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.

G force scale comparison