All posts by Keren_F

An amateur volcanology enthusiast with an interest in volcanic and seismic activities, viewed from the comfort and safety of my armchair. Meandered into this from using publicly available earthquake data to improve my Excel modelling skills. Then had to research the results both to understand and verify them. :)

Hekla – The Hood

June 12, 2020 Keren_F

Good Afternoon!

Having visited Katla, let’s go further north to the Eastern Volcanic Zone and take a look at Hekla.

Hekla is an active snow-covered elongated stratovolcano, lying at the southern end of the Eastern Volcanic Zone in Iceland in a rift transform junction. Following the 1104 AD eruption, Hekla was called “The Gateway to Hell” – a name that stuck until the 19^th century.

Fig 1: Hekla 22 June 2014; photo by Evgenia Ilynskya (see below for source).

The Hekla volcanic system comprises a 1490m high central volcano and a 60 km fissure swarm. The Heklugjá fissure, 5.5 km long cutting across the central volcano, is the site of many eruptions and gives Hekla its elongated shape. The Vatnafjöll fissure system, 40 km long and 9 km wide is considered part of the Hekla volcanic system. Hekla may have a small magma reservoir 4 km below the surface. She has permanent snow cover but no large glacier.

Fig 2: Hekla’s central volcano, fissure system and some lava flows. Retrieved from Icelandic Volcanoes (see Sources below). GPS and seismic stations are included so we can identify earthquake locations in later plots.

Hekla’s lavas differ from the rift zone volcanoes; her lavas are andesite, basaltic andesite, basalt / picro basalt, rhyolite and dacite. She erupts tephra and silicic to intermediate lavas from the central volcano. Eruptions tend to be a short plinian / subplinian phase followed by lava flows. Larger explosive silicic eruptions have produced enough tephra for the deposits to act as time markers in dating other eruptive activity in Iceland. She is a large fluorine producer which is hazardous to livestock. The hazards listed are tephra fallout, fluorine gas, pyroclastic flows and lava flows; the absence of a large glacier means that jökulhlaups are not a major hazard for Hekla.

The fissure system produces basaltic lavas and a small amount of tephra; its hazards are listed as lava flows and volcanic gas pollution.

Eruptive History

Fig 3: 1980 Eruption of Hekla by oxonhutch at English Wikipedia: *Hekla* *,shared under* *CC-BY-2.5*

According to GVP, Hekla has had 65 Holocene eruptions ranging from VEI 1 to 5. Larsen and Thordarson state that there have been 100 eruptions in the past 9000 years, 23 of which occurred in the last millennium with VEIs ranging from 0 to 5. The central volcano produces eruptions of VEI 2 to 6, VEI 3 to 4 being the most frequent; the longer the repose time between eruptions, the larger the ensuing eruption. The fissure system produces less explosive eruptions (VEI 1 to 2). Hekla’s largest known eruption, a VEI 6, occurred between 3000 to 4300 years ago. Traces of ash from Hekla have been found in Scandinavia, Germany, Ireland and the UK.

Hekla’s eruptive style has changed over time: from effusive basalt 9000 to 7000 years ago; to large explosive silicic eruptions between 7000 to 3000 years ago; and, then smaller more frequent mixed silicic and basaltic eruptions from 3000 years ago to the present day.

The most recent eruption was in 2000, with a VEI 2 to 3, 0.01km^{3 of}airborne tephra 0.01km³and 0.12km³ of lava; it thought that magma rose through a conduit from a depth of more than 10km to 1km below before heading towards a fissure on the Hekla ridge.

Hekla has an unusually low level of seismic activity. Her largest earthquakes are in the order of 2M when dormant and 3M during an eruption. She does not give much warning of an eruption: known precursors are earthquakes 25 to 90 minutes beforehand. Monitoring has increased since the last eruption so there should be more information about any precursors to future eruptions.

Recent Seismic Activity

From the data set of earthquakes downloaded from IMO’s site for period 1 January 2016 to 31 May 2020, we extracted those for the Hekla – Vatnafjöll area: 63.7578°N, 19.4687°W to 64.0952°N, 19.9399°W. We found 1,018 earthquakes, compared to Katla’s 6,505 for the same period. The largest quake was 2.62 and the deepest 25.11km.

Fig 4: Latitude v Longitude and Latitude v Depth scatter plots of earthquakes at the Hekla – Vatnfjöll volcanic system from 1 January 2016 to 31 May 2020, plotted by the author. Green dots denote earthquakes of <2M; yellow dots, earthquakes >2M; black triangles GPS stations; and, orange triangles, volcanoes. © Copyright remains with the author; all rights reserved, 2020.

In our latitude v longitude scatter plot we can see that most earthquake activity is scattered along the fissure systems, with an E-W “cluster” to the south of GPS station HEK3, north of Hekla. The latitude v depth plot shows the activity near Vatnafjöll is occurring in the lithosphere, whereas there appears to be a conduit under Hekla. A close up of the cluster confirms this impression.

Fig 5: Latitude v Longitude and Latitude v Depth scatter plots of the cluster south of HEK3. Green dots denote earthquakes of <2M; yellow dots, earthquakes >2M; black triangles GPS stations; and, orange triangles, volcanoes. © Copyright remains with the author; all rights reserved, 2020.

Our geodensity plot of the earthquakes shows a hot spot south of HEK3, north of Hekla.

*Fig 6: Geodensity plot of earthquakes Hekla – Vatnfjöll volcanic system from 1 January 2016 to 31 May 2020, plotted by the author. © Copyright remains with the author; all rights reserved, 2020*.

We looked for swarms in the data set to see if these account for the hot spot but did not find any large ones; the hot spot appears to be an accumulation of activity over the period. Hekla seems to have a slow magma feed.

