Location
Location ANSS
The ANSS event ID is us60007f42 and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/us60007f42/executive.
2020/01/24 21:35:29 48.549 -125.184 10.0 4.5 BC, Canada
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2020/01/24 21:35:29:0 48.55 -125.18 10.0 4.5 BC, Canada
Stations used:
CC.CARB CN.BFSB CN.CBB CN.CLRS CN.GDR CN.HOPB CN.NCSB
CN.NLLB CN.PABB CN.PGC CN.PHC CN.PNT CN.SHB CN.SNB CN.VGZ
CN.WPB CN.WSLR UW.BERY UW.DONK UW.FORK UW.GNW UW.HOPR
UW.LUMI UW.OMAK UW.RPW2 UW.SHUK UW.WAT2
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 = 5.75e+22 dyne-cm
Mw = 4.44
Z = 31 km
Plane Strike Dip Rake
NP1 348 86 -145
NP2 255 55 -5
Principal Axes:
Axis Value Plunge Azimuth
T 5.75e+22 21 116
N 0.00e+00 55 354
P -5.75e+22 27 217
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.91e+22
Mxy -4.18e+22
Mxz 1.02e+22
Myy 2.38e+22
Myz 3.13e+22
Mzz -4.71e+21
###-----------
########--------------
###########-----------------
#############-----------------
###############-------------------
#################-------------------
################--################----
############--------###################-
#########-----------####################
#######--------------#####################
#####-----------------####################
###-------------------####################
##---------------------###################
----------------------##################
----------------------########### ####
----------------------########## T ###
------- -----------########## ##
------ P -----------##############
---- ------------###########
------------------##########
---------------#######
-----------###
Global CMT Convention Moment Tensor:
R T P
-4.71e+21 1.02e+22 -3.13e+22
1.02e+22 -1.91e+22 4.18e+22
-3.13e+22 4.18e+22 2.38e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20200124213529/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 = 255
DIP = 55
RAKE = -5
MW = 4.44
HS = 31.0
The NDK file is 20200124213529.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 2020/01/24 21:35:29:0 48.55 -125.18 10.0 4.5 BC, Canada
Stations used:
CC.CARB CN.BFSB CN.CBB CN.CLRS CN.GDR CN.HOPB CN.NCSB
CN.NLLB CN.PABB CN.PGC CN.PHC CN.PNT CN.SHB CN.SNB CN.VGZ
CN.WPB CN.WSLR UW.BERY UW.DONK UW.FORK UW.GNW UW.HOPR
UW.LUMI UW.OMAK UW.RPW2 UW.SHUK UW.WAT2
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 = 5.75e+22 dyne-cm
Mw = 4.44
Z = 31 km
Plane Strike Dip Rake
NP1 348 86 -145
NP2 255 55 -5
Principal Axes:
Axis Value Plunge Azimuth
T 5.75e+22 21 116
N 0.00e+00 55 354
P -5.75e+22 27 217
Moment Tensor: (dyne-cm)
Component Value
Mxx -1.91e+22
Mxy -4.18e+22
Mxz 1.02e+22
Myy 2.38e+22
Myz 3.13e+22
Mzz -4.71e+21
###-----------
########--------------
###########-----------------
#############-----------------
###############-------------------
#################-------------------
################--################----
############--------###################-
#########-----------####################
#######--------------#####################
#####-----------------####################
###-------------------####################
##---------------------###################
----------------------##################
----------------------########### ####
----------------------########## T ###
------- -----------########## ##
------ P -----------##############
---- ------------###########
------------------##########
---------------#######
-----------###
Global CMT Convention Moment Tensor:
R T P
-4.71e+21 1.02e+22 -3.13e+22
1.02e+22 -1.91e+22 4.18e+22
-3.13e+22 4.18e+22 2.38e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20200124213529/index.html
|
Regional Moment Tensor (Mwr)
Moment 6.498e+15 N-m
Magnitude 4.48 Mwr
Depth 31.0 km
Percent DC 78%
Half Duration -
Catalog US
Data Source US 2
Contributor US 2
Nodal Planes
Plane Strike Dip Rake
NP1 258 59 -9
NP2 352 82 -149
Principal Axes
Axis Value Plunge Azimuth
T 6.080e+15 N-m 15 121
N 0.768e+15 N-m 58 5
P -6.848e+15 N-m 27 219
|
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.
mLg Magnitude
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.
