2008/04/18 09:37:00 38.45 -87.89 11.6 5.2 Illinois
USGS Felt map for this earthquake
SLU Moment Tensor Solution
2008/04/18 09:37:00 38.45 -87.89 11.6 5.2 Illinois
Best Fitting Double Couple
Mo = 8.81e+23 dyne-cm
Mw = 5.23
Z = 14 km
Plane Strike Dip Rake
NP1 25 90 -175
NP2 295 85 0
Principal Axes:
Axis Value Plunge Azimuth
T 8.81e+23 4 160
N 0.00e+00 85 25
P -8.81e+23 4 250
Moment Tensor: (dyne-cm)
Component Value
Mxx 6.72e+23
Mxy -5.64e+23
Mxz -3.25e+22
Myy -6.72e+23
Myz 6.96e+22
Mzz 0.00e+00
##############
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--#####################---------------
--------###############-----------------
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---------------##############--------
P --------------##################-----
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---############# ###
############ T
Harvard Convention
Moment Tensor:
R T F
0.00e+00 -3.25e+22 -6.96e+22
-3.25e+22 6.72e+23 5.64e+23
-6.96e+22 5.64e+23 -6.72e+23
Details of the solution is found at
http://www.eas.slu.edu/Earthquake_Center/MECH.NA/20080418093700/index.html
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STK = 295
DIP = 85
RAKE = 0
MW = 5.23
HS = 14.0
The waveform inversion is preferred. The surface-wave data were very good for the Love wave, which had observations to 100 seconds
The following compares this source inversion to others
SLU Moment Tensor Solution
2008/04/18 09:37:00 38.45 -87.89 11.6 5.2 Illinois
Best Fitting Double Couple
Mo = 8.81e+23 dyne-cm
Mw = 5.23
Z = 14 km
Plane Strike Dip Rake
NP1 25 90 -175
NP2 295 85 0
Principal Axes:
Axis Value Plunge Azimuth
T 8.81e+23 4 160
N 0.00e+00 85 25
P -8.81e+23 4 250
Moment Tensor: (dyne-cm)
Component Value
Mxx 6.72e+23
Mxy -5.64e+23
Mxz -3.25e+22
Myy -6.72e+23
Myz 6.96e+22
Mzz 0.00e+00
##############
###################---
#####################-------
#####################---------
#######################-----------
#######################-------------
--#####################---------------
--------###############-----------------
-------------#########------------------
------------------####--------------------
---------------------#--------------------
---------------------#####----------------
--------------------##########------------
---------------##############--------
P --------------##################-----
-------------######################-
-------------#######################
-----------#######################
---------#####################
-------#####################
---############# ###
############ T
Harvard Convention
Moment Tensor:
R T F
0.00e+00 -3.25e+22 -6.96e+22
-3.25e+22 6.72e+23 5.64e+23
-6.96e+22 5.64e+23 -6.72e+23
Details of the solution is found at
http://www.eas.slu.edu/Earthquake_Center/MECH.NA/20080418093700/index.html
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USGS Body-Wave Moment Tensor Solution
08/04/18 09:36:57.31
ILLINOIS
Epicenter: 38.523 -87.884
MW 5.2
USGS MOMENT TENSOR SOLUTION
Depth 18 No. of sta: 7
Moment Tensor; Scale 10**16 Nm
Mrr=-0.41 Mtt= 7.31
Mpp=-6.91 Mrt=-1.11
Mrp=-0.54 Mtp= 5.83
Principal axes:
T Val= 9.55 Plg= 7 Azm=160
N -0.56 82 347
P -9.00 0 250
Best Double Couple:Mo=9.3*10**16
NP1:Strike=296 Dip=84 Slip= 4
NP2: 205 86 174
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---------------########----------
------------############-------
P -----------################---
----------####################
----------###################
-------##################
----########## ####
--########## T ##
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April 18, 2008, ILLINOIS, MW=5.3