The Armchair Volcanologist

12 June 2020

Sources and Further Reading

“Hekla”, Guðrún Larsen (Institute of Earth Sciences – Nordvulk, University of Iceland) and Thor Thordarson (Faculty of Earth Sciences, University of Iceland). In: Oladottir, B., Larsen, G. & Guðmundsson, M.T., Catalogue of Icelandic Volcanoes. IMO, UI and CPD-NCIP. Retrieved from Icelandic Volcanoes: http://icelandicvolcanos.is/?volcano=HEK

Fig 1: Photo: Ilynskaya,E. (2014 June 22). Hekla: photo 1 of 5. Retrieved from Retrieved from Icelandic Volcanoes: http://icelandicvolcanos.is/?volcano=HEK

Fig 2: Map: After Jóhannesson and Einarsson (1992), Jóhannesson and Saemundsson (1998a), Larsen et al (2013a), Base data, Iceland Geo Survey, IMO, NLSI | Base map: IMO. In: Oladottir, B., Larsen, G. & Guðmundsson, M.T., Catalogue of Icelandic Volcanoes. IMO, UI and CPD-NCIP. Retrieved from Icelandic Volcanoes: http://icelandicvolcanos.is/?volcano=HEK

Smithsonian Institution Natural History Museum Global Volcanism Program (GVP): https://volcano.si.edu

Earthquake data: Icelandic Meteorology Office: IMO https://en.vedur.is/earthquakes-and-volcanism/earthquakes

Plots are the author’s own work.

Eastern Volcanic Zone, Iceland, Mýrdalsjökull Region, South Iceland Seismic Zone

The Katla Volcanic System, Mýrdalsjökull – the not so cuddly Katla

June 9, 2020 Keren_F 1 Comment

Good Afternoon!

Continuing our theme of seismicity in Iceland, we have now reached the Mýrdalsjökull Region and are heading towards the Fire Districts in the Eastern Volcanic Zone.

We took a slight detour to set up a Glossary to explain some of the terms used here to help out. You can find it on the Menu bar.

Mýrdalsjökull lies at the southern end of the Eastern Volcanic Zone, near its junction with the South Iceland Seismic Zone. The South Iceland Seismic Zone is a transform fault system that links the West and East Volcanic Zones. The Eastern Volcanic Zone accommodates 40 to 100% of the spreading between the North American and the Eurasian Plates; the Western Volcanic Zone takes up the remainder. Active rifting on the Eastern Volcanic Zone terminates at Torfajökull volcano at the rift’s southern end. Katla, Eyjafjallajökull and more southerly volcanoes are on the Eurasian Plate.

Fig 1: Katla 1918 eruption. Image by RicHard-59 Public Domain

We updated our earthquake dataset so we are now looking at the period from 1 January 2016 to 31 May 2020. Apart from the continued swarm on the Reykjanes Peninsula, there has not been any unusual activity (to the untrained eye, at least). We used IMO’s latest earthquake map for Mýrdalsjökull as an indicator for the coordinates to extract the data for the region. This picked up five seismically active volcanic systems (Eyjafjallajökull, Hekla, Katla, Torfajökull and Vatnafjöll) and three inactive areas (Krakatindur, Þórólfsfell and Tindfjallajökull).

Fig 2: Earthquake activity in Mýrdalsjökull plotted by the author for 01/01/2016 to 31/05/2020. Red dots denote epicentres for earthquakes with magnitude under 3.0; black stars denote earthquake over 3.0; blue triangles are volcanic centres. © Copyright remains with the author; all rights reserved, 2020.

Let’s start by taking a closer look at Katla; the other volcanic centres will be covered in later posts.

The Katla Volcanic System

Katla is one of Iceland’s most active volcanoes. The volcanic system is 80 km long, made up of a 30 km wide central volcano and fissure systems. The central volcano has a 10km by 14km wide, 600m to 750m deep caldera with a 5km wide magma reservoir at a depth of 1.5 km. The Katla fissure, Kötlugjá, is located in the caldera. At the north west of the system is the Hólmsá fissure and to the north east, the Eldgjá fissure. There are also inactive fissures to the south. The Mýrdalsjökull ice cap covers most of the central volcano.

Katla’s lavas are basalt/picro basalt, rhyolite and dacite, with a few intermediate hybrids, andesite and basaltic andesite. The basaltic eruptions are the most voluminous , sourced from the mantle via a spreading rift. She has also had many dacite eruptions. Lavas from the Eldgjá fissure are basaltic.

Eruptive History

Volcanism at Mýrdalsjökull began over 800,000 years ago and at Katla, 200,000 years ago. Studies of tephra have identified 200 basaltic and 14 silicic eruptions in the last 8,500 years; unfortunately, no more is known about what happened before the end of the last ice age.

Katla’s largest known eruption was a rhyolitic VEI 6 in 10600 BC which produced more than 10 km³ of rhyolite in pyroclastic flows and airborne tephra that reached 1,300 km from the volcano. The Sólheimar ignimbrite formed from the pyroclastic flows; and the tephra is referred to as the Skógar tephra (Iceland) or Vedde Ash (Norway, after the place where it was discovered).

GVP notes 132 Holocene eruptions for Katla, which range from VEI 3 to VEI 5. All Holocene eruptions occurred in the caldera, except for the 934 AD to 940 AD eruption of the Eldgjá fissure to the north east and the Hólmsá Fires in 6600 BC. Her recent eruptive style tends to be explosive basaltic eruptions from the caldera with tephra volumes up to 2km³, accompanied by jökulhlaups (glacial outburst floods). Water from melting ice cap contributes to the explosivity of the eruptions.

The last eruption to break the ice-cap was a VEI 4 in 1918 which produced an ash column up to 14 km in height, 0.7 km³of airborne tephra, 1 km³of debris from jökulhlaups and a small volcanic fissure; no lava emission was reported. The 1625 and 1755 eruptions, both VEI 5s, produced more tephra which reached further than 1,000 km from the central volcano.

The average time between eruptions has been cited as between 40 and 80 years on average. On that basis, Katla is expected to be gearing up for another eruption in the near future.

A period of unrest started in 1999 with a jökulhlaup, seismic tremors, geothermal activity and cauldron formation. There have been more recent subglacial eruptions: a jökulhlaup occurred in 2011, accompanied by a harmonic tremor and the formation of several ice-cauldrons, was thought to be indicative of a sub-glacial eruption; and the most recent jökulhlaup was in 2017, it is not clear if this was accompanied by a harmonic tremor.

