Mauna Loa
Mauna Loa has become increasingly active over the last 13 months.Joe Parks

The world's biggest active volcano Mauna Loa appears to be waking up from a 30 years slumber, experts have said.

The volcano, in Hawaii, could be "slowly stirring to life" after it showed an increased level of seismic activity on the flanks and summit.

Over the last year, the Hawaiian Volcano Observatory has identified four distinct "earthquake swarms", which are clusters of earthquakes that take place in similar times and locations.

The earthquakes took place northwest of the volcano summit and all were less than magnitude 2.2, except for one on 9 May that measured 3.5.

Mauna Loa last erupted in 1984 and was preceded by a swarm of long-period deep earthquakes, about 45km under the volcano.

Before this eruption, seismic activity was elevated for about three years, with 28 earthquakes greater than magnitude three. A magnitude 6.7 earthquake struck on 16 November 1983 and may have been triggered by the pressurising magma system of the volcano.

A statement from Hawaiian Volcano Observatory said: "The recent swarms have not been associated with the deformation of the ground surface that we would expect from intrusion of significant amounts of magma into shallow levels beneath Mauna Loa.

Mauna Loa
Mauna Loa last erupted in 1984.R.W. Decker/USGS

"However, ground deformation indicating shallow magma accumulation has been measured during many of the past 30 years, including two episodes of very rapid inflation immediately following the most recent eruption in 1984, and again from 2004–2006."

It noted that the seismic network monitoring Mauna Loa has been much improved since 1984 and can better detect and locate small earthquakes that previous systems could not – many of which would not have been detected 30 years ago.

"Though the size and number of earthquakes observed over the past 13 months have been significantly smaller than those observed in the 3 years prior to the 1984 eruption, they have been occurring in the same general areas of the volcano ... Interestingly, the March 2013 and April 2014 swarms occurred on the northwest flank in areas that were not active prior to the 1975 or 1984 eruptions."

Explaining why the earthquakes are taking place, the Observatory added: "Mauna Loa consists of a hot and plastic core that is composed of overlapping dikes intruded during the construction of the volcano's edifice.

"Stresses arising from an intrusion of magma or gases under the volcano may be transmitted through the mushy core to the cooler and more brittle crust outside the core of the volcano. The earthquake activity present during the past 13 months has consisted entirely of earthquakes arising from brittle failure of the crust."