Volcano Watch — Kīlauea's south flank earthquakes more common than hurricanes

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Early Wednesday morning, as most island residents slept peacefully in the knowledge that we were to be spared by hurricane Flossie, some of us were jolted awake by yet another earthquake. This was a magnitude-4.4 quake beneath the south flank of Kīlauea. A larger earthquake earlier in the week—a magnitude 5.4 on Monday evening—also occurred beneath Kīlauea's south flank.

Kīlauea's south flank earthquakes more common than hurricanes...

Kīlauea's south flank earthquakes more common than hurricanes

(Public domain.)

Both of these earthquakes were located at about the same depth (9-10 km; about 6 miles) and at about the same distance south from Kīlauea's east rift zone, but the second quake was about 10 miles west of the first. These earthquakes bracket the east-west extent of the increased activity on the rift zone during the last two months.

Monday's earthquake occurred south of the current eruptive vent, which opened on July 21, in the middle part of the rift zone. Wednesday's earthquake was closer to Kīlauea's summit, centered just south of Pauahi Crater, where magma first forcibly intruded the rift zone in June.

This summer's activity originated way back in late 2003, when the summit area of Kīlauea started to rise and expand, while the eruption at Pu`u `O`o continued unchanged. More magma was being supplied to Kīlauea than could be delivered through the existing plumbing system within the rift zone. The volcano continued to inflate over the next several years, generating many small earthquakes in the process.

In May of this year, there was a renewed surge of seismicity in the summit area. On May 24, several earthquakes occurred along faults that mark the outer boundaries of Kīlauea's caldera. The largest of these was a magnitude 4.7, which was the largest summit area earthquake in the past 50 years. So much magma had accumulated beneath the summit that some serious rock breaking was required to relieve the pressure!

Still the swelling continued, until 02:16 a.m. on Father's Day, June 17th, when magma abruptly started leaving the summit reservoir and intruding the upper east rift zone, near Pauahi. About six hours later, magma migrated farther down the rift and accumulated near Makaopuhi Crater for the next day-and-a-half. By the time the intrusion was over, the rift zone near Makaopuhi had widened by almost a meter (about 3 ft).

Magma supply to Pu`u `O`o was cut off during this event, and lava did not return there until July 2. Even after a new lava lake formed in Pu`u `O`o Crater, the summit continued to inflate, until July 21. Just after midnight, eruptive fissures propagated downrift from the Pu`u `O`o cone, feeding lava flows and perched lava ponds. By early August, the eruption had localized at the lowest fissure, and has been building a perched, open lava channel that feeds `a`a flows advancing northeast.

Since the opening of the new eruptive fissures on July 21, the summit has been subsiding and contracting as magma moves from the reservoir to the eruption site. Seismicity quieted considerably-until the south flank events this week.

Did all this exciting activity cause this week's earthquakes? As usual, the answer isn't quite that simple. It may be more accurate to say that the intrusion triggered these earthquakes, but didn't exactly "cause" them.

Moderate-to-large earthquakes on the south flank after east rift zone intrusions are fairly common. The most recent example was a magnitude 5.1 in 1997, which occurred several months after an intrusion similar to the Father's Day event. That earthquake originated in exactly the same place as Monday evening's quake.

The areas of Monday's and Wednesday's quakes, however, have also generated numerous earthquakes not associated with intrusions. For example, the 1989 magnitude-6.2 earthquake was also located in almost exactly the same place as Monday's quake.

We also observe steady motion of the south flank towards the sea at about 6 cm (2.4 inches) per year. Obviously, there are forces that cause the south flank to move even in the absence of shallow rift zone intrusions. In addition to the inescapable force of gravity causing seaward slumping, magma accumulating in the deep core of the rift zone may be pushing the south flank seaward.

It is likely that the south flank of Kīlauea, being perpetually under some degree of stress, was already poised for an earthquake. Intrusions, such as the Father's Day sequence of events, may provide the proverbial last straw that triggers the release of at least some of the stress in the form of earthquakes large enough to serve as wake-up calls.

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Volcano Activity Update

The July 21 fissure eruption remains active. Of the four original fissure segments, only fissure D-the fissure farthest east-is erupting lava. The lava enters an open channel up to as much as 200 feet across that heads toward the northeast. Prior to last week, the lava stream transitioned into `a`a about a mile from the fissure. Poorly developed lava tubes beneath the `a`a carried the lava forward. Last weekend though, just upstream from that transition point, the north wall of the channel failed. Most, if not all, of the lava was diverted into a new `a`a flow. For the past week, this new flow has been creeping along the north edge of the initial `a`a flow fed by the fissure.

Prior to the channel diversion, the terminus of the flow had reached up to just over a mile and a half from the fissure. As of this writing (Thursday, August 16), it is not known if the channel diversion has robbed the entire lava supply or if some lava is still making it through to the end of the original `a`a flow. It is also not known if the lava flow being fed by the diverted channel has regained lost ground and caught up to the terminus of the original flow. Regardless, it does not appear that the lava flow has significantly advanced in the past week.

Though the July 21 fissure eruption has stolen the show, Pu`u `O`o is still alive. Steam and fume obscure the crater most of the time, but occasional glimpses of incandescence are still seen at four separate places on the crater floor and in the West gap. As has been seen in years past, Pu`u `O`o could be acting as temporary storage as lava passes beneath the cone on its way to the erupting fissure.

Mauna Loa is not erupting. One earthquake was located beneath the summit. Extension between locations spanning the summit, indicating inflation, continues at steady, slow rates, which have slowed further since May 2007.

Four earthquakes beneath Hawai`i Island and one earthquake beneath Maui were reported felt within the past week. A magnitude-2.2 earthquake occurred at 00:54 a.m. H.s.t. on Friday, August 10, and was located 7 km (4 miles) south-southeast of Keokea, Maui at a depth of 13 km (8 miles). The largest earthquake of the past week, a magnitude-5.4 event, occurred at 7:38 p.m. H.s.t. on Monday, August 13, 7 km (4 miles) south of the Pu`u `O`o crater at a depth of 10 km (6 miles). A magnitude-2.1 earthquake occurred at 1:33 a.m. on Tuesday, August 14, and was located 8 km (5 miles) south of Pu`u `O`o at a depth of 10 km (6 miles). A magnitude-4.4 earthquake occurred at 2:23 a.m. H.s.t. on Wednesday, August 15, 12 km (7 miles) southeast of Kīlauea's summit at a depth of 10 km (6 miles). The final felt earthquake this past week was a magnitude 3.8 event at 3:02 a.m. H.s.t. on Thursday, August 16, located 10 km (6 miles) north of Volcano at a depth of 28 km (17 miles).