Volcano Watch — M5.6 Pahala earthquake

After an earthquake felt by residents of Hawai`i County, we at the U S Geological Survey's Hawaiian Volcano Observatory are often asked what the earthquake means or indicates in terms of the volcano's behavior. It is beyond our ability to know the detailed implications of a single earthquake event for the complex and inter-related volcanic and tectonic processes that shape our island. In response to public inquiries about specific earthquakes, then, sometimes our best response is, "It's a gentle reminder that we live in a geologically active region and that we can expect, on occasion, larger and potentially damaging earthquakes in the future."

Unfortunately, our reminders are not always so gentle - like the M5.6 that occurred beneath Pahala at 2:56 p. m. on Friday, April 16. As described in newspaper and television reports, this earthquake destroyed homes and interrupted the delivery of power to the affected region. Elsewhere, there were reports of small landslides or rockfalls near steep cliffs.

Seismologically speaking, a M5.6 earthquake is not a large earthquake. It's not even large for Pahala, which lies where the Ka`oiki and Hilea fault zones on Mauna Loa's southeast flank come together. In 1868, the strongest earthquake historically recorded in Hawaii, estimated to be M7.9, is thought to have originated near April 16's M5.6 epicenter. While the M5.6 caused damage restricted to the earthquake's epicentral region, larger earthquakes like the November 16, 1983, M6.6 Kaoiki earthquake, the November 29, 1975, M7.2 Kalapana earthquake, as well as the 1868 earthquake can cause widespread damage, possibly affecting all of Hawai`i County.

Sometimes, our telephone conversations with the general public end with, "You'll let us know when the Big One is coming, won't you?" It would be terrific if we could. However, it has proven to be difficult, if not impossible, to correctly and consistently predict the times, locations, and magnitudes of earthquakes.

With the knowledge that we've had damaging—and devastating—earthquakes in the past, and our gentle and not-so-gentle reminders that the volcanoes and island continue to be very active geologically, we can reasonably expect that more Big Ones will come. So, what can we do to try to protect ourselves and our communities against future earthquakes?

If we choose to live with the hazards posed by damaging earthquakes, we ought to be prepared to experience them and be prepared to respond, if necessary, after they occur. There are numerous steps that we can take to mitigate against the hazards that we are exposed to and to minimize the hardships and costs of putting our homes and communities back together after a disaster. A great deal of information regarding personal earthquake safety is available from the Hawai`i County Civil Defense Agency, the Center for the Study of Active Volcanoes at UH-Hilo, and public libraries, as well as the USGS.

Other important tools in earthquake hazards and risk management and post-disaster recovery relate to appropriate zoning, planning, and design. Currently, the building code for Hawai`i County is based on the 1991 Uniform Building Code (UBC). In the 1997 edition of the UBC, the International Conference of Building Officials has upgraded its seismic zonation of Hawai`i County from Seismic Zone 3, as in the 1991 UBC, to Seismic Zone 4. This determination was based in part on recent USGS studies of earthquake occurrence and how seismic energy propagates through the island, and it signifies that Hawai`i lies in a region exposed to the highest seismic hazards considered in the UBC.

We expect that, later this spring, the Hawai`i County Council will be looking into adopting parts of the 1997 UBC. These will relate to the 1997 UBC seismic provisions for earthquake-resistant design with Hawai`i County included in Seismic Zone 4. As we examine the effects of April 16's earthquake and recall our experiences from other damaging earthquakes, it is essential that we use what we know to develop effective mitigation programs designed to prevent or minimize the casualties and losses that might result from future earthquakes that we know will occur.

Volcano Activity Update

Lava continues to erupt from Pu`u `O`o and flow through a network of tubes from the vent to the sea near Kamokuna, in Hawai`i Volcanoes National Park. A lava flow that oozed from a tube on the coastal plain has remained active; although sluggish, it has advanced to about 500 m of the coastline. Lava flows are also commonly visible at the bench, where lava enters the sea. Explosive activity there has been less frequent than in previous weeks. The public is reminded that the ocean-entry area is extremely hazardous, with explosions accompanying unpredictable collapses of new land. The steam clouds are highly acidic and laced with glass particles.

There were numerous earthquake reports for the week ending on April 22. The April 16 magnitude-5.6 earthquake, which occurred at 2:56 p.m. 7 km (4 mi) north of Pahala, was felt islandwide. Consequently, there were many felt aftershocks of magnitude less than, or equal to, 3.5. On April 17 at 10:11 p.m., a magnitude-3.7 earthquake occurred near Pu`ulena Crater at a depth of 1.5 km (.9 mi) and was felt in lower Puna. On April 18 at 1:06 a.m., a magnitude-4.0 earthquake, which occurred beneath Hana at a depth of 16.7 km (10 mi), was felt in Olinda, Haiku, Kihei, and Hana, Maui. On April 19, a magnitude-3.1 earthquake 7 km northwest of Pahala, located at a depth of 10 km (6.2 mi), was felt in Pahala and Hilo.