The ANSS event ID is ak0192hihfd0 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/ak0192hihfd0/executive.
2019/02/23 12:54:02 66.287 -156.976 38.2 3.5 Alaska
USGS/SLU Moment Tensor Solution
ENS 2019/02/23 12:54:02:0 66.29 -156.98 38.2 3.5 Alaska
Stations used:
AK.ANM AK.COLD AK.NEA2 AT.TTA TA.C18K TA.D19K TA.D22K
TA.D23K TA.E19K TA.E22K TA.E23K TA.F17K TA.F19K TA.F24K
TA.G16K TA.G19K TA.G23K TA.G24K TA.H17K TA.H18K TA.H21K
TA.J16K TA.J17K TA.J18K TA.J19K TA.J20K TA.K17K 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
br c 0.12 0.25 n 4 p 2
Best Fitting Double Couple
Mo = 1.53e+21 dyne-cm
Mw = 3.39
Z = 10 km
Plane Strike Dip Rake
NP1 170 85 -30
NP2 263 60 -174
Principal Axes:
Axis Value Plunge Azimuth
T 1.53e+21 17 220
N 0.00e+00 60 341
P -1.53e+21 24 122
Moment Tensor: (dyne-cm)
Component Value
Mxx 4.56e+20
Mxy 1.26e+21
Mxz -1.71e+19
Myy -3.23e+20
Myz -7.63e+20
Mzz -1.33e+20
---###########
-------###############
-----------#################
------------##################
--------------####################
---------------#####################
----------------######################
--------------####---------------#######
---------#########--------------------##
------#############----------------------#
----################----------------------
--##################----------------------
####################----------------------
####################--------------------
####################------------ -----
###################------------ P ----
###################----------- ---
#### ###########----------------
## T ############-------------
# ############------------
##############--------
##########----
Global CMT Convention Moment Tensor:
R T P
-1.33e+20 -1.71e+19 7.63e+20
-1.71e+19 4.56e+20 -1.26e+21
7.63e+20 -1.26e+21 -3.23e+20
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20190223125402/index.html
|
STK = 170
DIP = 85
RAKE = -30
MW = 3.39
HS = 10.0
The NDK file is 20190223125402.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.
 |
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 3 br c 0.12 0.25 n 4 p 2The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 350 85 10 3.04 0.4297
WVFGRD96 2.0 350 90 5 3.17 0.6077
WVFGRD96 3.0 170 90 -5 3.21 0.6651
WVFGRD96 4.0 170 85 -15 3.25 0.6965
WVFGRD96 5.0 165 75 -35 3.32 0.7165
WVFGRD96 6.0 170 80 -25 3.31 0.7312
WVFGRD96 7.0 170 80 -25 3.33 0.7410
WVFGRD96 8.0 170 80 -30 3.36 0.7475
WVFGRD96 9.0 170 85 -30 3.38 0.7505
WVFGRD96 10.0 170 85 -30 3.39 0.7511
WVFGRD96 11.0 170 85 -25 3.40 0.7507
WVFGRD96 12.0 170 85 -25 3.41 0.7490
WVFGRD96 13.0 170 85 -25 3.42 0.7455
WVFGRD96 14.0 175 90 -20 3.43 0.7423
WVFGRD96 15.0 175 90 -20 3.44 0.7387
WVFGRD96 16.0 175 90 -20 3.45 0.7346
WVFGRD96 17.0 175 90 -15 3.46 0.7297
WVFGRD96 18.0 355 90 -15 3.46 0.7190
WVFGRD96 19.0 355 90 -15 3.47 0.7151
WVFGRD96 20.0 175 80 15 3.48 0.7178
WVFGRD96 21.0 355 90 -15 3.49 0.7031
WVFGRD96 22.0 175 80 15 3.50 0.7047
WVFGRD96 23.0 175 80 15 3.51 0.6975
WVFGRD96 24.0 175 80 15 3.51 0.6888
WVFGRD96 25.0 175 80 15 3.52 0.6796
WVFGRD96 26.0 175 80 15 3.53 0.6701
WVFGRD96 27.0 175 80 15 3.54 0.6595
WVFGRD96 28.0 175 80 15 3.55 0.6487
WVFGRD96 29.0 175 75 15 3.55 0.6385
WVFGRD96 30.0 175 75 15 3.56 0.6276
WVFGRD96 31.0 175 75 15 3.57 0.6166
WVFGRD96 32.0 175 75 15 3.58 0.6055
WVFGRD96 33.0 175 75 15 3.59 0.5936
WVFGRD96 34.0 175 80 15 3.59 0.5809
WVFGRD96 35.0 175 80 15 3.60 0.5700
WVFGRD96 36.0 175 80 15 3.62 0.5593
WVFGRD96 37.0 175 80 15 3.63 0.5490
WVFGRD96 38.0 350 80 -10 3.62 0.5365
WVFGRD96 39.0 355 80 -5 3.65 0.5265
WVFGRD96 40.0 350 75 -15 3.67 0.5177
WVFGRD96 41.0 350 85 -20 3.68 0.5095
WVFGRD96 42.0 175 90 -45 3.73 0.5055
WVFGRD96 43.0 175 85 -40 3.74 0.4987
WVFGRD96 44.0 175 85 -40 3.74 0.4920
WVFGRD96 45.0 175 85 -40 3.75 0.4855
WVFGRD96 46.0 0 85 45 3.76 0.4769
WVFGRD96 47.0 0 90 40 3.77 0.4702
WVFGRD96 48.0 175 85 -40 3.76 0.4674
WVFGRD96 49.0 175 80 -35 3.77 0.4636
WVFGRD96 50.0 175 80 -35 3.78 0.4603
WVFGRD96 51.0 85 80 10 3.75 0.4601
WVFGRD96 52.0 85 80 10 3.76 0.4605
WVFGRD96 53.0 85 80 10 3.77 0.4605
WVFGRD96 54.0 85 75 10 3.78 0.4610
WVFGRD96 55.0 85 75 10 3.78 0.4630
WVFGRD96 56.0 85 75 10 3.79 0.4652
WVFGRD96 57.0 85 75 10 3.80 0.4665
WVFGRD96 58.0 85 75 10 3.80 0.4678
WVFGRD96 59.0 85 75 10 3.81 0.4686
The best solution is
WVFGRD96 10.0 170 85 -30 3.39 0.7511
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 br c 0.12 0.25 n 4 p 2
|
| 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