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Seismology

Seismology

Palu 2018 – Science and surprise behind the earthquake and tsunami

On September 28, 2018, a powerful 7.5-magnitude earthquake and an unexpected tsunami shook the Indonesian island of Sulawesi, leaving behind catastrophic results and open questions among geoscientists. How come this event is having such an impact on the scientific community?

Figure 1. Map of Indonesia, showing the four Greater Sunda Islands, the location of the 2018-09-28, Mw7.5 Palu earthquake and its focal mechanism.

 

What we know so far

On Friday afternoon (at around 5pm Western Indonesian Time) the Minahassa Peninsula on Sulawesi island was struck by a Mw7.5 earthquake (Figure 1) [1]. The main shock occurred on the strike-skip Palu-Koro fault, one of the most active structure in Sulawesi [5]. The epicenter of the quake is located 27 kilometers northeast of Donggala, at a depth of 10 km [1]. By looking at the aftershocks distribution from within 4 days after the Mw7.5 event (Figure 2), we can approximately estimate a rupture extent ranging from a few kilometers to the North of the epicenter over about 200 km to the South [2]. The rupture area is thus roughly 200x20km, mainly located South of the hypocenter and ending in the city of Palu (Figure 2). Nothing surprising by now, but the intriguing part is coming.

Figure 2. Aftershocks distribution (coloured circles) from within 4 days after the Palu earthquake event occurred on 28 September. Hypocentral depth and magnitude are shown respectively with the colour scale and the circle size. The Palu-Koro fault is indicated in black. The mainshock event lies in the centre of a large aftershock cloud stretching over more than 200 km in N-S direction. Picture taken from Global Earthquake Monitoring Processing Analysis (GEMPA)[2].

By looking at the source time function of the Palu earthquake (Figure 3), we can deduce that the event took about 30s to rupture a length of 200km, suggesting a supershear rupture velocity. While most seismic ruptures propagate at speeds lower than the S-wave speed, supershear ruptures propagate at speeds between the P and S waves. Supershear ruptures are extremely rare events which, in the past, mostly propagated on very smooth part of strike-slip faults [12]. Coincidentally, first estimates of surface displacements along the Palu-Koro fault show a very sharp offset between the two sides of the fault, suggesting that the rupture propagated on a linear, very smooth segment of the fault.

 

“This is the most sharp and linear earthquake surface displacement I’ve seen since 2013 Balochistan eq! says geologist Sotiris Valkaniotis on his twitter account. And the image [Figure 3] has already gone viral on the net. Further study will be necessary to confirm if this event is in fact supershear.

 

Figure 3. On the left: source time function of the Mw7.5 Palu earthquake and its focal mechanism. The Palu earthquake took about 30s to rupture a length of about 200km along the Palu-Koro fault segment. Picture from GEOSCOPE Web Portal [3]. On the right: displacement map of the Palu-Koro fault. Colorscale from red to blue represents the North-South displacement in meters. Picture realized and posted by geologist Sotiris Valkaniotis on his twitter account.

 

Tectonic setting

Sulawesi is one of the Greater Sunda Islands [4], a group of four large islands within the Malay Archipelago including Java, Sumatra and Borneo (Figure 1). An intricate pattern of faulting scars the land: collisional orogenies, subduction zones, rift systems and transform faults pile up in this area [5], making the island prone to earthquakes.

Figure 4. Tectonic setting of the Sunda–Australia–Philippine–Pacific plates junction area. Arrows depict the far-field velocities of the plates with respect to Eurasia. Picture taken from Socquet et al. (2006)[5]

 

From a tectonic point of view, Sulawesi lies within the triple junction of the Australian, Philippine and Sunda plates and accommodates the convergence of continental fragments with the Sunda margin [5]. In particular, the southwestern part of Sulawesi rotates anticlockwise respect to the Sunda Plate; the northeastern Manado Block and the central North Sula Block move toward the NNW and rotate clockwise; the East Sulawesi is pinched between the North Sula and Makassar blocks (Figure 4).

 

Palu
The left-lateral strike-slip Palu-Koro fault is the main active structure in Sulawesi: it bisects the island and connects to the Minahassa Trench, where subduction occurs. The Palu-Koro fault zone accommodates 42 mm/yr and shows a transtensive behavior more complex than a simple strike [5]. This deformation is most likely explained by the presence of a pull-apart structure that may be localized around the Palu area (Figure 5)[5].

Figure 5. Sketch showing a transtensional state of a rock. It is unlikely that a deforming body experiences ‘pure’ extension or ‘pure’ strike-slip. Many tectonic regimes that were previously defined as simple strike-slip shear zones are by now assumed as transtensional. As such, transtensional regions are characterised by both extensional structures (normal faults, grabens) and wrench structures (strike-slip faults).

