1995 JAN18 Garvin County, Oklahoma Earthquake

Origin time 15:51:39.90 UTC/GMT, Latitude 34.712degrees N, Longitude 97.542degrees W

Magnitude(s): mbLg 4.2, and aftershock mbLg=2.9


The above seismogram was recorded at 40 samples per second from a GS-13 short
period vertical (sz) seismometer in the walk in vault near Leonard, Oklahoma.

The first P and S are from a magnitude 4.2 (mbLg) earthquake which was felt
widely in Garvin County. The P and S which occurred 170.5 seconds later
was apparently a small movement along part of the fault plane which slipped
to cause the mbLg 4.2 earthquake. 

The difference in mbLg magnitude between the two earthquakes is 1.3. Because
magnitudes increase one unit for each increase of 10 (ten) in seismic wave
amplitude, we expect the amplitude of the main shock to be more than 10 times
larger than the aftershock amplitudes (actually a magnitude difference of 1.3
should produce an amplitude difference of 20 because 10 raised to the power
1.3 is 20). In the displayed seismogram the mainshock/aftershock amplitude
ratio is 14.3 for P and 15.0 for S. However mbLg, by definition, is determined
from 1.0 hertz S waves. When this seismogram is filtered with a 0.6 to 1,5
Hertz bandpass, the measured S ratio is 20.0.


The above seismogram was recorded on a Geotech KS-54000 seismometer 119 meters
deep in the Glasnost borehole near Leonard. It is broadband vertical motion digitized
at 20 samples/second. Slightly over 10 seconds, including the P arrival of
of the main shock are shown. There are two distinct P arrivals, Pn, and Pg
about 1.7 seconds apart. Pn travelled downward through the crust, was 
refracted along the very top of the upper mantle, and travelled back up 
through the crust. Pg travelled directly through the upper crust.
Because of energy loss at each refraction, and a longer travel path, Pn is
smaller than Pg. However, because P waves travel faster in the upper mantle
than in the crust, Pn arrives before Pg. 

Around the turn of the century, Mohorovicic (accents on both c ommitted)
saw the same pattern on smoked paper seismograms of local earthquakes in
Yugoslavia. He concluded that the earth must have a crust with higher
velocity material below the crust. The boundary between crust and mantle
is called the Moho, or the M discontinuity, or the Mohorovicic discontinuity.
As of September 1998, no drill hole has reached the Moho or top of the
mantle. The only evidence of the existance of crust and mantle is seismograms
such as the one above.

RAY PATHS OF Pn and Pg for the 1995 JAN18 OKLAHOMA EARTHQUAKE also Pb ray pathjl

The above diagram is a cross section of a model (approximation) of the crust
in Oklahoma. It is modeled as two layers each 20 kilometers thick (kilometer
scale is on the left). The material below the second crustal layer (below 40
kilometers) is the upper mantle. The crust is certainly more complicated than
this model. The model does not even include sedimentary rock found at the
surface in most of Oklahoma. The scales on the left and at the bottom are in 

The upper layer is called "granitic" because it includes rocks in which 
seismic waves travel at velocities similar to their velocity in granite.
It certainly includes granite but probably many other types of igneous rocks
are also found in the upper layer. The lower layer has seismic wave velocities
similar to those in velocities in basalt or gabbro. It is called the "basaltic"
layer, but is probably more complicated than a layer made entirely
of basalt or gabbro.

The * at 5 kilometers depth on the left represents the Garvin County earthquake
focus or hypocenter (the epicenter is the surface point directly above the
hypocenter or focus). The triangle, on the surface, 207 kilometers from the 
epicenter is the reciever/seismometer near Leonard. The sloped path in the
upper layer is the Pg ray path. The five segment line with its third segment
just below the Moho is the Pn ray path. 

The horizontal line at 20 kilometers depth is called the Conrad discontinuity.
The Conrad is less sharp and less universal than the Moho. In the above
seismogram of Pn and Pg, there is no sign of a ray path reflected just below
the Conrad, although such a ray has been seen in some Oklahoma and regional
earthquakes. The Pb ray path is also marked on the above diagram.