Goran Ekstrom
CENTROID-MOMENT-TENSOR SOLUTION
GCMT EVENT: C200804180936A
DATA: II IU MN CU IC G GE
L.P.BODY WAVES: 74S, 119C, T= 40
SURFACE WAVES: 103S, 186C, T= 50
TIMESTAMP: Q-20080418094937
CENTROID LOCATION:
ORIGIN TIME: 09:37:02.2 0.1
LAT:38.55N 0.01;LON: 87.91W 0.01
DEP: 15.0 FIX;TRIANG HDUR: 1.2
MOMENT TENSOR: SCALE 10**24 D-CM
RR=-0.011 0.021; TT= 0.983 0.019
PP=-0.972 0.021; RT= 0.067 0.057
RP=-0.154 0.062; TP= 0.736 0.017
PRINCIPAL AXES:
1.(T) VAL= 1.229;PLG= 1;AZM=342
2.(N) 0.012; 82; 77
3.(P) -1.241; 8; 251
BEST DBLE.COUPLE:M0= 1.24*10**24
NP1: STRIKE= 27;DIP=84;SLIP=-175
NP2: STRIKE=296;DIP=85;SLIP= -6
T #########
### ###########--
##################-----
###################--------
####################---------
---#################-----------
--------###########------------
-------------######--------------
-----------------#---------------
-----------------####------------
-------------########---------
P ------------############-----
-----------################--
-----------##################
---------##################
------#################
---################
###########
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The focal mechanism was determined using broadband seismic waveforms. The location of the event and the and stations used for 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 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:
hp c 0.02 n 3 lp c 0.10 n 3The results of this grid search from 0.5 to 19 km depth are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 0.5 115 75 -10 4.88 0.3906
WVFGRD96 1.0 115 75 -10 4.93 0.4333
WVFGRD96 2.0 295 90 0 4.99 0.5044
WVFGRD96 3.0 295 90 -5 5.03 0.5354
WVFGRD96 4.0 295 90 15 5.06 0.5492
WVFGRD96 5.0 295 90 10 5.08 0.5697
WVFGRD96 6.0 295 75 -5 5.09 0.5901
WVFGRD96 7.0 115 85 -10 5.11 0.6156
WVFGRD96 8.0 295 90 10 5.13 0.6411
WVFGRD96 9.0 295 85 5 5.15 0.6708
WVFGRD96 10.0 295 80 0 5.17 0.6988
WVFGRD96 11.0 295 85 5 5.19 0.7243
WVFGRD96 12.0 295 85 5 5.20 0.7432
WVFGRD96 13.0 295 85 5 5.21 0.7558
WVFGRD96 14.0 295 85 0 5.23 0.7634
WVFGRD96 15.0 115 90 -5 5.23 0.7627
WVFGRD96 16.0 295 85 0 5.25 0.7626
WVFGRD96 17.0 295 85 0 5.26 0.7547
WVFGRD96 18.0 115 90 0 5.27 0.7421
WVFGRD96 19.0 295 85 0 5.27 0.7293
WVFGRD96 20.0 295 90 0 5.28 0.7114
WVFGRD96 21.0 295 90 0 5.29 0.6927
WVFGRD96 22.0 295 90 0 5.29 0.6719
WVFGRD96 23.0 295 90 0 5.29 0.6510
WVFGRD96 24.0 295 90 0 5.30 0.6287
WVFGRD96 25.0 295 90 0 5.30 0.6063
WVFGRD96 26.0 115 85 0 5.30 0.5870
WVFGRD96 27.0 115 85 0 5.30 0.5667
WVFGRD96 28.0 115 80 0 5.30 0.5483
WVFGRD96 29.0 115 80 0 5.31 0.5316
The best solution is
WVFGRD96 14.0 295 85 0 5.23 0.7634
The mechanism correspond 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 componnet is plotted to the same scale and peak amplitudes are indicated by the numbers to the left of each trace. The number in black at the rightr of each predicted traces 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 and because the velocity model used in the predictions may not be perfect. 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 bandpass filter used in the processing and for the display was
hp c 0.02 n 3 lp c 0.10 n 3
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| Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to thewavefroms. 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. |
The following figure shows the stations used in the grid search for the best focal mechanism to fit the surface-wave spectral amplitudes of the Love and Rayleigh waves.
