1998 APR28 Comanche County, Oklahoma Earthquake

Origin time 14:13:02.48 UTC/GMT, Latitude 34.858degrees N, Longitude 98.483degrees W

Magnitude(s): mbLg 4.2

SHORT PERIOD VERTICAL AND THREE COMPONENT LONG PERIOD SEISMOGRAMS OF THE 1998 APR28 OKLAHOMA EARTHQUAKEjl


The top seismogram (EHZ) is vertical earth velocity recorded at 200 samples
per second. The other seismograms are earth velocity recorded along three
axes at 20 samples per second, then filtered through three pole butterworth
bandpass filters to pass frequencies between 0.01 Hertz, and 0.0625 Hertz.
This is equivalent to wave periods of 16 to 100 seconds. The bottom two traces
were earth velocity recorded in North-South and East-West directions. A
software "rotation" program used simple trigonometry to rotate these traces
to show radial velocity (br), which is velocity toward and away from the
epicenter, and transverse velocity (bt) which is motion perpendicular to
(at a right angle to) the line from the seismometer to the epicenter.

The upper trace shows clearly the Pn and Sg phases. Due to a software 
problem the Pn and Sg tags appear on the bottom traces where these phases 
are not seen at all.

The three low frequency traces (BHZ, br, and bt) show Love and Rayleigh
surface waves, which usually can't be seen on such a near earthquake unless
it is at least magnitude 4. Only Rayleigh waves show on the EHZ (vertical)
component. Because the horizontal traces are rotated to radial and transverse,
the Love and Rayleigh waves are separated on these components. Rayleigh
waves appear on the radial motion, and Love waves on the transverse motion.

The vertical traces were detected, digitized, and transmitted by optical fiber
from a Guralp CMG-1TD in at 830 meters depth in the Crosstimbers borehole.
The Guralp seismometer produces 200 sample/second, 20 sample/second, and 4
sample/second data streams. The horizontal signals were from a Geotech
KS54000-0103 seismometer at 119 meters depth in the Glasnost borehole. The
analog (continuously varying voltage) signals were digitized at 20 samples/
second by a remote data acquisition system in the Observatory vault.

EXTEEMELY SHORT PERIOD E/W SEISMOGRAM AND SPECTROGRAM OF THE 1998 APR28 OKLAHOMA EARTHQUAKEjl


The above is an East-West earth velocity seismogram and spectrogram of the 
1998 APR28 Comanche County earthquake. The data was obtained from a Guralp
CMG-1TD seismometer at 200 samples/second. The seismometer is at 830 meters
depth in the Crosstimbers Borehole.

The seismogram (black, top) clearly shows P and S arrivals. The 7400 to 
7600 scale is in seconds from the arbitrary beginning of the file.

The spectrogram shows significant power to 40 Hz from P and 90 Hz from S.
The "layer" around 90 Hz. is probably due to backscattering of the earthquake
waves from irregularities in the earths crust surrounding Leonard. As the
P. and later S, phases cover a larger arc there is more area from which
scattering occurs. This could explain why the "layer" starts slowly and
builds up after the P and S arrivals. 

The spectrogram was produced by LLNL SAC2000 spectrogram default but with
square root scaling. For more information on spectrograms see the comments
on the  1996 AUG03 Pushmataha County earthquake

FELT DATA: This earthquake was felt over 46753 square kilometers, which includes
15747 square kilometers in Texas. The felt area, which was typically of an 
irregular shape extended north to Pawnee, west to Greer County, and south and
east to the outskirts of Dallas, TX.

Nuttli and Zollweg (1974) developed an emperical formula connecting felt area and
magnitude (mbLg) for earthquakes in the Central United States. Where f is the felt 
in square kilometers:

mb=2.65+0.098*log10(f)+0.054*(log10(f))**2

For the felt area of 46753 square kilometers, this formula gives an mb of 4.3,
in very close agreement with the mbLg of 4.2 calculated from Lg wave amplitudes,

Reference: Nuttli, O. W. and J. E. Zollweg (1974). The relation between felt
area and magnitude for Central United States earthquakes, Bulletin of the Seismological 
Society of America, V. 64, pages 73-86.

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