The various magnitude scales represent different ways of deriving magnitude from such information as is available. Additional adjustments are made for distance, kind of crust, and the characteristics of the seismograph that recorded the seismogram. Determination of an earthquake's magnitude generally involves identifying specific kinds of these waves on a seismogram, and then measuring one or more characteristics of a wave, such as its timing, orientation, amplitude, frequency, or duration. For shallow earthquakes – less than roughly 60 km deep – the surface waves are stronger, and may last several minutes these carry most of the energy of the quake, and cause the most severe damage.Īn earthquake radiates energy in the form of different kinds of seismic waves, whose characteristics reflect the nature of both the rupture and the earth's crust the waves travel through.

Surface waves are smaller for deep earthquakes, which have less interaction with the surface. Following the S-waves are various kinds of surface-waves – Love waves and Rayleigh waves – that travel only at the earth's surface. Both of these are body-waves, that pass directly through the earth's crust. S-waves may take an hour to reach a point 1000 km away. The sideways-shaking S-waves (following the green lines) arrive some seconds later, traveling a little over half the speed of the P-waves the delay is a direct indication of the distance to the quake.

The compressive P-waves (following the red lines) – essentially sound passing through rock – are the fastest seismic waves, and arrive first, typically in about 10 seconds for an earthquake around 50 km away. A similar effect channeled seismic waves between the other major faults in the area. Geological structures were also significant, such as where seismic waves passing under the south end of San Francisco Bay reflected off the base of the Earth's crust towards San Francisco and Oakland. This is why, in the 1989 Loma Prieta earthquake, the Marina district of San Francisco was one of the most damaged areas, though it was nearly 100 km from the epicenter. For instance, thick layers of soft soil (such as fill) can amplify seismic waves, often at a considerable distance from the source, while sedimentary basins will often resonate, increasing the duration of shaking. The intensity of local ground-shaking depends on several factors besides the magnitude of the earthquake, one of the most important being soil conditions. With an isoseismal map of the observed intensities (see illustration) an earthquake's magnitude can be estimated from both the maximum intensity observed (usually but not always near the epicenter), and from the extent of the area where the earthquake was felt. Intensity refers to the strength or force of shaking at a given location, and can be related to the peak ground velocity. It is 'approximately related to the released seismic energy.' Magnitude is an estimate of the relative 'size' or strength of an earthquake, and thus its potential for causing ground-shaking. When this stress becomes great enough to rupture the crust, or to overcome the friction that prevents one block of crust from slipping past another, energy is released, some of it in the form of various kinds of seismic waves that cause ground-shaking, or quaking. The Earth's crust is stressed by tectonic forces. The irregular distribution of shaking arises from variations of geology and/or ground conditions. Different magnitude scales are necessary because of differences in earthquakes, the information available, and the purposes for which the magnitudes are used.Įarthquake magnitude and ground-shaking intensity Isoseismal map for the 1968 Illinois earthquake.

Magnitude scales vary on what aspect of the seismic waves are measured and how they are measured. Magnitudes are usually determined from measurements of an earthquake's seismic waves as recorded on a seismogram. These are distinguished from seismic intensity scales that categorize the intensity or severity of ground shaking (quaking) caused by an earthquake at a given location. Seismic magnitude scales are used to describe the overall strength or 'size' of an earthquake.

Moment magnitude scale chart