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The surface-wave determined focal mechanism is shown here.
NODAL PLANES
STK= 199.11
DIP= 85.08
RAKE= 169.97
OR
STK= 289.98
DIP= 80.00
RAKE= 5.00
DEPTH = 15.0 km
Mw = 5.32
Best Fit 0.8952 - P-T axis plot gives solutions with FIT greater than FIT90
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The P-wave first motion data for focal mechanism studies are as follow:
Sta Az Dist First motion
Surface wave analysis was performed using codes from Computer Programs in Seismology, specifically the multiple filter analysis program do_mft and the surface-wave radiation pattern search program srfgrd96.
Digital data were collected, instrument response removed and traces converted
to Z, R an T components. Multiple filter analysis was applied to the Z and T traces to obtain the Rayleigh- and Love-wave spectral amplitudes, respectively.
These were input to the search program which examined all depths between 1 and 25 km
and all possible mechanisms.
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| Pressure-tension axis trends. Since the surface-wave spectra search does not distinguish between P and T axes and since there is a 180 ambiguity in strike, all possible P and T axes are plotted. First motion data and waveforms will be used to select the preferred mechanism. The purpose of this plot is to provide an idea of the possible range of solutions. The P and T-axes for all mechanisms with goodness of fit greater than 0.9 FITMAX (above) are plotted here. |
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| Focal mechanism sensitivity at the preferred depth. The red color indicates a very good fit to the Love and Rayleigh wave radiation patterns. 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. Because of the symmetry of the spectral amplitude rediation patterns, only strikes from 0-180 degrees are sampled. |
The distribution of broadband stations with azimuth and distance is
Listing of broadband stations used
Since the analysis of the surface-wave radiation patterns uses only spectral amplitudes and because the surfave-wave radiation patterns have a 180 degree symmetry, each surface-wave solution consists of four possible focal mechanisms corresponding to the interchange of the P- and T-axes and a roation of the mechanism by 180 degrees. To select one mechanism, P-wave first motion can be used. This was not possible in this case because all the P-wave first motions were emergent ( a feature of the P-wave wave takeoff angle, the station location and the mechanism). The other way to select among the mechanisms is to compute forward synthetics and compare the observed and predicted waveforms.
The fits to the waveforms with the given mechanism are show below:
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This figure shows the fit to the three components of motion (Z - vertical, R-radial and T - transverse). For each station and component, the observed traces is shown in red and the model predicted trace in blue. The traces represent filtered ground velocity in units of meters/sec (the peak value is printed adjacent to each trace; each pair of traces to plotted to the same scale to emphasize the difference in levels). Both synthetic and observed traces have been filtered using the SAC commands:
hp c 0.02 n 3 lp c 0.10 n 3
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Should the national backbone of the USGS Advanced National Seismic System (ANSS) be implemented with an interstation separation of 300 km, it is very likely that an earthquake such as this would have been recorded at distances on the order of 100-200 km. This means that the closest station would have information on source depth and mechanism that was lacking here.
Dr. Harley Benz, USGS, provided the USGS USNSN digital data. The digital data used in this study were provided by Natural Resources Canada through their AUTODRM site http://www.seismo.nrcan.gc.ca/nwfa/autodrm/autodrm_req_e.php, and IRIS using their BUD interface.
Thanks also to the many seismic network operators whose dedication make this effort possible: University of Alaska, University of Washington, Oregon State University, University of Utah, Montana Bureas of Mines, UC Berkely, Caltech, UC San Diego, Saint L ouis University, Universityof Memphis, Lamont Doehrty Earth Observatory, Boston College, the Iris stations and the Transportable Array of EarthScope.
The CUS used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows:
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
Here we tabulate the reasons for not using certain digital data sets
The following stations did not have a valid response files:
DATE=Mon Apr 21 12:34:38 CDT 2008
