Location
Location ANSS
2021/07/08 20:40:16 59.164 -136.259 6.9 4.4 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2021/07/08 20:40:16:0 59.16 -136.26 6.9 4.4 Alaska
Stations used:
AK.BARN AK.BCP AK.BESE AK.GRNC AK.LOGN AK.PIN AK.R32K
AK.S31K AK.S32K AK.TABL AK.U33K AK.WAX AT.SIT AT.SKAG
AT.YKU2 CN.HYT CN.WHY TA.P32M US.WRAK
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 0.07 n 3
Best Fitting Double Couple
Mo = 7.33e+22 dyne-cm
Mw = 4.51
Z = 16 km
Plane Strike Dip Rake
NP1 158 62 101
NP2 315 30 70
Principal Axes:
Axis Value Plunge Azimuth
T 7.33e+22 71 92
N 0.00e+00 10 332
P -7.33e+22 16 240
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.73e+22
Mxy -2.98e+22
Mxz 9.00e+21
Myy -4.24e+22
Myz 3.97e+22
Mzz 5.96e+22
--------------
###-------------------
##---###########------------
------###############---------
--------##################--------
---------####################-------
----------#####################-------
-----------#######################------
-----------########################-----
-------------#######################------
-------------############ #########-----
--------------########### T ##########----
--------------########### ##########----
--------------#######################---
--- ---------######################---
-- P ----------#####################--
- -----------###################--
----------------#################-
---------------###############
----------------############
----------------######
--------------
Global CMT Convention Moment Tensor:
R T P
5.96e+22 9.00e+21 -3.97e+22
9.00e+21 -1.73e+22 2.98e+22
-3.97e+22 2.98e+22 -4.24e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210708204016/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 = 315
DIP = 30
RAKE = 70
MW = 4.51
HS = 16.0
The NDK file is 20210708204016.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 2021/07/08 20:40:16:0 59.16 -136.26 6.9 4.4 Alaska
Stations used:
AK.BARN AK.BCP AK.BESE AK.GRNC AK.LOGN AK.PIN AK.R32K
AK.S31K AK.S32K AK.TABL AK.U33K AK.WAX AT.SIT AT.SKAG
AT.YKU2 CN.HYT CN.WHY TA.P32M US.WRAK
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 0.07 n 3
Best Fitting Double Couple
Mo = 7.33e+22 dyne-cm
Mw = 4.51
Z = 16 km
Plane Strike Dip Rake
NP1 158 62 101
NP2 315 30 70
Principal Axes:
Axis Value Plunge Azimuth
T 7.33e+22 71 92
N 0.00e+00 10 332
P -7.33e+22 16 240
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.73e+22
Mxy -2.98e+22
Mxz 9.00e+21
Myy -4.24e+22
Myz 3.97e+22
Mzz 5.96e+22
--------------
###-------------------
##---###########------------
------###############---------
--------##################--------
---------####################-------
----------#####################-------
-----------#######################------
-----------########################-----
-------------#######################------
-------------############ #########-----
--------------########### T ##########----
--------------########### ##########----
--------------#######################---
--- ---------######################---
-- P ----------#####################--
- -----------###################--
----------------#################-
---------------###############
----------------############
----------------######
--------------
Global CMT Convention Moment Tensor:
R T P
5.96e+22 9.00e+21 -3.97e+22
9.00e+21 -1.73e+22 2.98e+22
-3.97e+22 2.98e+22 -4.24e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210708204016/index.html
|
Regional Moment Tensor (Mwr)
Moment 6.453e+15 N-m
Magnitude 4.47 Mwr
Depth 14.0 km
Percent DC 99%
Half Duration -
Catalog US
Data Source US 2
Contributor US 2
Nodal Planes
Plane Strike Dip Rake
NP1 297° 35° 51°
NP2 161° 63° 114°
Principal Axes
Axis Value Plunge Azimuth
T 6.461e+15 N-m 64° 111°
N -0.017e+15 N-m 21° 330°
P -6.445e+15 N-m 15° 234°
|
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.
|
|
Location of broadband stations used for waveform inversion
|
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 0.07 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 1.0 350 50 90 4.18 0.2963
WVFGRD96 2.0 170 40 90 4.27 0.3347
WVFGRD96 3.0 85 60 -30 4.20 0.2663
WVFGRD96 4.0 280 25 0 4.29 0.2813
WVFGRD96 5.0 280 15 25 4.33 0.3437
WVFGRD96 6.0 285 15 30 4.33 0.3958
WVFGRD96 7.0 300 15 50 4.34 0.4361
WVFGRD96 8.0 305 15 55 4.42 0.4633
WVFGRD96 9.0 305 20 55 4.43 0.5015
WVFGRD96 10.0 315 20 65 4.44 0.5343
WVFGRD96 11.0 315 25 65 4.46 0.5621
WVFGRD96 12.0 310 30 65 4.48 0.5846
WVFGRD96 13.0 310 30 65 4.48 0.6021
WVFGRD96 14.0 315 30 70 4.49 0.6136
WVFGRD96 15.0 315 30 70 4.50 0.6201
WVFGRD96 16.0 315 30 70 4.51 0.6222
WVFGRD96 17.0 310 30 65 4.51 0.6208
WVFGRD96 18.0 310 30 65 4.52 0.6164
WVFGRD96 19.0 310 30 65 4.53 0.6096
WVFGRD96 20.0 310 30 65 4.53 0.6003
WVFGRD96 21.0 310 30 65 4.55 0.5902
WVFGRD96 22.0 305 30 60 4.55 0.5782
WVFGRD96 23.0 305 30 60 4.56 0.5644
WVFGRD96 24.0 300 30 55 4.57 0.5493
WVFGRD96 25.0 300 30 55 4.57 0.5337
WVFGRD96 26.0 305 25 60 4.58 0.5172
WVFGRD96 27.0 300 25 55 4.58 0.5004
WVFGRD96 28.0 300 25 55 4.59 0.4833
WVFGRD96 29.0 300 25 55 4.59 0.4651
The best solution is
WVFGRD96 16.0 315 30 70 4.51 0.6222
The mechanism correspond to the best fit is
|
|
Figure 1. Waveform inversion focal mechanism
|
The best fit as a function of depth is given in the following figure:
|
|
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 0.07 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.
|
|
|
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.
|
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 Thu Jul 8 16:26:39 CDT 2021
