This is so because the energy released in an earthquake , and thus its magnitude , is proportional to the area of the fault that ruptures and the stress drop . Therefore , the longer the length and the wider the width of the faulted area , the larger the resulting magnitude . The topmost , brittle part of the Earth 's crust , and the cool slabs of the tectonic plates that are descending down into the hot mantle , are the only parts of our planet which can store elastic energy and release it in fault ruptures . Rocks hotter than about flow in response to stress ; they do not rupture in earthquakes . The maximum observed lengths of ruptures and mapped faults ( which may break in a single rupture ) are approximately . Examples are the earthquakes in Chile , 1960 ; Alaska , 1957 ; Sumatra , 2004 , all in subduction zones . The longest earthquake ruptures on strike-slip faults , like the San Andreas Fault ( 1857 , 1906 ) , the North Anatolian Fault in Turkey ( 1939 ) and the Denali Fault in Alaska ( 2002 ) , are about half to one third as long as the lengths along subducting plate margins , and those along normal faults are even shorter
Sumatra , 2004
2004 Indian Ocean earthquake and tsunami
Earthquake