seismic activity
On August 19, 2006, As reported by the USGS, a magnitude 5.5 earthquake occurred 43 miles NNW of Puerto Escondido, 53 miles SW of Oaxaca City at 12:41 am CDT at a depth of 27 km. [1]
These waves may cause faults, or fractures, as well as folds, or bends in the rock structure, which manifest on the surface in the form of earthquakes, volcanoes, and other varieties of seismic activity. [2]
During such periods of heightened earthquake activity, scientists work around the clock to detect subtle and significant variations in the type and intensity of seismic activity and to determine when an eruption is occurring, especially when a volcano cannot be directly observed. [3]
The moment magnitude of an earthquake is conventionally reported, or the related and mostly obsolete Richter magnitude, with magnitude 3 or lower earthquakes being mostly imperceptible and magnitude 7 causing serious damage over large areas. [4]
The results from nature were devastating; the earthquake caused a tsunami, or tidal wave, that flooded the Tagus River even as a fire, also caused by the earthquake, raged through the city. [...] An area of geophysics known as seismology is the study of these waves and their effects, which often can be devastating when experienced in the form of earthquakes. [2]
In its most generic sense, the word earthquake is used to describe any seismic event’whether a natural phenomenon or an event caused by humans’that generates seismic waves. [4]
Earthquake activity beneath a volcano almost always increases before an eruption because magma and volcanic gas must first force their way up through shallow underground fractures and passageways. [...]… when cracks vibrate either low-frequency earthquakes or a continuous shaking called volcanic tremor is triggered. [3]
Popocatepetl, an active volcano east of Mexico City erupted December 18, 2000 (and many other times in recent history). [1]
They measure earthquakes according to their magnitude or energy as well as their intensity or human impact. [2]
Having enough of the right instruments located in strategic places is especially important for detecting earthquakes smaller than magnitude 1 or 2; sometimes, these tiny earthquakes represent the only indication that a volcano is becoming restless. [3]
Tectonic earthquakes will occur anywhere within the earth where there is sufficient stored elastic strain energy to drive fracture propagation along a fault plane. [4]
There was extensive damage to buildings in Colima, capital of Colima, and in Manzanillo, Colima, as well as in the state of Jalisco. [1]
Dramatic improvements in computer technology and increased scientific experience with volcano seismicity from around the world have improved our ability to provide eruption warnings and to characterize eruptions in progress. [3]
Sources:
[1] Seismic Activity
[2] seismology: Definition from Answers.com
[3] Monitoring Volcano Seismicity
[4] Earthquake - Wikipedia