University:
California State University, Northridge
Course:
GEOL 548/L | Seismology and Lab
Academic year:

2024
Views:

124

Pages:

34
Author:

estorrii16i

History of Seismology  Early science  Seismology before computers   1880-1960 Seismology since computers  1960-present Earthquake mythology – ancient beliefs Early understanding of earthquakes was based on folklore, written stories, and were somewhat superstitious. Earthquake mythology – ancient beliefs India: The earth is held up by 4 elephants that stand on the back of a turtle. The turtle is balanced on top of a cobra. When the animals move, the earth trembles and shakes Earthquake mythology – ancient beliefs Mexico: El Diablo, an Indian devil god, made a giant rip in the ground so that he and his cohorts did not have to take the long way around, whenever they wanted to stir up mischief on the earth. Siberia: The Earth rests on a sled driven by the god named Tuli. The dogs have fleas, When they stop to scratch, the Earth shakes Earthquake mythology – ancient beliefs Japan: A great catfish, or namazu, lies curled up under the sea, with the islands of Japan resting on its back. A demigod, or daimyojin, holds a heavy stone over his head to keep him from moving. Once in a while, though, the daimyojin is distracted, the namzu moves and the earth trembles Native tales and the Cascadia megathrust earthquakes Stories from the Hoh and Quillette tribes of the Olympic Peninsula of north west Washington describe an epic battle between the supernatural beings Thunderbird and Whale. “The great Thunderbird finally carried the weighty animal to its nest in the lofty mountains and there was a final and terrible contest fought. There was shaking, jumping up and down and trembling of the earth beneath, and the rolling up of the great waters.” A reference to the Cascadia Megathrust earthquake of 1705? Cascadia megathrust earthquakes Cascadia Megathrust 1705 Estimated at Mg 8.7 – 9.2 January 26, 1705 Rupture length: 1000 km The earthquake caused tsunami that struck the coast of Japan, and may also be linked to the Bonneville Slide. The first seismic instrument   The Chinese Seismoscope Invented 132 AD The instrument is reported to have detected a four-hundred-mile distant earthquake which was not felt at the location of the seismoscope. Early Detectors of Ground Movement Zhang Heng was a celebrated astronomer of ancient China. An inventor of the seismograph and armillary sphere, he made great contributions to the development of astronomy in ancient China. 132 AD, Eastern Han Dynasty Physics, Wave energy: Sir Isaac Newton (1642-1727)  Newton is ranked as the most influential figure in the history of Western science (Simmons,1996)  Best known for his 3 laws of motion.  (i) The law of inertia      (ii) An object's acceleration is directly proportional to the object's mass (F=ma) (iii) To every action there is an equal and opposite reaction Law of Gravity:   A body in motion moves with constant velocity unless acted upon by some force. A body at rest remains at rest unless acted upon by some force. The gravitational force between 2 bodies is proportional to the product of their masses and inversely proportional to the square of the distance between them Invented Calculus Physical Properties: Robert Hooke (1635 - 1703)   Natural philosopher, inventor Robert Hooke is one of the most neglected natural philosophers of all time. The inventor of:      the the the the iris diaphragm in cameras, universal joint used in motor vehicles, balance wheel in a watch originator of the word 'cell' in biology Best known for Hooke's Law  “stress is proportional to strain” Wave theory: Christian Huygens (1629-1695)  Improved telescope and resolved numerous astronomical questions  Invented pendulum clock and balance clocks to improve the measurement of time  worked on wave theory of light  discovered polarized light  deduced laws of reflection and refraction  Huygen's Principal: “ Every point on a wave front can be regarded as a new source of waves” The first scientifically studied earthquake The Lisbon Earthquake (Nov 1, 1755)  Probably magnitude 9 with a 3 large tsunamis, thousands killed  epicentre 200 km off SW corner of Portugal  destroyed the city of Lisbon, Portugal  tsunami's struck England and were detected across the Atlantic Ocean in North America  Its widespread physical effects aroused a wave of scientific interest and research into earthquakes. The Lisbon Earthquake J. Mitchel (1761) and J. Drijhout (1765)  Noted the separation of the earthquake source from the effects that it produced    proposed that the distant motion was caused by a wave propagating from a specific location. Mitchel suggested that the vibrations close to the source were related to wave propagating through the elasticity of the rocks Suggested the cause of the earthquake itself was caused by water vaporized by sudden contact with underground fires. Earthquake Studies – before 1880 1800's Couchy Poisson Stokes Lord Rayleigh A.E. H. Love } Found the major waves that pass through solid material. - Body waves (P,S) - Surface waves (Rayleigh, Love) Developed theory on elastic wave propagation “ahead” of any observations (no seismometers!). Earthquake Studies – before 1880  Robert Mallet (1810-1881 ) Irish geologist and engineer  Visited Naples, Italy to study damage from a large earthquake Earthquake Studies – before 1880  Robert