When I recently noticed earthquakes listed day after day for the Halmahera region (northeastern part of Indonesia) I got my typical ‘how-come?’ feeling. So I went looking for a reason, and I read that an earthquake swarm had begun in the Jailolo region on Sept. 27, 2017. Although it has diminished by now, there are still quakes coming.
While Halmahera lies within a conundrum of subduction zones and major faults, none of those seem to affect the Jailolo area specifically. Halmahera lies on the western boundary of the Philippine Sea Plate, which extends to the E, and to the N along the Philippine Trench.
Next from the N, the Eurasian Plate reaches into the West Halmahera Thrust zone. To the W of the island is the Molucca Sea plate. This one got so encased and distressed in tectonic history that it had no other choice than to dive east under Halmahera and west under the Sangihe Arc in an inverted U-shape, thus creating a double-subduction zone. In the Halmahera subduction area, the Benioff zone (where magma is created) would be at ~200–300 km depth, causing volcanism on the island. To the S, the E-W trending Sorong fault is too far away; it can not be connected to the Jailolo EQ swarms.
TWO EARTHQUAKE SWARMS, 2015 & 2017
The 2017 swarm
The present swarm is perpendicular to the subduction zone, also crossing the N-S line of Halmahera arc volcanoes. It covers an area ~100 km long in SE-NW direction and ~35 km wide. EQs are up to M 5; depths are 1 – 15 km. It is really hard to find any EQ data for Indonesian lower magnitude earthquakes. As long as not people are affected or houses fall apart, they seem not to be available to the wider public. Which is probably understandable with the zillions of EQs happening in that country, but hey – what are databases for? Only very few of the swarm appeared in a regularly updated list of the last 20 earthquakes, with magnitudes M2 to M5. Their depths were almost all less than 10 km, some even just 1 km deep.
Due to the recurrence of an EQ swarm in the Jailolo area the BMKG issued two reports. Here the excerpts (via Google translate):
29/09/2017: An earthquake swarm that occurred around Jailolo is mainly of shallow depth with a variety of magnitudes. From the evening of 27/9 2017 to morning of 29/9 2017 there were 988 events [that’s in one and a half days]. Of those, 412 could be analysed; 74 were felt by the population. Magnitudes were < M 5.0. The earthquakes were felt in Jailolo, Ternate, and Sofifi. Cause of Earthquakes: Based on the parameters, viewed from the depth of the hypocenters, the earthquakes are of a shallow earthquake type. This swarm mostly has a horizontal fault source mechanism (strike slip). This type of seismic phenomenon has occurred in Jailolo before, in November-December 2015.
01/10/2017: From 27/9 2017 to morning of 01/10 an earthquake swarm in Jailolo has had 1582 events.
A few days later, a journalist of the SSI could squeeze out more – well, not a big lot more – from an official at Ternate Geophysics Station: “…These earthquakes occurred due to the deformation of rocks under the crust of the North Halmahera, while the swarm was investigated … It is said to be extreme because they are more numerous. In 2015 [a similar swarm] had been researched but that ended when the swarm stopped. It is researched again at this time.”
In both reports it is stressed that “A quake swarm is a series of small magnitude earthquake activity with a very high frequency of occurrence that lasts for a relatively long time in an area, and without any major earthquake (mainshock).”
The 2015 swarm
If the EQ swarm was researched I should find some of the results on the interwebs? Yes, found the “Field Investigation…” in a short paper hidden behind a paywall (link in the sources list below): The West Halmahera Swarm event occurred in Sept./Nov./Dec. /2015. A seismic station located in Ternate City had detected at least 1000 shocks of magnitude >1 and 11 shocks of M >4.5.
Estimations yielded a cumulative seismic moment that was equivalent to a single M 8.3 earthquake.
During this series of events, residents in Jailolo city reported continuous ground shaking as well as cracks or part destruction in their houses. Also damage of infrastructure and environmental effects were obvious. For the residents it was so bad that they chose to sleep in tents provided by the Indonesian National Board for Disaster Management. As many as 1593 damaged buildings were recorded. – Unfortunately, when Endra Gunawan et al. conducted their field survey during the time of the swarm, no GPS stations were available for more specific research.
TECTONIC OR VOLCANIC ?
If most of the quakes have a horizontal fault source mechanism (“strike slip”), this *could* be an indicator for tectonic movements. I imagine it would give a similar picture if there was a large fault rupturing. With all the motions of all those plates (and a few microplates) relative to one another in the area I would not dismiss that easily. Moreover, earthquakes caused by magma movement would very often produce a different type, the vertical (“normal” or “reverse”) fault quakes.
