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 ak019ndzw4e and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak019ndzw4e/executive.
2019/01/14 14:23:14 69.599 -145.069 15.8 4.1 Alaska
USGS/SLU Moment Tensor Solution
ENS 2019/01/14 14:23:14:0 69.60 -145.07 15.8 4.1 Alaska
Stations used:
AK.COLD AK.FYU AK.PPD TA.C27K TA.D24K TA.E23K TA.E25K
TA.F25K TA.F26K TA.F28M TA.G22K TA.G23K TA.G31M TA.H24K
TA.H27K TA.I23K TA.I27K TA.POKR
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 = 1.26e+22 dyne-cm
Mw = 4.00
Z = 13 km
Plane Strike Dip Rake
NP1 195 80 35
NP2 98 56 168
Principal Axes:
Axis Value Plunge Azimuth
T 1.26e+22 31 62
N 0.00e+00 54 209
P -1.26e+22 16 322
Moment Tensor: (dyne-cm)
Component Value
Mxx -5.24e+21
Mxy 9.41e+21
Mxz -2.65e+19
Myy 2.77e+21
Myz 7.02e+21
Mzz 2.47e+21
------------##
---------------#######
-- ------------###########
--- P -----------#############
----- ----------################
-------------------#################
-------------------########### #####
-------------------############ T ######
-------------------############ ######
#------------------#######################
##-----------------#######################
###---------------########################
#####-------------########################
#######---------#######################-
###########-----###################-----
###############-#############---------
#############-----------------------
############----------------------
##########--------------------
#########-------------------
######----------------
##------------
Global CMT Convention Moment Tensor:
R T P
2.47e+21 -2.65e+19 -7.02e+21
-2.65e+19 -5.24e+21 -9.41e+21
-7.02e+21 -9.41e+21 2.77e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190114142314/index.html
|
STK = 195
DIP = 80
RAKE = 35
MW = 4.00
HS = 13.0
The NDK file is 20190114142314.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 20 60 -30 3.67 0.2778
WVFGRD96 2.0 175 45 -50 3.80 0.3445
WVFGRD96 3.0 5 75 -45 3.81 0.3735
WVFGRD96 4.0 5 75 -45 3.84 0.4131
WVFGRD96 5.0 5 80 -45 3.85 0.4428
WVFGRD96 6.0 10 85 -45 3.87 0.4706
WVFGRD96 7.0 190 90 40 3.87 0.4892
WVFGRD96 8.0 5 80 -45 3.94 0.5119
WVFGRD96 9.0 5 80 -45 3.95 0.5226
WVFGRD96 10.0 10 85 -40 3.96 0.5290
WVFGRD96 11.0 195 80 35 3.97 0.5334
WVFGRD96 12.0 10 90 -35 3.98 0.5335
WVFGRD96 13.0 195 80 35 4.00 0.5375
WVFGRD96 14.0 195 75 35 4.01 0.5368
WVFGRD96 15.0 195 75 35 4.02 0.5342
WVFGRD96 16.0 195 75 35 4.03 0.5298
WVFGRD96 17.0 195 70 35 4.05 0.5246
WVFGRD96 18.0 195 70 35 4.06 0.5187
WVFGRD96 19.0 195 70 35 4.07 0.5122
WVFGRD96 20.0 195 70 35 4.08 0.5043
WVFGRD96 21.0 195 70 40 4.09 0.4966
WVFGRD96 22.0 195 70 40 4.10 0.4884
WVFGRD96 23.0 200 70 45 4.11 0.4796
WVFGRD96 24.0 200 70 45 4.12 0.4708
WVFGRD96 25.0 200 70 45 4.13 0.4619
WVFGRD96 26.0 200 70 45 4.14 0.4520
WVFGRD96 27.0 200 70 50 4.15 0.4421
WVFGRD96 28.0 200 70 50 4.16 0.4313
WVFGRD96 29.0 200 70 50 4.17 0.4195
WVFGRD96 30.0 200 70 50 4.18 0.4078
WVFGRD96 31.0 195 75 45 4.18 0.3964
WVFGRD96 32.0 195 75 50 4.18 0.3857
WVFGRD96 33.0 195 75 45 4.20 0.3759
WVFGRD96 34.0 195 80 45 4.20 0.3680
WVFGRD96 35.0 195 80 45 4.21 0.3610
WVFGRD96 36.0 195 80 45 4.21 0.3547
WVFGRD96 37.0 10 80 25 4.23 0.3478
WVFGRD96 38.0 10 75 20 4.25 0.3460
WVFGRD96 39.0 10 75 20 4.26 0.3448
WVFGRD96 40.0 10 75 30 4.31 0.3454
WVFGRD96 41.0 10 75 25 4.33 0.3428
WVFGRD96 42.0 110 60 30 4.34 0.3422
WVFGRD96 43.0 110 60 30 4.35 0.3443
WVFGRD96 44.0 110 65 25 4.37 0.3462
WVFGRD96 45.0 110 65 25 4.38 0.3484
WVFGRD96 46.0 110 65 25 4.39 0.3488
WVFGRD96 47.0 110 65 25 4.40 0.3497
WVFGRD96 48.0 110 65 25 4.41 0.3492
WVFGRD96 49.0 110 65 25 4.42 0.3475
WVFGRD96 50.0 110 65 25 4.43 0.3457
WVFGRD96 51.0 280 60 5 4.44 0.3497
WVFGRD96 52.0 280 65 5 4.45 0.3529
WVFGRD96 53.0 280 65 0 4.45 0.3564
WVFGRD96 54.0 280 65 0 4.46 0.3591
WVFGRD96 55.0 280 65 0 4.47 0.3619
WVFGRD96 56.0 275 65 -5 4.46 0.3646
WVFGRD96 57.0 275 65 -5 4.47 0.3667
WVFGRD96 58.0 275 65 -5 4.48 0.3686
WVFGRD96 59.0 275 65 -5 4.48 0.3698
The best solution is
WVFGRD96 13.0 195 80 35 4.00 0.5375
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