Two significant shudders under Kilauea's south flank last week — a magnitude 5.4 quake Monday night and a magnitude 4.4 quake Wednesday morning — punctuated anxiety over what exactly is going on down there.

The area clearly is geologically unstable, and in the past — notably in 1975 — it has produced damaging earth movements.

The Nov. 29, 1975, Big Island quake has been calculated at magnitude 7.5. It knocked down houses and caused a tsunami that killed two people and injured 19 others who were camping at Halape.

Volcano scientists say they're working hard to develop a clear idea of the geology of the area, and they're still not there.

"It's still a topic of active research to understand the details of the behavior," said Hawaiian Volcano Observatory seismologist Paul Okubo.

A nearly 44-acre lava bench in Hawai'i Volcanoes National Park collapsed into the ocean last week, scientists reported.

The collapse of the East Lae'apuki delta, which was created by lava from Kilauea Volcano entering the sea and then hardening, could have been related to either the passing of Hurricane Flossie or the 5.4-magnitude earthquake on Monday, officials at the observatory said.

Officials said they do not know exactly when the collapse occurred because access to the site and viewing conditions had been limited by weather and surf conditions related to the storm. The collapse was first reported by a tour pilot flying over the area.

No one was injured in the collapse of the lava bench.

Okubo said that the best guess is that the quakes represent the continued movement of the entire southern portion of Kilauea's East Rift Zone seaward — and away from the rest of the island.

The south flank is moving relatively constantly seaward at a rate of about 2.4 inches a year. Occasionally it speeds up, as it did during the period after the June 17 episode, which saw significant earth cracking, some eruptive activity and a great deal of molten rock being forced into the east rift zone.

The magma intrusions may be acting like a giant wedge, forcing the island apart along a rift zone. The immense mass of Mauna Loa to the north doesn't move, so the relatively smaller slope of Kilauea's south flank does. One way to think of it is that the tapering slope of the volcano is sliding across the ocean floor, and when it catches and jerks, an earthquake is produced.

The recent quakes are measured at about six miles deep — at the boundary between the Earth's crust and the layers of the Big Island's volcanic rock.

Geologists hesitate to make direct cause-and-effect comparisons between the quakes last week and the latest volcanic episode. The volcano is too complex — and too incompletely understood — to do that, Okubo said.

Thursday's Volcano Watch column, produced by the Hawaiian Volcano Observatory, said this: "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."

The same areas sometimes produce strong quakes even when there isn't any magma intrusion going on, it said.

"The 1989 magnitude 6.2 earthquake was also located in almost exactly the same place as Monday's quake," the column said.

Still, although intrusions of magma may not be the only cause of the slope's jerky seaward slide, it's difficult to ignore the eruptive activity of the past two months.

"It would be almost reasonable to think that there were a connection, given the time scenario," Okubo said.

But generally, the seaward movement of the south flank has been going on for a very long time, and "the basic behaviors haven't changed in any significant way," he said.

There have been suggestions the entire south flank could at some point catastrophically collapse into the sea. But the more reasonable near-term risk is powerful earthquakes like the 1975 event, he said.

"A repeat of the 1975 earthquake is out there. I think it's a matter of when, not if," Okubo said.

But no one can reasonably predict the time of that repeat earthquake.

Reach Jan TenBruggencate at jant@honoluluadvertiser.com.