Earthquakes worldwide: significant earthquakes

Earthquakes are generated when tensions built up due to relative movement between rocks along faults are suddenly released. The seismic energy from these events travels through the Earth in the form of waves and causes the tremors observed as earthquakes.

^{Aerial view of the San Andreas Fault, California (USA) © Ian Kluft 2007}

Significant earthquakes

Earthquakes are considered significant according to the NOAA (National Centres for Environmental Information) if they meet at least one of the following criteria: Moderate damage (approximately $1 million or more), 10 or more deaths, Magnitude 7.5 or greater, Modified Mercalli Intensity X or greater, or the earthquake generated a tsunami.

Strength of an earthquake

The first scale for measuring earthquake magnitudes was developed by the physicist and seismologist Charles F. Richter in 1935. Even today earthquakes are commonly classified in units on the Richter scale (local magnitude). However, over time it became apparent that the Richter scale was only suitable for earthquakes of certain magnitudes and occurring within specific distances because it does not accurately reflect the quantity of energy released by very large or extremely distant earthquakes. For this reason, other magnitude scales were developed.

Moment magnitude [Mw] is the most revealing type of magnitude, but can be calculated only for earthquakes with a magnitude higher than about 4. It is the only type of magnitude that does not top out, regardless of how powerful the earthquake is. The Mw scale was developed in the 1970s to succeed the Richter scale and retains a continuum of magnitude values similar to the older one, ranging from 0.0 to 9.9. To span the wide range of possible values, the scale is logarithmic, meaning an earthquake of magnitude of 7.0 releases about 32 times as much energy as one of 6.0 and nearly 1,000 times that of 5.0.

Severity of an earthquake

The consequences of an earthquake of a given magnitude can vary greatly depending on where it happens. For this reason, at the beginning of the twentieth century, the Italian volcanologist Giuseppe Mercalli revised some existing scales to create the Mercalli Intensity scale, a seismic intensity scale used for measuring the effects of an earthquake. Nowadays, the severity of an earthquake is measured by means of the European Macroseismic Scale 1998 (EMS-98) using numbers between I (not felt) and XII (complete destruction) for the individual classification of earthquakes. The intensity describes the strength of an earthquake based on the extent of the destruction (buildings, landscape) and the subjective perception of an observer. The intensity of an earthquake is location-specific and is defined by the magnitude, the distance to the hypocenter and the geology of the subsurface.

^{Mercalli intensity scale}

^{Magnitude - Intensity comparison}

Location of an earthquake

As for the location of an earthquake's point of initial rupture, and for the point at ground level, the two terms hypocenter and epicenter are used.

The hypocenter is the point of seismic focus in the ground (i.e. the point from which seismic waves originate) and is generally designated by its depth, longitude and latitude.

The epicenter is the vertical projection of a hypocenter onto the Earth’s surface (i.e. the point on the Earth’s surface directly above the hypocenter) and is generally designated by its longitude and latitude.

^{Hypocenter and epicenter of an earthquake}

Frequency of earthquake occurrence

It is estimated that around 500,000 earthquakes occur each year, detectable with current instrumentation. About 100,000 of these can be felt. Minor earthquakes occur nearly constantly around the world in areas of high seismicity. Larger earthquakes occur less frequently, the relationship being roughly exponential:

^{Earthquake occurrence frequency}

Nowadays, many more earthquakes are reported than in the past, but this is because of the vast improvement in instrumentation, rather than an increase in the number of earthquakes.

The 1960 Chilean earthquake is the largest earthquake that has been measured on a seismograph, reaching 9.5 magnitude. The energy released was approximately twice that of the next most powerful earthquake, the Good Friday earthquake (Alaska, 1964). The 1556 Shaanxi earthquake (China) is the deadliest earthquake on record, killing approximately 830,000 people. However, from the ten largest recorded earthquakes only the 2004 Indian Ocean earthquake is simultaneously one of the deadliest earthquakes in history. A tsunami may occur, when the epicenter of a large earthquake is located offshore, and the seabed is thereby displaced heavily.

^{2004 Indian Ocean earthquake tsunami © David Rydevik 2004}