The United States Geological Survey recorded a Magnitude 3.6 Earthquake at a depth of 7.7 km, 41 km to the south of Hilo, slightly after 8.35 am local time (slightly after 6.35 pm GMT) on Thursday 26 September 2013. This is not a large quake, and is unlikely to have caused any damage or injuries, but was felt across much of the east of the island.
The approximate location of the 26 September 2013 Big Island Earthquake. Google Maps.
Big Island, Hawai'i, sits on top of a volcanic hotspot, an area where magma from deep inside the Earth is welling up through the overlying plate (in this case the Pacific) to create volcanism at the surface. The island comprises five overlapping shield volcanoes, two of which are currently active and another two of which are considered to be dormant with the possibility of reactivating in the future. Volcanoes move as they erupt, swelling as magma enters their chambers from bellow, then shrinking as that magma is vented as lava. The movements of a group of volcanoes close to one another can place considerable strain on layered rocks, and the islands of Hawai'i, and in particular Big Island, are very prone to Earthquakes, though these tend to be small and frequent rather than large, rare and destructive.
Witness accounts of quakes can help geologists to understand these events and the rock structures that cause them. If you felt this quake (or if you were in the area but did not, which is also useful information) you can report it to the USGS here.
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