(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)
The Southwest Rift Zone (SWRZ) of Kilauea is often overlooked as a source of eruptions, because the volcano’s East Rift Zone is so much more active. The SWRZ, however, is far from dead. It has had recorded eruptions in 1823, 1868, 1919-1920, 1971, and 1974. These eruptions occurred before the advent of GPS, so most of the information we have on pre-eruptive sequences and conduit geometry comes from seismology.
To get an estimate of the pathways that magma takes in the Southwest Rift Zone, we can look at the location of shallow earthquakes recorded by HVO since 1959. Those earthquakes outline a structure that emerges from the south part of Kilauea Caldera, trending due south toward the Koa`e Fault Zone. The earthquakes then trace a structure that trends southwest along the Koa`e Fault Zone to a region about halfway to the coast. Interestingly, the structure that is outlined by shallow earthquakes is offset to the south by 3–4 km (2-2.5 mi) from the line of eruptive vents at the surface of the Southwest Rift Zone.
By looking at the earthquakes that occurred prior to eruptions in 1971 and 1974, we may be able to gain clues into what to expect before the next SWRZ eruption. Prior to the September 1971 eruption, several swarms occurred months ahead of time, especially in the summit caldera and in the area to the south. Seismicity also extended sparsely to Mauna Iki and, at times, south to the coast. This was interpreted as a slow magma intrusion into the SWRZ.
The summit was highly inflated, and a short-lived summit eruption occurred just a month before the 1971 SWRZ eruption. When the 1971 SWRZ eruption finally started, earthquakes were shifted west of the pre-eruption earthquakes in locations between surface fissures. The earthquakes migrated downrift at about 500 m/hr (0.3 mi/hr). The pattern of seismicity and summit tilt suggests that the 1971 eruption was actually fed from the shallow part of the summit reservoir to the west of the typical SWRZ earthquake source area.
In contrast, the 1974 eruption follows the structure most obviously outlined by historical seismicity. About 7 days before the eruption, a vigorous earthquake swarm—associated with deflationary tilt and, very likely, a small intrusion into one or both rift zones—occurred in the Upper SWRZ and Upper East Rift Zone. Beginning on December 31, another swarm began at the eruption site south of the summit caldera at about 2.5-km (1.6 mi) depth.
Tremor soon replaced discrete earthquakes, and about 9 hours after the swarm began, an eruption occurred on the uppermost SWRZ. Seismicity continued downrift, following a propagating dike, for approximately 14 km (9 mi) at a rate of about 1.3 km/h (0.8 mi/hr) to the Kamakai`a Hills. No extrusion of lava was recorded away from the initial eruption site, though seismicity continued downrift of the dike terminus for weeks after the eruption.
Several earthquake swarms have been recorded on the SWRZ since 1974. In 2006, inflation associated with an increase in shallow seismicity in the SWRZ was detected using GPS and satellite data. Even over the past month, there has been a small increase in the number of earthquake to the south of the summit caldera (hvo.wr.usgs.gov/seismic/volcwe…).
In the future, when eruptive activity returns to the SWRZ, scientists will look to past eruptions to give them clues to look for. Since the last SWRZ eruption in 1974, the seismic network has improved dramatically, and many new technologies, such as GPS and InSAR, have become available. That leaves HVO with many tools to forecast and monitor any future eruption in ways not previously possible. Out of those observations will come a better understanding of the mechanics of the SWRZ.