Recent Seismicity at Loihi Volcano


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The 1996 Loihi Earthquake Swarm
    On July 16th, 1996, Loihi seamount began to shake with earthquakes. In the two weeks that followed, over 4000 Loihi earthquakes were detected by the Hawaii Volcano Observatory's (HVO) seismic network, making this the largest earthquake swarm ever recorded on a Hawaiian volcano.

What IS an Earthquake Swarm?
   Earthquakes are common on many active volcanoes. They may result from movement of magma within the volcano or from tectonic processes associated with the volcano readjusting itself. A swarm occurs when a large number of earthquakes are recorded in a short period time on a volcano.

What are swarms on Loihi like?
   Low level seismic activity has been documented on Loihi since 1959. In a typical month, between 2 and 10 Loihi earthquakes are detected on the Hawaii Volcano Observatory's seismic network. However, Loihi occasionally experiences an earthquake "swarm", wherein up to several hundred quakes may occur on the volcano in a matter of days. The number of Loihi earthquakes that have been recorded by the HVO network since 1959 are shown in the plot below. Large swarms took place on Loihi in 1971, 1972, 1975, 1990 and 1996.

Obviously, the swarm of 1996 was unlike any previously measured swarm.

The figure to the right shows epicentral locations for approximately 1000 of the events which took place during the 1996 swarm. The majority of events seem to cluster near Loihi's summit, while others spread over the south and west flanks of the volcano. On this plot, the red circles represent earthquakes which took place in the first few days of the swarm. Then there was a day of quiet on earthquakes were recorded on July 19th. But on July 20th, the swarm started up again with a vengeance. The yellow circles show the locations of events which took place in the later phase of activity. Notice that the early (red) earthquakes were located nearer the southwest flank of the volcano while the later (yellow) events cluster near the summit. Other events were either too small to accurately locate, or occurred almost simultaneously with other earthquakes and were therefore difficult to time.

Some of the questions we hope to answer with this data are:

Why were there two phases of activity?

Why did earthquake locations change during those two phases?

How do the earthquakes relate to the eruption and to the formation of a pit crater on Loihi's summit?

Loihi Velocity Models

   In order to determine where an earthquake took place, we need to know how fast the seismic waves move within the ground. A "velocity model" describes what we think the speed is at different depths under the ground. Not only is it one of the most important tools used in earthquake location, it is also very useful in examining the geology of those parts of the volcano that we cannot see. For example, an anomalously slow area may indicate the presence of a magma chamber (because molten rock retards the rate at which seismic waves travel) while a high-velocity region may be an area with dense, cool intrusive rocks.
Initially, we located the Loihi earthquakes using
the same velocity model used beneath the Big
Island of Hawaii and believed that the quakes
were spread along the south rift of Loihi. These
locations are shown below:

Using a new velocity model, in which seismic waves
move much more slowly beneath Loihi, we find that
the earthquake locations change. The new locations
are shown below. Notice that they are clustered
much nearer the summit.

Depths can only be constrained for a few of the 1200 events, but for those earthquakes we find that they lie at depths of 78 km beneath Loihi's summit. These depths are consistent with petrologic studies of Loihi rocks which suggest that Loihi's magma chamber lies just below same depth. This was one of our first clues that the earthquake swarm may have been directly associated with the movement of magma within the volcano.


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Last page update on 22 Jul 1998