 

‘Surprise Tsunami’

Following the Mw7.5 earthquake, a series of tsunami waves hit Palu, Donggala and Mamuju decimating the coastline, flattening homes, destroying several inland areas in Central Sulawesi [watch the video below]. Indonesian Agency for Meteorology, Climatology and Geophysics (BMKG) initially issued a tsunami warning but eventually revoked it.

<< The decision was based on visual monitoring and further monitoring using the equipment [tsunami detection buoys] at sea for 30 minutes. The BMKG did not see any significant change in the sea level. That’s why they ended the alert >> BNPB spokesman Sutopo Purwo Nugroho said in a statement [6].

 

 

 

The eastern Indian Ocean basin is a region of high earthquake and volcanic activity, so tsunamis should come as no surprise. Yet, geoscientists are doubtful. Could the Palu earthquake alone have generated such a big tsunami wave?

 

Eighteen tsunamis have crashed in the area since 1900 by large shallow earthquakes [7] and among them we can’t forget the tsunami that hit the Indian Ocean following the December 26, 2004, earthquake in Sumatra [8]. The controversy about the ‘surprise’ tsunami however arises when we look at the tectonic setting of Sulawesi island and the devastating effects documented on the coastline.

 

Tsunami waves are generated by a sudden vertical motion of the seafloor. They are therefore mostly triggered by dip-slip earthquakes where the block of rock on top of the fault, also called the hanging wall, moves up, which pushes the overlying water column up [watch the animation below]. But the Palu earthquake occurred along a strike-slip fault, meaning the motion was mainly horizontal. It is therefore very unlikely that such a large tsunami could have been originated by earthquake rupturing alone.

This simplified animation illustrates the tsunamis generation in sudbuction zones. Tsunamis generally occur after a megathrust earthquakes and thus follow vertical movement in the crust. (© Marina Corradini)

 

Scientists’ best guesses?

¤ A submarine landslide: triggered by the main shock, a submarine tectonic displacement may have generated or augmented the tsunami waves.
¤ A complex bathymetry: the seafloor in the Palu Bay is not flat. If there is a slope, even an horizontal motion of the seafloor can produce an apparent vertical motion [9].

 

There is still no evidence that any of those hypotheses (landslide or seafloor slope) is the actual reason. They could be both too. Besides, the intensity of the tsunami seems to be more related to the shape of the Bay, which naturally acts as a funnel for the tsunami waves. As yet, a straightforward interpretation seems hasty and further analysis should come in aid.

 

 

Rescue and damages: what now?

The confirmed death toll from the Palu earthquake and tsunami has risen to 1,407 [10] together with thousand of buildings destroyed and over 61,000 people displaced [11]. The full scale of damages in the region can already be guessed by watching the dramatic pictures and videos spread online. First aid has already been mobilized through several nonprofit organizations and charities, and everyone’s help is essential in these circumstances.

 

This post was written by Marina Corradini, with revisions from Maria Tsekhmistrenko and Lucile Bruhat

 

 

Maria Tsekhmistrenko is a PhD student at the University of Oxford. She works on the velocity structures beneath the La Reunion Island from the surface to the core mantle boundary. You can reach her at mariat[at]earth.ox.ac.uk

Lucile Bruhat is a post-doctoral researcher at the Ecole Normale Supérieure (ENS) in Paris. She works on the description and understanding of the physical processes that underlie the earthquake cycle, through geodetic/seismological data analysis and numerical modeling. You can reach her on Twitter at @seismolucy

Acknowledgements:
The author thanks the IPGP Seismology and Tectonic Labs for the useful material and discussion.

__________________________________________________________________________

References :

[1] https://ds.iris.edu/wilber3/find_stations/10953070

[2] https://gempa.de/news/2018/9/28/tsunami-m75-earthquake-palu-indonesia/
[3] http://geoscope.ipgp.fr/index.php/en/
[4]  https://en.wikipedia.org/wiki/Greater_Sunda_Islands
[5] Socquet, A., W. Simons, C. Vigny, R. McCaffrey, C. Subarya, D. Sarsito, B. Ambrosius, and W.Spakman (2006), Microblock rotations and fault coupling in SE Asia triple junction (Sulawesi, Indonesia) from GPS and earthquake slip vector data, J. Geophys. Res., 111, B08409, doi:10.1029/2005JB003963
[6] http://www.thejakartapost.com/news/2018/09/29/central-sulawesi-quake-what-we-know-so-far.html
[7] Prasetya, G.S., De Lange, W.P. & Healy, T.R. Natural Hazards (2001) 24: 295. https://doi.org/10.1023/A:1012297413280
[8] https://en.wikipedia.org/wiki/2004_Indian_Ocean_earthquake_and_tsunami
[9] http://adsabs.harvard.edu/abs/2003AGUFMOS21D..06L
[10] https://www.reuters.com/article/us-indonesia-quake-toll/death-toll-from-indonesia-quake-tsunami-rises-to-1407-agency-idUSKCN1MD0KH
[11] https://www.nytimes.com/interactive/2018/10/01/world/asia/map-tsunami-indonesia.html
[12] Bouchon, M., et al., Faulting characteristics of supershear earthquakes, Tectonophysics (2010), doi:10.1016/j.tecto.2010.06.011