Recent Seismicity

We extracted the earthquakes for Katla from the above data set using the coordinates 63.785°N, 19.4987°W to 63.4547°N,18.6608°W. This produced 6,505 earthquakes for the period 1 January 2016 to 31 May 2020.

Our plots show most activity in the caldera, some at the Goðabunga cryptodome and a low level of activity to the south and east of the caldera. Activity in the caldera is fed from a depth of 32km.

Fig 4: Earthquake activity in Katla plotted by the author. Green dots denote epicentres for earthquakes with magnitude under 2.0M; orange circles denote earthquakes between 2.0M and 3.0M; yellow stars denote earthquake over 3.0M; blue triangles are volcanic centres; black triangles are local GPS stations. © Copyright remains with the author; all rights reserved, 2020.

Fig 5: Geodensity plot of earthquake activity in Katla plotted by the author. Black triangles are GPS stations. Orange triangle is the volcanic centre. ENTC and AUST are on the caldera rim. GOLA is near the Goðabunga cryptodome. © Copyright remains with the author; all rights reserved, 2020.

So what is causing the hot spots? We looked for earthquake swarms to see whether they are the cause of the hot spots in the geodensity plot. We used the criterion of 30 or more earthquakes in one day or in two consecutive days, which is a higher level of activity than Mýrdalsjökull’s “normal” activity. This showed 16 swarms (ref Appendix), of which five had more than 100 earthquakes; four of the five swarms were clustered (Swarms 4, 6, 7 and 12, shown below) and one was more spread out. Note: our analysis of swarms is not intended to pick up any small magma intrusions that have fewer than 30 earthquakes attached to them.

*Fig 6: Swarms 6 and 7 plotted by the author; dots denote earthquakes less than 3.0M, black stars denote earthquakes of 3.0M or more, black triangles are GPS stations*. *© Copyright remains with the author; all rights reserved, 2020.*

*Fig 7: Swarms 4 and 12 plotted by the author; dots denote earthquakes less than 3.0M, black stars denote earthquakes of 3.0M or more, black triangles are GPS stations*. *© Copyright remains with the author; all rights reserved, 2020.*

We see that Swarm 4 and Swarm 12 contributed to the hotter spots in the geodensity plots; the remaining hot spots seem to be caused by an accumulation of activity over four plus years in the data set. Without a clear map of the fissures within the caldera (Googling around did not find one), we cannot tell if the swarms coincide with known fissures. However, swarm 12 coincides with the 2017 jökulhlaup. Most of the swarms in the set appear to be part of the run up to a potential subglacial eruption. It is interesting to note that there do not appear to be any larger swarms after July 2017.

Earthquake swarms are precursors to eruptive activity. Unfortunately, as the last eruption to break through the ice-cap preceded any modern volcano monitoring, there is less certainty over what would precede another subaerial eruption, notably in respect of the intensity of swarms, magnitude of the earthquakes, jökulhlaups, and the time-frames.

It has been a while since Katla produced a large eruption. Let’s hope she sleeps for a bit longer; the world has enough to contend with at the moment.

The Armchair Volcanologist

8 June 2020.

Appendix

*Fig 8: Summary of swarms identified by the author; Swarm 0 is all earthquakes not associated with a particular swarm.* *© Copyright remains with the author; all rights reserved, 2020.*

Sources and Further Reading

Raw earthquake data: Icelandic Meteorology Office: IMO https://en.vedur.is/earthquakes-and-volcanism/earthquakes

Smithsonian Institution Natural History Museum Global Volcanism Program (GVP): https://volcano.si.edu/

“Iceland”, Thor Thordarson & Armann Hoskuldsson, Classic Geology in Europe 3, Terra Publishing, Third Impression, 2009

“Katla”, Guðrún Larsen and Magnús T. Guðmundsson (2016 March 7). In: Oladottir, B., Larsen, G. & Guðmundsson, M.T., Catalogue of Icelandic Volcanoes. IMO, UI and CPD-NCIP. Retrieved from Icelandic Volcanoes: http://icelandicvolcanos.is/?volcano=KAT

Fig 3: Map: After Jóhanneson and Saemundsson (1998a), Björnsson et al (2000) and Larsen (2000), Base data, Iceland Geo Survey, IMO, NLSI | Base map: IMO. In: Oladottir, B., Larsen, G. & Guðmundsson, M.T., Catalogue of Icelandic Volcanoes. IMO, UI and CPD-NCIP. Retrieved from Icelandic Volcanoes: http://icelandicvolcanos.is/?volcano=KAT

“Katla and Eyjafjallajökull Volcanoes”, Erik Sturkell, Páll Einarsson, Freysteinn Sigmundsson, Andy Hooper, Benedikt G. Ófeigsson, Halldór Geirsson and Halldór Olafsson, Developments in Quaternary Sciences, Volume 13, ISSN 1571-0866

Plots are the author’s own work.

Famous Eruptions, North America

Mount St Helens : 18 May 1980 Eruption

June 2, 2020 Keren_F

Good Afternoon!

The 40th anniversary of the catastrophic eruption of Mount St Helens was on 18 May 2020. On 18 May 1980, 57 people are known to have been killed, most probably by the initial lateral blast, not taking into account anyone unwise enough to have ignored the exclusion zone. In addition, significant damage was done to the surrounding area.

Geologists Keith and Dorothy Stoffel, on a photography expedition, witnessed the catastrophic failure of the northern flank of the volcano: the north side of the volcano slid down then disintegrated in a massive debris avalanche. This was followed by the horizontal blast and the vertical ash column which reached a height of 20 km within 10 minutes. The threat of being engulfed in the ash cloud forced them to return to base.

*Fig 1: Mount St Helens and Spirit Lake before the May 1980 eruption. Image cropped from one by Jim Nieland – US Forest Service,* *Public Domain*

Geological Setting

Mount St Helens lies at the front of the Cascades volcanic arc in Washington. She is the most active volcano in the arc. Her lavas are dacite, andesite / basaltic andesite, basalt / picro basalt, trachybasalt / tephrite basanite and trachyandesite / basaltic trachyandesite(¹). The lavas are sourced from depths of 7 km – 14 km beneath the base of the edifice. Eruptions occurring after 1980 were fed from magma at a depth of less than 4 km.

