Location
Location ANSS
The ANSS event ID is uw61114971 and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/uw61114971/executive.
2015/12/30 07:39:29 48.587 -123.300 52.4 4.79 BC, Canada
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2015/12/30 07:39:29:0 48.59 -123.30 52.4 4.8 BC, Canada
Stations used:
CC.CIHL CC.CPCO CC.JRO CC.NORM CC.SHRK CC.SWNB CN.LLLB
CN.PNT CN.WALA IU.COR IW.PLID MB.JTMT TA.A04D TA.B05D
TA.C06D TA.D03D TA.E04D TA.F04D TA.F05D TA.G03D TA.G05D
TA.I02E TA.I03D TA.I05D TA.J05D TA.K02D TD.TD009 TD.TD012
TD.TD028 UO.BUCK UO.DBO UO.PINE US.BMO US.HAWA US.NEW
US.NLWA UW.BABR UW.BLOW UW.BRAN UW.CCRK UW.DAVN UW.DDRF
UW.DOSE UW.FORK UW.GNW UW.HEBO UW.HOOD UW.KENT UW.LEBA
UW.LTY UW.OMAK UW.PASS UW.PHIN UW.RATT UW.SP2 UW.STOR
UW.TOLT UW.TREE UW.TUCA UW.UMAT UW.WISH UW.WOLL
Filtering commands used:
cut o DIST/3.3 -30 o DIST/3.3 +80
rtr
taper w 0.1
hp c 0.02 n 3
lp c 0.06 n 3
Best Fitting Double Couple
Mo = 1.23e+23 dyne-cm
Mw = 4.66
Z = 52 km
Plane Strike Dip Rake
NP1 325 60 -109
NP2 180 35 -60
Principal Axes:
Axis Value Plunge Azimuth
T 1.23e+23 13 69
N 0.00e+00 17 335
P -1.23e+23 69 195
Moment Tensor: (dyne-cm)
Component Value
Mxx -6.17e+15
Mxy 3.53e+22
Mxz 5.04e+22
Myy 1.00e+23
Myz 3.64e+22
Mzz -1.00e+23
----##########
-----#################
######-#####################
######-----###################
#######---------##################
#######------------#################
#######---------------############ #
#######------------------########## T ##
#######-------------------######### ##
########--------------------##############
#######----------------------#############
#######-----------------------############
########---------- ----------###########
#######---------- P -----------#########
#######---------- -----------#########
#######------------------------#######
#######-----------------------######
#######----------------------#####
######---------------------###
######--------------------##
#####-----------------
####----------
Global CMT Convention Moment Tensor:
R T P
-1.00e+23 5.04e+22 -3.64e+22
5.04e+22 -6.17e+15 -3.53e+22
-3.64e+22 -3.53e+22 1.00e+23
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20151230073929/index.html
|
Preferred Solution
The preferred solution from an analysis of the surface-wave spectral amplitude radiation pattern, waveform inversion or first motion observations is
STK = 180
DIP = 35
RAKE = -60
MW = 4.66
HS = 52.0
The NDK file is 20151230073929.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 |
USGSMT |
USGSW |
USGS/SLU Moment Tensor Solution
ENS 2015/12/30 07:39:29:0 48.59 -123.30 52.4 4.8 BC, Canada
Stations used:
CC.CIHL CC.CPCO CC.JRO CC.NORM CC.SHRK CC.SWNB CN.LLLB
CN.PNT CN.WALA IU.COR IW.PLID MB.JTMT TA.A04D TA.B05D
TA.C06D TA.D03D TA.E04D TA.F04D TA.F05D TA.G03D TA.G05D
TA.I02E TA.I03D TA.I05D TA.J05D TA.K02D TD.TD009 TD.TD012
TD.TD028 UO.BUCK UO.DBO UO.PINE US.BMO US.HAWA US.NEW
US.NLWA UW.BABR UW.BLOW UW.BRAN UW.CCRK UW.DAVN UW.DDRF
UW.DOSE UW.FORK UW.GNW UW.HEBO UW.HOOD UW.KENT UW.LEBA
UW.LTY UW.OMAK UW.PASS UW.PHIN UW.RATT UW.SP2 UW.STOR
UW.TOLT UW.TREE UW.TUCA UW.UMAT UW.WISH UW.WOLL
Filtering commands used:
cut o DIST/3.3 -30 o DIST/3.3 +80
rtr
taper w 0.1
hp c 0.02 n 3
lp c 0.06 n 3
Best Fitting Double Couple
Mo = 1.23e+23 dyne-cm
Mw = 4.66
Z = 52 km
Plane Strike Dip Rake
NP1 325 60 -109
NP2 180 35 -60
Principal Axes:
Axis Value Plunge Azimuth
T 1.23e+23 13 69
N 0.00e+00 17 335
P -1.23e+23 69 195
Moment Tensor: (dyne-cm)
Component Value
Mxx -6.17e+15
Mxy 3.53e+22
Mxz 5.04e+22
Myy 1.00e+23
Myz 3.64e+22
Mzz -1.00e+23
----##########
-----#################
######-#####################
######-----###################
#######---------##################
#######------------#################
