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)