The NWS’s Pacific Tsunami Warning Center (PTWC) mitigates tsunami hazards in Hawaii produced by large, distant earthquakes throughout the Pacific Ocean, but PTWC also issues warnings for tsunamis generated by earthquakes within the State of Hawaii itself. The last such dangerous tsunami was generated by a 7.7 magnitude earthquake on the Big Island of Hawaii in 1975. Small earthquakes generated by volcanic activity are far more common, and typically have a magnitude less than 3.0 and occur a few times a day. That changed on the afternoon of April 30. 2018, when an earthquake “swarm” began within Kilauea Volcano such that earthquakes began to occur far more frequently, about 100 per day.
This animation begins a month earlier on April 1 to start with a more typical earthquake pattern and proceeds forward in time at a rate of one day per second. The size of the circles indicate their magnitude, and color represents their depth. Three days before the swarm began the lava within the “Overlook crater” inside of Halema`uma`u Crater overflowed then retreated back below its rim. Then on April 30 the 35 year-old Pu`u `O`o eruption ceased and its cone partially collapsed. This event coincided with the start of a swarm of volcanic earthquakes on Kilauea’s East Rift Zone, a feature extending from the volcano’s summit that carries magma underground through its flank. Eruptions can take place anywhere along this rift zone as well as at the volcano’s summit. Earthquake occurrence then moved northeast along the rift zone away from the summit, suggesting the movement of magma below ground in this direction. Magma reached the surface and erupted as lava on the afternoon of May 3 and is still ongoing (1 July 2018).
At the same time lava began to drain from the “Overlook crater” and by May 15 its lava lake had dropped 100s of feet then produced steam-driven explosions when it interacted with ground water. Some of these explosions were strong enough to register as magnitude 5.0 earthquakes and send ash clouds to 30,000 ft. above sea level. By the end of May, however, these explosions changed their character such that they no longer produced large steam-driven ash clouds. The walls of Halema`uma`u had begun to collapse, thus widening Halema`uma`u and burying the “Overlook crater” as magma continued to drain from the summit to feed the flank eruption. These collapses seem to release trapped volcanic gas rather than groundwater steam and produce only small ash clouds, though they still release explosive energy greater than magnitude 5.0 earthquakes and occur about once per day. Black circles represent these volcanic explosions in the animation, with their size indicating their energy release in equivalent earthquake magnitude. This explosive activity also continues today.
This animation includes charts showing some statistics about the earthquake activity. The top graph shows the maximum magnitude per day for earthquakes in white and explosions in black. The bottom graph shows the total number of seismic events per day. Since the eruption began the frequency of earthquakes initially increased to about 100 per day and their magnitudes exceeded magnitude 4.0. The largest earthquake struck on the afternoon of May 4 with a magnitude of 6.9 and produced numerous aftershocks. This largest earthquake also moved the flank of Kilauea Volcano as much as 20 inches seaward. With the subsidence of Kilauea Caldera and the collapse of Halema`uma`u the number of earthquakes dramatically increased to more than 700 per day.
Though PTWC monitors all earthquakes in Hawaii, including this ongoing activity, the primary responsibility for mitigating volcanic hazards in the State of Hawaii rests with the USGS’s Hawaiian Volcano Observatory who publishes updates daily and advises local emergency managers.
That earthquake video was amazing. For you too?