WOMEESA (Women in Earth & Environmental Science Australasia)

WOMEESA (Women in Earth & Environmental Science Australasia)
The condition of women in the workplace has not always been easy. Even though the mentality of the people is changing towards inclusion and equality, it is a long way before things really change. We always observe unbalanced number of women versus number of men, unbalanced competition among colleagues for career promotion and responsibility and unequal wages between women and men in many places. Moreover, the situation can even take a turn for the worse when the woman becomes pregnant and needs to handle her family life and her professional career at the same time. In this context the support of female colleagues in sharing experience becomes very important. By developing networking, we can get information about how to manage professional but also personal situation, discuss about inclusive workplace environment and relationship with colleagues.
Recently, WOMEESA (Women in Earth and Environmental Science Australasia) network organized a meeting at Macquarie University in Sydney for sharing experiences among different generations of women geoscientists ranging from industry to academia. The meeting started first with two invited speakers from Oil search (Julianne Lamb) and Pells Sullivan Meynink (Felicia Weir) representing industry. They shared their own experience from undergraduate to professional life and how they managed the difficulties -professional and personal- met during their lives. Then, the discussion focused on the career pathways of 4 different generations of geoscientists: Jessica Walsh (PhD candidate, University of Wollongong), Shari Gallop (Lecturer in Environmental science, Macquarie University), Kelsie Dadd (Curious Minds) and Anita Andrew (Environmental Isotopes). The panel showed the obstacles and complications that a female geoscientist can face (as for instance, competition with a male colleague, promotion/monitoring PhD candidate, find a balance between professional career and family life). The panel tried to reply these questions from their own experiences. Their conclusion pointed out the importance of developing a strong female network where to speak up, share and provide advices to common issues.
In the last part of the evening, working groups were formed to think at how WOMEESA can help the new generation of geoscientists. Among the suggestions: to encourage girls to study Earth Sciences during events such as the National Science Week, to organize social and gathering events where female geoscientists can develop a strong network, to let female students be monitored by female professors. Some of these ideas have already been implemented in other institutions such as the EGU or the AGU. Our wish is to have the opportunity to exchange in the future with WOMEESA.

 

This event was proposed by Heather Handley, Volcanologist at Macquarie University and co-founder of WOMEESA. We got the chance to ask her some questions:

 

Q: Why and when was the idea of WOMEESA born?

Heather : The idea to form a network of women working in the fields of earth and environmental sciences was born at the Dorothy Hill Women in Earth Science Symposium held at the University of Queensland in November 2017. We realised that despite some existing industry and field-specific networks, there was not a single overall network that included everyone working in these fields across industry, academia and government.

Q: In your opinion, how long will it take it to change the situation?

Heather: We need to change the present culture of equity and inclusion and so this will likely be a gradual change, but hopefully we will get there by providing continued pressure for change.

Q: What do we need to change the situation:

Heather: We need continued and greater support at national and institutional levels to ensure a culture of equity is supported and implemented. Initiatives such as Science in Australia Gender Equity (SAGE; http://www.sciencegenderequity.org.au/) and Superstars of STEM (https://scienceandtechnologyaustralia.org.au/what-we-do/superstars-of-stem/) are critical to this cause and are definitely making a positive difference.

Q: What is the most important thought we need to change first?

Heather: There has been much recent media coverage of the challenges and issues facing women in STEM careers and now it is time to see change. We need to see innovative solutions and support for women at all career stages, as well as those in the early stages in their career. Through our networking events, WOMEESA aims to identify these solutions and then work to bring them to reality.

Q: What will be the impact of WOMEESA on the evolution of the situation of women in academia or industry?

Heather: We hope that through the WOMEESA network we can do a multitude of things to improve the situation of women in academia, industry and government. We aim to:

  • Create a supportive network for all women in Earth and Environmental Sciences in Australasia

  • Develop new nodes in the region without current support and link to existing networks/societies

  • Facilitate greater collaboration between academia, government and industry in these fields

  • Provide role models of women in Earth and Environmental Sciences

  • Provide key support for women in the early stages of their career and those with carer responsibilities

  • Raise awareness of current gender equity issues

  • Develop strategies to support indigenous Australasians (in Australia, NZ and beyond)

  • Promote the importance of Earth and Environmental Science research and activities through social networks and the media

Q: Which public are you targeting and are you happy so far about the first events of WOMEESA?

