Earthquakes occur when underground rocks break and suddenly move. The size of an earthquake is expressed using two scales.
Basic Knowledge
The underground location where an earthquake occurs is called the focus, and the point on the surface directly above it is the epicenter. There are two types of waves transmitted from the focus: the longitudinal P wave, which arrives first, and the transverse S wave, which arrives later.
The time from the arrival of the P wave to the arrival of the S wave is called the duration of the preliminary tremor and is approximately proportional to the distance from the focus(Omori's formula: distance from focus ≒ 6〜8×duration of preliminary tremor[seconds]).
The strength of shaking at a location is expressed by seismic intensity(0, 1, 2, 3, 4, 5-lower, 5-upper, 6-lower, 6-upper, and 7, in 10 levels), while the size of the earthquake itself is expressed by magnitude(M). When magnitude increases by 1, the energy becomes about 32 times greater; when it increases by 2, the energy becomes about 1000 times greater.
Focus(The underground location where an earthquake occurs)
Epicenter(The point on the surface directly above the focus)
P wave(A longitudinal wave. About 5〜7 km/s. Causes the preliminary tremor)
S wave(A transverse wave. About 3〜4 km/s. Causes the principal motion)
Seismic intensity(The strength of shaking at a particular location. 10 levels)
Magnitude(The size of the earthquake itself. An increase of 1 makes the energy about 32 times greater)
In Depth (Background and Significance)
The earthquake early warning system uses the time difference between the arrivals of P and S waves. Seismometers near the focus detect the P wave and issue warnings to various areas before the S wave arrives. Electrical signals travel at about 300,000 km/s, so information can be transmitted faster than seismic waves.
Because earthquake energy is proportional to 10^(1.5M), an M9 earthquake has about 1000 times the energy of an M7 earthquake. The 2011 Tohoku earthquake had a magnitude of M9.0 and released about 1400 times the energy of the Great Hanshin-Awaji Earthquake(1995, M7.3).
Seismic waves also serve a role like an X-ray for learning about underground structures. The internal structures of Earth, including the mantle, outer core, and inner core, have been clarified from the way seismic waves travel.
- Focus = underground point of origin / epicenter = directly above it
- P wave = longitudinal and fast / S wave = transverse and slow
- Duration of preliminary tremor ∝ distance from focus(Omori's formula)
- Seismic intensity has 10 levels(0,1,2,3,4,5-lower,5-upper,6-lower,6-upper,7)
- An increase of 1 in magnitude makes the energy about 32 times greater
- An increase of 2 in magnitude makes the energy about 1000 times greater
- Earthquake early warning = application of detecting the P wave first
Points to Note (Common Confusions)
① Distinguish seismic intensity(shaking at each location) from magnitude(the size of the earthquake). Even a place far from the focus may experience an intensity of only 1〜2 during a large earthquake. ② Do not confuse the focus and the epicenter. ③ Remember that P means Primary(first) and S means Secondary(later). ④ Intensities 5 and 6 each have two levels, lower and upper; intensity 0 and intensity 1 are different.
Practice
- What is the name of the wave that causes the preliminary tremor?
- How many times greater does the energy become when magnitude increases by 2?
- How many levels are included in the Japan Meteorological Agency's seismic intensity scale from 0 to 7?