waves travel at about 90 percent of the velocity of S waves QUESTIONS (II-3, Part A1) because the surface waves are traveling through lower velocity materials located at the Earth's surface. Epicenter If arrival times are available from several seismograph stations, the distances given by the travel-time curves may be 1 . In Figure II-3.2, use the time scale to determine the lag in used to determine the earthquake's location. The distance arrival time between the P and S waves at four stations: St. provides the radius of a circle about the seismograph station. Louis, Missouri (SLM); Bloomington, Indiana (BLO); MinThe epicenter is located somewhere on that circle. With at neapolis, Minnesota (MNM); and Buwling Green, Ohio least three stations, the location of the epicenter may be de- (BGO). The first major impulse on the left in the seismotermined as the point where the three circles intersect. gram indicates the arrival of the first P wave at the station, We can also arrive at the distance to the epicenter by us- the second impulse, the arrival of the first S wave. The lag ing simple subtraction and a proportional relationship. Be- time, T , is given by the difference between S and P times. Eacause of their different velocities, there is a time lag between ter the lag time value for each station below: arrival of the first P and first S wave at a seismograph station. The time lag (time of S minus time of P ) can be determined from seismograms. This time ing can be used to compute the distance to the epicenter, provided the average velocity of each wave type is known. In the first part of the exercise, ive will use seismograms from four different stations to locate the epicenter and time of an earthquake. FIGURE 11-3.2 Partial seismograms for an earthquake. The P wave arrived at the St. Louis seismograph at 10 minutes and 50 seconds after 3:00 p.m. CST. The second disturbance on the seismogram represents the arrival of the S waves..

1. waves travel at about 90 percent of the velocity of S waves QUESTIONS (II-3, Part A1) because
the surface waves are traveling through lower velocity materials located at the Earth's surface.
Epicenter If arrival times are available from several seismograph stations, the distances given by
the travel-time curves may be 1 . In Figure II-3.2, use the time scale to determine the lag in used
to determine the earthquake's location. The distance arrival time between the P and S waves at
four stations: St. provides the radius of a circle about the seismograph station. Louis, Missouri
(SLM); Bloomington, Indiana (BLO); MinThe epicenter is located somewhere on that circle.
With at neapolis, Minnesota (MNM); and Buwling Green, Ohio least three stations, the location
of the epicenter may be de- (BGO). The first major impulse on the left in the seismotermined as
the point where the three circles intersect. gram indicates the arrival of the first P wave at the
station, We can also arrive at the distance to the epicenter by us- the second impulse, the arrival
of the first S wave. The lag ing simple subtraction and a proportional relationship. Be- time, T ,
is given by the difference between S and P times. Eacause of their different velocities, there is a
time lag between ter the lag time value for each station below: arrival of the first P and first S
wave at a seismograph station. The time lag (time of S minus time of P ) can be determined from
seismograms. This time ing can be used to compute the distance to the epicenter, provided the
average velocity of each wave type is known. In the first part of the exercise, ive will use
seismograms from four different stations to locate the epicenter and time of an earthquake.
FIGURE 11-3.2 Partial seismograms for an earthquake. The P wave arrived at the St. Louis
seismograph at 10 minutes and 50 seconds after 3:00 p.m. CST. The second disturbance on the
seismogram represents the arrival of the S waves.