Volcanism here is driven by the subduction of the Juan de Fuca plate under the North American plate. The arc starts in northern California and reaches up to British Columbia, including composite volcanoes and volcanic centres, calderas and back arc basalt shield volcanoes. Lava compositions range from intermediate to high silica lava domes to low silica lave flows in the southern end of the arc. The presence of low silica lavas at the southern end of the arc is attributed to extensional tectonics permitting the rise of more fluid magmas.

Seismicity in the Cascades

We downloaded the earthquakes between 39.470°N, 130.808°W and 50.870°N, 118.696°W from 1975 to 31 May 2020 to see what the subduction zone below the Cascades looks like. There were 18,173 quakes over 2.5, of which 48 were between 6.0 and 7.0 and 4 had magnitudes in excess of 7.0. We found two areas where there was some evidence of subduction, one at the northern end of the region (north of latitude 45°N) and the other in the south (south of latitude 42°N), otherwise the area was relatively quiet in this period.

*Fig 2: Seismicity and some volcanoes of the Cascades plotted by the author. ©* *Copyright remains with the author; all rights reserved, 2020.*

In order to see the subduction zone clearly we have had to removed the “noise” from the Blanco Fracture Zone.

*Fig 3: The subduction zone north of latitude 45N from the above data set, plotted by the author. © Copyright remains with the author, all rights reserved, 2020.*

*Fig 4: The subduction zone south of latitude 42N from the above data set, plotted by the author. © Copyright remains with the author, all rights reserved, 2020.*

Eruptive History

Mount St Helens is a relatively young volcano, formed around 40,000 to 50,000 years ago. The current edifice was built in the last 2,200 years. Before the eruption Mount St Helens was conical stratovolcano made up of lava, ash, pumice and other deposits with layers of basalt and andesite. Dacite lava domes extruded from the summit and on the northern flank at Goat Rocks.

Mount St Helens has had 44 Holocene eruptions according to GVP(¹), of which one was VEI 6, and five were VEI 5, including the 1980 eruption. Two years prior to the1980 eruption USGS volcanologists produced a hazard assessment, predicting an eruption with the next couple of decades.

The 18 May 1980 Eruption

A 4.2 earthquake on 20 March 1980 and increased seismic activity showed that the volcano was reawakening. Ash eruptions followed from 27 March 1980. A harmonic tremor, indicating magma ascent, set in. Between the initial earthquake and to the earthquake preceding the eruption on 18 May 1980 there were 228 earthquakes over 2.5 and thousands more smaller quakes.

By the end of April, a 2 km wide cryptodome (bulge) had deformed the northern flank, swelling upwards at a rate of 1 metre per day to 150 metres above the existing topography by 12 May 1980. Ground deformation was now presenting a serious hazard.

*Fig 5 The cryptodome on the north flank 27 April 1980, cropped from an image by Peter Lipman in – CVO Photo Archives Mount St. Helens: A General Slide Set,* *Public Domain*

On the morning of 18 May 1980 magma had reached a level of 2 km below the edifice. The slope failure was triggered by a 5.1 earthquake on the morning of 18 May 1980 (or vice versa). A debris avalanche of rocks, glacier ice, soil and other debris ensued, reaching Spirit Lake and the Toutle River valley and ending up east of the Camp Baker logging base, 20 km further down the valley.

Depressurisation of the system, resulted in the violent explosive lateral blast. The blast outran the initial avalanche, felling trees in an area of 600km² and causing the most loss of life. The blast included heated old lava from the volcano and left a relatively thin ash layer.

After the blast, pumice was erupted in a vertical eruption column and pyroclastic currents covered the northern side of the volcano. Pyroclastic deposits reached a depth of 40 metres in the Upper Toutle Valley. It is estimated that at the time of the eruption the pyroclastic flows had temperatures in the region of 700°C. Hot ash reaching Spirit Lake caused secondary eruptions as the water flashed to stream.

During the eruption, Mount St Helens lost its pristine cone and around 400 metres height; it was left with a 1.6 km wide crater, open on the northern side.

Fig 6 The day before the eruption and a few days later. Both images are from Johnson’s Ridge (named after the volcanologist who lost his life in the eruption) and are by Harry Glicken – USGS Cascades Volcano Observatory, *Public Domain*

Post Eruption

From mid-June 1980, lava emerged into the crater forming a lava dome; subsequent eruptions have been crater-based lava dome.

Could Mount St Helens produce another catastrophic eruption?

As she has a history of VEI 5s and a VEI 6, the answer is probably, yes. Her lava types do produce explosive eruptions, including the dacite lava domes. If any of the remaining slopes are unstable, there could be future slope failure should ascending magma cause enough deformation but it’s possible that she may have to rebuild the cone first.

In the meantime, a risk would be to the unwary straying too near to the crater should she have a smaller e.g. phreatic eruption, which are unpredictable. Phreatic eruptions occur when ground water in the edifice heated by hot rocks or magma flashes to steam.

GVP(¹) have added a commemorative feature to its website giving a lot more information, including footage of the eruption. We recommend that you take a look; it is well worth the visit(²).

The Armchair Volcanologist

2 June 2020

References & Further Reading

Smithsonian Institution Natural History Museum Global Volcanism Program (GVP): https://volcano.si.edu/
Mount St Helens 40th Anniversary, GVP: https://volcano.si.edu/projects/sthelens40/
Mount St Helens, Wikipedia: https://en.wikipedia.org/wiki/Mount_St._Helens
1980 Eruption of Mount St Helens, Wikipedia, https://en.wikipedia.org/wiki/1980_eruption_of_Mount_St._Helens

Iceland, Tjornes Fracture Zone

Seismic Activity in the Tjörnes Fracture Zone

May 29, 2020 Keren_F

Good Afternoon!

It’s back to Iceland to finish off a post I started before being diverted by the earthquake swarm in Nevada.

