Seismographs are sensitive instruments that can detect, amplify, and record ground vibrations, especially earthquakes, producing a seismogram. Numerous seismographs have been installed in the ground throughout the world and form a seismograph network, monitoring earthquakes, explosions, and other ground-shaking events.
The first very crude seismograph was constructed in 1890. While the seismograph could tell that an earthquake was occurring, it was unable to actually record the earthquake. Modern-era seismographs display movements of the Earth by means of an ink-filled stylus on a continuously turning roll of graph paper. When the ground shakes, the needle wiggles and leaves a characteristic zigzag line on the paper. In early models, the ink-filled stylus recorded real movement between the ground and the stylus, and was not very accurate. In recent models, the ink-filled stylus records motions that are detected through modern, ultra-sensitive seismographs, and converted into an electronic signal that is used to move the stylus and make the trace on the paper on the moving drum.
Seismographs are built using a few simple principles of physics. To measure the vibrations of the Earth during an earthquake, the point of reference must be separate from the ground and free from shaking. To this end, engineers have designed an instrument known as an inertial seismograph. These make use of the principle of inertia, which is the resistance of a large mass to sudden movement. When a heavy weight is hung from a string or thin spring, the string can be shaken and the heavy weight will remain stationary. Using an inertial seismograph, the ink-filled stylus is attached to the heavy weight, and remains stationary during an earthquake. The continuously turning graph paper is attached to the ground, and moves back and forth during the quake, resulting in the zigzag trace of the record of the earthquake motion on the graph paper.
seismographs are used in groups, each recording a different type of motion of the ground. Some seismographs are set up as pendulums and some others as springs, to measure ground motion in many directions. Engineers have made seismographs that can record motions as small as one hundred-millionth of an inch, about equivalent to being able to detect the ground motion caused by a car driving by several blocks away. The ground motions recorded by seismographs are very distinctive, and geologists who study them have methods of distinguishing between earthquakes produced along faults and earthquake swarms associated with magma moving into volca noes, and even between explosions from different types of construction, accidents, and nuclear blasts. It is even possible to infer the size and other characteristics of different nuclear and other explosions with detailed analysis of the seismic signal from a specific event. Interpreting seismograph traces has therefore become an important aspect of nuclear test ban treaty verification.
In the late 19 th century, seismologist and engineer E. Wiechert introduced a seismograph with a large, damped pendulum used as the sensor, with the damping reducing the magnitude of the pendulum's oscillations. This early seismograph recorded horizontal motions using a photographic recording device. Wiechert soon introduced a new seismograph with a mechanical recording device, with an inverted pendulum that could vibrate in all horizontal directions. The pendulum was supported by springs that helped stabilize the oscillations and furthered the productivity of the seismograph. Wiechert's assistant, named Schluter, introduced a vertical recording device. He moved the mass horizontally away from the axis of rotation and maintained it there with a vertical spring. In doing so he was able to record vertical displacement, which helped record many of the complex movements associated with earthquakes.
In the 20th century, seismographs that recorded movements using a pen on a rotating paper-covered drum were introduced, with alternative devices including those that recorded movements using a light spot on photographic film. More sophisticated seismographs can record movements in three directions (up-down, north-south, and east-west), and electronic recording of relative motions is now commonplace.
See also convection and the earth's mantle; earthquakes; mantle; plate tectonics.
Keary, P., Keith Klepeis, and Frederick J. Vine. Global Tectonics. Oxford: Blackwell Publishers, 2008. Shearer, Peter M. Introduction to Seismology. Cambridge:
Cambridge University Press, 2009. Sheriff, Robert E. Encyclopedic Dictionary of Applied Geophysics, 4th ed. Tulsa, Okla.: Society of Exploration Geophysicists, 2002. Stein, S. Introduction to Seismology. Oxford: Blackwell
Publishing, 2000. Turcotte, Donald L., and Gerald Schubert. Geodynam-ics. 2nd ed. Cambridge: Cambridge university Press, 2002.
