# Volcano Watch — Natural hazards in the Kau District

March 5, 1998

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

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

Most of Kau 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. Kau 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 Kau 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 Mokuaweoweo (Mauna Loa's caldera) to Highway 11, passing northeast of Wood Valley and including the western part of Kapapala ahupuaa. The other zone-6 area extends from Ka Lae northeast to Honuapo and mauka to about the 900 m (3,000 ft) elevation; it includes Naalehu, Waiohinu, and much of the Kaunamano and Kioloku ahupuaa. Lava flows covered the two zone-6 areas about 750-1,500 years ago and more than 3,000 years ago, respectively. Kamaoa Puueo and the area between Honuapo and Waiohinu 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 Kaalela.

Lava flows are only one hazard in Kau. Earthquakes are another. The largest earthquake in Hawaii in at least the past 175 years was centered in Kau, 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 Hawaii, 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 Kau. 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 Kau. Many are grouped into what is called the Kaoiki-Honuapo fault system, which reaches from the Naalehu-Waiohinu 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, Kau stands out in terms of its natural hazards. Past eruptions and earthquakes have shaped the land of Kau 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 Puu Oo 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).