Location
Location ANSS
The ANSS event ID is ak020bo3ssdp and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ak020bo3ssdp/executive.
2020/09/10 02:18:00 61.459 -149.936 31.7 4 Alaska
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2020/09/10 02:18:00:0 61.46 -149.94 31.7 4.0 Alaska
Stations used:
AK.CAST AK.CUT AK.GHO AK.KNK AK.L20K AK.M19K AK.M20K
AK.PPLA AK.PWL AK.RC01 AK.SAW AK.SCM AK.SKN AK.SSN AT.PMR
AV.STLK TA.M22K
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 = 1.50e+22 dyne-cm
Mw = 4.05
Z = 46 km
Plane Strike Dip Rake
NP1 195 65 -80
NP2 352 27 -110
Principal Axes:
Axis Value Plunge Azimuth
T 1.50e+22 19 278
N 0.00e+00 9 11
P -1.50e+22 68 125
Moment Tensor: (dyne-cm)
Component Value
Mxx -4.21e+20
Mxy -7.83e+20
Mxz 3.51e+21
Myy 1.17e+22
Myz -8.86e+21
Mzz -1.13e+22
#########--###
#############----#####
###############-------######
##############-----------#####
###############-------------######
###############----------------#####
###############-----------------######
###############-------------------######
## ##########--------------------#####
### T #########---------------------######
### #########---------------------######
##############---------- ---------######
##############---------- P ---------######
#############---------- ---------#####
############-----------------------#####
###########----------------------#####
##########---------------------#####
#########---------------------####
########------------------####
#######-----------------####
#####--------------###
#-----------##
Global CMT Convention Moment Tensor:
R T P
-1.13e+22 3.51e+21 8.86e+21
3.51e+21 -4.21e+20 7.83e+20
8.86e+21 7.83e+20 1.17e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20200910021800/index.html
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Preferred Solution
The preferred solution from an analysis of the surface-wave spectral amplitude radiation pattern, waveform inversion or first motion observations is
STK = 195
DIP = 65
RAKE = -80
MW = 4.05
HS = 46.0
The NDK file is 20200910021800.ndk
The waveform inversion is preferred.
Moment Tensor Comparison
The following compares this source inversion to those provided by others. The purpose is to look for major differences and also to note slight differences that might be inherent to the processing procedure. For completeness the USGS/SLU solution is repeated from above.
| SLU |
USGSMWR |
USGS/SLU Moment Tensor Solution
ENS 2020/09/10 02:18:00:0 61.46 -149.94 31.7 4.0 Alaska
Stations used:
AK.CAST AK.CUT AK.GHO AK.KNK AK.L20K AK.M19K AK.M20K
AK.PPLA AK.PWL AK.RC01 AK.SAW AK.SCM AK.SKN AK.SSN AT.PMR
AV.STLK TA.M22K
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 = 1.50e+22 dyne-cm
Mw = 4.05
Z = 46 km
Plane Strike Dip Rake
NP1 195 65 -80
NP2 352 27 -110
Principal Axes:
Axis Value Plunge Azimuth
T 1.50e+22 19 278
N 0.00e+00 9 11
P -1.50e+22 68 125
Moment Tensor: (dyne-cm)
Component Value
Mxx -4.21e+20
Mxy -7.83e+20
Mxz 3.51e+21
Myy 1.17e+22
Myz -8.86e+21
Mzz -1.13e+22
#########--###
#############----#####
###############-------######
##############-----------#####
###############-------------######
###############----------------#####
###############-----------------######
###############-------------------######
## ##########--------------------#####
### T #########---------------------######
### #########---------------------######
##############---------- ---------######
##############---------- P ---------######
#############---------- ---------#####
############-----------------------#####
###########----------------------#####
##########---------------------#####
#########---------------------####
########------------------####
#######-----------------####
#####--------------###
#-----------##
Global CMT Convention Moment Tensor:
R T P
-1.13e+22 3.51e+21 8.86e+21
3.51e+21 -4.21e+20 7.83e+20
8.86e+21 7.83e+20 1.17e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20200910021800/index.html
|
Regional Moment Tensor (Mwr)
Moment 1.927e+15 N-m
Magnitude 4.12 Mwr
Depth 46.0 km
Percent DC 80%
Half Duration -
Catalog US
Data Source US 3
Contributor US 3
Nodal Planes
Plane Strike Dip Rake
NP1 348 27 -124
NP2 206 68 -74
Principal Axes
Axis Value Plunge Azimuth
T 1.819e+15 N-m 21 284
N 0.202e+15 N-m 15 19
P -2.020e+15 N-m 64 142
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Magnitudes
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.
ML Magnitude
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.
