HVO reported continuing unrest at Mauna Loa during 19-25 October. The seismic network detected 10-46 daily small-magnitude (below M 3) earthquakes 3-5 km beneath Mokua’weoweo caldera and 6-8 km beneath the upper NW flank of Mauna Loa. Data from Global Positioning System (GPS) instruments at the summit and flanks showed continuing inflation, though data from tiltmeters at the summit did not show significant surface deformation over the past week. A M 3.1 earthquake was recorded at 2035 on 23 October at a depth of 4 km beneath Mokua’weoweo caldera. The Volcano Alert Level remained at Advisory (the second lowest level on a four-level scale) and the Aviation Color Code remained at Yellow (the second lowest level on a four-color scale).

Geological Summary. Massive Mauna Loa is a basaltic shield volcano that rises almost 9 km from the ocean floor to form the world's largest Holocene volcano. Flank eruptions typically occur from the lengthy NE and SW rift zones, and from the Moku'aweoweo summit is caldera, which is within an older and larger 6 x 8 km caldera. Two of the youngest large debris avalanches documented in Hawaii traveled nearly 100 km from Mauna Loa; the second of the Alika avalanches was emplaced about 105,000 years ago (Moore et al., 1989). Almost 90% of the surface of the volcano is covered by lavas less than 4,000 years old (Lockwood and Lipman, 1987). Beginning about 1,500 years ago, a series of voluminous overflows from a summit lava lake covered about 25% of the volcano's surface. Over the last 750 years, from shortly after the formation of Moku'aweoweo caldera until the present, an additional 25% of the volcano has been covered with lava flows, mainly from summit and NW rift zone vents.

Source: US Geological Survey Hawaiian Volcano Observatory (HVO)