The ANSS event ID is nn00804187 and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/nn00804187/executive.
2021/04/02 19:08:02 38.177 -117.872 8.3 4.3 Nevada
USGS/SLU Moment Tensor Solution
ENS 2021/04/02 19:08:02:0 38.18 -117.87 8.3 4.3 Nevada
Stations used:
BK.DANT BK.EAGL BK.HELL BK.SUTB BK.WELL BK.YUBA CI.BFS
CI.CLC CI.CWC CI.FUR CI.GRA CI.HAR CI.ISA CI.LRL CI.OSI
CI.PASC CI.TEH CI.TIN CI.TPO CI.VES GS.MCA04 IM.NV31 LB.BMN
LB.TPH NC.AFD NC.MDPB NN.BEK NN.CMK6 NN.GMN NN.KVN NN.LHV
NN.MPK NN.MZPB NN.PAH NN.PIO NN.PNT NN.PYM2 NN.Q09A NN.QSM
NN.S11A NN.UNVG NN.WAK NN.WDEM NN.WLDB NN.YER SN.HEL US.ELK
US.TPNV UU.FOR1 UU.LCMT UU.SZCU UU.VRUT
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 = 9.23e+21 dyne-cm
Mw = 3.91
Z = 10 km
Plane Strike Dip Rake
NP1 193 85 155
NP2 285 65 5
Principal Axes:
Axis Value Plunge Azimuth
T 9.23e+21 21 146
N 0.00e+00 65 3
P -9.23e+21 14 242
Moment Tensor: (dyne-cm)
Component Value
Mxx 3.59e+21
Mxy -7.37e+21
Mxz -1.50e+21
Myy -4.21e+21
Myz 3.62e+21
Mzz 6.16e+20
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--------------------#################-----
-------------------####################---
------------------######################
--- -----------#######################
-- P -----------######################
- -----------#####################
-------------############ ######
-----------############ T ####
----------############ ###
-------###############
--############
Global CMT Convention Moment Tensor:
R T P
6.16e+20 -1.50e+21 -3.62e+21
-1.50e+21 3.59e+21 7.37e+21
-3.62e+21 7.37e+21 -4.21e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20210402190802/index.html
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STK = 285
DIP = 65
RAKE = 5
MW = 3.91
HS = 10.0
The NDK file is 20210402190802.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 -40 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 1.0 105 85 0 3.43 0.3477
WVFGRD96 2.0 285 85 5 3.57 0.4586
WVFGRD96 3.0 285 60 10 3.67 0.5198
WVFGRD96 4.0 285 55 10 3.73 0.5804
WVFGRD96 5.0 285 60 10 3.76 0.6307
WVFGRD96 6.0 285 60 5 3.79 0.6705
WVFGRD96 7.0 285 65 10 3.82 0.7024
WVFGRD96 8.0 285 60 5 3.87 0.7233
WVFGRD96 9.0 285 60 0 3.89 0.7359
WVFGRD96 10.0 285 65 5 3.91 0.7406
WVFGRD96 11.0 285 65 5 3.93 0.7388
WVFGRD96 12.0 285 65 5 3.94 0.7319
WVFGRD96 13.0 285 65 0 3.96 0.7192
WVFGRD96 14.0 285 65 0 3.97 0.7031
WVFGRD96 15.0 285 70 5 3.98 0.6840
WVFGRD96 16.0 285 70 5 3.99 0.6637
WVFGRD96 17.0 285 70 5 4.00 0.6421
WVFGRD96 18.0 285 70 0 4.01 0.6188
WVFGRD96 19.0 285 70 0 4.01 0.5964
WVFGRD96 20.0 285 70 0 4.02 0.5749
WVFGRD96 21.0 285 70 0 4.02 0.5538
WVFGRD96 22.0 285 70 0 4.03 0.5349
WVFGRD96 23.0 285 70 0 4.03 0.5177
WVFGRD96 24.0 285 70 0 4.04 0.5013
WVFGRD96 25.0 285 70 0 4.04 0.4865
WVFGRD96 26.0 285 70 0 4.04 0.4728
WVFGRD96 27.0 285 70 0 4.05 0.4605
WVFGRD96 28.0 285 70 0 4.05 0.4481
WVFGRD96 29.0 285 65 0 4.06 0.4373
The best solution is
WVFGRD96 10.0 285 65 5 3.91 0.7406
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 -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. |
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