Volcano Watch article by USGS Hawaiian Volcano Observatory
Astute watchers of recent earthquake activity at Kilauea have noticed a marked increase of earthquakes, both within the summit caldera and in a northwest-trending linear zone to the east of the caldera. Are these earthquakes associated with the high lava levels in Halema`uma`u? Why aren’t there earthquakes in the rest of the East Rift Zone?
On Hawaiian volcanoes, the summit caldera and rift zones are intimately linked in the transport of magma, and Kilauea is no exception. The summit acts as a control center, initially storing magma, regulating the flow of magma down the rift zones.
Since 1983, Kilauea has had a well-developed pathway along the East Rift Zone to Pu`u `O`o. A typical batch of magma first rises from depth under the summit of Kilauea, where it is briefly stored. After losing most of its CO2 gas, the magma is denser than fresh magma coming from depth, so it sinks and is transported into the East Rift Zone, where it eventually erupts at Pu`u `O`o.
One remarkable feature of the magma transport between the summit and Pu`u `O`o is that it typically occurs without any earthquakes at all. During periods of background effusion at Pu`u `O`o, the entire East Rift Zone between the summit and Pu`u `O`o is seismologically quiet, except for tremor associated with the lava lakes at Halema`uma`u and Pu`u `O`o.
Recently, the level of the lava lake at Halema`uma`u has increased significantly. Much like the hydraulic head of a well or municipal water system, the level of the lava lake is representative of pressure in the system. High lava-lake levels mean that the pressure in the magma system is high within the caldera and in the East Rift Zone. As the lava lake rises, so does the seismicity in the Upper East Rift region between Keanakako`i and Mauna Ulu. This is not a coincidence.
Prior to many historical eruptions, including those in March, August, and September 2011, earthquakes along the Upper East Rift increased in numbers and size. During the Kamoamoa eruption in March 2011, some of these earthquakes were as large as magnitude 4. The interesting part of these events is that they look like typical earthquakes that you might find on a tectonic fault—not the type of long-period events that we are used to associating with subsurface magma movement in the summit. So what is going on?
Earthquakes on the Upper East Rift occur only when the pressure in the system is high, as evidenced by the high lava levels in Halema`uma`u. Our best explanation to account for the types of earthquakes we see in the Upper East Rift is that there is a strike-slip fault above the magma pathway that is activated when the pressure in the pathway is high. The earthquakes occur at approximately 3 km (2 mi.) below the surface and, thus, the magma pathway likely exists at some deeper level. The crust around the pathway and below it is probably too hot to support earthquakes.
So why doesn’t the Middle East Rift (between Mauna Ulu and Kupaianaha) also exhibit this behavior? The East and Southwest Rift Zones of Kilauea outline a block of the island that moves nearly due south at a rate of 8 cm per year (3 in. per year). The Middle East Rift is oriented perpendicular to that movement and, thus, the north side of the rift is moving away from the south side, creating space at depth and relieving any stored pressure within the rift. This explains the frequency of eruptions in this area as the crust is being pulled apart and is relatively weak. It also explains the paucity of earthquakes in this area, as magma can flow through the weak crust without disturbing its surroundings. The only time that seismicity is high in the Middle East Rift is immediately prior to a fissure eruption or intrusion as new crust is broken ahead of a dike as it steamrolls to the surface.
The Lower East Rift (between Kupaianaha and Cape Kumukahi) acts in many ways like the Middle East Rift, except that movement of the south flank is significantly slower than in the Middle East Rift and, thus, the rate of eruptions is relatively lower.