Heather: Those interested in a career in earth and environmental sciences are encouraged to join WOMEESA. We support diversity and welcome all genders to the network. The network will be defined by the needs of its members and so this will be dynamic based upon our growing member profile. We currently have 280 members from 6 countries, with 96% being women. The majority of our members are postgraduate students and those at early to mid-career stage (75%) but we also have undergraduate students and members with greater than 15 years experience. At present, 64% of WOMEESA members are from academia, 17% from industry, 13% from government with the remainder employed in other sectors.

We are thrilled to have held the first WOMEESA networking event in Sydney. We are also hosting networking events at the inaugural Australian Geoscience Council Convention, AGCC 2018, in Adelaide, Australia in October 2018 and at the Geoscience Society of New Zealand (GSNZ) conference in Napier, New Zealand in November 2018. We are excited that the NZ Prime Minister’s Chief Science Advisor, Juliet Gerrard will be attending our event in New Zealand.

Q: A last word to encourage people to join WOMEESA?

Heather: We will be much more effective to create change and support women in earth and environmental sciences if we form a critical mass with a unified voice. Through the hard work of some of its members, WOMEESA have already shown the power of the network by successfully lobbying for the provision of childcare at the Australian Geoscience Council Convention (AGCC 2018) in October.

If you want to learn more about WOMEESA do not hesitate to check their website (https://www.womeesa.net).

 

 

 

This post was written by Walid Ben Mansour, with revisions from Maria Tsekhmistrenko and Marina Corradini

 

Walid Ben Mansour is a post-doctoral research fellow at Macquarie University. He works on multi-observable probabilistic tomography for different targets (mining, seismic hazard). You can reach him at walid.benmansour[at]mq.edu.au

Maria Tsekhmistrenko is a PhD student at the University of Oxford. She works on the velocity structures beneath the La Reunion Island from the surface to the core mantle boundary. You can reach her at mariat[at]earth.ox.ac.uk

 

What’s Shaking? – Facts and figures of latest earthquakes

Haven’t you heard about the recent major earthquakes which stroke the globe? No worries, the ECS-reps will post a quarterly report with basic information about the latest earthquakes you can’t miss. Let’s start with June, July and August 2018. 

 

Figure 1. Location of Mw> 5.0 earthquakes (small circles with colour scale showing the depth). Harvard CMT of death tolls events and the greatest event of magnitude Mw8.1 recorded in the period June-July-August.

 

Between the 1st June and the 31st August, 425 earthquakes with magnitude Mw above 5.0 were recorded by the international network broadband seismic instruments and listed in the USGS catalogue (figure 1). Among them, 32 events showed a magnitude Mw> 6.0 and 4 large earthquakes (Mw>7.0) were observed.

Among these 425 earthquakes, we can count 200 shallow events (<20 km) and 76 deep earthquakes (>100 km). During this last quarterly, the seismic activity in the lesser Sunda islands (red square on figure 1) was very active, as we will see later in this report.

 

A zoom on South East Asia and Central America

Subduction zones in South East Asia and Central America knew a strong activity this summer in comparison with other regions. Let’s have a look at South America: we can observe 2 large earthquakes occurred in August (Venezuela on the 21st and Peru on the 24th) few days after the Fiji earthquake (Mw8.1) on the 18th (Table 1). Could we argue that the Fiji earthquake triggered the Venezuela earthquake (Mw7.3) and that, in turn, the Venezuela earthquake triggered the Peru earthquake (Mw7.1)? A recent paper by O’Malley et al. [2018] supports the idea that a large earthquake can trigger teleseismic events[1].

 

Table 1. List of death tolls in the period June-July-August.
Location Date Time (UTC) Mw Mechanism
Osaka Prefecture* 17/06/2018 22:58:35 5.5 Strike slip
Lombok island* 28/07/2018 22:47:37 6.4 Dip slip (thrust)
Lombok island* 05/08/2018 11:46:37 6.9 Dip slip (thrust)
Lombok island* 19/08/2018 14:56:26 6.9 Dip slip (thrust)
Fidji islands 19/08/2018 00:19:37 8.1 Dip slip (thrust)
Venezuela 21/08/2018 21:32:42 7.3 Dip slip (thrust)
Peru-Brazil 24/08/2018 08:04:07 7.0 Dip slip (thrust)
Kermansha* 25/08/2018 23:06 6.0 Strike slip

 

In spite of the progress in engineering seismology and a better awareness of the rules of conduct in case of earthquake, some of these events always kill relatively too many people. That’s why we call them  “death toll”.During this summer, 564 persons were killed after death tolls in Osaka in June (Mw5.5), in Lombok-Sumbawa in July (Mw6.4) and Kermanshah on August (Mw6.0).