This, however, depends on the setting within a magmatic system, and strike-slip swarms have been known in connection with magma ascent. For example, volcanic EQs near G. Guntur volcano have shown a strike-slip mechanism, and were somewhat deeper at 5-10 km (than the usual 0-8 km). So, how can we discern between purely tectonic quakes and volcano-tectonic ones? Here from the textbook:
Deep (>2 km) Volcano-Tectonic EQs (VT-A): “The name of this event type implies a well-known source mechanism, namely a common shear failure caused by stress buildup and resulting in slip on a fault plane similar to a tectonic earthquake source. The only difference from the latter is the frequent occurrence of swarms of VT events which do not follow the usual main-and-after-shock distribution. An earthquake swarm is a sequence where the largest events are similar in size, and not necessarily at the beginning of the sequence.”
Here you are. Thousands of EQs between 15 and 1 km deep, with magnitudes up to M 5. The strongest ones occurred sometime within the swarm, not at the beginning. The location is very defined, above a subduction zone, within a volcanic arc. Moreover, we have the conclusion arrived at by scientists in the above mentioned paper: They concluded “from observations, as well as from the geologic and tectonic settings, that the swarm was strongly associated with the volcanic activity of Jailolo“.
Our friend dfm made a great job of plotting some of the earthquakes of the Jailolo swarm for us – thank you! These are only the ones that could be drawn from the EMSC database for 27/09-12/10/2017, i.e. some 75 quakes. All but a few are of a depth < 10 km and magnitudes below M 5. The few stray ones are deeper and/or have a higher M. Might be purely tectonic, or perhaps an existing fault being utilized?
Of course, this being only a small fraction of all the quakes, it is hard to read much from it. I am a bit amazed that the activity seems to arrive from the southeast on a shallow “plain” instead from greater depth – like a long sheet or sill filling in, rather than a dike rising upwards. I’m afraid I am not great at interpreting – I would like to hear what others think, any ideas? Here’s the video:
Back in 2015, a newspaper reported about the expert’s discussion at the time: the cause of the earthquake sequence at Jailolo was apparently a rise in pressure in the transition zone at depth, in 5-15 kilometers underground, due to fluid intrusion. This could mean magmatic activity of volcanoes. According to the Head of Volcano Monitoring, Gede Suantika, the results of the Jailolo earthquake discussion were just in the phase of scientific hypothesis. It was thought that an earthquake swarm in the Jailolo region could affect the increase in volcanic activity of one or a number of other volcanoes. I.e. of type A volcanoes (active): Dukono, Gamkonora, Gamalama, Kiebesi, as well as two type B volcanoes (dormant): Jailolo and Todako.
The Jailolo volcanic complex forms a peninsula west of Jailolo Bay on the western coast of Halmahera Island. The N-S trending West Halmahera volcanic arc is composed of 13 volcanoes. Jailolo’s volcanic neighbors are the very active G. Gamalama on Ternate Island in the S., and to the N a row of volcanoes along the western edge of Halmahera. The latter include active G. Gamkonora and very active G. Ibu. Somewhat off the line to the NE stands the ever-busy G. Dukono.
Not much is known about this stratovolcano at the center of Jailolo complex, and its troublesome past. It must have had a very busy history, and it might have been active not too long ago. It is classified as a Holocene volcano. It has youthful looking lava flows on its E flank, and hot mudflows were reported ~1883. However, no eruptions are known during historical time. The rock types produced by Jailolo are Andesite/Basaltic Andesite and Basalt/Picro-Basalt. Several small calderas are located to the W and SW. The westernmost caldera, Idamdehe, truncates an older twin volcano of Jailolo. Hot springs also occur along the NW coast of the caldera. The Kailupa cinder cone forms a small volcanic peninsula to the south of the volcano. And, looking at Jailolo Bay in the SE, might this be another caldera of the complex? After the 2015 EQ swarm, new hot springs have been reported.
We have two earthquake swarms of over thousand events. There are no ‘main shocks’ that would define the rest as ‘aftershocks’. They happened in a relatively short time span, i.e. with a high frequency of appearance. Almost all are located at depths between 1 km and 15 km in an area straddling the Jailolo peninsula. Scientists find that the first swarm was strongly associated with volcanic activity of Jailolo volcano.
Now, no panic please! These findings suggest no more than that there has been an intrusion of magma below the area. That does not mean the volcano is going to erupt soon. It may never erupt in our life-time. Or, if it were going to erupt, there would be different and clear signs of magma rising into the edifice. Nevertheless, it is now yet another location for Indonesian volcanologists and authorities to monitor more closely. And be prepared for any further, perhaps unexpected developments.
