Location
Location ANSS
2021/01/25 22:02:00 60.237 -145.940 16.5 4.8 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2021/01/25 22:02:00:0 60.24 -145.94 16.5 4.8 Alaska
Stations used:
AK.BARN AK.BMR AK.BRLK AK.CAPN AK.CNP AK.CRQ AK.CUT AK.DHY
AK.DIV AK.DOT AK.EYAK AK.FIRE AK.GHO AK.GLB AK.GLI AK.GRNC
AK.HIN AK.K24K AK.KAI AK.KLU AK.KNK AK.L22K AK.LOGN AK.M20K
AK.M26K AK.M27K AK.MCAR AK.MCK AK.P23K AK.PAX AK.PWL AK.RAG
AK.RC01 AK.RIDG AK.RND AK.SAW AK.SCM AK.SCRK AK.SKN AK.SLK
AK.TGL AK.TRF AK.VRDI AT.MENT AT.PMR TA.L27K TA.M22K
TA.O22K TA.O28M
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 = 2.45e+23 dyne-cm
Mw = 4.86
Z = 19 km
Plane Strike Dip Rake
NP1 245 80 -80
NP2 20 14 -135
Principal Axes:
Axis Value Plunge Azimuth
T 2.45e+23 34 326
N 0.00e+00 10 63
P -2.45e+23 54 167
Moment Tensor: (dyne-cm)
Component Value
Mxx 3.58e+22
Mxy -5.87e+22
Mxz 2.09e+23
Myy 4.69e+22
Myz -8.93e+22
Mzz -8.27e+22
##############
#####################-
##########################--
###### ####################-
######## T #####################--
######### ######################--
###################################-##
#############################---------##
########################--------------##
#####################------------------###
#################----------------------###
##############-------------------------###
##########-----------------------------###
######-------------------------------###
####---------------- --------------###
#------------------ P -------------###
------------------ ------------###
-------------------------------###
---------------------------###
------------------------####
------------------####
---------#####
Global CMT Convention Moment Tensor:
R T P
-8.27e+22 2.09e+23 8.93e+22
2.09e+23 3.58e+22 5.87e+22
8.93e+22 5.87e+22 4.69e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210125220200/index.html
|
Preferred Solution
The preferred solution from an analysis of the surface-wave spectral amplitude radiation pattern, waveform inversion and first motion observations is
STK = 245
DIP = 80
RAKE = -80
MW = 4.86
HS = 19.0
The NDK file is 20210125220200.ndk
The waveform inversion is preferred.
Moment Tensor Comparison
The following compares this source inversion to others
| SLU |
USGSMWR |
USGS/SLU Moment Tensor Solution
ENS 2021/01/25 22:02:00:0 60.24 -145.94 16.5 4.8 Alaska
Stations used:
AK.BARN AK.BMR AK.BRLK AK.CAPN AK.CNP AK.CRQ AK.CUT AK.DHY
AK.DIV AK.DOT AK.EYAK AK.FIRE AK.GHO AK.GLB AK.GLI AK.GRNC
AK.HIN AK.K24K AK.KAI AK.KLU AK.KNK AK.L22K AK.LOGN AK.M20K
AK.M26K AK.M27K AK.MCAR AK.MCK AK.P23K AK.PAX AK.PWL AK.RAG
AK.RC01 AK.RIDG AK.RND AK.SAW AK.SCM AK.SCRK AK.SKN AK.SLK
AK.TGL AK.TRF AK.VRDI AT.MENT AT.PMR TA.L27K TA.M22K
TA.O22K TA.O28M
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 = 2.45e+23 dyne-cm
Mw = 4.86
Z = 19 km
Plane Strike Dip Rake
NP1 245 80 -80
NP2 20 14 -135
Principal Axes:
Axis Value Plunge Azimuth
T 2.45e+23 34 326
N 0.00e+00 10 63
P -2.45e+23 54 167
Moment Tensor: (dyne-cm)
Component Value
Mxx 3.58e+22
Mxy -5.87e+22
Mxz 2.09e+23
Myy 4.69e+22
Myz -8.93e+22
Mzz -8.27e+22
##############
#####################-
##########################--
###### ####################-
######## T #####################--
######### ######################--
###################################-##
#############################---------##
########################--------------##
#####################------------------###
#################----------------------###
##############-------------------------###
##########-----------------------------###
######-------------------------------###
####---------------- --------------###
#------------------ P -------------###
------------------ ------------###
-------------------------------###
---------------------------###
------------------------####
------------------####
---------#####
Global CMT Convention Moment Tensor:
R T P
-8.27e+22 2.09e+23 8.93e+22
2.09e+23 3.58e+22 5.87e+22
8.93e+22 5.87e+22 4.69e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210125220200/index.html
|
Regional Moment Tensor (Mwr)
Moment 3.246e+16 N-m
Magnitude 4.94 Mwr
Depth 9.0 km
Percent DC 85%
Half Duration -
Catalog US
Data Source US 3
Contributor US 3
Nodal Planes
Plane Strike Dip Rake
NP1 197 3 41
NP2 66 88 92
Principal Axes
Axis Value Plunge Azimuth
T 3.108e+16 N-m 47 338
N 0.260e+16 N-m 2 246
P -3.368e+16 N-m 43 153
|
Magnitudes
ML Magnitude
(a) ML computed using the IASPEI formula for Horizontal components; (b) 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.