 

June 2018

June was characterized by 120 seismic events with magnitude Mw above 5.0 and only 1 event with Mw6.1 in Vanuatu. This month was also characterized by a strong volcanic activity in Hawaii (26 events with Mw above 5.0 were recorded).

 

 

Figure 2.a. On the left: Location and CMT Harvard Solution of Osaka quake (17 June 2018) and Kobe quake (19 January 1995). Red dots correspond to earthquakes with magnitude Mw>5.0. On the right: Seismograms of the Osaka earthquake from the global seismic network.

 

 

On 17 June, a Mw5.5 hit the northern region of Osaka Prefecture (figure 2.a.) where more than 9 million people live [2]. In this area, the seismicity is controlled by a system of strike slip faults [3] which accommodate the convergence of the Philippine plate and the Eurasian plate. The seismic activity of this region is relatively quiet in comparison with the northern part of the islands and subduction earthquakes. Nevertheless, the region was struck by 2 great historical earthquakes (Mw>7.0) in the last 500 years (1995 in Kobe Mw7.2 and 1596 in Keicho Fushimi M7.5) [4,5]. In 1995, the Mw 7.2 Kobe earthquake broke the 40 km long and 20 km wide fault plane of the Arima-Takatsuki fault zone (ATFZ). This quake killed 6,433 people and cost $100 billion.

 

 

Figure 2.b. Site amplification factor map of Osaka Prefecture (source: http://www.j-shis.bosai.go.jp/)

 

 

The 17 June event was located in the same area where a large earthquake was expected in the last 30 years after the 1995 earthquake [6]. Seismic hazard studies in this region show a strong site amplification of the area to ground motion (figure 2.b.). The relatively low magnitude event was amplified in some areas [7] and became a death toll due to collapsing concrete walls [8]: a danger knew since the Miyagi earthquake (Mw7.4) in 1978 [9], where several aspects of engineering seismology and soil behaviour started to be developped in Japan.

 

July 2018

July was characterized by 132 seismic events with magnitude Mw above 5.0 and 9 events higher than 6.0. Volcanic activity in Hawaii continued during this month with about 22 events recorded. As in June, July also knew a death toll event.

 

Figure 3. Location and Harvard CMT solution of the Lombok earthquake (Mw6.4) and historical great earthquakes (Mw>7.0) in the area. Local seismicity (red dot) during the month. Death tolls are shown in grey.

 

On the 28th July, a magnitude Mw6.4 event [10] shook Lombok island (Indonesia) killing 20 persons and injuring 401 (figure 3). Lombok island is a paradisiac island east of Bali with a population of about 3.4 million people. The region had previously known large earthquakes along the Sunda subduction zone (Table 2): in 1977 in Sumba (Mw8.3), where 177 people were killed and 1,100 injured; in 1994 in Java (Mw7.8), with 250 killed and a triggered tsunamis [11,12]; in 1992 in Flores islands (Mw7.8) with 2,500 deads.

 

Table 2. Death tolls along the Sunda subduction zone.
Location  Date  Mw Mechanism 
Sumbawa islands 19/08/1977 8.3 Dip slip (Normal)
Flores islands 12/12/1992 7.8 Dip slip (thrust)
Java islands 03/06/1994 7.8 Dip slip (thrust)
Lombok islands 28/07/2018 6.4 Dip slip (thrust)

 

A zoom on the Lombok island
The Mw6.4 Lombok event nucleated within the megathrust subduction zone in the south and the Flores back arc thrust fault in the north [13]. Even if this event occurred in a shallow part of the crust (about 15 km deep), no tsunami was reported afterwards. Recently, Koulali et al. (2016) [14] show that, from GPS measurement analysis, this region could expect a large earthquake (Mw>7.0) within the next years, implying that the Mw6.4 event could change the recurrence time of this kind of events in the future.

 

Figure 4.a. Seismicity across the Lombok island and the location of the Mw6.4 event (28th July 2018).

 

Lombok island is also characterized by the presence of an explosive volcano (Rinjano, 3726 m) which knew its last eruption in 2016 [15]. The 2018 earthquake did not trigger any eruption but rather a landslide of the northern face where the seismicity was mainly concentrated (figure 4.a.). More than 1,200 hikers were stuck on the volcano [16], 5,448 houses were destroyed and more than 10,000 were displaced [17] (figure 4.b.).

 

Figure 4.b. Damages after the Lombok event (source: AFP)

 

 

August 2018

August was characterized by 168 seismic events with magnitude Mw above 5.0 and 22 events higher than 6.0 (with 4 large earthquakes). The low volcanic activity in Hawaii continued during this month with about 1 event recorded. On the 5th August a  Mw6.9 event (figure 5) stroke Indonesia again in the same area [18,19].

