Volcano Watch — Recent activity highlights Kīlauea Volcano's restless nature

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As of this writing, the lava lake level has dropped and is no longer visible from Jaggar Museum, and the earthquake frequency has decreased to background levels.

So, what exactly happened over the past month, and what does it mean?

First, let's review what happened. Unusual activity began on April 21, 2015, with an abrupt onset of rapid summit inflation and a rise of Kīlauea's summit lava lake. By April 24, the rising lava lake, which normally is too deep within its crater to be seen by Park visitors, came into view from the Jaggar Museum vantage point. Inflation continued and the lake kept rising, eventually spilling lava onto the floor of Halema‘uma‘u Crater for the first time on April 28. Along with the inflation and high lava lake levels, earthquakes in Kīlauea's upper East Rift Zone were also increasing in rate and size.

The inflation, rising lava lake level, and increased earthquake activity all pointed to increasing pressure in Kīlauea's summit magma reservoir and upper East Rift Zone. Increased pressure in a magma storage system can be explained by either more magma coming into it (due to an increase in magma supply) or less going out of it (for instance, due to a blockage of magma flowing out to the Pu‘u ‘Ō‘ō vent on the East Rift Zone). Preliminary interpretations suggest that the recent increased pressure was driven by a brief increase in magma supply.

Regardless of the cause, increased pressure at the summit means a higher likelihood of new activity on Kīlauea, such as an intrusion (injection) of magma into the rock that makes up the volcano—an intrusion that might, or might not, make its way to the surface and form a new eruptive vent. The most recent example of this occurred in March 2011, when building summit pressure produced an East Rift Zone intrusion that resulted in new vents opening west of Pu‘u ‘Ō‘ō and erupting fountains of lava for about five days. Intrusions (and associated eruptions) act as a pressure release "valve" for the summit, and are generally accompanied by deflation of Kīlauea's summit as magma leaves the reservoir.

High lava lake levels and inflation were sustained through the first week of May, but around May 10, the summit began to deflate and the lava lake level began to drop. On May 14, HVO recorded the onset of a sharp increase in small earthquakes in the southern part of Kīlauea's summit caldera. Around the same time, a nearby electronic tiltmeter (an instrument that measures tiny changes in the slope of the ground) began to tilt rapidly away from the south caldera.

Earthquake activity in Kīlauea's south caldera peaked on May 15, with more than one earthquake occurring each minute. Rapid ground tilting also continued, but diminished to low levels by May 17.

Now, let's address what all this means. Our preliminary interpretation is that, beginning in late April, an increase in magma supply to the shallow magma storage system beneath Kīlauea's summit resulted in inflation and a rising lava lake level. By mid-May, the summit reservoir could no longer accommodate the influx of magma. This triggered an intrusion of magma from the reservoir into the southern part of Kīlauea's caldera, and, as the intrusion reduced pressure in the summit, the lava lake level dropped.

This pattern of inflation, seismicity, and subsequent intrusion in response to excess magma pressure is a common phenomenon at Kīlauea. Although the heightened activity diminished to background levels earlier this week, more changes could still occur at Kīlauea in the coming days to weeks. Because of this, HVO scientists continue to keep a close eye on Kīlauea. Daily eruption updates are posted on the HVO website at http://hvo.wr.usgs.gov/activity/kilaueastatus.php. You can also sign up to receive email notifications of volcanic activity at http://volcanoes.usgs.gov/vns/.

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Volcano Activity Update

Kīlauea's summit lava lake level dropped rapidly beginning on May 11 in response to summit deflation and then the intrusion of magma beneath the southern caldera region. By May 15, the lake level had dropped 62 m (203 ft) below the highest point reached on May 8. As of this writing (May 21), the lake level was about 51 m (167 ft) below the high point (the top of the new vent rim formed by lava overflowing the Overlook crater). By May 17, seismicity and deformation in the summit and southern caldera returned to normal, background levels, where they remained as of May 21.

Kīlauea's East Rift Zone lava flow continues to feed widespread breakouts northeast of Pu‘u ‘Ō‘ō. The most distant active lava was about 8.5 km (5.3 mi) from Pu‘u ‘Ō‘ō when mapped on May 21.

Four earthquakes were reported felt on the Island of Hawai‘i in the past week. On Friday, May 15, 2015, at 8:37 a.m., HST, a magnitude-3.2 earthquake occurred 2.6 km (1.6 mi) southwest of Kīlauea Summit at a depth of 2.9 km (1.8 mi). On Saturday, May 16, at 9:14 a.m., a magnitude-3.3 earthquake occurred 4.7 km (2.9 mi) southeast of Kīlauea Summit at a depth of 1.5 km (1.0 mi). On Sunday, May 17, at 8:42 a.m., a magnitude-3.3 earthquake occurred 5.2 km (3.2 mi) southeast of Kīlauea Summit at a depth of 0.4 km (0.3 mi). On Sunday, May 17, at 10:17 a.m., a magnitude-3.4 earthquake occurred 8.2 km (5.1 mi) south of Hāwī at a depth of 25.6 km (15.9 mi).