVFGRD96 27.0 160 80 -40 3.64 0.3371
WVFGRD96 28.0 155 75 -45 3.64 0.3271
WVFGRD96 29.0 155 75 -45 3.65 0.3176
The best solution is
WVFGRD96 12.0 155 85 -45 3.51 0.5155
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