(a) ML computed using the IASPEI formula for Vertical components (research); (b) 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.
Context
The next figure presents the focal mechanism for this earthquake (red) in the context of other events (blue) in the SLU Moment Tensor Catalog which are within ± 0.5 degrees of the new event. This comparison is shown in the left panel of the figure. The right panel shows the inferred direction of maximum compressive stress and the type of faulting (green is strike-slip, red is normal, blue is thrust; oblique is shown by a combination of colors).
Waveform Inversion using wvfgrd96
The focal mechanism was determined using broadband seismic waveforms. The location of the event and the
and stations used for the waveform inversion are shown in the next figure.
|
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Location of broadband stations used for waveform inversion
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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 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 3
The results of this grid search from 0.5 to 19 km depth are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 55 45 90 4.33 0.2024
WVFGRD96 2.0 235 45 90 4.49 0.2727
WVFGRD96 3.0 55 55 -90 4.53 0.2339
WVFGRD96 4.0 75 85 -60 4.52 0.2573
WVFGRD96 5.0 165 15 10 4.55 0.3128
WVFGRD96 6.0 170 15 15 4.57 0.3680
WVFGRD96 7.0 170 15 15 4.58 0.4134
WVFGRD96 8.0 170 15 15 4.67 0.4459
WVFGRD96 9.0 170 15 15 4.69 0.4852
WVFGRD96 10.0 165 15 10 4.71 0.5189
WVFGRD96 11.0 140 15 -15 4.73 0.5469
WVFGRD96 12.0 140 15 -15 4.75 0.5707
WVFGRD96 13.0 140 15 -15 4.76 0.5898
WVFGRD96 14.0 135 15 -20 4.78 0.6053
WVFGRD96 15.0 140 20 -20 4.80 0.6169
WVFGRD96 16.0 245 80 -80 4.82 0.6274
WVFGRD96 17.0 245 80 -80 4.83 0.6334
WVFGRD96 18.0 245 80 -80 4.85 0.6364
WVFGRD96 19.0 245 80 -80 4.86 0.6365
WVFGRD96 20.0 245 80 -80 4.87 0.6344
WVFGRD96 21.0 245 80 -80 4.89 0.6309
WVFGRD96 22.0 245 80 -80 4.90 0.6247
WVFGRD96 23.0 245 80 -80 4.91 0.6165
WVFGRD96 24.0 245 80 -80 4.92 0.6068
WVFGRD96 25.0 245 80 -80 4.93 0.5959
WVFGRD96 26.0 245 80 -80 4.94 0.5840
WVFGRD96 27.0 245 80 -80 4.95 0.5703
WVFGRD96 28.0 245 80 -80 4.96 0.5559
WVFGRD96 29.0 245 80 -80 4.96 0.5399
The best solution is
WVFGRD96 19.0 245 80 -80 4.86 0.6365
The mechanism correspond to the best fit is
|
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Figure 1. Waveform inversion focal mechanism
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The best fit as a function of depth is given in the following figure:
|
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Figure 2. Depth sensitivity for waveform mechanism
|
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 and because the velocity model used in the predictions may not be perfect.
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
|
|
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.
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Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to thewavefroms.
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.
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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:
- The origin time and epicentral distance are incorrect
- The velocity model used for the inversion is incorrect
- The velocity model used to define the P-arrival time is not the
same as the velocity model used for the waveform inversion
(assuming that the initial trace alignment is based on the
P arrival time)
Assuming only a mislocation, the time shifts are fit to a functional form:
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.
Discussion
Acknowledgements
Thanks also to the many seismic network operators whose dedication make this effort possible: University of Nevada Reno, University of Alaska, University of Washington, Oregon State University, University of Utah, Montana Bureau of Mines, UC Berkely, Caltech, UC San Diego, Saint Louis University, University of Memphis, Lamont Doherty Earth Observatory, the Oklahoma Geological Survey, TexNet, the Iris stations, the Transportable Array of EarthScope and other networks.
Velocity Model
The WUS.model used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows:
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
Quality Control
Here we tabulate the reasons for not using certain digital data sets
The following stations did not have a valid response files:
Last Changed Mon Jan 25 18:38:40 CST 2021
