The ANSS event ID is nc72895971 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/nc72895971/executive.
2017/09/21 22:15:21 37.977 -122.066 16.1 2.7 Alaska
USGS/SLU Moment Tensor Solution
ENS 2017/09/21 22:15:21:0 37.98 -122.07 16.1 2.7 Alaska
Stations used:
AK.BMR AK.BPAW AK.BWN AK.CAST AK.CCB AK.CRQ AK.CTG AK.CUT
AK.DHY AK.DIV AK.FID AK.GHO AK.GLB AK.GLI AK.HDA AK.HIN
AK.KAI AK.KLU AK.KNK AK.KTH AK.MCAR AK.MCK AK.MDM AK.MLY
AK.NEA2 AK.PPD AK.PPLA AK.PWL AK.RC01 AK.RIDG AK.RND AK.SAW
AK.SCM AK.SCRK AK.SKN AK.SSN AK.TRF AK.VRDI AK.WRH AT.MENT
AT.MID AT.PMR AT.SVW2 AT.TTA IM.IL31 TA.H21K TA.H23K
TA.H24K TA.J20K TA.J25K TA.J26L TA.K20K TA.L19K TA.L26K
TA.L27K TA.M19K TA.M22K TA.M24K TA.M26K TA.M27K TA.N19K
TA.N25K TA.O22K TA.POKR TA.Q23K
Filtering commands used:
cut o DIST/3.3 -30 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 = 1.72e+22 dyne-cm
Mw = 4.09
Z = 18 km
Plane Strike Dip Rake
NP1 195 60 60
NP2 64 41 131
Principal Axes:
Axis Value Plunge Azimuth
T 1.72e+22 62 56
N 0.00e+00 26 211
P -1.72e+22 10 306
Moment Tensor: (dyne-cm)
Component Value
Mxx -4.58e+21
Mxy 9.66e+21
Mxz 2.22e+21
Myy -8.30e+21
Myz 8.30e+21
Mzz 1.29e+22
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Global CMT Convention Moment Tensor:
R T P
1.29e+22 2.22e+21 -8.30e+21
2.22e+21 -4.58e+21 -9.66e+21
-8.30e+21 -9.66e+21 -8.30e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20170921221521/index.html
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STK = 195
DIP = 60
RAKE = 60
MW = 4.09
HS = 18.0
The NDK file is 20170921221521.ndk The waveform inversion is preferred.
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.
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.
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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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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 +50 rtr taper w 0.1 hp c 0.03 n 3 lp c 0.10 n 3The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 1.0 225 45 -85 3.62 0.2969
WVFGRD96 2.0 40 45 -95 3.76 0.3871
WVFGRD96 3.0 245 45 -65 3.77 0.2939
WVFGRD96 4.0 0 40 0 3.72 0.2788
WVFGRD96 5.0 -5 40 -5 3.74 0.3128
WVFGRD96 6.0 200 85 -65 3.79 0.3446
WVFGRD96 7.0 65 25 -45 3.81 0.3810
WVFGRD96 8.0 65 25 -50 3.90 0.4088
WVFGRD96 9.0 65 30 -50 3.93 0.4407
WVFGRD96 10.0 65 35 -50 3.95 0.4679
WVFGRD96 11.0 195 65 65 3.97 0.5075
WVFGRD96 12.0 195 60 60 3.99 0.5461
WVFGRD96 13.0 195 60 60 4.01 0.5781
WVFGRD96 14.0 195 60 60 4.03 0.6033
WVFGRD96 15.0 195 60 60 4.05 0.6217
WVFGRD96 16.0 195 60 60 4.06 0.6342
WVFGRD96 17.0 195 60 60 4.08 0.6409
WVFGRD96 18.0 195 60 60 4.09 0.6420
WVFGRD96 19.0 195 60 60 4.10 0.6382
WVFGRD96 20.0 195 60 60 4.11 0.6297
WVFGRD96 21.0 195 60 60 4.13 0.6167
WVFGRD96 22.0 190 60 60 4.14 0.6021
WVFGRD96 23.0 190 60 60 4.14 0.5854
WVFGRD96 24.0 190 60 60 4.15 0.5670
WVFGRD96 25.0 190 60 60 4.15 0.5472
WVFGRD96 26.0 190 60 60 4.16 0.5261
WVFGRD96 27.0 190 60 60 4.16 0.5044
WVFGRD96 28.0 190 60 60 4.17 0.4817
WVFGRD96 29.0 190 60 60 4.17 0.4581
The best solution is
WVFGRD96 18.0 195 60 60 4.09 0.6420
The mechanism corresponding to the best fit is
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The best fit as a function of depth is given in the following figure:
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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 +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. |
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:
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.
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