LECTURE 9 NOTES - EARTH'S STRUCTURE (updated 11/10/97)
Instructor: Professor Barbara Romanowicz
Director of Seismological Laboratory
Office Hours: Thursday 2-4 pm , upon appointment only
475 Mc Cone Hall
EARTH'S STRUCTURE
Introduction
Ancient times: center of the Earth--> mysterious underworld of volcanic
explosions and fiery furnaces.
1st knowledge: radius of the Earth from astronomical measurements (6371 km):
size and shape of the Earth.
Until seismological observations (turn of the century) not much was known
about the interior of the Earth with any accuracy.
What was known:
Density
Newton --> theory of gravity --> means to measure the Earth's density
compare that with rock densities at the surface ---> first approximation of
Earth's composition.
Actually done by Cavendish 1798: measured the twist of torsion rod caused by
the attraction of two sets of leaden balls (actually he was able to obtain
gravitational constant G from Newton's law => Earth's mass => Earth's density)
---> measurement of the average density of the Earth ---> rho ~ 5,45 g/cm3,
which is about twice that of common rocks.
From there came the realisation that there are no great cavities deep inside the
Earth and that the deep interior is made of very dense material.
Study of ocean tides:
If interior liquid: the surface would rise and faull like the oceans ---> but no
tides are seen (actually small ones) --> overall rigidity is considerable.
It was possible to obtain simple curves relating density to pressure
(hydrostatic theory) and simple models of the interior.
~1897: outer shell desnity ~ 3.2 g/cm3 (igneous rocks)
core ~ 8.2 g/cm3 (10% less than iron meteorites)
to match the average density, central core had to have a radius of about 4500 km.
No resolution whatsoever about quantitative details of model:
long debate about whether liquid or solid:
Picture at the end of the 19th century:
slightly flattened sphere with a solid crust (thickness unknown) floating on
an elastic or plastic substratum, beneath which solid or liquid nucleus, several
thousand km thick.
20th century:
much progress, almost entrely using a single tool: analysis of earthquake waves.
By exampling earthquakes from all over the world it was possible to locate
accurately the main structural boundaries inside the Earth and define the
composition of the different layers plus made several substantal discoveries:
-core liquid
-solid inner core
How can we see inside the Earth using Earthquake waves?
-Examples
-Comment on ScS wave: sharp = large reflection coefficient.
Key: identification of phases
aided by their accumulated knowledge about the comparative speeds of the
different wave types and the distinctive directions of shaking for each waves.
Using the most clear prominent phases P and S --> location and origin time
already available, from that compute predicted arrival times of different types
of phases that have followed more complicated paths.
Names of phases
Classification in terms of paths, conversions and reflections
-mantle: P, S
-into core: K (P-type wave and no S waves)
-into inner core: I (P waves) PKIKP
J (S-waves) PKJKP (never observed)
-reflections on surface: PP PPP pP
SS SSS
-conversions: SP
-reflections on core mantle boundary: PcP, ScS
-i = reflection inner core boundary: PKiKP
-additional notations: P'P' = PKPPKP (useful because they arrive long after the
other waves, 39 mn after the origin time) - P'650P' (precursors to P'P')
-Pdiff, Sdiff: diffracted along the core-mantle boundary
Identification of phases made using travel time tables: empirical times know
now to within a second of scatter. Main effort nowadays is to determine any
significant differences for various regions of the world:
departures from radial symmetry: can be substantial differences of 5 sec for waves
traveling through subduction zones.
-examples of Peter Shearer's stacks
Discovery of the Earth's liquid core : Oldham 1906
Plotted P S wave travel times from a number of Earthquakes sources,
providing distances from 20 degrees to almost 160 degrees,
Noted two important discontinuities.
1- about 130 degrees, P waves began to be delayed by about a minute compared
to the trend at shorter distances.
2- S waves could be followed only to 120 degrees. Beyond that arrival time
delayed by 10 minutes or more.
Hypothesized that S waves penetrated a central core that transmits S waves at
much slower speed, he deduced:
120 degrees maximum depth about half radius, hence core radius not more
than 0.4R
We now know that there are some problems with identification of phases in
Oldham study and arithmetic approximate (rays are not straight lines).
Crucial test: observation of waves bouncing from the core mantle boundary:
ScS and PcP provided first accurate estimate of depth of the CMB:
Gutenberg 1914: 2900 km (only within a few km from modern value)
Question: waves penetrating the core delayed because core fluid or because
rocks softened by great T and P?
1930's: beyond 105 degrees, Earthquake P waves become weak and stronger
waves arrive 3 minutes later than extension of P travel time curve.
S waves could not be identified on seismograms beyond 105 degrees:
Core shadow could be explained by assuming that the core was liquid rock and
temperature greater than 5000 degrees C.
Since then, no S waves have been found that propagated through core:
we are now very confident that the outer core is liquid
Discovery of the Earth's inner core
-1936 Lehmann:
Inner core about the size of the Moon within the outer core.
Copenhagen well located to record waves that pass through the core of the
Earth from large earhtquakes in Pacific subduction zones.
Looked at many seismograms, found waves that could not be explained by contemporary
models of the Earth's interior: could be explained if waves had reflected from a
solid interface within the core --> radius found that agreed with observed travel
times.
Radius about 1500 km.
Sharp or spread boundary of IC? Gradual boundaries would turn back reflected
energy weakly.
Thickness ~5 km (Bolt) and radius 1216 km (1960's).