Having looked at the recent activity at the Reykjanes Peninsula, let’s now look at the Tjörnes Fracture Zone, where the Mid Atlantic Ridge leaves Iceland to head northwards. Here, current seismic activity is predominantly tectonic. Our study is based on the same data set used for the introduction to Iceland and the Reykjanes Peninsula (earthquake data downloaded from the Icelandic Meteorological Office(¹) from January 2016 to April 12, 2020, updated to May 3, 2020).

Fig 1: Earthquakes in the Tjörnes Fracture Zone Region from January 2016 to April 12, 2020, plotted by the author. NVB is the Northern Volcanic Belt. © Copyright remains with the author, all rights reserved, 2020.

Geological Setting

The Tjörnes Fracture Zone (TFZ) is a complex area of transform and extensional faulting connecting the Kolbeinsey Ridge, the Western Volcanic Zone and the Northern Volcanic Zone. The Kolbeinsey Ridge, itself, is slow spreading at a rate of 10mm per year. The main faults in the area are: the EyjaFjarðaráll Rift, the Húsavík-Flatey Fault (the TFZ, itself), the Grímsey Oblique Rift and the Dalvik Fault. Both hydrothermal and seismic activity cluster on the faults. The Húsavík-Flatey Fault has produced earthquakes with magnitudes in the region of 7.0.

Grímsey is an inhabited island on the Arctic Circle. Its main industries are fishing and tourism(²).

*Fig 2: Grímsey Cliffs. Cropped from an image by MosheA, published under* *CC BY-SA 2.5*

Flatey is a small island in Skjálfandi Bay in northern Iceland. It is inhabited in the summer for the tourist season, being home to puffins, terns whimbrels and plovers, amongst others(³).

According to GVP(⁴) a submarine eruption occurred in 1868 on the Manareyjar Ridge, north of Manareyjar Island, at the south eastern end of the system; the lavas were basalt / picro basalt. A submarine eruption or dyke intrusion in 1999 caused an earthquake swarm 180km north of Grimsey and 100km north of Kolbeinsey Island on the Southern Kolbeinsey Ridge. Volcanic activity occurred in 1372 and 1755, but its whereabouts is unclear.

Seismic Activity

In the period from January 2016 to May 3, 2020, there were 26,762 earthquakes reported by the Icelandic Meteorological Office (IMO)(²) for the region. 131 earthquakes had a magnitude of 3.0 or more; 67 occurred in month 26 (February 2018) on the Skajálfandadujúp Rift, 52 miles ENE of Grímsey, the largest of which was 5.21M.

Fig 3: Earthquakes in the Tjörnes Fracture Zone Region, February 2018, plotted by the author. Black stars denote earthquakes of magnitude 3.0 or more; blue triangles are the approximate locations of volcanoes or volcanic islands. *© Copyright remains with the author, all rights reserved, 2020.*

A depth plot of the February 2018 swarm shows that most earthquakes over 3.0M occur in the lithosphere.

Fig 4: Depth v Longitude plot of earthquakes April 2018. Green dots denote earthquakes less than 2.0M, yellow circles denote earthquakes over 2.0M; red stars denote earthquakes over 3.0M; blue triangles are the approximate locations of volcanoes or volcanic islands. © Copyright remains with the author, all rights reserved, 2020.

According to IMO, these swarms have occurred before; the most recent being in May & September 1969, December 1980, September 1988 and April 2013. The data for most of the earlier swarms is not publicly available on IMO’s website, but we can get data for the April 2013 swarm. In that swarm, there were 84 earthquakes with a magnitude of 3 or more; the largest of which had a magnitude of 5.37.

Fig 5: Earthquakes in the Tjörnes Fracture Zone Region April 2013, plotted by the author. Black stars denote earthquakes of magnitude 3.0 or more; blue triangles are the approximate locations of volcanoes or volcanic islands. *© Copyright remains with the author, all rights reserved, 2020.*

A depth plot of this swarm shows that most earthquakes over 3.0M also occurred in the lithosphere.

Fig 6: Depth v Longitude plot of earthquakes in the April 2013 swarm. Yellow circles denote earthquakes over 2.0M; red stars denote earthquakes over 3.0M; the blue triangle is the approximate location of Grímsey. © Copyright remains with the author, all rights reserved, 2020

How does this compare to the activity on the Reykjanes Peninsula?

In the Tjörnes Fracture Zone, most seismic activity is occurring in the lithosphere. There is no reported volcanic activity associated with the two swarms we looked.

Apart from the recent large swarm, the Reykjanes Peninsula shows much less activity in the same period; again, most activity was in the lithosphere. The recent swarm, itself, was atypical (still ongoing at the time of writing, but at a reduced rate) and accompanied by ground uplift – hence the increased monitoring put in place there.

I am not Icelandic so apologies for any typos in Icelandic names.

The Armchair Volcanologist

29 May 2020

References & Further Reading:

Icelandic Meteorological Office: https://en.vedur.is
Grímsey: https://en.wikipedia.org/wiki/Grímsey
Flatey, Skjálfandi: https://en.wikipedia.org/wiki/Flatey,_Skjálfandi
Smithsonian Institution Natural History Museum Global Volcanism Program (GVP): https://volcano.si.edu
“Present Kinematics of the Tjörnes Fracture Zone North Iceland, from campaign and continuous GPS measurements”, Sabrina Metzger, Sigurjón Jónsson, Gillis Danielsen, Sigrún Hreinsdóttir, François Jouanne,Domenico Giardini, Thierry Villemin, Geophysical Journal International, Volume 192, Issue 2, 1 February 2013, Pages 441–455, https://doi.org/10.1093/gji/ggs032

Credits:

Raw earthquake data downloaded from the Icelandic Met Office: https://en.vedur.is

Plots are the author’s own work.

North America

Wobbles in the Walker Lane Deformation Belt

May 27, 2020 Keren_F

Good Afternoon,

An earthquake of magnitude of 6.5, 56 km west of Tonopah, Nevada, started an earthquake swarm on 15 May 2020; at the time of writing there had been 1,032 quakes. Last year there was a large swarm to the south in the Eastern California Sheer Zone. This prompted me to take a look at earthquake activity in both California and Nevada.

