Richter scale
Due to the variance in earthquakes, it is essential to understand the Richter scale uses common logarithms simply to make the measurements manageable (i.e., a magnitude 3 quake factors 10³ while a magnitude 5 quake factors 105 and has seismometer readings 100 times larger).
[5] The Richter magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs.
The original formula is:[6] where A is the maximum excursion of the Wood-Anderson seismograph, the empirical function A0 depends only on the epicentral distance of the station,
In practice, readings from all observing stations are averaged after adjustment with station-specific corrections to obtain the ML value.
Events with magnitudes greater than 4.5 are strong enough to be recorded by a seismograph anywhere in the world, so long as its sensors are not located in the earthquake's shadow.
)[14] The intensity and death toll depend on several factors (earthquake depth, epicenter location, and population density, to name a few) and can vary widely.
Millions of minor earthquakes occur every year worldwide, equating to hundreds every hour every day.
[16] Seismologist Susan Hough has suggested that a magnitude 10 quake may represent a very approximate upper limit for what the Earth's tectonic zones are capable of, which would be the result of the largest known continuous belt of faults rupturing together (along the Pacific coast of the Americas).
[17] A research at the Tohoku University in Japan found that a magnitude 10 earthquake was theoretically possible if a combined 3,000 kilometres (1,900 mi) of faults from the Japan Trench to the Kuril–Kamchatka Trench ruptured together and moved by 60 metres (200 ft) (or if a similar large-scale rupture occurred elsewhere).
Wood and John A. Anderson developed the Wood–Anderson seismograph, one of the first practical instruments for recording seismic waves.
[21] He also recruited the young and unknown Charles Richter to measure the seismograms and locate the earthquakes generating the seismic waves.
[22] In 1931, Kiyoo Wadati showed how he had measured, for several strong earthquakes in Japan, the amplitude of the shaking observed at various distances from the epicenter.
He then plotted the logarithm of the amplitude against the distance and found a series of curves that showed a rough correlation with the estimated magnitudes of the earthquakes.
[23] Richter resolved some difficulties with this method[24] and then, using data collected by his colleague Beno Gutenberg, he produced similar curves, confirming that they could be used to compare the relative magnitudes of different earthquakes.
[25] Additional developments were required to produce a practical method of assigning an absolute measure of magnitude.
Magnitude was then defined as "the logarithm of the maximum trace amplitude, expressed in microns", measured at a distance of 100 km (62 mi).
The scale was calibrated by defining a magnitude 0 shock as one that produces (at a distance of 100 km (62 mi)) a maximum amplitude of 1 micron (1 μm, or 0.001 millimeters) on a seismogram recorded by a Wood-Anderson torsion seismometer.
ML is the scale used for the majority of earthquakes reported (tens of thousands) by local and regional seismological observatories.
For large earthquakes worldwide, the moment magnitude scale (MMS) is most common, although Ms is also reported frequently.
The seismic moment, M0 , is proportional to the area of the rupture times the average slip that took place in the earthquake, thus it measures the physical size of the event.
Mw is derived from it empirically as a quantity without units, just a number designed to conform to the Ms scale.
[39] New techniques to avoid the saturation problem and to measure magnitudes rapidly for very large earthquakes are being developed.