#######---------------############ #
#######------------------########## T ##
#######-------------------######### ##
########--------------------##############
#######----------------------#############
#######-----------------------############
########---------- ----------###########
#######---------- P -----------#########
#######---------- -----------#########
#######------------------------#######
#######-----------------------######
#######----------------------#####
######---------------------###
######--------------------##
#####-----------------
####----------
Global CMT Convention Moment Tensor:
R T P
-1.00e+23 5.04e+22 -3.64e+22
5.04e+22 -6.17e+15 -3.53e+22
-3.64e+22 -3.53e+22 1.00e+23
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20151230073929/index.html
|
Regional Moment Tensor (Mwr)
Moment 1.278e+16 N-m
Magnitude 4.67
Depth 52.0 km
Percent DC 98%
Half Duration –
Catalog US (us10004adm)
Data Source US3
Contributor US3
Nodal Planes
Plane Strike Dip Rake
NP1 195 40 -47
NP2 324 62 -120
Principal Axes
Axis Value Plunge Azimuth
T 1.285 12 75
N -0.015 26 339
P -1.271 61 189
|
W-phase Moment Tensor (Mww)
Moment 1.495e+16 N-m
Magnitude 4.72
Depth 45.5 km
Percent DC 83%
Half Duration –
Catalog US (us10004adm)
Data Source US3
Contributor US3
Nodal Planes
Plane Strike Dip Rake
NP1 181 39 -66
NP2 331 55 -108
Principal Axes
Axis Value Plunge Azimuth
T 1.558 8 74
N -0.135 15 342
P -1.423 73 193
|
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 -30 o DIST/3.3 +80
rtr
taper w 0.1
hp c 0.02 n 3
lp c 0.06 n 3
The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 2.0 160 40 90 4.04 0.3498
WVFGRD96 4.0 160 50 -85 4.11 0.3102
WVFGRD96 6.0 190 60 -50 4.07 0.2675
WVFGRD96 8.0 150 80 70 4.15 0.2920
WVFGRD96 10.0 150 80 70 4.17 0.3280
WVFGRD96 12.0 215 25 -15 4.18 0.3671
WVFGRD96 14.0 210 30 -25 4.20 0.4003
WVFGRD96 16.0 210 30 -25 4.22 0.4311
WVFGRD96 18.0 210 30 -25 4.24 0.4567
WVFGRD96 20.0 210 30 -25 4.26 0.4787
WVFGRD96 22.0 210 30 -20 4.29 0.4980
WVFGRD96 24.0 210 30 -20 4.31 0.5155
WVFGRD96 26.0 210 30 -20 4.33 0.5317
WVFGRD96 28.0 210 30 -20 4.35 0.5482
WVFGRD96 30.0 210 35 -20 4.37 0.5633
WVFGRD96 32.0 210 35 -20 4.39 0.5767
WVFGRD96 34.0 205 35 -30 4.40 0.5897
WVFGRD96 36.0 195 35 -40 4.41 0.6035
WVFGRD96 38.0 190 30 -50 4.42 0.6174
WVFGRD96 40.0 185 30 -55 4.56 0.6543
WVFGRD96 42.0 180 30 -60 4.58 0.6708
WVFGRD96 44.0 180 30 -60 4.60 0.6822
WVFGRD96 46.0 180 35 -60 4.61 0.6909
WVFGRD96 48.0 180 35 -60 4.63 0.6998
WVFGRD96 50.0 180 35 -60 4.65 0.7052
WVFGRD96 52.0 180 35 -60 4.66 0.7079
WVFGRD96 54.0 180 35 -60 4.67 0.7076
WVFGRD96 56.0 185 40 -55 4.69 0.7046
WVFGRD96 58.0 185 40 -55 4.70 0.7026
WVFGRD96 60.0 185 40 -55 4.71 0.6985
WVFGRD96 62.0 185 40 -55 4.72 0.6913
WVFGRD96 64.0 185 40 -55 4.73 0.6819
WVFGRD96 66.0 185 40 -55 4.74 0.6702
WVFGRD96 68.0 190 45 -50 4.75 0.6596
WVFGRD96 70.0 190 45 -50 4.76 0.6489
WVFGRD96 72.0 190 45 -50 4.76 0.6365
WVFGRD96 74.0 190 45 -50 4.77 0.6228
WVFGRD96 76.0 190 45 -50 4.77 0.6085
WVFGRD96 78.0 190 45 -45 4.78 0.5939
WVFGRD96 80.0 190 50 -45 4.79 0.5804
WVFGRD96 82.0 190 50 -45 4.79 0.5688
WVFGRD96 84.0 190 50 -45 4.80 0.5560
WVFGRD96 86.0 190 50 -45 4.80 0.5422
WVFGRD96 88.0 190 50 -45 4.80 0.5282
WVFGRD96 90.0 195 50 -40 4.80 0.5147
WVFGRD96 92.0 195 50 -40 4.80 0.5014
WVFGRD96 94.0 195 55 -40 4.81 0.4879
WVFGRD96 96.0 195 55 -40 4.81 0.4766
WVFGRD96 98.0 200 55 -30 4.81 0.4655
The best solution is
WVFGRD96 52.0 180 35 -60 4.66 0.7079
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 -30 o DIST/3.3 +80
rtr
taper w 0.1
hp c 0.02 n 3
lp c 0.06 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. 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 Sat Apr 27 02:24:44 AM CDT 2024