Disclaimer: I am not a scientist, all information in this (and any of my other posts) is gleaned from the www and/or from books I have read, so hopefully from people who do get things right! 🙂 If you find something not quite right, or if you can add some more interesting stuff, please leave a comment.
Enjoy! – GRANYIA
SOURCES & FURTHER READING
– Field Investigation of the […] EQ Swarm in West Halmahera (2016, paywalled, DOI: 10.1007/s10706-016-0117-4)
– Basin formation by volcanic arc loading (PDF, 2008, paywalled)
– Earthjay Science (educational materials)
– BMKG reports: 29/9 | 01/10 2017
– BMKG list of last 20 EQs
– Volcano Seismology (2012, PDF)
– Source Mechanisms of Volcanic Earthquakes […] (2001, PDF)
– News from 12/2015 about eq swarm (senyumperawat.com)
– Earthquake Focal Mechanisms (USGS, Education)
reminds me of the swarm at Matata that was long thought to be just tectonic (it’s on the western edge of the Whakatane graben) but was later found to be caused by a sill developing.
M5 seems very large though and strike-slip also doesn’t sound too much like a sill forming. Given the bizarre tectonic environment, there may be something else happening like a transform fault developing or something.
OTOH the depth does seem to be relatively constant over the entire length of the sequence.
Nicely done, Guys. Gut reaction is we are seeing a magma chamber being charged. Probably completely wrong. Sure would like to see a low velocity zone map of the region. Half expecting to see something interesting.
Given the jumbled tectonics of the surrounding area, I wonder what they resolve to. In other words, what happens over the millions of years to such a jumbled region? Western Antarctica started out as 4 – 5 microplates jostling one another, but there is very little tectonic movement and they kind of froze into place. E of Sulawesi moves a lot as it is being squeezed from all sides, primarily from the south via the Australian Plate on its way north to China. What a mess. What a fascinating mess. Cheers –
great new article out on Mauna Loa:
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Whoa, the geology of the spot seems really very complex.
Plus the data is hard to come by – for instance only one quake show for the Agung area (just in case you’d ask -:) )
Thanks to Granyia for pointing me out to the catalog. Unfortunalety this is not iceland or Spain and data is not as complete, there are only the higher intensity quakes.
Please be indulgent this is my first shot while using Python, as my previous Octave/Matlab scripts do not work anymore because of an obscure graph engine problems in Linux.
I’ll try to better the plot system in the weeks to come, but I’m travelling for work right now, so it’s not easy.
Cheers and thanks for a great article (I’ll need some time to digest it)
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Plus I see some paywalled article – if you want them I have a way to get them, most of the time, just say
I have only difficulties (sometimes) finding very new papers of, say, the last week or so. Otherwise I use http://sci-hub.cc/, which is also pretty good and can deliver most of the papers, even pre-releases. – Also, a new browser extension has been developed earlier this year – Unpaywall – but I find it not being very effective yet. Perhaps it needs some more time becoming known to authors and building up a good database. http://unpaywall.org/. Thank you for offering help!
I know of the first one, but there are others.
Meanwhile in La Palma….
As the pythoncode is done it is relatelively easy to adapt.
Now this is one neat intrusion, coming up straight from depth as one would expect! Is this when the swarm began, early October, or just the time when the bulk of the quakes appeared? Most of them at 20-25 km deep, way to go. But that most shallow one came up last, so the dike is still progressing upwards. Thank you, dfm, and keep at it!
it’s since the beginning of October.
Neat intrusion, but nothing to fear for the moment we will see what happens next.
I’ll try to perfect the code as we go. I’ll probably do some updates if there are some new developments.
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Nature paper on volcanic impact on Nile flooding over the last 2400 years. Despite the headlines blaming this on Russian, Alaskan and Greenland volcanoes, I could find no specific volcanoes mentioned. At first glance, looks like the work is based on models, ice core data, and a record of Nile water levels called the Nilometer. Link to actual paper follows. Cheers –
First tornillos and HT starting to show up on the Agung plot. Still very sporadic but a good sign that magma is getting shallow enough for bubbles to start forming in it and allow resonance to occur in the conduit(s)/dykes (he says, sounding like he knows what he is talking about but doesn’t) 😉
Has anyone heard of any recent inflation data?
They still call them non-harmonic tremors in today’s report… Volcanologist Devy Kamil Syahbanaan gives an explanation of the N-HT in this interview http://ubudnowandthen.com/interview-vulcanologist-devy-kamil-syahbana/.
