A Brief History of Seismology
Every Day: There are about 50 earthquakes strong enough to be Every day: There are about 50 earthquakes strong enough to be felt locally; several of these produce distant seismic felt locally; several of these produce distant seismic waves that can be measured with sensitive waves that can be measured with sensitive instruments anywhere on the globe. instruments anywhere on the globe.
Every Few Days: There is an earthquake strong enough to damage structures. Every few days: There is an earthquake strong enough to damage structures.“ Seismology is the scientific study of the seismic waves generated by earthquakes. Seismology is the scientific study of the seismic waves generated by earthquakes.
Scientific & Practical Objectives of Seismology: Scientific & Practical Objectives of Seismology: To learn about the structure of the earth (direct observation of the deep earth is impossible) and the physics of earthquakes Seismology is a young science, only about 150 years old. Seismology is a young science, only about 150 years old. Before scientific studies began, ideas about earthquakes were largely based on myth and superstition. Before scientific studies began, ideas about earthquakes were largely based on myth and superstition.
Early 1800s : “ The theory of elastic wave propagation in solid materials is developed by Cauchy, Poisson, Stokes, Rayleigh The theory of elastic wave propagation in solid materials is developed by Cauchy, Poisson, Stokes, Rayleigh , and others. They describe primary and secondary body waves (P- and S-waves) and surface , and others. They describe primary and secondary body waves (P- and S-waves) and surface waves. (Theory is way ahead of observation.) waves. (Theory is way ahead of observation.)
1857 : “ R. Mallet, an Irish engineer, travels to Italy to study damage caused by an earthquake near Naples. His R. Mallet, an Irish engineer, travels to Italy to study damage caused by an earthquake near Naples. His work is generally considered to be the first serious work is generally considered to be the first serious attempt at observational seismology. His contributions: attempt at observational seismology. His contributions:
- earthquake waves radiate from a central focus
- earthquakes can be located by projecting these waves backward toward the source
- observatories should be established to monitor earthquakes
1875 : “ F. F. Cecchi Cecchi in Italy. in Italy. builds the first time-recording seismograph builds the first time-recording seismograph Higher quality instruments are then developed by British scientists in Japan. Higher quality instruments are then developed by British scientists in Japan. These early instruments are These early instruments are undamped undamped , and therefore inaccurate after the first few cycles of shaking. , and therefore inaccurate after the first few cycles of shaking.
1897 : “ First seismograph in North America is installed at Lick Observatory near San Jose, California. This First seismograph in North America is installed at Lick Observatory near San Jose, California. This instrument will later record the 1906 San Francisco earthquake. instrument will later record the 1906 San Francisco earthquake. E. E. Wiechert develops the first seismometer with develops the first seismometer with viscous damping, capable of producing a useful Wiechert viscous damping, capable of producing a useful record for the entire duration of ground shaking. record for the entire duration of ground shaking
Early 1900s : “ B. B. B. B. Galitzen develops the first electromagnetic develops the first electromagnetic seismograph in which a moving pendulum generates Galitzen seismograph in which a moving pendulum generates electric current in a coil, and establishes a network of electric current in a coil, and establishes a network of seismic stations across Russia. seismic stations across Russia. The new design will prove to be much more accurate and reliable than previous mechanical instruments; all The new design will prove to be much more accurate and reliable than previous mechanical instruments; all modern seismographs are electromagnetic. modern seismographs are electromagnetic.
1906 : “ H. F. Reid, an American engineer, studies survey lines across the San Andreas fault measured before H. F. Reid, an American engineer, studies survey lines across the San Andreas fault measured before and after the 1906 San Francisco earthquake. He proposes an and after the 1906 San Francisco earthquake. He proposes an “ elastic rebound elastic rebound ” theory for the origin of earthquakes, where accumulated elastic energy is theory for the origin of earthquakes, where accumulated elastic energy is released suddenly by slip on the fault.
1900-1910 : Seismograms from many earthquakes recorded at many distances become widely available. Seismograms from many earthquakes recorded at many distances become widely available. R. Oldham identifies P-, S-, and surface waves in earthquake records, and detects liquid earth R. Oldham identifies P-, S-, and surface waves in earthquake records, and detects liquid earth s core from the absence of direct body waves at certain s core from the absence of direct body waves at certain distances. distances. A. Mohorovicic identifies velocity boundary between identifies velocity boundary between earth earth ’ s crust and mantle ( scrust and mantle ( Moho)
1914 : “ B. Gutenberg publishes travel-time tables that include core phases (seismic waves that penetrate or reflect B. Gutenberg publishes travel-time tables that include core phases (seismic waves that penetrate or reflect from the core), and accurately estimates the depth of the earth ’ from the core), and accurately estimates the depth of the earth s fluid core (2900 km). s fluid core (2900 km).
1920s : “ Seismic surveying methods using explosions and other artificial sources are developed in the United Seismic surveying methods using explosions and other artificial sources are developed in the United States for exploring for oil and other resources in the shallow crust. States for exploring for oil and other resources in the shallow crust.
1935 : “ C. Richter proposes a magnitude scale for specifying the sizes of earthquakes in southern California. The C. Richter proposes a magnitude scale for specifying the sizes of earthquakes in southern California. The logarithmic Richter scale allows a huge range of logarithmic Richter scale allows a huge range of earthquake sizes to be conveniently measured. earthquake sizes to be conveniently measured. Defined for a specific region, specific distance range, specific wave type and period, and specific Defined for a specific region, specific distance range, specific wave type and period, and specific instrument, the idea is quickly adapted for other cases. instrument, the idea is quickly adapted for other cases. The smallest felt earthquakes are about magnitude 3, while rare great earthquakes are magnitude 8-9+. The smallest felt earthquakes are about magnitude 3, while rare great earthquakes are magnitude 8-9+
1936: “ I. I. Lehmann Lehmann ” discovers the earth ’ s solid inner core.
1940 : “ H. Jeffries and K. Bullen Bullen publish final versions of their travel-time tables for many seismic phases. publish final versions of H. Jeffries and K. their travel-time tables for many seismic phases. They are accurate enough to still be in use today. They are accurate enough to still be in use today.
1950s & 1960s The Cold War The Cold War : Soviet nuclear tests in the early 1950s generate intense interest by the U.S. military in detection and Soviet nuclear tests in the early 1950s generate intense interest by the U.S. military in detection and measurement of nuclear explosions, and funding for measurement of nuclear explosions, and funding for government and academic seismology programs government and academic seismology programs surges during the Cold War.
1966 : The disadvantages of traditional magnitude measures are widely recognized: saturation, inconsistency The disadvantages of traditional magnitude measures are widely recognized
1960s : “ The increased number of seismic stations established after ~1900 allowed large earthquakes to be routinely The increased number of seismic stations established after ~1900 allowed large earthquakes to be routinely located, leading to the discovery that earthquakes are located, leading to the discovery that earthquakes are not randomly located, but rather are concentrated in not randomly located, but rather are concentrated in narrow belts around the globe. The significance of narrow belts around the globe. The significance of this observation was not appreciated until the plate this observation was not appreciated until the plate tectonics revolution of the 1960s. Earthquakes are generated where tectonics revolution of the 1960s. Earthquakes are generated where crustal crustal plates spread apart (e.g., midAtlantic Ridge), are consumed at plates spread apart (e.g., midAtlantic Ridge), are consumed at zones (e.g., Japan, Aleutians), or slide past each other at subduction subduction zones (e.g., Japan, Aleutians), or slide past each other at transform boundaries (e.g., San Andreas fault). transform boundaries (e.g., San Andreas fault).
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