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.
The ANSS event ID is us2000fw8c and the event page is at https://earthquake.usgs.gov/earthquakes/eventpage/us2000fw8c/executive.
2018/07/03 20:15:22 41.398 -100.142 3.1 3.6 Nebraska
USGS/SLU Moment Tensor Solution
ENS 2018/07/03 20:15:22:0 41.40 -100.14 3.1 3.6 Nebraska
Stations used:
IU.RSSD N4.K30B N4.N35B N4.R32B TA.BGNE TA.KSCO US.CBKS
US.ECSD US.KSU1 US.OGNE XU.KE01
Filtering commands used:
cut o DIST/3.3 -30 o DIST/3.3 +40
rtr
taper w 0.1
hp c 0.03 n 3
lp c 0.10 n 3
Best Fitting Double Couple
Mo = 2.85e+21 dyne-cm
Mw = 3.57
Z = 11 km
Plane Strike Dip Rake
NP1 76 72 154
NP2 175 65 20
Principal Axes:
Axis Value Plunge Azimuth
T 2.85e+21 31 34
N 0.00e+00 58 224
P -2.85e+21 5 127
Moment Tensor: (dyne-cm)
Component Value
Mxx 4.16e+20
Mxy 2.33e+21
Mxz 1.18e+21
Myy -1.16e+21
Myz 5.26e+20
Mzz 7.47e+20
----##########
-------###############
---------###################
----------############ #####
-----------############# T #######
------------############# ########
-------------#########################
--------------##########################
--------------##########################
--------------#########################---
---------------#####################------
---------------#################----------
---------------#############--------------
--------------######--------------------
###############-------------------------
##############------------------------
##############------------------ -
#############------------------ P
############-----------------
############----------------
##########------------
########------
Global CMT Convention Moment Tensor:
R T P
7.47e+20 1.18e+21 -5.26e+20
1.18e+21 4.16e+20 -2.33e+21
-5.26e+20 -2.33e+21 -1.16e+21
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20180703201522/index.html
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STK = 175
DIP = 65
RAKE = 20
MW = 3.57
HS = 11.0
The NDK file is 20180703201522.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: 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.
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 +40 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 1.0 350 70 -15 3.41 0.4125
WVFGRD96 2.0 345 65 -30 3.47 0.4322
WVFGRD96 3.0 345 70 -25 3.48 0.4222
WVFGRD96 4.0 175 70 20 3.47 0.4189
WVFGRD96 5.0 175 65 20 3.49 0.4272
WVFGRD96 6.0 175 65 20 3.50 0.4369
WVFGRD96 7.0 175 65 15 3.51 0.4452
WVFGRD96 8.0 175 65 15 3.52 0.4519
WVFGRD96 9.0 175 65 15 3.54 0.4579
WVFGRD96 10.0 175 65 20 3.56 0.4611
WVFGRD96 11.0 175 65 20 3.57 0.4615
WVFGRD96 12.0 175 65 15 3.58 0.4596
WVFGRD96 13.0 175 65 15 3.59 0.4550
WVFGRD96 14.0 180 60 15 3.59 0.4490
WVFGRD96 15.0 180 60 15 3.60 0.4434
WVFGRD96 16.0 180 60 15 3.61 0.4356
WVFGRD96 17.0 180 60 15 3.62 0.4279
WVFGRD96 18.0 180 55 15 3.63 0.4191
WVFGRD96 19.0 180 55 15 3.64 0.4108
WVFGRD96 20.0 185 50 20 3.66 0.4037
WVFGRD96 21.0 185 50 15 3.66 0.3962
WVFGRD96 22.0 185 50 15 3.67 0.3884
WVFGRD96 23.0 185 50 15 3.68 0.3823
WVFGRD96 24.0 185 50 15 3.69 0.3743
WVFGRD96 25.0 185 50 20 3.70 0.3669
WVFGRD96 26.0 185 50 20 3.71 0.3593
WVFGRD96 27.0 265 75 25 3.72 0.3541
WVFGRD96 28.0 265 80 30 3.73 0.3518
WVFGRD96 29.0 265 80 30 3.74 0.3517
The best solution is
WVFGRD96 11.0 175 65 20 3.57 0.4615
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 +40 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 CUS.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 CUS Model with Q from simple gamma values 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 1.0000 5.0000 2.8900 2.5000 0.172E-02 0.387E-02 0.00 0.00 1.00 1.00 9.0000 6.1000 3.5200 2.7300 0.160E-02 0.363E-02 0.00 0.00 1.00 1.00 10.0000 6.4000 3.7000 2.8200 0.149E-02 0.336E-02 0.00 0.00 1.00 1.00 20.0000 6.7000 3.8700 2.9020 0.000E-04 0.000E-04 0.00 0.00 1.00 1.00 0.0000 8.1500 4.7000 3.3640 0.194E-02 0.431E-02 0.00 0.00 1.00 1.00