Richter scale

Charles Richter, co-inventor and namesake of the Richter scale

The Richter scale is a magnitude scale . It is based on amplitude measurements from seismogram recordings that were obtained at a relatively short distance of a few hundred kilometers from the epicenter . It is therefore also known under the term local earthquake magnitude .

To determine the strength of earthquakes , records from measuring devices are used today, which are distributed over the entire surface of the earth. The value determined from this is usually indicated on the torque magnitude scale as the torque magnitude . The press often mistakenly speak of values on the Richter scale. ${\ displaystyle M _ {\ mathrm {w}}}$

Emergence

The scale was developed by Charles Francis Richter and Beno Gutenberg at the California Institute of Technology in 1935 and was initially referred to as the M _L scale (Magnitude Local). In his seminal publication An instrumental Earthquake Magnitude Scale in the Bulletin of the Seismological Society of America , Charles Francis Richter applied the basic idea of an instrumental earthquake scale first published by K. Wadati in 1931 to California earthquakes.

Basics

Due to its definition, the Richter scale has no upper limit, but the physical properties of the earth's crust make an earthquake of magnitude 9.5 or higher almost impossible, since the rock cannot store enough energy and discharges before it reaches this strength. The term “open to the top”, which is often used in the media, is intended to distinguish the instrumental Richter scale from the intensity scales, which are often used to characterize the strength and destructive power of an earthquake.

Most magnitude scales reach saturation in the upper value range: If the energy released during the quake increases further, the magnitude changes only slightly and the scale loses its linearity . The Richter scale is also subject to this phenomenon, so it is not suitable for values above magnitude 6.5. Values beyond this generally refer to other magnitude scales.

Derivation

The specified value, the magnitude or size class, is derived from the decadic logarithm of the maximum amplitude (deflection) in the seismogram . The magnitude is determined according to the following relationship:

{\ displaystyle M _ {\ mathrm {L}} = \ log _ {10} \ left ({\ frac {A _ {\ max}} {A_ {0}}} \ right)}

,

where A _max indicates the maximum deflection in micrometers (μm) with which a short-period standard seismometer (Wood-Anderson seismograph ) would record an earthquake at a distance of 100 km from the epicenter . For the purpose of correction, the reference may have to be adapted to the conditions for earthquakes at different distances. For this purpose, the damping of the amplitude is taken into account, which in turn depends on the regional velocity and damping structure, the age of the earth's crust and its composition, the depth of the focal point and the heat flow conditions. Strictly speaking, these calibration functions according to Richter are only valid for southern California and must be determined separately for other regions of the world. ${\ displaystyle A_ {0}}$

Because of the decadic logarithm, the increase in magnitude by one point on the scale means an approximately ten times higher deflection (amplitude) in the seismogram and approximately 32 times the energy release ( exponential growth ) in the earthquake focus. A magnitude of two or less is called a micro-earthquake because it often cannot be perceived by humans and is only recorded by local seismographs. Quakes around 4.5 and higher in magnitude are strong enough to be captured by seismographs around the world. However, the magnitude must be greater than 5 to be considered a moderate earthquake.

Classification of the scale values

Typical effects in the area of the epicenter can be assigned to the magnitude values. It should be noted that the intensity and thus the ground effects depend not only on the magnitude, but also on the distance to the epicenter, the depth of the earthquake focus below the epicenter and the local geological conditions.

Richter magnitudes	Classification of the earthquake strength	Earthquake effects	Frequency of events worldwide	released energy (TNT equivalent in t (energy in J))
<2.0	Micro	Micro earthquake ^** , not noticeable	≈ 8000 times per day (> magnitude 1.0)	up to 1 t (<4.2 GJ)
2.0 ... <3.0	extremely light	Generally not noticeable, but measured	≈ 1500 times a day	1 to 32 t (4.2 to 135 GJ)
3.0 ... <4.0	very easy	Often noticeable, but damage is very rare	≈ 49,000 times per year (estimated)	32 to 1,000 t (135 to 4,200 GJ)
4.0 ... <5.0	light	Visible movement of room objects, vibration noises; mostly no damage	≈ 6200 times per year (estimated)	1 to 32 kt (4.2 to 135 TJ)
5.0 ... <6.0	medium strength	Serious damage to fragile buildings, slight or no damage to robust buildings	≈ 800 times a year	32 to 1,000 kt (135 to 4,200 TJ)
6.0 ... <7.0 ^*	strong	Destruction within a radius of up to 70 km	≈ 120 times a year	1 to 50 Mt (4.2 to 210 PJ)
7.0 ^* ... <8.0 ^*	big	Destruction over large areas	≈ 18 times a year	50 to 1,000 Mt (210 to 4,200 PJ)
8.0 ^* ... <9.0 ^*	very large	Destruction in areas of a few hundred kilometers	≈ once a year	1 to 5.6 Gt (4.2 to 23.5 EJ)
9.0 ^* ... <10.0 ^*	extremely big	Destruction in areas of a thousand kilometers	≈ every 20 years	5.6 to 1,000 Gt (23.5 to 4,200 EJ)
≥ 10.0 ^*	global catastrophe	Never registered	unknown	> 1,000 Gt (> 4,200 EJ)

^* The Richter scale is metrologically limited to an upper limit of magnitude 6.5. Higher magnitudes of stronger earthquakes are determined with the moment magnitude scale (M _W ).

