Editing Earthquake (section)

==Measurement and location==
{{Main|Seismic magnitude scales|Seismology}}

The instrumental scales used to describe the size of an earthquake began with the [[Richter scale]] in the 1930s. It is a relatively simple measurement of an event's amplitude, and its use has become minimal in the 21st century. [[Seismic waves]] travel through the [[Earth's interior]] and can be recorded by [[seismometer]]s at great distances. The [[surface-wave magnitude]] was developed in the 1950s as a means to measure remote earthquakes and to improve the accuracy for larger events. The [[moment magnitude scale]] not only measures the amplitude of the shock but also takes into account the [[seismic moment]] (total rupture area, average slip of the fault, and rigidity of the rock). The [[Japan Meteorological Agency seismic intensity scale]], the [[Medvedev–Sponheuer–Karnik scale]], and the [[Mercalli intensity scale]] are based on the observed effects and are related to the intensity of shaking.

=== {{anchor|Magnitude}}Intensity and magnitude ===
The shaking of the earth is a common phenomenon that has been experienced by humans from the earliest of times. Before the development of strong-motion accelerometers, the intensity of a seismic event was estimated based on the observed effects. Magnitude and intensity are not directly related and calculated using different methods. The magnitude of an earthquake is a single value that describes the size of the earthquake at its source. Intensity is the measure of shaking at different locations around the earthquake. Intensity values vary from place to place, depending on the distance from the earthquake and the underlying rock or soil makeup.<ref>{{Cite book |last1=Earle |first1=Steven |date=September 2015 |title=Physical Geology |edition=2nd |chapter=11.3 Measuring Earthquakes |chapter-url=https://opentextbc.ca/geology/chapter/11-3-measuring-earthquakes/|access-date=2022-10-22 |archive-date=2022-10-21 |archive-url=https://web.archive.org/web/20221021040843/https://opentextbc.ca/geology/chapter/11-3-measuring-earthquakes/ |url-status=live }}</ref>

The [[Seismic magnitude scales#Richter|first scale for measuring earthquake magnitudes]] was developed by [[Charles Francis Richter]] in 1935. Subsequent scales ([[seismic magnitude scales]]) have retained a key feature, where each unit represents a ten-fold difference in the amplitude of the ground shaking and a 32-fold difference in energy. Subsequent scales are also adjusted to have approximately the same numeric value within the limits of the scale.<ref>{{Harvnb|Chung|Bernreuter|1980|p=1}}.</ref>

Although the mass media commonly reports earthquake magnitudes as "Richter magnitude" or "Richter scale", standard practice by most seismological authorities is to express an earthquake's strength on the [[seismic scale#Mw|moment magnitude]] scale, which is based on the actual energy released by an earthquake, the static seismic moment.<ref>{{cite web |title=USGS Earthquake Magnitude Policy (implemented on January 18, 2002) |url=https://earthquake.usgs.gov/aboutus/docs/020204mag_policy.php |publisher=United States Geological Survey |url-status=dead |archive-url=https://web.archive.org/web/20160504144754/http://earthquake.usgs.gov/aboutus/docs/020204mag_policy.php |archive-date=2016-05-04 }} A copy can be found at {{cite web |title=USGS Earthquake Magnitude Policy |url=http://dapgeol.tripod.com/usgsearthquakemagnitudepolicy.htm |access-date=2017-07-25 |archive-date=2017-07-31 |archive-url=https://web.archive.org/web/20170731230704/http://dapgeol.tripod.com/usgsearthquakemagnitudepolicy.htm |url-status=live }}</ref><ref>{{Cite journal |last1=Bormann |first1=P |last2=Di Giacomo |first2=D |date=2011 |title=The moment magnitude Mw and the energy magnitude Me: common roots and differences |url=https://doi.org/10.1007/s10950-010-9219-2 |journal=Journal of Seismology |volume=15 |issue=2 |pages=411–427 |doi=10.1007/s10950-010-9219-2 |via=Springer Link}}</ref>

=== Seismic waves ===
Every earthquake produces different types of seismic waves, which travel through rock with different velocities:
* Longitudinal [[P waves]] (shock- or pressure waves)
* Transverse [[S waves]] (both body waves)
* [[Surface wave]]s – ([[Rayleigh wave|Rayleigh]] and [[Love wave]]s)

==== Speed of seismic waves ====
[[Propagation velocity]] of the seismic waves through solid rock ranges from approx. {{Convert|3|km/s|mi/s|abbr=on}} up to {{Convert|13|km/s|mi/s|abbr=on}}, depending on the [[density]] and [[Elasticity (physics)|elasticity]] of the medium. In the Earth's interior, the shock- or P waves travel much faster than the S waves (approx. relation 1.7:1). The differences in travel time from the [[epicentre|epicenter]] to the observatory are a measure of the distance and can be used to image both sources of earthquakes and structures within the Earth. Also, the depth of the [[hypocenter]] can be computed roughly.

