Location
Location ANSS
The ANSS event ID is ci10368325 and the event page is at
https://earthquake.usgs.gov/earthquakes/eventpage/ci10368325/executive.
2008/12/28 05:17:07 32.568 -115.543 10.0 4.52 Baja California, Mexico
Focal Mechanism
USGS/SLU Moment Tensor Solution
ENS 2008/12/28 05:17:07:0 32.57 -115.54 10.0 4.5 Baja California, Mexico
Stations used:
AZ.CRY AZ.KNW AZ.LVA2 AZ.RDM AZ.TRO CI.BAR CI.GLA CI.GSC
CI.ISA CI.LDF CI.OSI CI.PASC II.PFO IU.TUC TA.113A TA.114A
TA.214A TA.218A TA.318A TA.U15A TA.U16A TA.V17A TA.W13A
TA.W17A TA.X18A TA.Y12C TA.Y17A TA.Y18A TA.Z13A TA.Z14A
TA.Z17A
Filtering commands used:
hp c 0.02 n 3
lp c 0.05 n 3
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 #################---------###########
## ##################-----##############
#########################-################
########################--################
####################------##############
#################-----------############
#############---------------##########
#######---------------------########
----------------------------######
---------------------------###
--------------------------##
----------------------
--------------
Global CMT Convention Moment Tensor:
R T P
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
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Preferred Solution
The preferred solution from an analysis of the surface-wave spectral amplitude radiation pattern, waveform inversion or first motion observations is
STK = 55
DIP = 75
RAKE = 10
MW = 4.25
HS = 16.0
The NDK file is 20081228051707.ndk
The waveform inversion is preferred.
Moment Tensor Comparison
The following compares this source inversion to those provided by others. The purpose is to look for major differences and also to note slight differences that might be inherent to the processing procedure. For completeness the USGS/SLU solution is repeated from above.
| SLU |
SCAL |
USGS/SLU Moment Tensor Solution
ENS 2008/12/28 05:17:07:0 32.57 -115.54 10.0 4.5 Baja California, Mexico
Stations used:
AZ.CRY AZ.KNW AZ.LVA2 AZ.RDM AZ.TRO CI.BAR CI.GLA CI.GSC
CI.ISA CI.LDF CI.OSI CI.PASC II.PFO IU.TUC TA.113A TA.114A
TA.214A TA.218A TA.318A TA.U15A TA.U16A TA.V17A TA.W13A
TA.W17A TA.X18A TA.Y12C TA.Y17A TA.Y18A TA.Z13A TA.Z14A
TA.Z17A
Filtering commands used:
hp c 0.02 n 3
lp c 0.05 n 3
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 #################---------###########
## ##################-----##############
#########################-################
########################--################
####################------##############
#################-----------############
#############---------------##########
#######---------------------########
----------------------------######
---------------------------###
--------------------------##
----------------------
--------------
Global CMT Convention Moment Tensor:
R T P
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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Magnitudes
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.
ML Magnitude
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.
Context
The left panel of the next figure presents the focal mechanism for this earthquake (red) in the context of other nearby events (blue) in the SLU Moment Tensor Catalog. The right panel shows the inferred direction of maximum compressive stress and the type of faulting (green is strike-slip, red is normal, blue is thrust; oblique is shown by a combination of colors). Thus context plot is useful for assessing the appropriateness of the moment tensor of this event.
Waveform Inversion using wvfgrd96
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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Location of broadband stations used for waveform inversion
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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:
hp c 0.02 n 3
lp c 0.05 n 3
The results of this grid search are as follow:
DEPTH STK DIP RAKE MW FIT
WVFGRD96 0.5 55 80 -10 3.87 0.3132
WVFGRD96 1.0 55 85 -5 3.89 0.3394
WVFGRD96 2.0 55 85 -10 3.99 0.4318
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 corresponding to the best fit is
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Figure 1. Waveform inversion focal mechanism
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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
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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
hp c 0.02 n 3
lp c 0.05 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.
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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:
- The origin time and epicentral distance are incorrect
- The velocity model used for the inversion is incorrect
- The velocity model used to define the P-arrival time is not the
same as the velocity model used for the waveform inversion
(assuming that the initial trace alignment is based on the
P arrival time)
Assuming only a mislocation, the time shifts are fit to a functional form:
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.
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
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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
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.
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Love-wave radiation patterns
Rayleigh-wave radiation patterns
