Volcano Watch — Volcano scientists monitoring rash of quakes

Beginning on October 21 and continuing through October 31, residents in parts of lower Puna have felt a sequence of small earthquakes. The earthquakes appear to have stopped, but we continue to monitor the area carefully. Following a particularly sharp event at 6:35 p.m. on Sunday, October 30, residents called and reported the activity to Hawaii County Civil Defense and the Hawaiian Volcano Observatory. The events were unusual in that several callers indicated that they heard a boom or explosion before the earthquake shook their homes. All the reports came from a small area along upper Nanawale Boulevard in the Nanawale subdivision. Several callers indicated that the noise came from the general direction of Puna Geothermal Ventures (PGV) property.

The earthquakes were located between zero and four kilometers beneath the north side of Kaliu Cone in Leilani Estates and were not related to any activities at PGV. All the earthquakes had magnitudes of less than 1.5 on the Richter scale, surprisingly small magnitudes to be felt at all, much less in Nanawale Estates, which is located nearly two miles to the north. No reports came from residents of Leilani Estates, located directly above the earthquakes. Earthquakes are common in this area, with several small ones occurring each day.

The similarity in reports between the earthquake sequence in the last two weeks and those that preceded the last two eruptions in lower Puna led us to inform Hawaii County Civil Defense that there were several possible causes of the earthquakes: they could be (1) minor, unimportant readjustments within the rift zone, (2) precursors to an eruption, or (3) triggered by changes in activity at PGV. The last possibility seemed unlikely because the earthquakes were not located around PGV production or re-injection wells. However, we wanted to eliminate PGV activities as a possible cause of the earthquakes; a call to PGV determined that there had been no changes in their activities that could trigger small earthquakes.

Like the present earthquakes, most of those early in the sequences before the 1955 and the 1960 eruptions had small magnitudes, yet many were felt by local residents, and many were reported to have made booming or explosion-like noises. The frequency and magnitudes of the earthquakes increased dramatically in the weeks and days preceding both eruptions. The previous eruption in lower Puna before 1955 occurred in 1840, so there are no records of precursory seismic activity.

Prior to the beginning of the eruption that began on February 28, 1955, infrequent earthquakes began during the preceding October; activity remained low until December, when several moderate earthquakes were felt in lower Puna near Pahoa. During that period, the Observatory recorded about three earthquakes each day. The eruptive fissures eventually extended along the rift from Heiheiahulu to Kapoho Crater, with most of the activity near the ends of the fissure.

The 1960 eruption, which began in Kapoho on January 13, was preceded by only three weeks of seismic activity in lower Puna. The earthquakes began in the last week of December 1959 but declined during the first week of January. The following week, the locations of the earthquakes migrated down rift towards the ocean at Kapoho; the earthquakes also became more frequent and larger.

Because the earthquakes were so small and infrequent, we monitored them continuously for any increase in frequency or in magnitude. We kept Civil Defense informed throughout the week. In addition, we made some adjustments and repairs to a seismic station in Nanawale Estates, one of five stations we operate in lower Puna, in order to locate the earthquakes more reliably. We also did a reconnaissance survey of all the roads in Leilani Estates to look for new cracks and found none. We occupied all our tilt stations in the area and remeasured ground tilt for any changes since we last made measurements there. This work served to familiarize several new staff members with the area in the (unlikely) event that activity increased. The earthquake sequence appears to be over; they appear to have been caused by minor adjustments within the rift zone - - no cause for alarm.

An interesting aspect of this earthquake sequence is that they were felt at all. The area along Nanawale Boulevard appears to have amplified the ground motion. The ground commonly responds to earthquake shaking very differently at nearby, even adjacent, locations. In other areas, like the San Francisco or Los Angeles regions, thick sediments amplify ground shaking, whereas solid rock engenders less intense shaking. In 1983, the magnitude 6.7 Kaoiki earthquake caused significant damage to the Kaumana section of Hilo, where houses are built on thick soils, whereas nearby homes on Mauna Loa lava flows were largely unaffected. The measured ground motions in Hilo for this earthquake varied by a factor of five. Such non-uniform response of the ground is why fine-scale zoning for earthquake shaking is so difficult to do. In Puna, all sites are essentially solid rock, because no areas have thick sediments or soils.However, non-uniform response still occurs, apparently caused by underground structures. Such variations in response are not yet well understood.

The response also varies with the location of the earthquake, so that earthquakes with different locations, each the same distance from the recording site, cause different responses. In other words, if the locations of the earthquakes these past two weeks had been slightly closer or farther from Nanawale Estates, the ground shaking might have been far less.

We will continue to closely monitor the earthquake activity in lower Puna until we are confident that the sequence is completely over. We appreciate the reports called in by citizens like Tony, Dennis,and Julian in Nanawale Estates; they provide valuable information. If you feel an earthquake, call us at 967-7328 and tell us or record on our after-hours answering machine your location, the time and day of the earthquake, and any other observations about the earthquake. Reports made several days after an event are still valuable.

Volcano Activity Update

The eruption along Kīlauea's east rift zone continues with numerous active surface flows visible along the Pulama Pali and on the coastal lowlands near Lae'apuki. By Friday, the flows had once again reached the ocean near the west edge of the flowfield. The tube that fed flows to the ocean in July 1993 has not been reoccupied below the top of Pulama Pali at 2,000 feet, after the pause in the eruption that ended on October 26. The early 'a'a flows on the pali have now been largely overrun by pahoehoe flows, and the flows entering the ocean are sheet-like pahoehoe flows. Within the next few days, a new tube will form beneath the pahoehoe crust, and the flows will consolidate to a narrower entry at the coast.