'''P wave speed'''
* Upper crust soils and unconsolidated sediments: {{Convert|2-3|km|mi|abbr=on}} per second
* Upper crust solid rock: {{Convert|3-6|km|mi|abbr=on}} per second
* Lower crust: {{Convert|6-7|km|mi|abbr=on}} per second
* Deep mantle: {{Convert|13|km|mi|abbr=on}} per second.

'''S waves speed'''
* Light sediments: {{Convert|2-3|km|mi|abbr=on}} per second
* Earths crust: {{Convert|4-5|km|mi|abbr=on}} per second
* Deep mantle: {{Convert|7|km|mi|abbr=on}} per second

==== Seismic wave arrival ====
As a consequence, the first waves of a distant earthquake arrive at an observatory via the Earth's mantle.

On average, the kilometer distance to the earthquake is the number of seconds between the P- and S wave arrival times, multiplied by 8.<ref>{{cite web |url=http://hypertextbook.com/facts/2001/PamelaSpiegel.shtml |title=Speed of Sound through the Earth |publisher=Hypertextbook.com |access-date=2010-08-23 |archive-date=2010-11-25 |archive-url=https://web.archive.org/web/20101125091130/http://hypertextbook.com/facts/2001/PamelaSpiegel.shtml |url-status=live }}</ref> Slight deviations are caused by inhomogeneities of subsurface structure. By such analysis of seismograms, the Earth's core was located in 1913 by [[Beno Gutenberg]].

S waves and later arriving surface waves do most of the damage compared to P waves. P waves squeeze and expand the material in the same direction they are traveling, whereas S waves shake the ground up and down and back and forth.<ref>{{cite web|url=https://newsela.com/articles/govt-science-earthquakes/id/26756/|title=Newsela {{!}} The science of earthquakes|website=newsela.com|access-date=2017-02-28|archive-date=2017-03-01|archive-url=https://web.archive.org/web/20170301005337/https://newsela.com/articles/govt-science-earthquakes/id/26756/|url-status=live}}</ref>

=== Location and reporting ===
{{main|Epicenter}}

Earthquakes are not only categorized by their magnitude but also by the place where they occur. The world is divided into 754 [[Flinn–Engdahl regions]] (F-E regions), which are based on political and geographical boundaries as well as seismic activity. More active zones are divided into smaller F-E regions whereas less active zones belong to larger F-E regions.

Standard reporting of earthquakes includes its [[Richter magnitude scale|magnitude]], date and time of occurrence, [[geographic coordinates]] of its [[epicenter]], depth of the epicenter, geographical region, distances to population centers, location uncertainty, several parameters that are included in USGS earthquake reports (number of stations reporting, number of observations, etc.), and a unique event ID.<ref>{{cite web |url=http://geographic.org/earthquakes/real_time_details.php?id=recenteqsww/Quakes/usc000f1s0.php&lat=-10.7377&lon=165.1378 |title=Magnitude 8.0 – SANTA CRUZ ISLANDS Earthquake Details |work=Global Earthquake Epicenters with Maps |author=Geographic.org |access-date=2013-03-13 |archive-date=2013-05-14 |archive-url=https://web.archive.org/web/20130514143205/http://geographic.org/earthquakes/real_time_details.php?id=recenteqsww/Quakes/usc000f1s0.php&lat=-10.7377&lon=165.1378 |url-status=live }}</ref>

Although relatively slow seismic waves have traditionally been used to detect earthquakes, scientists realized in 2016 that gravitational measurement could provide instantaneous detection of earthquakes, and confirmed this by analyzing gravitational records associated with the [[2011 Tōhoku earthquake and tsunami|2011 Tohoku-Oki]] ("Fukushima") earthquake.<ref>{{cite web|url=https://www.researchgate.net/blog/post/changes-to-earths-gravity-offer-early-earthquake-warning|title=Earth's gravity offers earlier earthquake warnings|access-date=2016-11-22|archive-date=2016-11-23|archive-url=https://web.archive.org/web/20161123201125/https://www.researchgate.net/blog/post/changes-to-earths-gravity-offer-early-earthquake-warning|url-status=live}}</ref><ref>{{cite web|url=https://cosmosmagazine.com/geoscience/gravity-shifts-could-sound-early-earthquake-alarm|title=Gravity shifts could sound early earthquake alarm|access-date=2016-11-23|archive-date=2016-11-24|archive-url=https://web.archive.org/web/20161124100006/https://cosmosmagazine.com/geoscience/gravity-shifts-could-sound-early-earthquake-alarm|url-status=dead}}</ref>