The earthquake had a maximum strength on the Mercalli intensity scale of VIII (Severe).[12] The earthquake was preceded by a smaller event, measuring 5.4, that was felt across the island and as far away as Oahu.[13]
The earthquake produced a minor tsunami that reached a maximum height of 40 cm (15.7 in.) in Kapoho, 20 cm (7.9 in.) in Hilo and 15 cm (5.9 in.) in Honuapo.[14]
Tectonic setting
Hawaii island is affected by a large number of minor earthquakes related to the movement of magma beneath its active volcanoes. Additionally there are less frequent tectonic earthquakes that are caused either by faulting within the volcanoes or by slip along the basal detachment surface at the top of the oceanic crust associated with the gradual collapse of the island's flanks as it continues to be enlarged.[15] Near Kilauea volcano, the largest structure formed by the collapse of the southeastern flank of the island is the Hilina slump. This slump moves seaward at an average rate of 10 cm/year (3.9 in/year). The slump moved during the May 4 earthquake by about 0.6 m (2.0 ft) and has moved during previous earthquakes, such as those in 1868 and 1975.[16]
Earthquake
The main earthquake was preceded almost exactly an hour earlier by a Mw 5.4 foreshock with a similar epicenter and focal mechanism.[17] Analysis of seismic waveforms suggest that the mainshock was most likely caused by slip on a thrust fault dipping at 20° to the northwest beneath the southeastern flank of the volcano.[18] The mainshock was followed by four aftershocks greater than Mw 4.5 over the next 30 minutes. Smaller aftershock continued to occur for months after the main earthquake.[19]
Further analysis of Love waves, whose radiation pattern gives greater discrimination on low-angle dips, indicate that the rupture occurred on a plane dipping at about 7°, consistent with it happening at the base of the Hawaiian volcanic rocks, where they overlie sedimentary rocks on the earlier seafloor. The calculated low rupture speed is also consistent with propagation along a relatively weak zone. This would make it similar to the 1975 Kalapana earthquake.[20]
Damage
The quake damaged many buildings, caused landslides that damaged shorelines, and caused cracks in a road, causing it to be shut down.[21]
^The "lurch" of the M6.9 earthquake was prompted by pressure in the east rift zone due to the magma intrusion.,"Kīlauea Eruption FAQs: General Questions". Archived from the original on May 22, 2018. Retrieved May 22, 2018 – via Facebook.{{cite web}}: CS1 maint: bot: original URL status unknown (link)
^"Hawaiian Volcano Observatory Update". United States Geological Survey. September 25, 2018. Retrieved September 26, 2018. Small aftershocks from the magnitude-6.9 earthquake in early May are still being generated on faults located on Kīlauea's South Flank.