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 1.0 165 65 -40 3.72 0.2046
WVFGRD96 2.0 35 45 90 3.99 0.2973
WVFGRD96 3.0 165 65 -35 3.93 0.3108
WVFGRD96 4.0 170 85 -20 3.94 0.3300
WVFGRD96 5.0 170 85 -20 3.97 0.3420
WVFGRD96 6.0 170 90 -20 4.00 0.3523
WVFGRD96 7.0 350 80 20 4.03 0.3638
WVFGRD96 8.0 355 70 30 4.09 0.3767
WVFGRD96 9.0 350 80 25 4.11 0.3837
WVFGRD96 10.0 350 80 25 4.13 0.3883
WVFGRD96 11.0 350 80 25 4.15 0.3912
WVFGRD96 12.0 350 80 25 4.16 0.3933
WVFGRD96 13.0 255 65 -10 4.18 0.4034
WVFGRD96 14.0 255 65 -10 4.21 0.4209
WVFGRD96 15.0 255 65 -10 4.23 0.4377
WVFGRD96 16.0 255 65 -10 4.25 0.4547
WVFGRD96 17.0 255 60 -10 4.26 0.4717
WVFGRD96 18.0 255 60 -10 4.28 0.4878
WVFGRD96 19.0 255 60 -10 4.30 0.5038
WVFGRD96 20.0 255 60 -10 4.31 0.5204
WVFGRD96 21.0 255 60 -10 4.33 0.5352
WVFGRD96 22.0 255 60 -10 4.35 0.5513
WVFGRD96 23.0 255 60 -10 4.36 0.5700
WVFGRD96 24.0 255 60 -10 4.38 0.5856
WVFGRD96 25.0 255 60 -10 4.39 0.6012
WVFGRD96 26.0 255 60 -10 4.40 0.6145
WVFGRD96 27.0 255 60 -10 4.41 0.6278
WVFGRD96 28.0 255 60 -10 4.42 0.6371
WVFGRD96 29.0 255 60 -10 4.43 0.6452
WVFGRD96 30.0 255 55 -5 4.44 0.6497
WVFGRD96 31.0 255 55 -5 4.44 0.6547
WVFGRD96 32.0 255 55 -5 4.45 0.6543
WVFGRD96 33.0 255 55 -5 4.46 0.6539
WVFGRD96 34.0 255 55 -5 4.46 0.6509
WVFGRD96 35.0 255 60 -10 4.47 0.6456
WVFGRD96 36.0 255 60 -10 4.48 0.6427
WVFGRD96 37.0 255 60 -10 4.49 0.6377
WVFGRD96 38.0 255 60 -10 4.50 0.6344
WVFGRD96 39.0 255 65 -10 4.51 0.6321
WVFGRD96 40.0 255 50 -5 4.57 0.6239
WVFGRD96 41.0 255 55 -5 4.58 0.6187
WVFGRD96 42.0 255 55 -5 4.58 0.6129
WVFGRD96 43.0 255 60 -10 4.59 0.6099
WVFGRD96 44.0 255 60 -5 4.60 0.6047
WVFGRD96 45.0 255 60 -5 4.60 0.6016
WVFGRD96 46.0 255 60 -5 4.61 0.5965
WVFGRD96 47.0 255 60 -5 4.62 0.5928
WVFGRD96 48.0 255 65 -5 4.62 0.5883
WVFGRD96 49.0 255 65 -5 4.63 0.5844
The best solution is
WVFGRD96 31.0 255 55 -5 4.44 0.6547
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 10:54:27 AM CDT 2024
