Volcano Watch — Three earthquakes felt in past two weeks: Hawaii Tribune-Herald

The largest earthquake in Hawai`i since January 26 occurred at 2:58 a.m. Tuesday. Many residents were awakened by the shaking from this magnitude-4.9 earthquake. The Jan. 26 earthquake, which occurred beneath Pahala, had a comparable magnitude. The foci of Tuesday morning's earthquake was beneath the south flank of Kīlauea, about 7 miles southeast of the summit and at a depth of about 5.6 miles.

There have been several other felt earthquakes in this same region in the previous 10 days. The first of these occurred on May 30 at 7:18 p.m. and had a magnitude of 3.6. The next took place on June 5 at 1:06 a.m., with a magnitude of 3.1. The June 5 earthquake was shallow and was located along the upper East Rift Zone; it was related to magma movement in the rift zone. On the other hand, the May 30 and June 8 earthquakes were deeper and were caused by slippage of the south flank of Kīlauea Volcano toward the sea along a horizontal fault plane near the base of the volcanic pile.

The 10-year long eruption along the East Rift Zone of Kīlauea Volcano continues with slightly less than 390,000 cubic yards of bubble-free lava being erupted each day. This lava is carried from the vents on the southwest side of the Pu`u `O`o cone downslope, entirely underground in a tube system, to the ocean entry near Kamoamoa. During the last week, a new, small surface flow of tube-fed pahoehoe had been advancing down the west side of the lava field and had reached the 450-foot contour by Thursday. The main flow of lava into the ocean is marked by the location of the huge steam plume at the coast. However, surface lava is rarely visible at this site, as lava flows directly into the ocean and is veiled by the plume of steam. Additional small steam explosions have occurred at the entry site in the last few weeks, and angular ballistic lava blocks are scattered up to 500-650 feet inland from the coast.

The surface level of the lava pond inside the Pu`u `O`o cone has been dropping during the past three weeks. Before this episode began on February 16, the surface of the pond rose to 213 feet below the rim of the cone. As the eruption began, the pond surface dropped to about 246 feet below the rim, where it remained until two weeks ago. For the last two weeks the pond surface was 275 feet below the rim, and on Thursday of this week, it was 308 feet below the rim.

The level of activity of the lava pond in Pu`u `O`o has also declined during the past two weeks. Prior to that time, the pond surface circulated actively, and spatter was even thrown over the rim of the cone, on occasion. The surface is now sluggish, and little spattering is occurring.

The magnitude-4.9 earthquake Tuesday morning did not cause any collapses on the rim of Pu`u `O`o, although several new concentric cracks formed near the lowest part of the rim. The largest of these new cracks is 65 feet long and about 4 inches wide.

During previous eruptive episodes 50 and 51, the pond level dropped and became less active during the eruption and then rose and became more active during the pause between eruptions. The earthquakes in the last week-and-a-half are probably indirectly related to the observed changes at Pu`u `O`o, as the earthquakes and the level of the pond are caused by changes in magma pressure in the rift zone. The occurrence of recent moderate-sized earthquakes near the upper east rift zone, the currently deepening pond and the decrease in pond activity may be an early warning that this eruptive episode is also near its end. This does not mean that the 10-year-long eruption is nearly over, simply that it may be close to another pause in activity.