Figure 5. At the top: Location and Harvard CMT solution of the Mw6.4 earthquake in July (in black) and the Mw6.9 in August (in blue). In red, Harvard CMT solution of historical great earthquakes (Mw>7.0) and local seismicity (red dots) during the month of August. At the bottom: Map of the ground deformation from satellite data analysis. (source: NASA/JPL Caltech)

 

August was the worst month of the year with 513 killed. Strong seismic activity [20,21] is observed on Lombok island generating a series of landslides but no eruption was reported. This Mw6.9 event also caused the uplift of the northern part of the island (maximum uplift of about 25 cm) [22,23]. Another event hit Lombok island 2 weeks later on the 19th August with a Mw6.9. This event also produced an uplift in the northen part, but affecting another fault.

 

Figure 6. Location and Harvard CMT of Kermansha earthquake (M6.0 on 25th August) and historical great earthquake in the area. Death tolls are shown in grey.

 

August also recorded the biggest event of the year with a Mw8.1 event (figure 1 and Table 1) near Fiji [21] without damaged, thanks to its depth (~ 560 km). Later in the month, on the 28th August, another death toll of Mw6.0 (3 deaths and 243 injured people) hit Iran (figure 6).

 

 

Table 3. List of death tolls in Iran
Location  Date Mw Mechanism 
Tabas 16/09/1978 7.3 Dip slip (thrust)
Manjil Rudbar 20/06/1990 7.4 Strike slip
Kermanshah 12/11/2017 7.4 Dip slip (thrust)
Kermnshah 25/08/2018 6.0 Strike slip

 

This region has already been stroke in the past: on the 12th November 2017, a Mw7.3 event killed about 630 persons [24,25]; eastern, the Manjil Rudbar quake (Mw7.4) in 1990 (figure 6) killed between 35,000 and 50,000 people [26,27]. This strong seismic activity in Iran is due to thecollision zone between the Arabia plate and the Eurasian plate, originating the Zagros Fold thrust belt, and the presence of several active strike slip faults. As we can see on figure 6, the region was stroke by 8 large earthquakes (Mw7.0) in less than 50 years and 4 death tolls listed in Table 3.

 

 

This post was written by Walid Ben Mansour, with revisions from Lucile Bruhat and Marina Corradini

 

 

Walid Ben Mansour is a post-doctoral researcher at Macquarie University, Australia. He works on multi-observable probabilistic tomography for different targets (mining, seismic hazard). You can reach him at walid.benmansour[at]mq.edu.au

Lucile Bruhat is a post-doctoral researcher at the Ecole Normale Supérieure (ENS) in Paris. She works on the description and understanding of the physical processes that underlie the earthquake cycle, through geodetic/seismological data analysis and numerical modeling. You can reach her on Twitter at @seismolucy

 

____________

References

[1] O’Malley et al.(2018). Evidence of Systematic Triggering at Teleseismic Distances Following Large Earthquakes. Scientific Reports, 8 (1) DOI: 10.1038/s41598-018-30019-2 

Osaka Earthquake
[2] Powerful quake shakes Osaka: three killed, 307 injured. The Asahi Shimbu. http://www.asahi.com/ajw/articles/AJ201806180009.html
[3] Powerful quake that jolted northern Osaka Pref occurred near fault zone.The Mainichi. https://mainichi.jp/english/articles/20180618/p2a/00m/0na/015000c
[4] Kanaori and Kawaka (1996). The 1995 7.2 magnitude Kobe earthquake and the ArimaTakatsuki tectonic line: implications of the seismic risk for central Japan. Engineering Geology,43, 135–150
[5] Lin et al. (1988). Paleoseismic events and the 1596 Keicho-Fushimi large earthquake produced by a slip on the Gosukebashi fault at the eastern Rokko Mountains, Japan.The Island Arc,7,621–636.
[6] Today’s deadly Japan earthquake could be related to the 1995 Kobe earthquake. Temblor blog- http://temblor.net/earthquake-insights/todays-deadly-japan-earthquake-could-be-related-to-the-1995-kobe-earthquake-7347/
[7] Soft soil added to intense shaking of quake on Osaka plain. The Mainichi. https://mainichi.jp/english/articles/20180619/p2a/00m/0na/022000c
[8] Osaka quake highlights dangers posed by concrete-block walls. The Japan Times- https://www.japantimes.co.jp/news/2018/06/19/national/osaka-quake-highlights-dangers-posed-concrete-block-walls/#.W5daOi2Q0Wo
[9] An investigation of the Miyagi ken oki, Japan, earthquake of June 12.1978. NBS Special publication-592.

