Volcano Watch — HVO maintains wide seismic network on the Big Island

The Hawaiian Volcano Observatory maintains a network of about 50 seismic stations on the Island of Hawai`i. The purpose of the network is twofold: to define the regional seismicity of the Island of Hawai`i, and to distinguish and study earthquakes as they relate to major fault zones and to the eruption and intrusion of magma. The seismic network forms the backbone of our capabilities to forecast eruptions and locate large earthquakes.

Station locations are shown on the attached map of Hawai`i. At each station, there are seismometers, sensitive devices that detect even weak ground motion, electronic radio equipment to transmit the ground motion data to the observatory, where all the data are gathered for analysis, and a power supply. Most of our stations are powered by small solar panels and a battery to store power for continued operation at night and during rainy periods. As seen on the map, most of the stations are located on Kīlauea and Mauna Loa volcanoes because they are where earthquake activity, also called seismicity, is concentrated. Moreover, many stations are located along the active rift zones of these volcanoes to allow us to track the movement of magma within the volcanoes. We plan to install additional stations on Mauna Loa Volcano in the next year to improve our ability to monitor seismic activity that should precede and forewarn us of the next eruption.

The seismic data transmitted to the observatory from our field stations are recorded on seismographs, or paper recorders, and simultaneously converted to digital data for storage on computers. The computer system is used to locate and to determine the magnitude, in near real time, of the larger earthquakes (those that trigger a number of stations). Display monitors located in the observatory and in the Jaggar Museum plot the locations of earthquakes within minutes of their occurrence.

Earthquake swarms commonly precede eruptions or intrusions of magma. The earthquake locations, or hypocenters, typically migrate ahead of the intruding dike of magma. This pattern of seismicity often reveals the probable location of the eruptive outbreak. The seismic swarm preceding the January 1983 eruption on the East Rift Zone of Kīlauea allowed seismologists to trace the magma as it moved down the East Rift Zone to the eventual eruptive site, where a ground observer was in position to view the eruption as it broke the ground surface just after midnight on the morning of January 3, 1983. Seismic swarms also occur as aftershock sequences following large earthquakes that are not directly related to magma movement. In the case of aftershocks, the earthquake locations generally do not migrate with time, but they define a broad region of adjustment about the hypocenter of the main shock.

Seismic monitoring at the observatory has distinguished two main classes of earthquakes, volcanic and tectonic. Although all earthquakes associated with active volcanoes are ultimately related to volcanic processes, volcanic earthquakes are directly related to magma movement, whereas tectonic earthquakes occur in zones not spatially associated with magma movement. Tectonic earthquakes in Hawai`i share characteristics with earthquakes in other non-volcanic systems, such as the San Andreas fault zone in California.

Most of the larger earthquakes in Hawai`i are tectonic in origin and are caused by seaward sliding of the flanks of the volcanoes. The fault plane that ruptures in these large events is similar to the San Andreas fault except that it is a near-horizontal plane, whereas the San Andreas fault is a near-vertical one. The zone where such ruptures occur is located roughly at the base of the volcano at a depth of about five to seven miles, between the overlying volcanic pile and the underlying lavas and sediments of the sea floor.

n a typical week, the seismic network records about 200 small earthquakes beneath Kīlauea Volcano, and another 150 beneath Mauna Loa Volcano. During an intrusive or eruptive swarm, such as occurred on March 3 of this year, we may record several thousand earthquakes in a single day. Few of these recorded earthquakes are large enough to be felt (generally, earthquakes larger than about magnitude 3.0 are felt).

The seismic stations also tell us whether the eruption at the episode 51 vents is active or not. The nearest station, located about one mile uprift from Pu`u `O`o, records the ground vibration, or tremor, associated with magma movement through the vent. This record of tremor tells us that the eruptive activity at the episode 51 vents has been continuous from before dawn on Sunday, June 21, to Thursday morning, July 2, when this column was written. Most flow activity has been directed toward the south, although during periods of large volume effusion, flows fed by overflows of the pond spread to the north and west as well. The lava lake inside Pu`u `O`o remains active at a depth of about 165 feet below the rim of the cone.