Volcano Watch — Natural hazards in the Ka`u District

In recent months we have discussed hazards from lava flows in the Hilo and Puna areas. Today we focus on the Ka`u District.

Most of Ka`u is in lava flow hazard zones 1, 2, and 3, according to the map prepared by HVO and State workers and available in public libraries or on the internet. By definition, more than 25 percent of zone 1 has been covered by lava since 1800 and more than 65 percent in the past 750 years. For zone 2, those numbers are 15-25 percent and 25-75 percent, respectively. For zone 3, 1-5 percent of the area has been inundated since 1800 and 15-75 percent in the past 750 years.

These comparatively high rates of inundation result from the obvious. Ka`u contains the calderas of Mauna Loa and Kīlauea, each volcano's southwest rift zone, and part of Mauna Loa's northeast rift zone and Kīlauea's east rift zone. These are the most likely areas to be covered by lava, so they are placed in zone 1. Zone 2 includes areas adjacent to and downslope of the rift zones. Zone 3 is gradationally less hazardous than zone 2, because of greater distance from recently active vents and/or because the topography makes it less likely that flows will cover the area. HOVE is in zones 1 and 2 and Pahala and Kanelelu Flat in zone 3.

Outside the National Park, only two parts of Ka`u are considered less susceptible to flow inundation than zone 3. Each is relatively protected by topography, has not been covered by lava in the past 750 years, and is placed in hazard zone 6. One area reaches southeastward from Moku`aweoweo (Mauna Loa's caldera) to Highway 11, passing northeast of Wood Valley and including the western part of Kapapala ahupua`a. The other zone-6 area extends from Ka Lae northeast to Honu`apo and mauka to about the 900 m (3,000 ft) elevation; it includes Na`alehu, Wai`ohinu, and much of the Kaunamano and Ki`oloku ahupua`a. Lava flows covered the two zone-6 areas about 750-1,500 years ago and more than 3,000 years ago, respectively. Kama`oa Pu`u`eo and the area between Honu`apo and Wai`ohinu have been free of lava flows for more than 10,000 years. Mahana Bay, site of the green sand beach, is located in one of these old flows, as is Ka`alela.

Lava flows are only one hazard in Ka`u. Earthquakes are another. The largest earthquake in Hawai`i in at least the past 175 years was centered in Ka`u, probably near Kapapala though the exact location is not known. This earthquake took place in 1868 and had a magnitude of 7.9 as estimated from damage reports. By comparison, the 1975 earthquake, the largest in recent memory in Hawai`i, had a magnitude of only 7.2. The 1868 earthquake, giant even by California standards, caused extensive damage on the Big Island and especially in Ka`u. It leveled houses, started a large landslide in Wood Valley that killed 31 people, and caused the distant coast of Kīlauea to suddenly sink into the ocean and generate a tsunami that destroyed villages near the shore.

The geologic map shows a number of faults in Ka`u. Many are grouped into what is called the Ka`oiki-Honu`apo fault system, which reaches from the Na`alehu-Wai`ohinu area northeastward to the Mauna Loa strip road. Many readers will recognize the faulted, stair-stepped slopes on Mauna Loa while driving from the National Park to Pahala. This system is seismically active and generates hundreds of small earthquakes yearly and damaging earthquakes every few years.

Another fault, the Kahuku fault, forms Pali o Kulani and Pali o Mamalu and extends another 20 km (12 mi) offshore. The pali forms a natural barrier to keep lava from spreading eastward, but it is also a potential source of earthquakes.

Of all the Big Island's districts, Ka`u stands out in terms of its natural hazards. Past eruptions and earthquakes have shaped the land of Ka`u and serve as our best guides to the future.

Volcano Activity Update

The east rift zone eruption of Kīlauea Volcano continued unabated during the past week. The lava enters tubes near the Pu`u `O`o vent and flows continuously to the coast. Lava escapes sporadically from the tube system along the coastal plain, but the flows have been of limited extent. The tube system discharges lava into the ocean at two locations - Waha`ula and Kamokuna. The public is reminded that the ocean entry areas are extremely hazardous, with explosions accompanying the frequent collapses of the lava delta. The steam clouds are highly acidic and laced with glass particles.

Two earthquakes were reported felt during the past week. On February 28 at 6:03 p.m., an earthquake located 2 km (1.2 mi) northwest of Kainaliu was felt by residents throughout Kona. The earthquake had a magnitude of 4.0 and originated from a depth of 13.8 km (8.2 mi). A resident of Orchid Isle Estates subdivision reported feeling an earthquake at 7:32 p.m. on Monday, March 2. The magnitude 1.6 earthquake was located 18 km (11 mi) southeast of Kīlauea summit at a depth of 4.5 km (2.7 mi).