Seismology and Earth’s Internal Structure

How do geologists know that the interior of the Earth is composed

of a number of concentric shells of rock with different compositions

and physical characteristics? The main way is through

studying earthquakes. Geologists have seismographs stationed

all around the world, and by studying single earthquake events,

changes in the properties of the earth in different places can be

determined.

In a very general sense, seismic wave velocity increases

smoothly with depth because increased density is equated with

higher seismic velocities. Over time, geologists began to note that

the velocity of seismic waves does not increase steadily with

depth, but that several dramatic changes occur. The positions and

degree of changes across these zones can be determined by noting

several different properties of seismic waves. Some waves are

reflected off interfaces, just as light is reflected off surfaces and

other waves are refracted or bent, changing the ray’s velocity and

path just like a straw appears bent in a glass of water because light

rays from it are bent across the water/air surface. The positions of

the main boundaries were calculated using observations of where

and at what depth these changes occur.

The core-mantle boundary at 2,000 miles (2,900 km) depth in the

Earth strongly influences both seismic velocities and properties—it

refracts P waves, causing a P-wave shadow in a belt around the

globe. Because liquids cannot transmit S waves, none get through

causing a huge S-wave shadow on the side of the Earth opposite the

earthquake event.

small amounts of partial melt in the rock, and this corresponds

to the asthenosphere, the weak sphere that the plates

move on, which is lubricated by partial melts.

There is another seismic discontinuity at 248.5 miles

(400 km) depth, where velocity again increases sharply, this

time caused by a rearrangement of the atoms of olivine in a

polymorphic transition, into spinel structure, corresponding

to an approximate 10 percent increase in density.

A major seismic discontinuity at 416 miles (670 km)

may be either another polymorphic transition or a compositional

change, the topic of many current investigations. Some

models suggest that this boundary separates two fundamentally

different types of mantle, circulating in different convection

cells, whereas other models suggest that there is more

interaction between rocks above and below this discontinuity.

The core-mantle boundary is one of the most fundamental

on the planet, with a huge density contrast from 5.5 g/cm3

above, to 10–11 g/cm3 below, a contrast greater than that

between rocks and air on the surface of the Earth. The outer

core is made dominantly of molten iron. An additional discontinuity

occurs inside the core at the boundary between the

liquid outer core and the solid, iron-nickel inner core.

The properties of seismic waves can also be used to

understand the structure of the Earth’s crust. Andrija

Mohorovicic (a Yugoslavian seismologist) measured slow and

fast arrivals from nearby earthquake source events. He proposed

that some seismic waves were traveling through the

crust, some along the surface, and that some were reflected

off a deep seismic discontinuity between seismically slow and

fast material at about 18–22 miles (30–35 km) depth. We

now recognize this boundary to be the base of the crust and

call it the Mohorovicic (or Moho) boundary and use its seismically

determined position to measure the thickness of the

crust, typically 6–45 miles (10–70 km).

See also CONVECTION AND THE EARTH’S MANTLE;

EARTHQUAKES; MANTLE; PLATE TECTONICS; SEISMOGRAPH.

seismometer See SEISMOGRAPH.

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