Geoscatter plot of the Tonopah swarm, May 2020 — *Fig 1: Latitude v Longitude plot of Nevada swarm 15 May to 21 May 2020 by the author. © Copyright remains with the author; all rights reserved, 2020*

The May 2020 swarm occurred in the Walker Lane Deformation Belt. According to USGS(¹), this area has produced two dozen earthquakes with magnitude over 5.0M, mostly to the west and south. In December 1968, there was a 6.8M quake 50 km to the north; and, in January 1934, a 6.5M 40km to the north west.

Geological Setting

The North American Plate and Pacific Plates slide past each other at the San Andreas Fault(²) on the west coast of North America. There are small plates, the Gorda Plate(³) and Juan de Fuca Plates to the north of California, believed to be the remnants of the much larger Farallon Plate which subducted under the North American Plate.

The San Andreas Fault, a transform fault, accommodates 75+% of the relative motion between the North American and Pacific Plates. The Walker Lane Deformation Belt(⁴), itself, takes up between 15% to 25% of the boundary motion.

The Mendocino Fracture Zone(⁵) links the junction between the San Andreas Fault and Cascadia Subduction Zone to the Gorda Ridge on the western boundary of the Gorda Plate.

The Walker Lane Deformation Belt is a roughly 800km long trough, roughly aligned with the California / Nevada state border: the northern end lies at the junction of the Honey Lake Fault Zone, Warm Springs Valley Fault Zone, the Pyramid Lake Fault Zone and the southern boundaries of the Modoc Plateau and Columbia Plateau; and, the southern end lies at the intersection of Death Valley and the Garlock Fault.

The Eastern California shear zone is a portion of the Walker Lane Deformation Belt that links Owens Valley to the San Andreas Fault. It is an area that has produced several quakes of 7+M: 1872 Lone Pine quake in Owens Valley; 1992 Landers Earthquake; 1999 Hector Mine earthquake; and, most recently, the 2019 Ridgecrest swarm.

Seismic Activity 1975 to May 21, 2020

This swarm is not the first in the area. If we look at seismic activity in California and Nevada, we should see the activity from the Walker Lane Deformation Belt to the San Andreas Fault.

We downloaded earthquake data between 43.860°N, 128.786°W and 33.68°N, 114.548°W with magnitude over 2.5 from USGS’ site(¹). In the plot below, we can see earthquakes clearly delineating the plate boundaries of the Gorda Plate, Pacific Plate and the North American Plate.

Fig 2: Latitude v Longitude plot of earthquakes occurring between 1975 and May 21, 2020. Yellow stars denote earthquakes over magnitude 6.0. Plotted by the author. © Copyright remains with the author; all rights reserved, 2020

A geodensity plot of the above data set, weighted by earthquake magnitude, shows the areas of most activity: Mammoth Lakes, an area to the west of Sacramento, and the Mendocino Fracture Zone, east of the southern boundary of the Gorda Plate.

*Fig 3: Geodensity plot of earthquakes occurring between 1975 and May 21, 2020 by the author. © Copyright remains with the author; all rights reserved, 2020*

We looked through the data set to find the larger earthquake swarms; we found 17 swarms (groups of earthquakes exceeding 30 per day). With the exception of two swarms in April 1992, this was successful. In April 1992, there appears to be two swarms occurring at the same time so we split them based on geographic location.

The swarms correlate with the areas of activity displayed in the geodensity plot, with the exception of the area to the west of Sacramento. The activity to the west of Sacramento has not experienced any large swarms or quakes over 6.0.

Fig 4: Earthquake swarms occurring in California and Nevada between 1975 and 21 May 2020 plotted by the author. The swarms are colour coded from magenta as the earliest, through red, orange, yellow, green, blue and black as the latest. Earthquakes with magnitude over 6.0 are denoted as yellow stars. © Copyright remains with the author; all rights reserved, 2020

Mammoth Lakes(⁶) is located near the Long Valley Caldera and Mammoth Mountain, an area of rhyolitic, rhyodacite and dacite lava domes and hot springs. Mammoth Mountain last erupted around 700 years ago, with a small phreatic eruption, but still produces large volumes of CO2.

Looking more closely at the area to the west of Sacramento (38.74°N, 122.9°W to 38.86°N,122.7°W), we found that it has had 2,746 earthquakes spread consistently throughout the period from 1975 to May 21, 2020, of which the maximum magnitude was 5.01. the activity is located near Clear Lake(⁷), California, and the Clear Lake Volcanic Field. The Clear Lake Volcanic Field has not erupted for thousands of years but is not wholly inactive, with volcanic type earthquakes, hot springs and seepage of volcanic gas.

*Fig 5: Location of the activity to the west of Sacramento by the author. Green dots represent earthquake locations. © Copyright remains with the author; all rights reserved, 2020*

Although 44 years of earthquakes feels like a long period to study, this is a very short period of time geologically-speaking so any apparent trends may not be representative and, while very interesting and worthy of investigation, may not be enough for future predictions.

So how would geologists make predictions or update risk assessments? In addition to the type of work above, they would look at ground deformation and movement (the ground may stretch before fracturing), and, for volcanoes, volcanic gas emissions, and gravitational and temperature changes.

The Armchair Volcanologist

21 May 2020

References & Further Reading

Earthquake data was downloaded from USGS; the rest from Wikipedia.

USGS https://earthquake.usgs.gov/
San Andreas Fault, Wikipedia: https://en.wikipedia.org/wiki/San_Andreas_Fault
Gorda Plate, Wikipedia: https://en.wikipedia.org/wiki/Gorda_Plate
Walker Lane, Wikipedia: https://en.wikipedia.org/wiki/Walker_Lane
Mendocino Fracture Zone, Wikipedia: https://en.wikipedia.org/wiki/Mendocino_Fracture_Zone
Mammoth Lakes, Wikipedia: https://en.wikipedia.org/wiki/Mammoth_Lakes,_California
Clear Lake, Wikipedia: https://en.wikipedia.org/wiki/Clear_Lake_(California)

Plots are the author’s own work.

Iceland, Reykjanes Peninsula

Recent Seismic Activity on the Reykjanes Peninsula

May 14, 2020 Keren_F

Good Afternoon!

