EARTHQUAKE FREQUENCY

The USGS typically reports 2000 earthquakes per month, for an average of about 70 earthquakes per day. There are actually many more quakes than this, but we are limited in our ability to detect small earthquakes that occur far away from seismic observing stations.
Due to measuring limitations, a plot of known seismicity worldwide includes some spatial bias. For example, the southern hemisphere is almost entirely ocean, and much of the land there is inhospitable, so there are more seismometers deployed in the northern hemisphere. Therefore a plot of one month's earthquakes shows more events in the northern hemisphere simply because smaller events in the southern hemisphere were not detected. In addition, very dense networks of seismometers are arrayed in certain regions of the developed world with high seismic risk, such as Japan and California, which means that disproportionately many small earthquakes are detected in these regions.

EARTHQUAKES DELINEATE THE EARTH'S TECTONIC PLATES
Because of the large number of earthquakes that occur on a daily basis, we can see a nearly complete set of plate boundaries with as little as 3-4 months of data. With six months worth of data, we can map the boundaries with a high degree of accuracy. The narrow bands of seismicity outline seven large plates and several smaller ones. The largest plate is the Pacific plate, followed by the African plate, Eurasian plate, Australian-Indian plate, Antarctic plate, North American plate, and South American plate. Smaller plates include the Nazca plate, Philippine plate, Caribbean plate, Cocos plate, and Juan de Fuca plate.

LITHOSPHERIC PLATES:
The Fundamental Unit of the Earth's Outer Surface
Most continental shorelines do not coincide with plate boundaries (one important exception is the west coast of South America). Most shorelines are not zones of earthquake activity. For this reason, they are referred to as "passive margins," places that may have been plate boundaries in the past, but are no longer active. The east coast of North America is a passive margin, as are the east, south and west coasts of African, northern and western Europe, etc. The eastern boundary of the North American plate is in the middle of the Atlantic Ocean, where there is a belt of active seismicity; thus the North American plate is an example of a plate that is made up of both continent and ocean. Most of the plates consist of both continental and oceanic material. There are several entirely oceanic plates (Nazca, Cocos), but no entirely continental plates (with the possible exception of the Arabian peninsula; it depends on the criteria one uses to define individual plates and how much ocean is required to be considered "ocean"). The fact that continents are included as part of plates made of both continent and ocean suggests that the continents do not move independently of the oceans as Wegener envisioned, but rather continent and ocean move together as part of a single plate. Thus from a geodynamical perspective, a "plate" appears to be a more fundamental unit than a continent or ocean.

 

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