The surface wave magnitude (
) scale is one of the
magnitude scales used in
seismology
Seismology (; from Ancient Greek σεισμός (''seismós'') meaning "earthquake" and -λογία (''-logía'') meaning "study of") is the scientific study of earthquakes (or generally, quakes) and the generation and propagation of elastic ...
to describe the size of an
earthquake
An earthquakealso called a quake, tremor, or tembloris the shaking of the Earth's surface resulting from a sudden release of energy in the lithosphere that creates seismic waves. Earthquakes can range in intensity, from those so weak they ...
. It is based on measurements of
Rayleigh surface waves that travel along the uppermost layers of the Earth. This magnitude scale is related to the
local magnitude scale proposed by
Charles Francis Richter in 1935, with modifications from both Richter and
Beno Gutenberg
Beno Gutenberg (; June 4, 1889 – January 25, 1960) was a German-American seismologist who made several important contributions to the science. He was a colleague and mentor of Charles Francis Richter at the California Institute of Technolo ...
throughout the 1940s and 1950s.
It is currently used in People's Republic of
China
China, officially the People's Republic of China (PRC), is a country in East Asia. With population of China, a population exceeding 1.4 billion, it is the list of countries by population (United Nations), second-most populous country after ...
as a
national standard (GB 17740-1999) for categorising earthquakes.
[
]
Recorded magnitudes of earthquakes through the mid 20th century, commonly
attributed to Richter, could be either
or
.
Definition
The formula to calculate surface wave magnitude is:
:
where A is the maximum
particle displacement
Particle displacement or displacement amplitude is a measurement of distance of the movement of a sound particle from its equilibrium position in a medium as it transmits a sound wave.
The SI unit of particle displacement is the metre (m). In m ...
in surface waves (
vector sum
In mathematics, physics, and engineering, a Euclidean vector or simply a vector (sometimes called a geometric vector or spatial vector) is a geometric object that has magnitude (or length) and direction. Euclidean vectors can be added and scal ...
of the two horizontal displacements) in
μm, T is the corresponding
period in
s (usually 20 2 seconds), Δ is the
epicentral distance in
°, and
:
Several versions of this equation were derived throughout the 20th century, with minor variations in the constant values.
Since the original form of
was derived for use with
teleseismic waves, namely shallow earthquakes at distances >100 km from the seismic receiver, corrections must be added to the computed value to compensate for epicenters deeper than 50 km or less than 20° from the receiver.
For official use by the Chinese government,
the two horizontal displacements must be measured at the same time or within 1/8 of a period; if the two displacements have different periods, a weighted sum must be used:
:
where A
N is the north–south displacement in μm, A
E is the east–west displacement in μm, T
N is the period corresponding to A
N in s, and T
E is the period corresponding to A
E in s.
Other studies
Vladimír Tobyáš and Reinhard Mittag proposed to relate surface wave magnitude to
local magnitude scale M
L, using
[
]
:
Other formulas include three revised formulae proposed by CHEN Junjie et al.:
[
]
:
:
and
:
See also
*
Seismic magnitude scales
Seismic magnitude scales are used to describe the overall strength or "size" of an earthquake. These are distinguished from seismic intensity scales that categorize the intensity or severity of ground shaking (quaking) caused by an earthquake at ...
Notes and references
External links
*
{{DEFAULTSORT:Surface Wave Magnitude
Seismic magnitude scales