There has been a large earthquake swarm on the Reykjanes Peninsula over recent months, still ongoing at the time of writing, albeit with reduced intensity.

So let’s take a look at what’s been going on.

*Fig 1: Earthquake epicentres January 2016 to April 12, 2020 plotted by the author.* *© All rights reserved, 2020.*

Geological Setting

The Reykjanes Peninsula lies at the south west tip of Iceland on the Mid Atlantic Ridge, the boundary between the North American and Eurasian Plates. The North American Plate is moving westwards in relation to the Eurasian Plate; transform and extension faulting accommodate the relative Plate motions. The Reykjanes Volcanic Belt lies on the Reykjanes Peninsula, comprising five north east trending volcanic systems: Reykjanes, Svartsengi, Krýsuvík, Brennisteinsfjöll and Hengill. The volcanic systems are fissure swarms.

Earthquakes

The line of earthquake epicentres in Fig 1 shows the path of the Mid Atlantic Ridge. These earthquakes were extracted from the data set used to generate the plots in the earlier post introducing Iceland. The raw earthquake data is publicly available data downloaded from the Icelandic Meteorological Office for the period January 2016 to 12 April 2020. The plot above is still data-heavy (too much data to see what is happening) so I have extracted the earthquakes by month to see where and when most activity occurred.

In the plots there is an impression of seismic activity trending along the Peninsula from the east to the west. This impression is born out most strongly in the activity from month 48 onwards. Although it should be noted that the level of activity for these months is unusually high.

Fig 2: Earthquake epicentres Month 48 (December 2019) plotted by the author. Earthquakes with magnitude ≥ 3.0 are shown as black stars. Blue triangles are the approximate location of the volcanic systems. © Copyright remains with the author; *all rights reserved, 2020.*

Fig 3: Earthquake epicentres Month 49 (January 2020) plotted by the author. Earthquakes with magnitude ≥ 3.0 are shown as black stars. Blue triangles are the approximate location of the volcanic systems. © Copyright remains with the author; all rights reserved, 2020.

Fig 4: Earthquake epicentres Month 50 (February 2020) plotted by the author. Earthquakes with magnitude ≥ 3.0 are shown as black stars. Blue triangles are the approximate location of the volcanic systems. © Copyright remains with the author; all rights reserved, 2020.

Fig 5: Earthquake epicentres Month 51 (March 2020) plotted by the author. Earthquakes with magnitude ≥ 3.0 are shown as black stars. Blue triangles are the approximate location of the volcanic systems. © Copyright remains with the author; all rights reserved, 2020.

Fig 6: Earthquake epicentres Month 52 (April 1 to April 12, 2020) plotted by the author. Earthquakes with magnitude ≥ 3.0 are shown as black stars. Blue triangles are the approximate location of the volcanic systems. © Copyright remains with the author; all rights reserved, 2020.

There are several possible reasons for the east to west trending of the earthquake swarms:

the plates do not move smoothly past each other, so friction generates faults and earthquakes;
rifting is occurring to accommodate the upward motion of land further to the east, generated by the mantle plume in the vicinity of Vatnajökull; and /or,
magma intrusion in local volcanoes.

Let’s take a look at depth plots of earthquakes under the Reykjanes / Svartsengi area. This shows that the earthquakes over 3 M are largely in the lithosphere.

*Fig 7: Analysis of earthquakes in the Svartsengi area by magnitude and depth by the author. © Copyright remains with the author; all rights reserved, 2020*

Plotting depth against longitude, effectively looking northwards through the swarms, also shows that most larger quakes are in the lithosphere but some over 2 M track towards the surface. On its own, this is not enough to draw any conclusions over the likelihood of an eruption; field observations, including gas emissions and ground deformation are required to determine how close to the surface magma may be.

Fig 8: Depth plot by the author of the earthquakes in the Reykjanes / Svartsengi area. Green circles are earthquakes less than 2.0 M, yellow circles are earthquakes between 2.00 and 3.00 M, red stars are earthquakes over 3.00 M. © Copyright remains with the author; all rights reserved, 2020.

These swarms were accompanied by local uplift, as shown by local GPS stations.

*Fig 9: Uplift in the vicinity of Mt Thorbjörn as shown in recent GPS plots published by IMO:* *https://en.vedur.is/. THOB moved south eastwards and ELDC moved westward; both showed uplift.*

IMO(³) has reported that the most likely explanation for the recent swarms and uplift is a magma intrusion near Mt Thorbjörn at depth; Mt Thorbjorn is located near the Blue Lagoon, Svartsengi. As to whether or not magma will reach the surface for an eruption and where it emerges, we will have to wait and see. In the meantime, let’s look at the recent activity in historical times.

Historic Volcanic Activity

As noted earlier, the volcanic systems are fissure swarms. Activity is driven by rifting which enables magma to reach the surface. The most recent onshore volcanic activity took place between 940 AD and 1340 AD; later activity has been offshore. Onshore lavas from these eruptions tend to be tholeiitic basalts (¹). The systems are still active as demonstrated by current geothermal activity.

Hengill

*Fig 10: Image cropped from one by Hansueli Krapf, published under cc licence:* *CC BY-SA 3.0*

The Hengill volcanic system is a series of fissure vents, crater rows and small shield volcanoes, with a highest point of 803m. It lies at the triple junction of the Reykjanes Peninsula volcanic zone, the Western volcanic zone and the South Iceland seismic zone. The lava types are basalt / picro-basalt, andesite / basaltic andesite and rhyolite. Hengill’s lavas are more complex that those to the west, reflecting its position at the triple junction. GVP(²) lists 13 Holocene eruptions ranging between VEI 0 and VEI 2; the last known eruption was a VEI 2 in 150 AD.

Brennisteinsfjöll

The Brennisteinsfjöll volcanic system is a series of crater rows and small shield volcanoes, with a highest point of 610m. Its lava types are basalt / picro-basalt. GVP(²) lists 9 Holocene eruptions ranging between VEI 0 and VEI 2. One eruption, previously attributed to Hengill, occurred during a meeting of the Icelandic parliament at Thingvellier in 1000 AD. The most recent eruption was a VEI 2 in 1341.