Left: GEarth image of Agung crater from 03/08/2014
Right: One of the first 400 images of a drone flight over crater on 2017/10/19. The crack in the E part (left side) reportedly widened. – Both images enhanced (sharpness, contrast) by me. To be honest, I can’t spot the cracks…
The Bali Expat reports that today a decision was expected on whether or not to downgrade the alert level from 4 to 3, based on declining seismicity. But no announcement has come through in the media so far, and it’s past midnight in ID now.
Overall EQ activity from 15/09/207 up to date (via BNPB on Twitter)
I guess the activity will go up every time it’s raining heavily over Bali.
Water finding hot rock perhaps? Cheers –
Yeah, that’s what it is, you can wait for the earthquakes coming in when you see it pouring on the webcams. I remember when Ubinas’ activity started a few years ago, they said, oh, it’s nothing, just rainwater causing little steam explosions, don’t worry. Ubinas has had some sizeable ashy phreatic eruptions in the following months and years.
Even the rough Ggl-translation makes it clear that there must have been a harsh discussion in Bali over the alert status of G. Agung. Mr. Kasbani (PVMBG/ESDM) confirmed that the status of Gunung Agung remains at Level IV (Awas).
He said in a press conference today: Our institution will not be rushed to change the status of Mount Agung, because the volcano’s activity is still high and fluctuating. Although currently the seismicity shows a decrease compared with previous days, the deformation data still show the opposite, i.e. there is still inflation. This means that this is not the right time to reduce the status of Mount Agung. We still have to watch its development, if all parameters show consistent slow decline, including the deformation, and when also the visual observation activities have decreased, it can be our guide to decrease the status of G. Agung. That means, the status is not made by me but by the information we get from the mountain itself.
Good on him! I really don’t envy volcanologists in that position. there are no doubt huge pressures on them to reduce no-go zones when an “imminent” eruption decides to take its time and people feel their lives are being unduly disrupted.
But only going on the signs we can see (the seismogram, the fumaroles, and the reports of inflation) it most definitely looks like magma has risen to shallow levels and this development is on the back of a long and sustained intrusion. It certainly hasn’t stopped yet and certainly has the potential to culminate in a life-threatening eruption. Just because the last eruption started with lava flows before ramping up into an explosive eruption doesn’t mean it is going to do the same again this time.
PS there is much more pronounced gas and fluid activity on the seismogram in the last 7 hours.
Plus, there are so many other factors in a seismic record, for one example that in some volcanoes earthquakes cease altogether shortly before an eruption.
But, as to the small noise (“thickening” of the lines) I believe that could be human activity as it begins abt. 8 am and diminishes after abt. 7pm. Don’t know if wind plays a role as well, depends how sensitive the sensor is.
Another volcano (that unanimously “roared to life” in various newspapers) erupted in the Solomon Islands yesterday: Tinakula is a 3.5-km-wide, now uninhabited island at the NW end of the Santa Cruz island group – and the exposed top of a large andesitic stratovolcano. It produced an 11 km high ash plume (acc. to Wellington VAAC) yesterday. More ash advisories have been issued since, so the eruption seems to continue.
The volcano sent black ash into nearby communities when it started erupting early Saturday morning, and authorities do not have a scientific way to monitor the situation and determine when it will end (SIBC, 22/10). – Tinakula is described to closely resemble Stromboli volcano as it has a breached summit crater, forming a scree slope from the summit to below sea level. Last activity was in 2012.
The northern side of Tinakula volcano.
(Image: GSSI, courtesy of CSIRO)
Tinakula: White patches of steam/gas (‘vapour trails’) were caused by red-hot boulders bouncing down the slope. (observation by Cook and others in 2012, via GVP).
I wonder, what might be the quite regular thermal anomalies within the 1.5-km-range of Pinatubo crater? Some travel agencies coming up with tourist groups and making a bonfire? I have not heard of any volcanic activity there since the 1991 eruption, has anyone else?
Howdy Granyia –
I remember seeing fumaroles on the flanks of Pinatubo in Jan 1979.
There were post eruption fumaroles in the crater around the crater lake following the eruption. Don’t know how their activity has declined over time. Will poke around a bit and see if I can find more current photos. Cheers –
This must be the study which the press had hyped up about a few weeks ago, and Erik wrote a blog post to straighten things out.
“Probing Magma Reservoirs to Improve Volcano Forecasts … using the Yellowstone plateau as an example, we review current understanding of magma storage regions as relevant to volcanic hazards. We focus on two popular techniques for characterizing magma reservoirs, one geophysical (seismic tomography) and the other petrological (geospeedometry). We then suggest a third option—scientific drilling—that has the potential to dramatically improve our understanding of magma dynamics…” – Highly interesting read!
And another worthwile paper – if I only had more time for reading!
sustained tremor now on the Agung plot. there is a rain shower going through so it might just be water entering the system and boiling.