when earth quakes, everything seems to wiggle -- even seismographs -- sort of.
Almost all of the seismograph jiggles with the exception of one part.
For simplicity -- that is the part holding "the pen." It remains steady
but the paper on which it makes a mark MOVES due to the quake.
Thus, the pen stays still - and the paper ( the earth ) moves.
This occurs because the attachment holding the "pen" is very heavy and is
supported on a unique balance system. This weight has a lot of inertia
and keeps the "pen" steady ( "objects at rest tends to stay at rest" ).
From the "lag time" between the P-Wave arrival time and the S-Wave
arrival time, you calculate how far away the quake occurred.
Period. That's all. You can't tell it's direction. Hmmmm.
"The EQ occurred 200 miles (320 km) from me -- in an unknown direction. So if I draw a circle around my campus with a radius of 200 mi -- the EQ was somewhere on this line."
"The EQ was 400 miles (640 km) from me -- so if I draw a circle around
my campus with a radius of 400 mi -- the EQ was somewhere on this
line." So Tommy hops on INTERNET and puts out a call:
who has an EQ radius ?
Wynonna Wave Responds:
"I compute 500 miles (800 km) --- and I'm in Seattle." When Seismic Suzie e-mails her data, all three scientists get out their maps and compasses ( circle-drawing contraptions ), and the EQ epicenter is plotted.