This seismic event occurred in the metropolitan Chicago region on
November 4, 2013 at 18:35 UTC (12:35 CST). It was widely felt
The USGS/NEIC source parameters are
||± 2.0 km
|Number of Stations
|Number of Phases Used
||46.6 km (0.42°)
|Travel Time Residual
Group Velocity Analysis
After converting all broadband recordings to ground velocity in
m/s, I used the program do_mft to determine group
velocities and spectral amplitudes. The stations that provided
useful spectral amplitudes are shown in relation to the epicenter
in the next figure.
These stations are well distributed in azimuth about the epicenter.
The spectral amplitudes were measured with periods of 0.5 to 5.5
seconds. This are very short periods. For similar studies of
larger events, periods in the range of 5 - 50 seconds are
typical. The use of low frequency spectral amplitudes yields a
robust estimate of the source mechanism.
The first step was to define a velocity model for use in the source
inversion. This was difficult because the dispersion in the period
range is very sensitive to variations in shallow geological
structure. The dispersion to TA stations L44A and M44A was used to
improve the velocity model. These stations are roughly 45 km
north and south of the epicenter and had very good surface
waves. Using the program surf96, I started with the
CUS velocity model and replaced the upper layer by many thin
layers. I did not permit the lower part of the model to change
for stability in the inversion and because long period dispersion
were not available. The resultant velocity model is named modl.out. A comparison of this velocity model
to the CUS model is shown in the next figure.
The significant difference between the two velocity models in in the
upper 1 km. The next figure shows a comparison between the observed
group velocities and the model prediction.
The Love wave dispersion is not fit very well. However since
spectral amplitudes will be used for the source inversion, this is
not a major problem, since spectral amplitude inversion is very
On the other hand, an attempt at waveform inversion would be
The srfgrd96 program of Computer Programs in Seismology was
used to find the best shear-dislocation and depth that bet fit the
observed spectral amplltudes. The results of the grid search over
strike, dip, rake and depth show that the source was shallow.
The best fit as a function of depth is given in the next figure.
The mechanism plots are the best fitting solution for each
depth. Since spectral amplitudes are used and since the source
is modeled as a shear dislocation, the other equivalent solutions
can be obtained by increasing the strike by 180 degrees and by
exchanging the pressure and tension quadrants. A tabulation of the
goodness of fit and moment magnitude for each depth is given in the
The best fit solution had the parameters
H(km) STK DIP RAKE Mw FIT
0.1 125. 50. 75. 3.01 0.8041
For this solution, the comaprison of the observed and predicted
radiation patterns as a function of period are given in the
In addition to the radiation patterns two figures present compare
the model predicted group velocity and anelastic attenuaiton
coefficient dispersion to that obtained from the observations.
The observed dispersion is compared to the observed, and the
predicted and observed spectral amplitudes are assumed relation by
Aobs = Apr exp ( - gamma r ) where r is the epicentral distance, Apr
is the amplitude predicted on the basis of the moment magnitude,
mechanism and infinite Q at the station, and gamma = pi f / Q U.
The plots contain all observations and represent an average over the
broad region shown on the map.
Since a new velocity model was derived, it was appropriate to use
the Computer Programs in Seismology program elocate. Using
a fixed depth solution with a depth of 0.1 km, the data and
results are given in the file elocate.txt
and the source parameters from that file are
Error Ellipse X= 0.5472 km Y= 0.9280 km Theta = 165.3228 deg
The P-wave arrivals were difficult to read and there were very few
reliable first motion data. Thus the specification of a preferred
focal mechanism (e.g., one of the four possible from the radiation
pattern fit) was not possible.
RMS Error : 0.079 sec
Latitude : 41.7931 +- 0.0052 N 0.5792 km
Longitude : -87.8679 +- 0.0110 E 0.9083 km
Depth : 0.10 +- 1.35 km
Epoch Time : 1383590133.993 +- 0.39 sec
Event Time : 20131104183533.993 +- 0.39 sec
Event (OCAL) : 2013 11 04 18 35 33 993
HYPO71 Quality : BC
Gap : 105 deg
This event was shallow, had an Mw = 3.0. The strong Love
waves indicate either an earthquake source or a significant
horizontal movement of mass as part of a mining operation.
Using a rough value of 3.0E+18 dyne-cm as the isotropic moment for a
1 ton explosion, this event near Chicago would have had an explosive
yield of about 100 tons (Mw=3.0 = seismic moment of 3 E+20 dyne-cm).
I, Robert B. Herrmann, believe that this equivalent source size is
too large for normal quarry operations and thus I prefer a shallow