Mallet (1810-1881 ) Irish geologist and engineer        Theorized that earthquakes radiate energy from a focal point Assumed that earthquakes were explosions and predicted P wave energy only. Caused by sudden expansion of steam as water contacted hot rock Determined EQ location by projecting energy back to original source Made an attempt to measure seismic velocities using explosive sources Suggested creation of observatories to monitor earthquakes Made one of the first comprehensive earthquake catalogues Earthquake Locations - Mallet, 1868. (Plate tectonics still not accepted at this date) Earthquake Instrumentation – before 1880 Early seismic instruments The first seismometers were undamped pendulums No recording of time.     1856: L. Palmiero built a seismoscope that also recorded time 1873: Verbeck first pendulum observations 1875: Filippo Cecchi, Italy built first seismometer with time 1880: Wegner constructed a common-pendulum seismometer which did not write records.   Wegner detected 27 earthquakes. None of above instruments worked very well J. Milne, J. Ewing and T. Gray (British working in Japan) developed first successful working seismometers with time recording in 1880-1885 period – Horizontal pendulum, rotating disk of smoked glass  Milne and Gray (1881) conducted experiments on the propagation of elastic waves    artificial sources such as dynamite blasts. obtained an apparent velocity of 500 feet /sec. The farthest station was 400 feet from the source. First recording of a distant earthquake April 18, 1889  In Potsdam, Germany, E. von-Rebeur-Pashwitz had built a sensitive horizontal pendulum seismograph for measuring tidal tilts, his interest was primarily astronomical.  The figure below is an engraving of the recording he made of a teleseismic event from the 1989 earthquake in Japan All instruments upto this date were undamped oscillators. (only measured ground motion accurately at start of EQ) Lick Observatory 1897 - 1906 – Lick Observatory built in San Jose, California – (Now operated by the UC university system) Recorded the 1906 earthquake in San Francisco - 280 miles displacement - Shaking ~1 minute Seismology - Instrumentation 1898 Emil. Weichert - Used viscous damping - Measured earthquake energy for entire EQ duration 1903 B.B. Galtizen - Developed electromagnetic seismometers. - Here a moving pendulum was used to generate electric current in a coil - Deployed a chain of stations across Russia. (now all seismometers are electromagnetic and are easier to deploy) Seismology Observations: Earth Structure (1900-) Increase and improvements in instrumentation recording multiple earthquakes at greater distances led to measurements of deep Earth structure.   Richard Oldham (1900) – observed P, S, and surface waves on a seismogram. Oldham (1906) noticed that direct P and S wave arrivals were absent at distances beyond 100o and confirmed the existence of the Earth's core. Seismology Observations: Earth Structure (1900-) Andrija Mohorovicic (1909) Discovered the Moho from meticulous analysis of seismic data at relatively short distance from the Kulpa Valley earthquake (in Croatia). Seismology Observations: Earth Structure (1900-) Travel Time Tables: Zoppritz (1907), produced firsts comprehensive travel time tables. Beno Gutenbert (1914), Travel times for core penetration and reflection Picture of travel times... Seismology Observations: Earth Structure (1900-)   I. Lehman (1936) showed that the core has a solid inner core. Introduced the “I” phase. Earth structure H. Jeffereys (1891-1989) and K. Bullen (19061976)  Used large volumes of data and improved analysis of epicentre locations and derived new standard earth model    The Jeffreys-Bullen Seismological Tables (1940) Showed earth nearly spherical symmetrical with only a few discontinuities. Showed core-mantle boundary separated a solid mantle from a liquid core. Jeffreys-Bullen Earth Model Earth's Interior Structure mantle Compositional Layers – Crust (~7-70m thick) • Very thin outer rocky shell of Earth – Mantle (~2900 km thick) • Solid interior that flows slowly over time – Core (~3400 km radius) The Earth's total radius is 6480 km • Outer core – liquid iron • Inner core - solid iron Preliminary Reference Earth Model (PREM), (Dziewonski & Anderson, 1981) Earthquake mechanism (1911-1950)  1911: Reid's strain rebound theory - earthquakes related to faults  1917: T. Shida first to show that the first motions could be divided into quadrants separated by nodal lines  1923: Nekano provided the first theoretical treatment of the fault source mechanism  1935: Richter developed the Richter magnitude scale 1984: The US National Science Foundation and US Geological Survey provide funding for a new, digital seismic network, the GSN. to replace the aging WWSSN. 1996: The International Data Center is established in Vienna and seismic monitoring is done through the International Monitoring System (IMS). The IMS makes use of many stations of the GSN. USArray: Passive experiments EarthScope http://www.usarray.org/     400 portable 3 component broadband instruments on a regular grid 400 portable 3 component short-period and broadband seismographs and 2000 single channel highfrequency recorders Permanent array of broadband 3 component stations across the country as part of the USGS Advanced National Seismic System Movie

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