Krýsuvík

*Fig 11: Image cropped from one by Reykholt, published under creative commons licence:* *CC BY-SA 3.0*

The Krýsuvík volcanic system is a series of crater rows and small shield volcanoes, with a highest point of 360m. Like Brennisteinsfjöll, its lava types are basalt / picro-basalt. GVP(²) lists 11 Holocene eruptions ranging from VEI 0 to VEI 2; the most recent of which was in 1340. The Krýsuvík Fires spanned a period between 1151 and 1188, producing 36 km² of lava.

Reykjanes & Svartsengi

*Fig 12: Image of Svartsengi Power Station from one by Jóhann Heiðar Árnason, published under creative commons licence:* *CC BY-SA 3.0*

The Reykjanes volcanic system is a series of crater rows and small shield volcanoes, which extends offshore and includes several small islands. Reykjanes highest point is 140m. The Reykjaneshryggur volcanic system is a submarine system which is considered part of Reykjanes. GVP includes Svartsengi as a crater row of Reykjanes. Reykjanes lava types are basalt / picro-basalt. GVP(²) lists 22 Holocene eruptions ranging between VEI 0 and VEI 4, of which the most recent was a VEI 0 in 1970; the VEI 4 was in 1226. The 1226 eruption was part of the Reykjanes Fires which started in 1210 and lasted until 1240.

The area is one that has had a period of intense volcanic activity, so the recent earthquake swarms have generated a lot of interest.

Regular updates on seismic and volcanic activity in Iceland are published by the Icelandic Meteorological Office(³).

The Armchair Volcanologist

14 May 2020

References & Further Reading:

David W Peate, Joel A. Baker, Sveinn P. Jakobsson, Tod E. Waight, Adam J. R. Kent, Nathalie V. Grassineau, Anna Cecile Skovgaard , 2009. “Historic Magmatism on the Reykjanes Peninsula”, Contrib Mineral Petrol (2009) 157:359-382
Smithsonian Institution Natural History Museum Global Volcanism Program (GVP): https://volcano.si.edu
Icelandic Meteorological Office: https://en.vedur.is

Credits:

Raw earthquake data downloaded from the Icelandic Met Office: https://en.vedur.is

Plots are the author’s own work.

Iceland

A Brief Introduction to Iceland

April 22, 2020 Keren_F 1 Comment

This is a test post as a starter for my blog. I hope you find this and later posts interesting.

Iceland, home to Eyjafjalljökull and Grimsvötn, whose eruptions in 2010 and 2011, resp., disrupted European airspace, is recommended as a great place to study volcanology, with many types of volcanic activity and relatively easily accessible. It is also a great holiday destination, not that I have been privileged enough to visit.

Iceland, located between 67.2°N 23.0°W and 63.0°N 13.0°W, has an area of 103,000 km² and a population of 364,000. Lying on both the North American Plate and the Eurasian Plate, it is the only large surface expression of the MAR where its volcanic activity can be easily studied by field volcanologists. Iceland, itself, was formed from magma and accretion.

The Mid Atlantic Ridge in the northern hemisphere is the boundary the North American Plate and the Eurasian Plates. As the plates separate, the ridge widens and allows rising magma to come to the surface. The magma rises under its own buoyancy; hotter magma is less dense than the colder surrounding rock.

In addition to the MAR, the Icelandic Hotspot, a mantle plume, contributes to both plate separation and volcanic activity. The Iceland Plateau, itself, is a large basaltic igneous province. The plume head is thought to be located in the region of the Vatnajökull icecap.

The MAR crosses Iceland in a series of transform and extensional faults, starting at the Reykjanes Peninsula in the south west to the Tjörnes Fracture Zone in the north. Plotting the earthquakes reported by the Icelandic Meteorological Office (IMO) from January 2016 to 12 April 2020 shows the path of the MAR.

*Fig 1. Earthquakes from 2016 to 2020 (12/04/2020). Circles roughly denote four of the many volcanic zones.* *© Copyright remains with the author; all rights reserved, 2020.*

If we take a look at a three-dimensional plot of the same earthquake looking from the south, we can see that there are deeper earthquakes under the volcanic regions Myrdalsjökull and Vatnajökull.

*Fig 2. Three-dimensional plot of earthquakes from 2016 to 2020 (12/04/2020).* *© Copyright remains with the author; all rights reserved, 2020*

A geodensity plot of the same earthquakes, weighted by magnitude, shows most activity (yellow areas) at the western end of the Reykjanes Peninsula and on the Tjörnes Fracture Zone to the north. Other areas of interest are Myrdalsjökull (Katla), Vatnajökull (Barðabunga) and Herðubreið.

*Fig 3. Geodensity plot for the same earthquake data set as above. © Copyright remains with the author; all rights reserved, 2020*

Iceland is often touted as having every form of volcanism going but it does not have an obvious active subduction zone; eruptions tend to be associated with fissures. Not to worry, Iceland has many active volcanoes, including: Askja, Bárðabunga, Eyjafjalljökull, Katla, Grimsvötn, Hekla and Surtsey – plenty to offer us in terms of volcanic and seismic activity.

Our plot is data-heavy (in the region of 104,000 earthquakes) so we cannot see much more without further analysis. We will look at each volcanic region in turn in later posts.

Thank you for reading this and I look forward to sharing the next post with you.

The Armchair Volcanologist

Sources:

All plots are the author’s own work.

Raw earthquake data: Icelandic Meteorological Office, IMO: https://en.vedur.is/

Software used for 3D and geoplots: MatLab

An Armchair Volcanologist

All posts by Keren_F

Hekla – The Hood

The Katla Volcanic System, Mýrdalsjökull – the not so cuddly Katla

Mount St Helens : 18 May 1980 Eruption

Seismic Activity in the Tjörnes Fracture Zone

Wobbles in the Walker Lane Deformation Belt

Recent Seismic Activity on the Reykjanes Peninsula

A Brief Introduction to Iceland

An amateur's observations on volcanoes and earthquakes

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An amateur's observations on volcanoes and earthquakes