Location

Earthquakes Canada Solution

/08/24 12:50:11 54.47 -117.47 3 4.8 Alberta, Canada

Note that the Earthquakes Canada solution is 17.89 km at an azimith of 231 degrees from the USGS NEIC solution. Interestingly the estimation of location error at the bottom of this page indicates that the waveforms would indicate that the true epicenter is 13 km at an azimuth of 226 degrees from the USGS NEIOC solution. The display at the bottom os approximate given the velocity model used and the simplicity of the technique.

Using the Earthquake Canada location, the best solution in shown in the beachball titled SLUFM in the Moment Tensor Comparison below. The depth is 2 km deepder than the USGS solution. The nodal planes are slightly different, but not significsntly. The estimate of the lcoation error is now ssignificantly smaller.

Location ANSS

The ANSS event ID is us7000n96l and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/us7000n96l/executive.

2024/08/24 12:50:11 54.571 -117.255 9.4 4.2 Alberta, Canada

Focal Mechanism

USGS/SLU Moment Tensor Solution ENS 2024/08/24 12:50:11:0 54.57 -117.25 9.4 4.2 Alberta, Canada Stations used: 1E.MONT1 1E.MONT3 1E.MONT4 1E.MONT5 1E.MONT7 1E.MONT8 1E.MONT9 1E.MONTA 1E.MONTB 1E.MONTC 1E.MONTD 1E.MONTE CN.EDM CN.FNSB CN.FSJB CN.HOPB CN.LLLB CN.PNT CN.WSLR PQ.NAB1 PQ.NBC4 PQ.NBC5 PQ.NBC8 RV.BDMTA RV.BLSTA RV.BRLDA RV.BSLNA RV.EGLEA RV.FAIRA RV.FOXCA RV.KAKWA RV.LGPLA RV.PKSKA RV.REDDA RV.SNUFA RV.STPRA RV.THORA RV.WTMTA RV.YELLA TD.TD002 TD.TD008 TD.TD009 UW.GOBBL XL.MG01 XL.MG04 XL.MG05 XL.MG07 XL.MG09 XL.MG10 XL.MG11 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 = 2.60e+22 dyne-cm Mw = 4.21 Z = 5 km Plane Strike Dip Rake NP1 181 76 164 NP2 275 75 15 Principal Axes: Axis Value Plunge Azimuth T 2.60e+22 21 138 N 0.00e+00 69 319 P -2.60e+22 0 228 Moment Tensor: (dyne-cm) Component Value Mxx 8.73e+20 Mxy -2.42e+22 Mxz -6.37e+21 Myy -4.24e+21 Myz 5.97e+21 Mzz 3.36e+21 #######------- ##########------------ ############---------------- #############----------------- ###############------------------- ###############--------------------- ################---------------------- #################----------------------- #################----------------------- ###--------------############------------- -----------------###################------ -----------------#######################-- -----------------######################### ----------------######################## ----------------######################## ----------------###################### ---------------############# ##### - ----------############# T #### P ----------############# ## -----------################ ---------############# ------######## Global CMT Convention Moment Tensor: R T P 3.36e+21 -6.37e+21 -5.97e+21 -6.37e+21 8.73e+20 2.42e+22 -5.97e+21 2.42e+22 -4.24e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20240824125011/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 = 275
      DIP = 75
     RAKE = 15
       MW = 4.21
       HS = 5.0

The NDK file is 20240824125011.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 SLUFM

USGS/SLU Moment Tensor Solution ENS 2024/08/24 12:50:11:0 54.57 -117.25 9.4 4.2 Alberta, Canada Stations used: 1E.MONT1 1E.MONT3 1E.MONT4 1E.MONT5 1E.MONT7 1E.MONT8 1E.MONT9 1E.MONTA 1E.MONTB 1E.MONTC 1E.MONTD 1E.MONTE CN.EDM CN.FNSB CN.FSJB CN.HOPB CN.LLLB CN.PNT CN.WSLR PQ.NAB1 PQ.NBC4 PQ.NBC5 PQ.NBC8 RV.BDMTA RV.BLSTA RV.BRLDA RV.BSLNA RV.EGLEA RV.FAIRA RV.FOXCA RV.KAKWA RV.LGPLA RV.PKSKA RV.REDDA RV.SNUFA RV.STPRA RV.THORA RV.WTMTA RV.YELLA TD.TD002 TD.TD008 TD.TD009 UW.GOBBL XL.MG01 XL.MG04 XL.MG05 XL.MG07 XL.MG09 XL.MG10 XL.MG11 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 = 2.60e+22 dyne-cm Mw = 4.21 Z = 5 km Plane Strike Dip Rake NP1 181 76 164 NP2 275 75 15 Principal Axes: Axis Value Plunge Azimuth T 2.60e+22 21 138 N 0.00e+00 69 319 P -2.60e+22 0 228 Moment Tensor: (dyne-cm) Component Value Mxx 8.73e+20 Mxy -2.42e+22 Mxz -6.37e+21 Myy -4.24e+21 Myz 5.97e+21 Mzz 3.36e+21 #######------- ##########------------ ############---------------- #############----------------- ###############------------------- ###############--------------------- ################---------------------- #################----------------------- #################----------------------- ###--------------############------------- -----------------###################------ -----------------#######################-- -----------------######################### ----------------######################## ----------------######################## ----------------###################### ---------------############# ##### - ----------############# T #### P ----------############# ## -----------################ ---------############# ------######## Global CMT Convention Moment Tensor: R T P 3.36e+21 -6.37e+21 -5.97e+21 -6.37e+21 8.73e+20 2.42e+22 -5.97e+21 2.42e+22 -4.24e+21 Details of the solution is found at http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20240824125011/index.html

