Earthquake-Generated Tsunami - Tsunami - page 81

5

Earthquake-Generated Tsunami

5.1 Introduction

consist of four types: P, S, Rayleigh, and Love waves (Geist
1997 ; Bryant 2005 ). P waves are primary waves that arrive
first at a seismograph. The wave is compressional, con-
sisting of alternating compression and dilation similar to
waves produced by sound traveling through air. These
waves can pass through gases, liquids, and solids. P waves
can thus travel through the of Earth; however, at the core-
mantle boundary, they are refracted, producing two
3,000 km wide shadow zones without any detectable
P waves on the opposite side of the globe from an epicenter.
To detect tsunami produced by earthquakes, seismic sta-
tions must be located outside these shadow zones. S or shear
waves behave very much like the propagation of a wave
down a skipping rope that has been shaken up and down.
These waves travel 0.6 times slower than primary waves.
The spatial distribution and time separation between the
arrival of P and S waves at a seismograph station can be
used to determine the location and magnitude of an earth-
quake. Love and Rayleigh waves spread slowly outwards
from the epicenter along the surface of the Earth's crust.
Love waves have horizontal motion and are responsible for
much of the damage witnessed during earthquakes. Ray-
leigh waves have both horizontal and vertical motions that
produce an elliptical rotation of the ground similar to that
produced in water particles by the passage of an ocean
wave. While it is logical to believe that earthquakes gen-
erate tsunami through the physical displacement of the
seabed along a fault line that breaches the seabed, tsunami
also obtain their energy from Rayleigh waves.

The most common cause of tsunami is seismic activity. Over
the past two millennia, earthquakes have produced 83.0 %
of all tsunami in the Pacific Ocean (National Geophysical
Data Center 2013 ). Displacement of the Earth's crust by
several meters during underwater earthquakes may cover
tens of thousands of square kilometers and impart tremen-
dous potential energy to the overlying water. These types of
events are common; however, tsunamigenic earthquakes are
rare. Between 1861 and 1948, over 15,000 earthquakes
produced only 124 tsunami (Lockridge 1988 ). Along the
west coast of South America, which is one of the most tsu-
nami-prone coasts in the world, 1,098 offshore earthquakes
have generated only 20 tsunami. This low frequency of
occurrence may simply reflect the fact that most tsunami are
small in amplitude and go unnoticed. Two thirds of dam-
aging tsunami in the Pacific Ocean region have been asso-
ciated with earthquakes with a surface wave magnitude of
7.5 or more. The majority of these earthquakes have been
teleseismic events affecting distant coastlines as well as local
ones. One out of every three of these teleseismic events has
been generated in the twentieth century by earthquakes in
Peru or Chile. This chapter discusses the mechanics of
tsunamigenic earthquakes, and presents evidence for a range
of events at the lower end of the seismic energy spectrum.
These events occurred in the 1990s and fueled a surge in
research on tsunami. Large scale events at the mega-tsunami
level will be described in Chap. 6 .

5.1.1 Seismic Waves 5.2 Magnitude Scales for Earthquakes

and Tsunami

Earthquakes occurring mainly in the upper 100 km of the
ocean's crust generate tsunami. However, earthquakes
centered over adjacent landmass have also produced tsu-
nami. Earthquakes produce seismic waves transmitted
through the Earth from an epicenter that can lie as deep as
700 km beneath the Earth's surface. These seismic waves

5.2.1 Earthquake Magnitude Scales

Earthquakes were originally measured using the Richter
scale, M L , defined as the logarithm to base ten of the maxi-
mum seismic-wave amplitude recorded on a seismograph at a