Today I’ve noticed that a swarm is taking place near the Grindavik in the Reykjanes Peninsula, this swarm is small but is probably related to the ongoing Reykjanes Fires. I will also use this chance to talk about the recently ended Kilauea eruption, its build-up, and what the future might hold for Kilauea.

Earthquakes north of Grindavik.

There have been some 70 earthquakes in the Reykjanes Peninsula today, September 18. I should specify; 70 earthquakes that are automatically located by the Icelandic Meteorological Office. The majority of these earthquakes are happening in an area that is north of Grindavik and east of the Blue Lagoon. The high frequency of earthquakes, that they are small (all less than M 3), and the way that they cluster tightly in a volcanic area means that either magma or hydrothermal fluids are probably involved. This is also where multiple inflation events have happened in the past few years.

Time and magnitude of earthquakes for the Reykjanes Peninsula in the past 48 hours.

Balls with 50% opacity are earthquakes. Red is today. Yellow is July 3.

The main reason these earthquakes caught my attention is that a similar cluster of seismic events, on July 3, preceded a dike intrusion at Fagradalsfjall, by about a day. This dike intrusion led to the last eruption of this volcano. Now we are seeing a similar swarm. What does it mean? Maybe that we are getting another eruption, maybe that Thorbjörn will start rising again, or maybe nothing. But I think it is some kind of magma-related unrest so it will be good to keep an eye on it. But isn’t it too soon to have a Fagradalsfjall eruption? Volcanoes don’t have periodicity, they never or almost never have it, this is something I’m sure of now, so I wouldn’t be surprised if the next eruption arrives earlier than expected, given how much unrest there is already.

Kilauea

Kilauea erupts again! And the eruption is already over. The fissure opened on July 10 (Honolulu time), effused lava for 5 days, then ended and at the same time changed to rapid inflation of the caldera floor, which has already recovered over a tenth of the total eruption deflation. It is the fifth summit eruption since 2018, and the third of this year. It is also the shortest. A dike cut E-W across the down dropped block, opening a fissure. Magma also got into concentric fractures of the 2018 collapse leading to smaller fissures perpendicular to the main one.

Image by HVO showing the eruption on September 10. The view is across the downdropped block, looking from the west.

Buildup to the last eruption of Kilauea

This year we’ve seen increasing rates of deformation at Kilauea, probably the highest in a long time. The interval between the last two eruptions saw intense inflation in both Halema’uma’u (summit crater) and the Southwest Rift Zone. The Uwekahuna tiltmeter is an instrument located on the SW rim of Kilauea caldera which picks up tilting of the ground as the summit caldera swells by magma accumulation beneath. The Uwekahuna tiltmeter monitors the Halema’uma’u magma chamber, the heart of Kilauea, and the most common source of deformation. During 2019-2020, the Uwekahuna tiltmeter would pick something like 6-8 microradians of tilting per month, this is also similar to typical rates of tilting in the 1960-1983 period, during times of inflation. However, after the end of the second-last eruption, in June, a very substantial tilting of 12 microradians had accumulated in merely 15 days of recovery. This rate changed over time, but totaled ~40 microradians in the 80 days that lasted the interval between the two eruptions.

The month of July featured inflation rates barely above normal. Then, towards August 10, a massive surge of magma took place and the Uwekahuna tiltmeter skyrocketed. Within six days, the tilt went up by about 15 microradians. On August 13, as Uwekahuna approached its steepest climb, two very small intrusions hit Kilauea in quick succession. The first intrusion was probably near the northern side of the 2018 caldera, given how it was best recorded by seismometers in the north greater caldera sector. The second intrusion had many located earthquakes and was a small magma injection near the southern margin of the 2018 caldera.

Things were just heating up. Within less than a day of the second intrusion, a large swarm of earthquakes commenced in what is known as the Southwest Rift Zone Connector. The SWRZ Connector is a highly seismogenic structure that forms the heart of the SWRZ, historical dike intrusions of this rift, like those of 1974 and 1981, initiated from the connector and propagated downrift from it. A major surge of activity in the SWRZ Connector on August 14 was a sign that something major was going on. This surge lasted 2-3 days at decreasing intensity. After the surge, earthquakes were down for 3-4 days, until another surge of seismicity hit on August 21, followed by an even more significant surge on August 23. Thereafter there was heightened activity until the September eruption happened. Seismicity propagated along the SWRZ, starting near the summit and making its way southwest. The images below illustrate the seismicity during this period. The upper part of the SWRZ Connector flares up from August 14 to August 21, although most of the earthquakes happened in a brief 2-3 day period. The rest of the SWRZ joins up gradually after August 21:

At the time of the earthquakes, swelling in the Southwest Rift Zone started to gradually accelerate. After August 25 a large area of inflation along the Southwest Rift Zone was visible; in a Sentinel interferogram covering 12 days, distributed by the MOUNTS project. There is at least a full fringe towards the satellite, 2.8 cm of line of sight inflation, perhaps a bit more, it’s hard to judge because the deformation falls partly outside the map and into vegetated areas with low coherence. The deformation that falls within the map is similar to that of the August 2021 sill, and to episodes of inflation around 2006, this would be a total area of about 10 km long and 6 km wide that would be affected by the tumescence in and outside the map. The inflation is a bull’s eye pattern which usually corresponds to sill or sill-like bodies, most likely sills along the Southwest Rift Zone filling with magma.

Sentinel interferogram distributed by MOUNTS project that shows inflation of the Southwest Rift Zone. The concentric pattern in the lower half of the image is the deformation.

So what happened exactly? We have earthquakes and we have inflation, but they don’t match in time. The graph below is the SDH tiltmeter. This tiltmeter is located immediately NW from the point of maximum inflation in the Southwest Rift Zone. The green line is the radial tilt, which means how much the ground slopes away from the center of deformation. So the green line shows how much deformation was going on in the Southwest Rift Zone. There was some magma accumulation going on before the crisis, and then there was a little bump around August 15, but it was not until around August 21 that inflation started to increase, and maximum deformation rates were not reached until just before the eruption happened. The strongest pulses of seismicity, are those of August 14, August 21, and August 23, which happen with very little inflation. That means the volume change driving the tumescence of the SWRZ is different from the process driving the earthquakes. There is probably more than one interpretation here, but what I think we see here is an accumulation of magma in a dike-like body, ~6-2 km below the surface, known as the deep rift, which is believed to underlie Kilauea and drive the flank slip. The earthquakes are caused by the stretching of the rock above this body as waves of magma start coming into it from the summit. As time goes on, more and more magma from the deep rift starts traveling upwards and inflating the shallow sills in the Southwest Rift Zone, I believe what we see here is a magma surge that sequentially activates different parts of the volcano, first affects the summit of Kilauea around August 10-13, gradually enters the deep rift to the SW of the summit, and finally starts filling up the shallow sills above the deep rift.

The future of Kilauea

Kilauea’s future is bright and hot right now. Kilauea’s supply seems to be rising this year thus far, so it’s not unlikely it will erupt a 4th time before the year is over. Right now the summit crater floor is at an elevation of 930 meters above sea level. Rift eruptions of Kilauea have taken place with an elevation of 950 meters or more at the summit crater. This is why I think Kilauea will probably try to fill those 20 meters of caldera first, and then we will likely see a resumption of rift activity. Another Halema’uma’u eruption then? Probably. However, the Southwest Rift Zone has seen some strong activity, so it is not unlikely it will see an eruption soon, or at least a non-eruptive dike intrusion. For now, the East Rift Zone is completely quiet and this is the only certainty there is, although even this could change relatively quickly.