Context
The left panel of the next figure presents the focal mechanism for this earthquake (red) in the context of other nearby events (blue) in the SLU Moment Tensor Catalog. 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). Thus context plot is useful for assessing the appropriateness of the moment tensor of this event.
Waveform Inversion using wvfgrd96
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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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'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 3
The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 195 45 95 3.17 0.1806
WVFGRD96 2.0 10 40 85 3.34 0.2527
WVFGRD96 3.0 5 30 75 3.40 0.2297
WVFGRD96 4.0 360 25 65 3.40 0.2327
WVFGRD96 5.0 135 70 -40 3.40 0.2438
WVFGRD96 6.0 135 70 -40 3.43 0.2683
WVFGRD96 7.0 140 70 -35 3.45 0.2906
WVFGRD96 8.0 135 65 -40 3.52 0.3014
WVFGRD96 9.0 135 65 -40 3.55 0.3161
WVFGRD96 10.0 135 55 -35 3.57 0.3318
WVFGRD96 11.0 135 55 -40 3.59 0.3438
WVFGRD96 12.0 135 55 -35 3.61 0.3524
WVFGRD96 13.0 130 55 -40 3.63 0.3588
WVFGRD96 14.0 235 55 35 3.66 0.3650
WVFGRD96 15.0 235 55 35 3.68 0.3683
WVFGRD96 16.0 70 60 40 3.67 0.3718
WVFGRD96 17.0 75 55 40 3.69 0.3787
WVFGRD96 18.0 75 55 40 3.70 0.3858
WVFGRD96 19.0 75 55 40 3.72 0.3919
WVFGRD96 20.0 75 55 40 3.73 0.3971
WVFGRD96 21.0 220 65 -65 3.72 0.4047
WVFGRD96 22.0 225 65 -50 3.73 0.4172
WVFGRD96 23.0 225 65 -50 3.75 0.4290
WVFGRD96 24.0 225 65 -50 3.76 0.4396
WVFGRD96 25.0 220 65 -55 3.77 0.4500
WVFGRD96 26.0 220 65 -60 3.78 0.4574
WVFGRD96 27.0 215 80 -75 3.80 0.4749
WVFGRD96 28.0 215 80 -75 3.81 0.4963
WVFGRD96 29.0 215 80 -75 3.83 0.5185
WVFGRD96 30.0 215 80 -75 3.84 0.5393
WVFGRD96 31.0 215 80 -75 3.85 0.5572
WVFGRD96 32.0 215 80 -75 3.86 0.5733
WVFGRD96 33.0 210 75 -75 3.87 0.5894
WVFGRD96 34.0 210 75 -75 3.87 0.6038
WVFGRD96 35.0 210 75 -75 3.88 0.6150
WVFGRD96 36.0 205 70 -70 3.88 0.6285
WVFGRD96 37.0 205 70 -70 3.89 0.6436
WVFGRD96 38.0 205 70 -70 3.89 0.6547
WVFGRD96 39.0 205 70 -70 3.90 0.6626
WVFGRD96 40.0 200 70 -80 4.01 0.6509
WVFGRD96 41.0 200 70 -75 4.02 0.6590
WVFGRD96 42.0 200 70 -80 4.03 0.6622
WVFGRD96 43.0 195 65 -80 4.03 0.6667
WVFGRD96 44.0 195 65 -80 4.04 0.6719
WVFGRD96 45.0 195 65 -80 4.05 0.6747
WVFGRD96 46.0 195 65 -80 4.05 0.6748
WVFGRD96 47.0 195 65 -80 4.06 0.6744
WVFGRD96 48.0 195 65 -80 4.06 0.6727
WVFGRD96 49.0 195 65 -80 4.07 0.6690
WVFGRD96 50.0 195 65 -80 4.07 0.6647
WVFGRD96 51.0 195 65 -80 4.07 0.6579
WVFGRD96 52.0 195 65 -80 4.07 0.6526
WVFGRD96 53.0 195 65 -80 4.08 0.6436
WVFGRD96 54.0 195 65 -80 4.08 0.6374
WVFGRD96 55.0 195 65 -80 4.08 0.6282
WVFGRD96 56.0 195 65 -80 4.08 0.6200
WVFGRD96 57.0 195 65 -80 4.08 0.6113
WVFGRD96 58.0 195 65 -80 4.08 0.6005
WVFGRD96 59.0 195 65 -80 4.08 0.5920
The best solution is
WVFGRD96 46.0 195 65 -80 4.05 0.6748
The mechanism corresponding 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
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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.
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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.
Velocity Model
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
Last Changed Thu Apr 25 09:23:33 PM CDT 2024