Lombok Earthquake 
[10] NTB declares seven-day state of emergency after earthquake that claims 17 lives- Jakarta Post- http://www.thejakartapost.com/news/2018/08/01/ntb-declares-seven-day-state-of-emergency-after-earthquake-that-claims-17-lives.html
[11] Gusman et al. (2009). Analysis of the tsunami generated by the great 1977 Sumba earthquakes that occurred in Indonesia. Bulletin of the Seismological Society of America,99 (4),2169–2179.
[12] Abercrombie et al. (2001). The 1994 Java tsunami earthquake: slip over a subducting seamount. Journal of Geophysical Research,106 (B4),6595–6607.
[13] Two types of tectonic plate activity create earthquake and tsunami risk on Lombok. The conversation- https://theconversation.com/two-types-of-tectonic-plate-activity-create-earthquake-and-tsunami-risk-on-lombok-101177
[14] Koulali et al. (2016). Crustal strain partitioning and the associated earthquake hazard in the eastern Sunda-Banda arc. Geophysical Research letter, 43(5)
[15] Indonesia evacuated tourists after volcano eruption. BBC news-https://www.bbc.com/news/world-asia-37492144
[16] Key route cleared for hundreds of trekkers stranded on quake hit Indonesian volcano. ABC news- http://www.abc.net.au/news/2018-07-30/key-route-cleared-for-trekkers-stranded-on-quake-hit-indonesian/10052138
[17] Major earthquake hits Indonesia’s Lombok island. News.com.au- https://www.news.com.au/technology/environment/major-earthquake-hits-indonesias-lombok-island/news-story/d5f1e8508c88e5e73423073cf922807c
[18] M=6.9 earthquake near Bali leaves at least 98 dead and 20,000 homeless. Temblor blog- http://temblor.net/earthquake-insights/m6-9-earthquake-near-bali-leaves-at-least-98-dead-and-20000-homeless-7477/
[19] Indonesian earthquakes cause death and panic. Sydney morning herald- https://www.smh.com.au/world/asia/indonesian-earthquakes-cause-deaths-panic-20180820-p4zymh.html
[20] Earthquake swarm lights up Indonesia- Temblor blog- http://temblor.net/earthquake-insights/earthquake-swarm-lights-up-indonesia-7531/
[21] Lombok and Fiji-or why a M6.9 earthquake can be worse news than a 8.2 events- EGU blog SM division- https://blogs.egu.eu/divisions/sm/
[22] Indonesian island lifted 10 inches by deadly quakes. Fox news http://www.foxnews.com/world/2018/08/11/indonesian-island-lifted-10-inches-by-deadly-quake.html
[23] New satellite map shows ground deformation after Indonesian quake. https://phys.org/news/2018-08-satellite-ground-deformation-indonesian-quake.html 

Kermanshah earthquake
[24] Magnitude 7.3 earthquake in Iran-Iraq is the deadliest of 2017-Forbes- https://www.forbes.com/sites/trevornace/2017/11/14/magnitude-7-3-earthquake-iran-iraq-deadliest-2017/#30cc902e76d0
[25] Damaging magnitude 7.3 earthquake along the Iran-Iraq border was preceded by magnitude 4.3 foreshock- Temblor blog- http://temblor.net/earthquake-insights/damaging-magnitude-7-3-earthquake-along-the-iran-iraq-border-was-preceded-by-magnitude-4-3-foreshock-5510/
[26] Berberian and Walker (2010). The Rudbar Mw7.3 earthquake of 1990 June 20; seismotectonics, coseismics and geomorphic displacements, and historic earthquakes of the western ‘High Alborz’, Iran, Geophysical Journal International, 182 (3): 1577–1602
[27] Toll in Iranian quake is put at 29,000. The New York Times- https://www.nytimes.com/1990/06/23/world/toll-in-iranian-quake-is-put-at-29000.html 

Lombok and Fiji – or why a M6.9 earthquake can be worse news than a M8.2 event

Two magnitude 6.9 earthquakes in Indonesia in the space of two weeks, 20 km apart. Meanwhile, a magnitude 8.2 event in the Pacific. Did you get any questions about the end of the world being upon us, how come all these quakes happen so close together and why the Fiji event was so harmless?

 

Seismicity in the years 2000-2018 around the Sunda and Banda Arcs, colour coded by depth. The location of Lombok (where the recent series of devastating earthquakes took place) is indicated by the red circle

 

 

Latitude Longitude Origin time depth Magnitude Region
8.2597° S 116.4363° E 2018-08-05 11:46:38 UTC 31.0 km Mww6.9 Sumbawa Region, Indonesia
8.324° S 116.6256° E 2018-08-19 14:56:28 UTC 25.62 km Mww6.9 Sumbawa Region, Indonesia
18.1777° S 178.1109° W 2018-08-19 00:19:37 UTC 563.41 km mww8.2 Fiji Islands Region

 

 

