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SPU-22: The Unity of Science from the Big Bang to the Brontosaurus and Beyond. Lecture 12 10 March 2014 Science Center Lecture Hall A. Another Blemish To Be Banished.
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SPU-22: The Unity of Science from the Big Bang to the Brontosaurus and Beyond Lecture 12 10 March 2014 Science Center Lecture Hall A
Another Blemish To Be Banished I was inconsistent in my reference to J.J. Thomson’s plum-pudding model of the atom: In at least one place, I referred to it as the “raisin-pudding model.” Why? In English usage in Thomson’s era, “plum” meant “raisin.” Samuel Johnson defined a “plum” as a “raisin; grape dried in the sun.” There were and are no plums in plum pudding. This seemingly strange usage has a long and interesting history.
And Then There Is The Last Lecture The previous lecture tried to pack too much information into one package and thereby was successful mostly in being confusing. I will be happy to entertain – and even answer - any questions that you might have on this lecture. Today’s lecture will be better.
Outline: The Question Format Goal: What is the earth like, inside? -Why do we want to know? -How can we find out? -What have we learned?
Why Do We Want To Know? Curiosity Understand and accurately predict earthquakes Understand distribution of below-surface, useful materials (only of interest for rather shallow depths)
How Do We Find Out? Drill holes (only useful now to depth of about 12 km, ≈ 1/500th distance to center). Can learn much of interest, e.g., past climate. We won’t pursue Use proxies (mainly signals emanating from earthquakes; some human produced). We will pursue
Premier: Un Peud’Histoire Earthquakes provide oldest comments on natural phenomena, back to ~ 4,000 years ago Drew interest of people for obvious reasons. Aristotle had theory with six types. Rev. John Michell, 18th century, apparently had first wholly scientific approach to earthquakes; reasoned that earthquakes were due to “waves set up by the shifting masses of rock miles below the surface” Scientific interest “took off” after 1755 Lisbon earthquake (felt all over Europe and beyond)
Instrumentation: Seismograph Earthquake causes ground motions (seismic waves). Want to detect these motions Instruments can now detect motions as small as 10 times the size of an atom, up to about 10 cm in amplitude Next slide shows replica of first known instrument to detect ground motion, followed by two slides that indicate principles of instrumentation and one slide of contemporary instrument
Types Of Seismic Waves Body waves (“traveling waves”): P (= primary, pressure); see demo S (= secondary, shear); see demo and next slide Whole-earth oscillations (“normal modes”): Oscillations of whole earth (excited by very powerful earthquakes); we will discuss little, though see two slides following next slide Surface: Rayleigh, Love,…; we will not discuss further
S & P Wave Characteristics Passages through earth: See next four slides Speeds of waves vs. depth (inferred from self- consistent analysis of suite of data): See next slide plus four
Two Earthquake Characteristics Location: Via triangulation (see next two slides) Strength: Energy associated with earthquake (Richter scale – log scale – shaky [pun intended]; mostly replaced, but still distant from reliable)
Global Properties From Seismic Data Global seismic network and earth seismicity (see next two slides) Density vs. depth (next slide plus two) Temperature vs. depth (next slide plus three)
Inferences On Earth Structure Top-down (see next slide): Crust Asthenosphere Moho (=Mohorovicic, with accent marks) Mantle Liquid outer core Solid inner core
