Types of Earthquakes
Not so very long ago the mechanics behind earthquakes remained a complete – and terrifying – mystery, even to scientists. Unexpectedly, the Earth would start to shake, the ground would split or seemingly “melt,” landslides would spill down hillsides, and in coastal regions the ocean would come racing across the land. Some regions of the world seemed particularly susceptible to these events. But that was the extent of our knowledge.
More recently, however, we’ve come to understand that the surface of the Earth is actually composed of a cracked outer shell (the lithosphere) that, for wont of a better description, ‘floats’ across the upper mantle (the aesthenosphere). The closest metaphor is a cracked hardboiled egg, wherein the shell can be slid around the inner egg. These oceanic and continental plates are in constant movement and are responsible for many of the unique geological features of our planet (most notably mountains and volcanoes).
Plate tectonics also explains the way the Earth renews itself. In the oceans, great undersea rifts are caused by plates pulling away from each other, resulting in magma rising up to create new mantle. Continental plates, meanwhile, smash together or slide along each other, forming vast mountain chains such as the Himalayas and Andes. In many cases, ‘subduction’ occurs where one plate is essentially driven beneath another plate, thereby enabling the Earth to consume old mantle even as new mantle is formed elsewhere.
Understandably, most earthquakes occur along these plate boundaries also known as ‘faults.’ California’s San Andreas fault is a particularly well-known example of just such a plate boundary, an 810-mile long stretch where the Pacific and North American plates are sliding past each other. Approximately 90% of all earthquakes occur along the Pacific’s “Ring of Fire,” so called because of a vast connection of tectonic plates constantly in movement around the Pacific Ocean. Rarely, an earthquake will occur in the middle part of a tectonic plate, a famous example being the 1811 and 1812 earthquakes in New Madrid, Missouri. The mechanics behind these events are not well understood.
Plate tectonics help to explain four different types of earthquakes, the first being the mid-ocean ridges where, as already noted, two tectonic plates are separating to allow magma to burble up and create new mantle. Because the lithosphere is thin in such locations, significant stress does not build, meaning earthquakes are rarely if ever particularly violent.
Strike-slip earthquakes occur where two plates are attempting to slide past each other, as in the case of the San Andreas. Over decades – sometimes centuries – the two plates ‘stick’ until pressure becomes so great that the plates “slip” past each other resulting in massive amounts of shaking. For obvious reasons, the longer the plates remain stuck in place the greater the seismic activity when at last they spring free of each other.
Subduction earthquakes are so named because they involve one plate pushing beneath another. Similar to strike-slip quakes, enormous pressure builds between the competing plates until they at last release. When this happens massive amounts of energy are released, causing some of the largest of all earthquakes as one plate is thrust upward. The magnitude-9 earthquake in Tohuku, Japan, is an example of a megathrust earthquake, the result of which was a massive displacement of water resulting in a tsunami.
The last kind of earthquake is caused by two plate boundaries shoving each other upward, producing vast mountain ranges that also create powerful earthquakes such as the 2005 earthquake in Pakistan’s mountainous Kashmir region.