Location
2008/12/28 05:17:07 32.5680 -115.5430 10.0 4.50
Arrival Times (from USGS)
Arrival time list
Felt Map
USGS Felt map for this earthquake
USGS Felt reports page for
Focal Mechanism
SLU Moment Tensor Solution
2008/12/28 05:17:07 32.5680 -115.5430 10.0 4.50
Best Fitting Double Couple
Mo = 2.99e+22 dyne-cm
Mw = 4.25
Z = 16 km
Plane Strike Dip Rake
NP1 322 80 165
NP2 55 75 10
Principal Axes:
Axis Value Plunge Azimuth
T 2.99e+22 18 278
N 0.00e+00 72 111
P -2.99e+22 4 9
Moment Tensor: (dyne-cm)
Component Value
Mxx -2.84e+22
Mxy -8.50e+21
Mxz -6.87e+20
Myy 2.58e+22
Myz -8.81e+21
Mzz 2.59e+21
--------- P --
------------- ------
##--------------------------
######------------------------
##########------------------------
#############---------------------##
###############-------------------####
##################---------------#######
# ###############------------#########
## T #################---------###########
## ##################-----##############
#########################-################
########################--################
####################------##############
#################-----------############
#############---------------##########
#######---------------------########
----------------------------######
---------------------------###
--------------------------##
----------------------
--------------
Harvard Convention
Moment Tensor:
R T F
2.59e+21 -6.87e+20 8.81e+21
-6.87e+20 -2.84e+22 8.50e+21
8.81e+21 8.50e+21 2.58e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20081228051707/index.html
|
Preferred Solution
The preferred solution from an analysis of the surface-wave spectral amplitude radiation pattern, waveform inversion and first motion observations is
STK = 55
DIP = 75
RAKE = 10
MW = 4.25
HS = 16.0
The waveform inversion is preferred. The surface-wave data does not have much sensitivity to depth. The depth is greater than the Southern California Earthquake Data Center solution.
Moment Tensor Comparison
The following compares this source inversion to others
| SLU |
SCAL |
SLU Moment Tensor Solution
2008/12/28 05:17:07 32.5680 -115.5430 10.0 4.50
Best Fitting Double Couple
Mo = 2.99e+22 dyne-cm
Mw = 4.25
Z = 16 km
Plane Strike Dip Rake
NP1 322 80 165
NP2 55 75 10
Principal Axes:
Axis Value Plunge Azimuth
T 2.99e+22 18 278
N 0.00e+00 72 111
P -2.99e+22 4 9
Moment Tensor: (dyne-cm)
Component Value
Mxx -2.84e+22
Mxy -8.50e+21
Mxz -6.87e+20
Myy 2.58e+22
Myz -8.81e+21
Mzz 2.59e+21
--------- P --
------------- ------
##--------------------------
######------------------------
##########------------------------
#############---------------------##
###############-------------------####
##################---------------#######
# ###############------------#########
## T #################---------###########
## ##################-----##############
#########################-################
########################--################
####################------##############
#################-----------############
#############---------------##########
#######---------------------########
----------------------------######
---------------------------###
--------------------------##
----------------------
--------------
Harvard Convention
Moment Tensor:
R T F
2.59e+21 -6.87e+20 8.81e+21
-6.87e+20 -2.84e+22 8.50e+21
8.81e+21 8.50e+21 2.58e+22
Details of the solution is found at
http://www.eas.slu.edu/eqc/eqc_mt/MECH.NA/20081228051707/index.html
|
** SCSN Moment Tensor Solution Message **
REAL-TIME SOLUTION: NOT REVIEWED
Inversion Method: Complete Waveform
Number of Stations used: 6
Stations: CI.BLY CI.GLA CI.SDR CI.MUR CI.BOR CI.TOR
Real-Time Solution:
-------------------
Event ID : 10368325
Magnitude : 4.60
Depth (km) : 23.1
Origin Time : 12/28/2008 05:17:07:370
Latitude : 32.54
Longitude : -115.52
Further Information at: http://pasadena.wr.usgs.gov/recenteqs/Quakes/ci10368325.htm
SCSN Moment Tensor Solution:
----------------------------
Moment Magnitude : 4.23
Depth (km) : 5
Variance Reduction(%): 91.15
Quality Factor : A
(A : Mw, MT good enough for distribution)
(B : Mw only good enough for distribution)
(C : Solution needs review before distribution)
Best Fitting Double Couple and CLVD Solution:
---------------------------------------------------
Moment Tensor: Scale = 10**21 Dyne-cm
Component Value
Mxx -21.8
Mxy -10.3
Mxz -3.82
Myy 29
Myz -1.81
Mzz -7.23
Best Fitting Double Couple Solution:
--------------------------------------------------
Moment Tensor: Scale = 10**22 Dyne-cm
Component Value
Mxx -2.431
Mxy -1.035
Mxz -0.598
Myy 2.577
Myz -0.221
Mzz -0.147
Principle Axes:
Axis Value Plunge Azimuth
T 2.786 2 281
N 0.000 76 183
P -2.786 13 11
Best Fitting Double-Couple:
Mo = 2.79E+22 Dyne-cm
Plane Strike Rake Dip
NP1 147 -169 82
NP2 55 -8 79
Moment Magnitude = 4.23
-------
----------- -----
##------------ P --------
####------------ ----------
#######--------------------------
#########-------------------------#
###########----------------------####
