Descriptionary

megatsunami a towering tsunami, reaching heights far above normal, caused by landslides, volcanic col­lapse, or an asteroid impact in water close to shore. The largest megatsunami, at an estimated 1,720 feet (524 m) in height, was caused when an earthquake dislodged a massive block of rock and ice directly into Lituya Bay in 1958.

Mercalli intensity scale a scale of earthquake inten­sity based on observed structural damage and people's responses in questionnaires after a local quake, unre­lated to the Richter scale, which measures magnitude.

microearthquake an earthquake with a magnitude of 2.0 or less as measured by the Richter scale, gener­ally not felt by people.

moment magnitude scale introduced in 1979, a scale that more accurately measures the energy released by earthquakes stronger than 7 on the Rich- ter scale, with the ability to measure more accurately from distances greater than 350 miles (600 km).

plate tectonics the interaction and subsequent effects of the Earth's continental plates colliding and scraping up against each other, as seen most notably in the San Andreas Fault in California.

precursor a geological event that immediately pre­cedes an earthquake, including a change in seismic velocities, and groundwater levels and a tilting of ground surface.

primary wave the fastest traveling earthquake wave, also known as a P-wave or compressional wave. It is a pressure wave that compresses and decompresses the Earth as it travels out from an epicenter. P-waves are faster moving but less destructive than S-waves. The roar often heard at the start of an earthquake is actually a burst of P-waves reaching the Earth's sur­face and agitating air molecules.

P-wave see primary wave.

Richter scale a scale determining the magnitude of an earthquake as recorded by seismographs. Due to the logarithmic basis of the scale, each whole number increase in magnitude represents a 10-fold increase in magnitude. For example, an earthquake measuring 6.7 on the Richter scale would be considered 10 times worse than one measuring 5.7.

right-lateral fault faulted ground that moves or has moved to the right as you face it.

sand blow a small, volcano-like formation made by the explosion of liquefied soil during an earthquake, also known as sand volcano.

seaquake see submarine earthquake.

earthquakes 125

secondary wave an earthquake wave, also known as an S-wave or shake wave, slower than a P-wavebut more powerful. An S-wave causes the Earth to shake back and forth as it travels out from an epicen­ter; however, it cannot move through water.

seismic referring to earthquake activity.

seismic prospecting a technique of determining the nature of an underground rock structure by setting off explosive charges and measuring the time it takes the shock waves to travel varying paths; a seismic profile is attained this way.

seismic wave a wave of movement that travels through the Earth.

seismograph an instrument that records vibrations of the Earth.

seismology the study of earthquakes and other vibrations of the Earth.

seismometer an electronic instrument that measures and records ground motion. Also known as a seismo­graph.

subduction zone any area where one tectonic plate descends and forces its way beneath another tectonic plate, a source of earthquake and volcanic activity.

submarine earthquake an earthquake originating beneath the seafloor.

S-wave see secondary wave.

temblor a tremor or earthquake.

tremor a small earthquake or vibration, often occur­ring before or after a major earthquake.

tsunami the Japanese term for harbor wave, for­merly and erroneously known as a tidal wave. A tsu­nami is actually any one of what is usually a series of waves that may reach a height of 100 feet (30.5 m) and can cross an entire ocean at speeds up to 600 miles per hour (966 km/h), usually caused by an earthquake, underwater landslide, or collapse of an onshore landmass, such as a volcano, into a large body of water.

tsunami detection buoy a surface buoy connected to an underwater pressure sensor that can detect tsu­namis and relay alerts via satellite.

tsunami warning network a series of tsunami detection buoys anchored throughout the ocean, capable of sending alerts via satellite.

FIELDS, MEADOWS, AND MARSHES

bracken any moist area with a heavy growth of a fern by the same name.

brake an area of thick brush.

everglade a marshland with tall stands of grass.

heath a large field or plain covered with small shrubs, such as heather.

moor a frequently elevated field covered with heather, bracken, and marshy areas.