Solution using Earthquakes Canada location. Focal Mechanism USGS/SLU Moment Tensor Solution ENS 2024/08/24 12:50:11:0 54.47 -117.47 3.0 4.2 Alberta, Canada Stations used: 1E.MONT1 1E.MONT3 1E.MONT4 1E.MONT5 1E.MONT7 1E.MONT8 1E.MONT9 1E.MONTA 1E.MONTB 1E.MONTC 1E.MONTD 1E.MONTE CN.EDM CN.FNSB CN.FSJB CN.HOPB CN.LLLB CN.PNT CN.WSLR PQ.NAB1 PQ.NBC4 PQ.NBC5 PQ.NBC8 RV.BDMTA RV.BLSTA RV.BRLDA RV.BSLNA RV.EGLEA RV.FAIRA RV.FOXCA RV.KAKWA RV.LGPLA RV.PKSKA RV.REDDA RV.SNUFA RV.STPRA RV.THORA RV.WTMTA RV.YELLA TD.TD002 TD.TD008 TD.TD009 UW.GOBBL XL.MG01 XL.MG04 XL.MG05 XL.MG07 XL.MG09 XL.MG10 XL.MG11 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 = 3.31e+22 dyne-cm Mw = 4.28 Z = 7 km Plane Strike Dip Rake NP1 90 80 20 NP2 356 70 169 Principal Axes: Axis Value Plunge Azimuth T 3.31e+22 21 315 N 0.00e+00 68 116 P -3.31e+22 7 222 Moment Tensor: (dyne-cm) Component Value Mxx -3.87e+21 Mxy -3.06e+22 Mxz 1.06e+22 Myy -2.68e+15 Myz -5.40e+21 Mzz 3.87e+21 #######------- ############---------- ################------------ ## ############------------- #### T #############-------------- ##### #############--------------- #######################--------------- ########################---------------- #########################--------------- ##########################---------------- ##########################---------------- ---#######################-------------### --------------------------################ -------------------------############### -------------------------############### ------------------------############## -----------------------############# -- ----------------############# P ----------------########### ----------------########## --------------######## ---------##### Global CMT Convention Moment Tensor: R T P 3.87e+21 1.06e+22 5.40e+21 1.06e+22 -3.87e+21 3.06e+22 5.40e+21 3.06e+22 -2.68e+15

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 M_L by application of the respective IASPEI formulae. As a research study, the linear distance term of the IASPEI formula for M_L is adjusted to remove a linear distance trend in residuals to give a regionally defined M_L. The defined M_L 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.

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'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 0.07 n 3

The results of this grid search are as follow:

           DEPTH  STK   DIP  RAKE   MW    FIT
WVFGRD96    1.0   270    80   -10   3.99 0.3481
WVFGRD96    2.0   270    75   -10   4.11 0.4310
WVFGRD96    3.0   275    85     5   4.14 0.4509
WVFGRD96    4.0   275    75    10   4.19 0.4567
WVFGRD96    5.0   275    75    15   4.21 0.4592
WVFGRD96    6.0   280    75    20   4.23 0.4564
WVFGRD96    7.0   280    75    15   4.24 0.4516
WVFGRD96    8.0   280    75    20   4.28 0.4474
WVFGRD96    9.0   280    75    20   4.29 0.4397
WVFGRD96   10.0   280    75    20   4.30 0.4311
WVFGRD96   11.0   280    75    15   4.31 0.4221
WVFGRD96   12.0   280    75    15   4.32 0.4128
WVFGRD96   13.0   280    75    15   4.33 0.4034
WVFGRD96   14.0   280    80    15   4.34 0.3949
WVFGRD96   15.0   280    80    15   4.35 0.3866
WVFGRD96   16.0   280    80    15   4.36 0.3785
WVFGRD96   17.0   280    80    15   4.37 0.3707
WVFGRD96   18.0   280    80    15   4.37 0.3634
WVFGRD96   19.0   275    80    10   4.38 0.3575
WVFGRD96   20.0   275    80    10   4.39 0.3512
WVFGRD96   21.0   280    75    10   4.39 0.3459
WVFGRD96   22.0   280    75    10   4.40 0.3413
WVFGRD96   23.0   280    75    15   4.41 0.3378
WVFGRD96   24.0   280    75    15   4.42 0.3349
WVFGRD96   25.0   280    75    15   4.43 0.3320
WVFGRD96   26.0   280    75    15   4.43 0.3285
WVFGRD96   27.0   280    75    15   4.44 0.3248
WVFGRD96   28.0   280    75    15   4.45 0.3202
WVFGRD96   29.0   190    80    15   4.45 0.3164

The best solution is

WVFGRD96    5.0   275    75    15   4.21 0.4592

The mechanism corresponding 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, 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 0.07 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. The time scale is relative to the first trace sample.

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.

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 Mon Aug 26 19:48:46 CDT 2024