Earthquakes - Geotechnical Engineering Investigation - page 905

ground velocity v y . ω A cos ω t (11.8)

ground acceleration: α ÿ - ω

2 A sin

ω t - ω

2 y

(11.9)

Acceleration is given in terms of the acceleration due to gravity g : 1 g 32 ft/sec 2 or 980

cm/sec 2 980 gal where 1 gal 1 cm/sec 2 .

Strong Ground Motion
Strong ground motion refers to the degree of ground shaking produced as the seismic
waves reach the surface. It has an effect on structures, and is applied in both the horizon-
tal and vertical modes.
Characteristics of ground motion are those of the waveform plus the duration of shaking.

Amplitude is the differential along the wavelength and causes differential dis-
placement of structures.

●

Wavelength , when much larger (long period) than the length of the structure, will
cause tall structures to sway, but differential displacement will be negligible.

●

Frequency causes the shaking of structures as the wave crests pass beneath, and
contributes to the acceleration magnitude.

●

Acceleration is a measure of the force applied to the structure.

●

Duration is the time of effective strong ground motion, and induces fatigue in
structures and pore pressures in soils ( Section 11.2.7 ).

●

Detecting and Recording
Seismographs
Seismic wave amplitudes are detected and recorded on seismographs . A seismometer, the
detection portion of the instrument, is founded on rock and includes a “steady mass” in
the form of a pendulum, which is damped. Seismic waves cause movement of the instru-
ment relative to the pendulum, which remains stationary. In modern instruments, the
movements are recorded electromechanically and stored on magnetic tape. Operation and
recording are continuous.
Recording stations have “sets” of instruments, each set having three seismographs.
Complete description of ground motion amplitude requires measurements of three com-
ponents at right angles: the vertical component, and the north-south and east-west com-
ponents. Instruments designed for different period ranges are also necessary, since no one
instrument can cover all of the sensitivity ranges required. In North America it is common
to have one set sensitive to periods of 0.2 to 2.0 sec, and another set sensitive to periods of
15 to 100 sec.
Seismograms are the records obtained of ground motion amplitudes; an example is
given in Figure 11.6. The Richter magnitude (see Section 11.2.4 ) is assigned from the max-
imum amplitude recorded. The distance between the epicenter and the recording seis-
mograph is determined from the arrival times of the P, S, and L waves. By comparing
records from several stations, the source of the waves can be located in terms of direction
and distance. Epicenters are calculated by NOAA from information received from the
worldwide network.
Seismographs are too sensitive to provide information of direct use in seismic design,
and a strong earthquake near the normal seismograph will displace the reading off scale
or even damage the instrument. Instruments are normally located on sound bedrock to
eliminate local effects of soils or weakened rock structure, and therefore do not provide
information on these materials.