# Volcano Watch - 30th earthquake and accelerogram anniversary

Release Date:

April 26, 2003, marks the 30th anniversary of the magnitude M6.2 Honomu earthquake. This earthquake occurred at 10:26 a. m. and was centered north of Hilo, beneath the town of Honomu. The earthquake caused nearly \$6 million in damage, to roads and highways, lifelines and utilities, and over 400 homes and businesses.

Photos of the accelerograph instruments. SMA instruments were installed and operated in Hawaii County, beginning in 1973. Earthquake strong motions are recorded on photographic film. Strong shaking triggers the operation of the instrument which includes turning on small lamps that trace the ground motions and advancing the film stored in a cannister. Records would be retrieved after a significant earthquake or, more, typically, yearly on scheduled maintenance visits. (Public domain.)

Photos of the accelerograph instruments. ANSS instruments will be installed in Hawaii County beginning in 2003. They are capable of transmitting their data continuously in real-time over digital radio links. Alternatively, they can record their data to on-board flash memory cards. HVO and the USGS will operate the ANSS strong motion instruments in continuous telemetry mode so that data from large earthquakes will be available in real time.

(Public domain.)

Long-time island residents can remember their whereabouts - at school, at work, perhaps shopping - when the earthquake struck. Fortunately, the earthquake did not take any lives, although a number of people were injured.

Quite memorable because of its effects, the 1973 Honomu earthquake is also historically significant in our studies of Hawaiian earthquakes. The first strong-motion accelerograms in Hawaii were recorded from this earthquake, obtained from two strong-motion accelerographs - which we will describe below - installed just months before the April 26 earthquake.

To explain what an accelerograph is, we first remind you that the Hawaiian Volcano Observatory's seismic monitoring program is built around our ability to record even the very smallest earthquakes or tremors that accompany magma movement within the volcanoes. We operate our instruments at high sensitivities and usually in very remote locations, where they are largely unaffected by cultural activity. This style of seismic monitoring in Hawaii dates back almost a century and has allowed scientists to develop fundamental understandings of how volcanoes and earthquakes work.

We can be thankful that we experience many, many more small earthquakes than large, damaging earthquakes, like the1973 Honomu M6.2, 1975 Kalapana M7.2, or 1983 Kaoiki (Volcano Golf Course) M6.6. However, when we do record larger earthquakes on our traditional volcano monitoring systems, our instruments saturate, or "clip," such that the full range of ground shaking is not recorded. This makes it impossible to describe in detail how the Earth breaks and subsequently shakes during large earthquakes. Even when we're too close to smaller earthquakes, out instruments lose track of the shaking because of the clipping.

Earthquake strong motions are recorded on very stiff instruments, termed "accelerographs" designed to be relatively insensitive to small earthquakes but capable of accurately recording ground motions produced by earthquakes up to magnitude 8 or greater. This also means that strong- motion accelerographs are not compromised by traffic or cultural activity and can therefore be operated within our communities and "built environment," where our concerns for life and safety are greatest.

There are now 27 strong-motion accelerographs in operation in Hawaii County. These are installed principally in public buildings, and through time, they have provided data that have proven to be critical to modern estimates of earthquake hazard in Hawaii. There are now dozens, if not hundreds, of strong-motion accelerographs operating in metropolitan areas of the Pacific Northwest and in California, affording greater insights into earthquake process and earthquake effect in those regions.

In Hawaii, additional buildings or other structures, such as bridges, ought to be equipped with strong-motion systems. When, and not if, we have another large earthquake, their data can help point to where emergency response efforts might be focused, which bridges might not be safe to cross, or which buildings might need to be evacuated.

It is also important to have such instruments installed on the volcanoes, within or close to the large earthquake source regions, to better describe what the larger earthquakes do and how they affect our volcanoes. Strong-motion data can also help improve our ability to assess potential local tsunami threat. Improvements in instrument design and technology now make it feasible for strong-motion instruments to be operated more efficiently and even placed in remote locations.

In 2001, the U S Geological Survey Earthquake Hazards Program started a project called the Advanced National Seismic System (ANSS). Among its goals are broader installation, implementation, and upgrade of strong-motion recording capabilities. If ANSS is supported, it will speed our progress toward our monitoring goals. We'll accept progress, however rapidly it occurs. But, we hope it is sooner, rather than later.

### Volcano Activity Update

Eruptive activity at the Puu Oo vent of Kilauea Volcano continued unabated during the past week. The flows on Pulama pali are frequently visible at night as streams of incandescence from the top of the pali down to the coastal flats. The east arm of the Mother's Day flow split in two with the western segment being more active. The new ocean entry near Laeapuki only lasted a day before the flow stagnated. Scattered surface breakouts are found throughout the inflating Kohola flow, especially on the west side. The National Park Service has marked trails out to the closest activity. Lava now enters the ocean only at the West Highcastle delta from two sites.

The public is reminded that the ocean entry areas are extremely hazardous, with explosions accompanying sudden collapses of the new land. The steam clouds are highly acidic and laced with glass particles. The National Park Service has erected a rope barricade to delineate the edge of the restricted area. Do not venture beyond this rope boundary and onto the lava deltas and benches.

No earthquakes were reported felt during the past week.

Mauna Loa is not erupting. The summit region continues to inflate slowly. Seismic activity remains low with only one earthquake located in the summit area during the last seven days.