sequence stratigraphy Sequence stratigraphy is the study of the large-scale three-dimensional arrangement of sedimentary strata and the factors that influence the geometry of these sedimentary packages. Sequences are defined as groups of strata that are bounded above and below by identifiable surfaces that are at least partly unconformities. Many sequence boundaries show up well in seismic reflection profiles, enabling their identification in deeply buried rock packages. Sequence stratigraphy differs from classical stratigraphy in that it groups together different sedimentary facies and depositional environments that were deposited in the same time interval, whereas classical stratigraphy would separate these units into different formations. By analyzing the three-dimensional shape of time-equivalent packages, the depositional geometry and factors that influenced the deposition are more easily identified. Some of the major factors that control the shape of depositional sequences include global sea-level changes, local tectonic or thermal subsidence or uplift, sediment supply, and differential biologic responses to subsidence in different climate conditions. For instance, carbonate reefs may be expected to keep pace with subsidence in tropical climates, but to be absent in temperate or polar climates. Sedi-mentologists and tectonicists use the techniques of sequence stratigraphy in the petroleum industry to understand regional controls on sedimentation and to correlate sequences of similar age worldwide.
See also passive margin; sedimentary rock, sedimentation; stratigraphy, stratification, cyclothem.
Silurian The Silurian refers to the third period of Paleozoic time ranging from 443 Ma to 415 Ma, falling between the Ordovician and Devonian Periods, and the corresponding system of rocks. From base to top it is divided into the Llandoverian and Wenlock-ian Ages or Series (comprising the Early Silurian) and the Ludlovian and Pridolian Ages or Series (comprising the Late Silurian). The period is named after a Celtic tribe called the Silures, who inhabited a region of Wales where rocks of the Silurian system are well exposed. The Silurian is also known as the age of fishes.
Rocks of the Silurian system are well exposed on most continents, with carbonates and evaporites covering parts of the Midwest of North America, the Russian platform, and China. Silurian clastic sequences form thick orogenic wedges in eastern and western North America, central Asia, western Europe, China, and Australia. Much of Gond-wana was together in the Southern Hemisphere, and included the present-day landmasses of South America, Africa, Arabia, India, Antarctica, Australia, and a fragmented China. North America, Baltica, Kazakhstan, and Siberia formed separate landmasses
Fossilized crinoid, or sea lily, in a mudstone deposit, from the Silurian period (Kaj R. Svensson/Photo Researchers, Inc.)
in equatorial and northern latitudes. Much of Gond-wana was bordered by convergent margins, and subduction was active beneath the Cordillera of North America. Baltica and Laurentia had collided during early stages of the Acadian-Caledonian orogeny, following an arc-accretion event in the Middle to Late Ordovician, known as the Taconic orogeny in eastern North America.
Land plants first appeared in the Early Silurian and were abundant by the middle of the period. Scorpionlike eurypterids and arthropods inhabited freshwater environments and may have scurried across the land. In the marine realm, trilobites, brachio-pods, cephalopods, gastropods, bryozoans, crinoids, corals, and echinoderms inhabited shallow waters. Stromatoporoids and rugose and tabulate corals built conspicuous reefs, while jawed fish fed on plankton and nekton.
See also Pleozoic; Phanerozoic.
Kious, Jacquelyne, and Robert I. Tilling. U.S. Geological Survey. "This Dynamic Earth: The Story of Plate
Tectonics." Available online. URL: http://pubs.usgs. gov/gip/dynamic/dynamic.html. Last modified March 27, 2007.
Prothero, Donald, and Robert Dott. Evolution of the Earth.
6th ed. New York: McGraw-Hill, 2002. Stanley, Steven M. Earth and Life through Time. New York: Freeman, 1986.
Smith, William (1769-1839) English Geologist
William Smith was a self-taught surveyor who recognized the regular succession of strata across England and proposed that lithologically similar rock beds could be distinguished by the groups of characteristic fossils embedded within. Using this information, he created the world's first large-scale geologic map of an entire country, showing more than 20 different units, topography, description of the stratigraphy, and structural cross sections. During the same time, Smith produced his works on "Strata identified by organic remains," in which he illustrated the fossils in the rocks through a series of wood engravings. In 1831, the Geological Society of London awarded William Smith the first Wollaston Medal, its highest honor. Since his death, William Smith has become known as the "Father of English Geology."
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