Lombok

The quakes in and around the island of Lombok, Indonesia, fall within a larger series of events. Apart from the two Mw 6.9 quakes on 5 and 19 August, there was a whole set of Mw 5+ and Mw 6+ in the surrounding days, not to mention the hundreds of smaller fore- and aftershocks (see Figure 1).
This huge seismic activity around Lombok is hardly surprising: Indonesia lies on the Pacific Ring of Fire, where the Australian oceanic crust subducts beneath the Eurasian plate along the Sunda Arc with a convergence rate of about 7 cm/yr [Nugroho et al, Tectonophysics 2009], consequently generating a lot of seismic and volcanic activity. To the east, subduction has progressed further, such that the Australian continent is now in collision with the Indonesian islands.
The recent quakes are, however, not direct subduction earthquakes – the main subduction zone lies south of the islands and at the location of these earthquakes (between 20km and 30km), the subducted Australian plate sits much deeper. The location and focal mechanism therefore point to a shallow south-dipping thrust fault that is part of the Flores Back-arc Thrust system [1, 2].

 

Seismicity in the Sunda/Banda arcs region, and the recent earthquakes around Lombok

Left: Map of seismicity in the Sunda / Banda arc region. Events were retrieved for the period 2000 to 21 August 2018 from the USGS NEIC catalog, and are colour-coded by depth. Lombok is indicated by a red circle. Right: The three Lombok events discussed in the post, as well as the fore-and aftershocks as recorded between 1 July and 21 August 2018. Fore- and aftershocks are colour coded by event magnitude

 

The series of earthquakes started with a Mw 6.4 event on 29 July, that killed around 20 people and injured hundreds [3]. About a hundred aftershocks followed, and roughly a week later, on 5 August, an even larger Mw 6.9 event shook the area again, thereby degrading the 29 July event to a foreshock. The Mw 6.9 event was much more destructive with a death toll of more than 450 and counting, not to mention the hundreds of thousands of people who lost their homes, properties and got displaced [4].
A tsunami warning was issued for a while, but was retracted when it became clear no dangerous waves were excited – otherwise the damage might have been even more widespread. Then, two weeks later and just 20 km to the west, another M 6.9 earthquake hit the island on 19 August [5]. Although of the same magnitude as the 5 August event, the 19 August earthquake resulted in far fewer casualties (around ten reported so far [6]) – presumably because of the destruction and evacuation come after the first earthquake.
As anyone living in an earthquake-prone region will tell you, it is common for a whole series of quakes to strike the same area within a short time: the release of stress on one part of a plate contact or system of faults causes another part to become instable, and some argue that this can even happen over very large distances [Stein & Toda, Science 2012]. That’s the reason why, so shortly after this series of destructive events, people on the Lombok island continue to remain on alert, with many preferring to sleep under shelters – even if their homes are still intact.

 

So what about the M 8.2 event near the Fiji islands?
It also happened on 19 August, was more than 30 times as strong, but barely made it to the news.

 

Fiji

The 19 August event is a typical deep-focus earthquake and falls within the Tonga subduction zone. At the Tonga Trench, part of the Pacific Ring of Fire, the Pacific plate subducts steeply beneath the Australian plate. The normal faulting focal mechanism, location and depth indicate deformation within the subducted slab (Figure 3).
19 August 2018 earthquake near Fiji and background seismicity in the Tonga trench region

Figure 3: The recent M8.2 earthquake in the Tonga trench region. Background seismicity is all events of magnitudes M>5 as reported by the USGS NEIC catalog.

Both strong events and deep events occur quite regularly along this trench, but what makes the 19 August earthquake quite unique is the fact that it is both strong and deep. However, the USGS has received very few “Did you feel it” reports*: fortunately for the people living on the Pacific islands, its depth of 560 km meant that any devastating surface waves were hardly excited at the surface and damage seems limited so far [8].
On Lombok, on the contrary, the matter cannot be addressed lightly. Efforts have started to clean the rubble and rebuilding houses, schools and hospitals. But this proves to be hard: most of the infrastructure, especially in the north of the island, has been gravely damaged by the earthquakes themselves, or ensuing landslides. Many people are living in temporary shelters, and it is difficult to supply food, clean water, and medicines to the area.

 

No, the Lombok and Fiji events are not likely to be related, in case someone asks you (although I’m probably preaching to the choir). There are roughly 7000 km between the islands and the only thing they share is a contact with the Australasian plate.

 

 

This post was written by Nienke Blom, with revisions from Marina Corradini

 

 

[*]: The largest ever recorded deep-focus event was the Mw 8.3 Sea of Okhotsk earthquake, which occurred off the Kamchatka Peninsula on May 24th, 2013 at 600 km depth. This earthquake attracted special attention because, among other issues, it was felt all over Asia [Chebrova et al, J Volc. Seismol. 2015] , despite the large epicentral distance.

 

Note: data were downloaded from the IRIS and NEIC catalogs using obspyDMT [Hosseini and Sigloch, Solid Earth, 2017] figures were produced using matplotlib based on obspyDMT scripts.