^**The term micro-earthquake or microquake is used inconsistently. It generally describes low-intensity tremors. The United States Geological Survey (USGS) defines microquakes as earthquakes up to a magnitude of 3.0. Other sources define them as earthquakes with a magnitude of up to 2.0. Microquakes are usually imperceptible to humans.

Negative values

At that time, Richter had related the magnitude 0 to a value of the ground vibration that appeared to him to be the smallest possible value that could ever be measured, so he set a seismometer reading of one micrometer 100 kilometers from the center of the earthquake as the zero point. Today, modern electronic seismographs can measure ground movements that are 1000 times smaller than in the 1930s. This means, however, that very weak earthquakes that can still be measured locally today can have negative magnitudes (up to around −2 to −3).

Relation to other scales

Despite the fundamentally different approach of the Richter scale, attempts are often made to relate it to the intensity scales, such as the modified and repeatedly further developed Mercallis scale by the Italian Giuseppe Mercalli (1850–1914). On a further intensity scale, the so-called MSK scale (Medvedev-Sponheuer-Karnik scale), the strength of an earthquake is given in twelve degrees of strength, for example. The gradation is based on both subjective and objective criteria . In Japan, the JMA scale is widely used as the intensity scale , while the JMA magnitude scale is used as the magnitude scale .

For some time now, the moment magnitude scale (abbreviation M _W ) has also been specified in many cases , the determinants of which are based on the physical parameters in the earthquake focus.

The logarithmic relationship between energy and magnitude can be roughly summed up with

{\ displaystyle M = 2 + {\ frac {2} {3}} \ log _ {10} W \ quad {\ textrm {or}} \ quad W = 10 ^ {{\ frac {3} {2} } (M-2)} = \ left ({\ frac {A _ {\ max}} {100 \, A_ {0}}} \ right) ^ {3/2} \ approx {\ frac {31.6 ^ {M }} {1000}} \ ,,}

where M is the magnitude and W is the equivalent (explosive) energy in tons of TNT .

literature

Charles F. Richter: An instrumental earthquake magnitude scale . In: Bulletin of the Seismological Society of America . Vol. 25, No. 1 , January 1935, ISSN 0037-1106 , p. 1-32 .
B. Gutenberg , CF Richter : Seismicity of the Earth and Associated Phenomena . Princeton University Press, Princeton NJ 1949 (English).

Web links

Individual evidence

↑ ^a ^b Peter Bormann (Ed.): IASPEI New Manual of Seismological Observatory Practice , GeoForschungsZentrum Potsdam 2002.

↑ USGS: FAQ- Measuring Earthquakes ( Memento of the original from November 15, 2016 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2
↑ http://www.meta-evolutions.de/pages/artikel-20070805-richterskala.html
↑ microearthquake. In: Merriam Webster Dictionary Online. Retrieved January 18, 2015 .
↑ USGS, after Kayal, JR: Microearthquake Seismology and Seismotectonics of South Asia . 2008, p. 1-3 .
↑ Press release: Deep geothermal energy and microquakes, Appendix: Definition of microquakes. In: Informationsdienst Wissenschaft idw-online. Geothermal Association - Bundesverband Geothermie e. V., September 23, 2009, accessed January 18, 2015 .
^ William Spence, Stuart A. Sipkin and George L. Choy: Measuring the Size of an Earthquake . In: Earthquakes and Volcanoes . tape 21 , no. 1 , 1989 ( excerpt from the pages of the USGS (HTML) [accessed January 18, 2015]). Excerpt from the pages of the USGS (HTML) ( Memento of the original from January 18, 2015 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2
↑ glossary - microearthquake. California Institute of Technology - Southern California Earthquake Data Center, accessed January 18, 2015 .
↑ Prof. Dr. Peter Bormann, GeoForschungsZentrum Potsdam (pdf)
↑ "If the top is open, it is easier to remember" The seismologist Thomas Kenkmann speaks in a TR interview about earthquake measurement methods and the accuracy of the Richter scale on August 31, 2012

[bormann-1] Peter Bormann (Ed.): IASPEI New Manual of Seismological Observatory Practice , GeoForschungsZentrum Potsdam 2002.