Big Island is no stranger to damaging quakes

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Prior to the magnitude-6.7 Kiholo Bay earthquake in October, 17 years had passed since we were last rattled by a magnitude-6 or greater earthquake-long enough for all of us to get complacent. That 17-year period was the longest in over 180 years in which the Big Island avoided any damaging earthquakes.

Big Island is no stranger to damaging quakes...

Two children were trapped in this damaged home about 7 km north of Hilo. Photograph by UPI. November 29, 1975.

(Public domain.)

Earthquakes occur every day on the island of Hawai`i. Most are less than magnitude-3, but rarely a week goes by without a quake large enough to be felt somewhere on the island. Most of them are associated with incremental movement of the south flank of Kīlauea Volcano towards the sea. However, large earthquakes can and do occur beneath nearly all parts of the island.

The magnitude-7.2 Kalapana earthquake of 1975 holds the record for Hawai`i's largest earthquake of the last century. It generated a local tsunami that claimed the lives of two campers at Halape on Kīlauea's south coast, where the run-up height of the waves was almost 15 m (50 ft) above sea level. The shaking also triggered a 17-hour-long eruption in Kīlauea's summit caldera.

The 1975 earthquake pales, however, beside the great earthquake of April 2, 1868, which had an estimated magnitude of 7.9. The epicenter was located about 5 miles north or northeast of Pahala. This event probably involved seaward slip of the south flanks of both Mauna Loa and Kīlauea. The main shock was preceded on March 28 by an earthquake with an estimated magnitude of 7.1 located near Wai`ohinu.

Reverend Titus Coan, who witnessed this great earthquake wrote, "In the district of Ka'u more than three hundred shocks were counted upon this terrible day; people were made seasick by their frequency. By the culminating shock, nearly every stone wall and house in Ka'u was demolished in an instant."

The death toll from the 1868 earthquake was 79. Thirty-one were killed by a mudslide in Wood Valley, two by falling rocks near Hilo, and the rest were victims of a local tsunami that erased several coastal villages.

The Ka`oiki fault zone, on the south slope of Mauna Loa, is also the site of recurring earthquakes. Five earthquakes greater than magnitude 6 occurred in this area between 1941 and 1983.

The largest of these was the 1983 magnitude-6.6 Ka`oiki quake, which caused widespread damage from Volcano to Hilo. Numerous landslides were triggered in nearby Hawai`i Volcanoes National Park and, as during the recent earthquake, along the Hamakua coast and at Kealakekua Bay.

While all of the quakes described above were located beneath the active volcanoes of Mauna Loa and Kīlauea, there have been plenty of precedents for damaging quakes on the west and north sides of the island.

In 1973, a magnitude-6.2 earthquake occurred just offshore of Honomu on the Hamakua coast, injuring 11 people. Like the October 15 Kiholo Bay quake, the Honomu quake was located at a depth of 39 km (24 miles). Both were probably responses to the immense load piled on the crust beneath Hawai`i by our ever-growing volcanoes.

The third largest earthquake in the last 180 years struck the central Kona coast in August 1951. This magnitude-6.9 earthquake was located several miles offshore of Kealakekua Bay at a depth of about 8 km (5 miles). Damage in Kona was widespread, particularly from Kealakekua to Ho`okena. Nearly 200 water tanks and miles of stone walls collapsed, and landslides blocked roads. Two people were injured, both by broken glass. This earthquake was apparently caused by seaward sliding of a large part of the western flank of Mauna Loa.

The aftershocks from the Kiholo Bay earthquake are gradually waning, but residents on the west and north sides of the island are unlikely to forget the effects of this event any time soon. For the rest of us, this was a wake-up call to take a good look at our homes and workplaces and do all we can to prepare for the next hard shake.

Inevitably, more earthquakes will come, and some will be stronger than last month's. Let's not count on another 17 years to get ready for the next big one.

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Volcano Activity Update

This past week, activity levels at the summit of Kīlauea Volcano have remained at background levels. The number of earthquakes located in the summit area is low (usually less than 10 per day that are large enough to locate). Widening of the summit caldera, indicating inflation, appears to have stopped since early October.

Eruptive activity at Pu`u `O`o continues. On clear nights, glow is visible from several vents within the crater. Lava is fed through the PKK lava tube from its source on the southwest flank of Pu`u `O`o to the ocean. About 1 kilometer south of Pu`u `O`o, the Campout flow branches off from the PKK tube. The PKK and Campout tubes feed two widely separated ocean entries, at East Lae`apuki and East Ka`ili`ili, respectively. Both entries are located inside Hawai`i Volcanoes National Park.

In the last week, intermittent breakouts from the Campout tube have been scattered from the 200-to-1,000-ft elevations.

Access to the sea cliff near the ocean entries is closed, due to significant hazards. The surrounding area, however, is open. If you visit the eruption site, check with the rangers for current updates, and remember to carry lots of water when venturing out onto the flow field.

There were four earthquakes beneath Hawai`i Island reported felt within the past week. A magnitude-2.7 earthquake occurred at 2:55 p.m. H.s.t. on Friday, November 3, and was located 2 km (1 mile) northeast of Pu`ulena Crater in Puna at a depth of 2 km (1 mile). A magnitude-3.2 earthquake occurred at 10:39 a.m. on Monday, November 6, and was located 11 km (7 miles) southeast of Waimea at a depth of 14 km (9 miles). A magnitude-2.9 earthquake occurred at 1:57 p.m. on Wednesday, November 8, and was located 10 km (6 miles) southwest of Waiki`i at a depth of 12 km (8 miles). A magnitude-2.5 earthquake occurred at 6:02 a.m. on Tuesday, November 9, and was located 22 km (14 miles) southwest of Waikoloa town at a depth of 30 km (19 miles).

Mauna Loa is not erupting. During the past week, earthquake activity remained low beneath the volcano's summit (one earthquake was located). Extension of distances between locations spanning the summit, indicating inflation, continues at slow rates.