Arrival Times (from USGS)
Arrival time list
Felt Map
USGS Felt map for this earthquake
USGS Felt reports page for Northern California
Focal Mechanism
The focal mechanism was determined using broadband seismic waveforms. The location of the event and the
station distribution are given in Figure 1.
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Figure 1. Location of broadband stations used to obtain focal mechanism
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UC Berkeley Moment Tensor Solution
Date: Tue, 28 Sep 2004 10:47:52 -0700 (PDT)
From: BDSN Moment Tensor Analyst
http://earthquake.usgs.gov/recenteqsUS/Quakes/nc51147892.htm
Reviewed by:
Dolenc
UCB Seismological Laboratory
Inversion method: complete waveform
Stations used: KCC, MHC, PACP, and TIN
Berkeley Moment Tensor Solution
Best Fitting Double-Couple:
Mo = 9.82E+24 Dyne-cm
Mw = 5.96
Z = 8. km
Plane Strike Rake Dip
NP1 57 -7 85
NP2 147 -175 83
Principal Axes:
Axis Value Plunge Azimuth
T 9.853 1 102
N -0.057 82 198
P -9.796 8 12
Source Composition:
Type Percent
DC 98.8
CLVD 1.2
Iso 0.0
Event Date/Time: September 28, 2004 at 17:15:24 UTC
Event ID: nc51147892
Moment Tensor: Scale = 10**24 Dyne-cm
Component Value
Mxx -8.776
Mxy -3.901
Mxz -1.395
Myy 9.034
Myz -0.131
Mzz -0.258
------
------------ P ----
##------------- -------
####-------------------------
#######--------------------------
#########-------------------------#
###########----------------------####
#############-------------------#######
##############---------------##########
################------------#############
#################--------################
##################-----##################
###################--##################
#################----################## T
#############---------################
##########-------------################
######-----------------##############
#----------------------############
------------------------#########
-----------------------######
-----------------------##
-------------------
-------
Harvard CMT
Date: Tue, 28 Sep 2004 15:17:21 -0400 (EDT)
Here is the solution for the recent event.
September 28, 2004, CENTRAL CALIFORNIA, MW=6.0
Natasha Maternovskaya
CENTROID, MOMENT TENSOR SOLUTION
HARVARD EVENT-FILE NAME C092804B
DATA USED: GSN
L.P. BODY WAVES: 29S, 64C, T= 40
SURFACE WAVES: 40S, 96C, T= 50
CENTROID LOCATION:
ORIGIN TIME 17:15:30.4 0.2
LAT 35.95N 0.01;LON 120.55W 0.01
DEP 12.0 FIX;HALF-DURATION 2.3
MOMENT TENSOR; SCALE 10**25 D-CM
MRR=-0.19 0.02; MTT=-0.92 0.02
MPP= 1.11 0.02; MRT= 0.16 0.06
MRP= 0.17 0.05; MTP= 0.21 0.02
PRINCIPAL AXES:
1.(T) VAL= 1.16;PLG= 8;AZM=276
2.(N) -0.19; 76; 43
3.(P) -0.97; 11; 185
BEST DOUBLE COUPLE:M0=1.1*10**25
NP1:STRIKE=321;DIP=77;SLIP=-178
NP2:STRIKE=230;DIP=88;SLIP= -13
-----------
-------------------
###--------------------
#######-------------------#
###########-------------#####
#############----------########
###############------##########
###############-##############
T ##############--##############
############------############
#############---------###########
#########-------------#########
#######----------------########
####-------------------######
#----------------------####
----------------------#
------- ---------
--- P -----
SLU Focal Mechanism
NODAL PLANES
STK= 139.99
DIP= 85.00
RAKE= -155.00
OR
STK= 47.66
DIP= 65.10
RAKE= -5.52
DEPTH = 6.0 km
Mw = 5.89
Best Fit 0.8335 - P-T axis plot gives solutions with FIT greater than FIT90
Focal Mechanism
First motion data
The P-wave first motion data for focal mechanism studies are as follow:
Sta Az(deg) Dist(km) First motion
CMB 360 246 i-
DAC 78 256 i+
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.
The velocity model used for the search is a
modified Utah model .
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. A nearly vertical strike-slip fault striking at 75 or 165 degrees is preferred. 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
Broadband station distributiuon
Sta Az(deg) Dist(km)
SAO 318 143
CMB 360 246
DAC 78 256
ELK 38 708
MVU 65 784
HWUT 47 999
HLID 29 1000
COR 347 1005
AHID 43 1110
BW06 47 1209
BOZ 31 1317
MSO 22 1336
SDCO 76 1343
OCWA 348 1363
ISCO 67 1371
NEW 10 1409
LAO 40 1688
LTX 110 1722
DGMT 38 1933
KSU1 73 2128
MIAR 86 2437
FVM 76 2669
JFWS 64 2697
PVMO 80 2751
MPH 83 2753
OXF 84 2813
LRAL 86 3069
ACSO 70 3296
ALLY 67 3526
MCWV 71 3568
NHSC 83 3680
DWPF 92 3753
CBN 73 3802
SDMD 71 3828
BINY 66 3866
LOZ 61 3965
NCB 62 3996
BRNJ 68 4001
ACCN 64 4045
PAL 68 4052
HRV 65 4226
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 velocity model used for the waveform fit is a
modified Utah model .
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 0.01 3
lp c 0.10 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.
Appendix A
The figures below show the observed spectral amplitudes (units of cm-sec) at each station and the
theoretical predictions as a function of period for the mechanism given above. The modified Utah model earth model
was used to define the Green's functions. For each station, the Love and Rayleigh wave spectrail amplitudes are plotted with the same scaling so that one can get a sense fo the effects of the effects of the focal mechanism and depth on the excitation of each.
Quality Control
Here we tabulate the reasons for not using certain digital data sets
The following stations did not have a valid response files:
Last Changed Tue Sep 28 15:32:12 CDT 2004