#############--------------------######
#############-----------------########
T ##############-------------############
###############----------##############
##################-------################
####################--###################
###################--####################
################-----##################
############----------#################
########---------------##############
##---------------------############
------------------------#########
-----------------------######
-----------------------##
-------------------
-------
Lower Hemisphere Equiangle Projection
============= Station Information ==============
Name Distance Azimuth VR ZCore
-------------------------------------------------
CI.BLY 163.247 34.434 95.807 20.00
CI.GLA 86.187 48.666 92.757 11.00
CI.SDR 135.206 279.612 87.871 19.00
CI.MUR 195.667 307.370 90.717 25.00
CI.BOR 116.459 314.134 96.644 15.00
CI.TOR 132.370 330.326 83.279 16.00
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Waveform Inversion
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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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 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 0.05 n 3
The results of this grid search from 0.5 to 19 km depth are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 0.5 55 80 -10 3.87 0.3132
WVFGRD96 1.0 235 90 5 3.89 0.3392
WVFGRD96 2.0 235 90 5 3.98 0.4314
WVFGRD96 3.0 55 80 -15 4.03 0.4688
WVFGRD96 4.0 55 80 -15 4.06 0.4965
WVFGRD96 5.0 55 80 -15 4.08 0.5183
WVFGRD96 6.0 55 80 -15 4.11 0.5364
WVFGRD96 7.0 55 80 -10 4.12 0.5530
WVFGRD96 8.0 55 80 -15 4.15 0.5686
WVFGRD96 9.0 235 85 15 4.16 0.5748
WVFGRD96 10.0 235 80 15 4.18 0.5793
WVFGRD96 11.0 55 75 10 4.19 0.5832
WVFGRD96 12.0 55 75 10 4.20 0.5874
WVFGRD96 13.0 55 75 10 4.21 0.5908
WVFGRD96 14.0 55 75 10 4.23 0.5931
WVFGRD96 15.0 55 75 10 4.24 0.5943
WVFGRD96 16.0 55 75 10 4.25 0.5946
WVFGRD96 17.0 55 75 10 4.25 0.5938
WVFGRD96 18.0 55 75 10 4.26 0.5918
WVFGRD96 19.0 55 75 10 4.27 0.5887
WVFGRD96 20.0 55 75 10 4.28 0.5847
WVFGRD96 21.0 55 75 10 4.29 0.5796
WVFGRD96 22.0 55 75 10 4.29 0.5738
WVFGRD96 23.0 55 75 10 4.30 0.5671
WVFGRD96 24.0 55 75 10 4.31 0.5599
WVFGRD96 25.0 55 75 10 4.31 0.5520
WVFGRD96 26.0 55 75 10 4.32 0.5435
WVFGRD96 27.0 55 75 10 4.33 0.5348
WVFGRD96 28.0 55 75 10 4.33 0.5260
WVFGRD96 29.0 55 75 10 4.34 0.5169
The best solution is
WVFGRD96 16.0 55 75 10 4.25 0.5946
The mechanism correspond to the best fit is
|
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Figure 1. Waveform inversion focal mechanism
|
The best fit as a function of depth is given in the following figure:
|
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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 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 0.05 n 3
|
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Figure 3. Waveform comparison for selected depth
|
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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.
|
Surface-Wave Focal Mechanism
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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Location of broadband stations used to obtain focal mechanism from surface-wave spectral amplitudes
|
The surface-wave determined focal mechanism is shown here.
NODAL PLANES
STK= 322.90
DIP= 71.25
RAKE= 158.83
OR
STK= 59.99
DIP= 70.00
RAKE= 20.00
DEPTH = 16.0 km
Mw = 4.35
Best Fit 0.8730 - P-T axis plot gives solutions with FIT greater than FIT90
First motion data
The P-wave first motion data for focal mechanism studies are as follow:
Sta Az Dist First motion
Surface-wave analysis
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.
Data preparation
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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Best mechanism fit as a function of depth. The preferred depth is given above. Lower hemisphere projection
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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.
|
Love-wave radiation patterns
Rayleigh-wave radiation patterns
Broadband station distribution
The distribution of broadband stations with azimuth and distance is
Listing of broadband stations used
Waveform comparison for this mechanism
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:
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 0.05 n 3
Discussion
The Future
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.
Acknowledgements
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.
Appendix A
Spectra fit plots to each station
Velocity Model
The WUS used for the waveform synthetic seismograms and for the surface wave eigenfunctions and dispersion is as follows:
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
Quality Control
Here we tabulate the reasons for not using certain digital data sets
The following stations did not have a valid response files:
DATE=Wed Dec 31 11:48:47 CST 2008
Last Changed 2008/12/28
