Geohazard report – Tokyo.

Following the catastrophic earthquake and tsunami in Northeast Japan in March 2011, is Tokyo at greater risk of destruction?

Plate tectonics as a theory to explain earthquakes, volcanism and continental migration over geological timescales is a mature theory that has developed and been refined since Alfred Wegener first proposed his theory in 1912 (Wegener, 1966), through to the development of the theory in the 1970’s (Dickinson, 1974). The movement of these tectonic plates, and the subsequent frictions in the upper 10 to 15 KM between them is the principle cause of energy release which manifests itself in the form of earthquakes and sudden displacement of large sections of these plates. Below the upper crust boundary (10 to 15 Km depth) the crust accommodates plate movements through ductile deformation, whilst elastic-brittle characteristics are dominant in the upper crust, thus most earthquakes originate from depths less than 15 Km (Maggi et al, 2000).

The friction between the boundaries of two joining tectonic plates and the speed of plate movement dictates the frequency and magnitude of the plate movement when it occurs (Lachenbruch, 1980). In a low friction environment frequent lower magnitude quakes can be a sign that energy is being released, thus preventing the accumulation and subsequent release of larger magnitude events (Kanamori and Anderson, 1975).

Release of energy in the form of earthquakes causes earth movement and shaking, but if the event occurs within a subaqueous setting, and there is either a substantial movement of seabed either in the form of plate thrust and uplift, or a major seafloor avalanche then the sudden displacement of large amounts of water can cause a tsunami (Song et al, 2017).

Tsunamis can travel through deep waters such as found in the open Pacific at tremendous speeds, and as experienced in the ‘boxing day tsunami’ of 2004 (Farrell et al, 2015), coastal areas physically remote from the location of the earthquake can be dramatically and devastatingly affected. Because of the remote nature of tsunami threats, the extensive seismic activity surrounding the north Pacific coastlines of East Asia and North America, the so called ‘ring of fire’, means the threat of a tsunami source is extensive (Kânoğlu and Synolakis, 2015).

Japan sits to the West of a number of major subduction zones, with the Pacific plate subducting beneath the Okhotsk plate, as well as the Philippine plate subducting beneath the Amuria and Okhotsk plates (Figure 1) (Schellart et al, 2011). The rate of plate movement in this region is amongst the highest in the world (Figure 2), with Tokyo itself being located on the Okhotsk plate and very close to all other plates’ subduction margins mentioned above (Uchida et al, 2016).

1

Figure 1. Map showing the tectonic plate configuration surrounding Japan. Plate movement direction (large red arrows) and subduction zones (red lines) marked. Also marked is the focal point of the 11 March 2011 earthquake and tsunami.

Source: Ozawa et al, 2011.

2

Figure 2. Global tectonic plate boundaries with plate movement direction shown by green arrows, blue arrows indicate the rate of subduction, and red arrows indicate the rate of trench migration.

Source: Schellart et al, 2011:Figure 2.

 

Due to the regions extensive and frequent experiencing of seismic activity, the Japanese have rigorous building control measures in place (Imamura et al, 2018), as well as public awareness programs (Esteban et al, 2018) and sea defences in preparation for earthquakes and subsequent tsunamis (Suppasri et al, 2016). The magnitude of the event is key however in deciding the degree of preparation required. The last decade alone has seen 15,343 earthquakes in and around Japan between 3.5 and 7.5 magnitude event (See figure 3 for area parameters: USGS, 2018), with 124 events in the last 100 years ranging from 7 to 7.9 magnitude (Figure 3) (United states geological survey, 2018).

3

Figure 3. Earthquakes occurring near to Japan in the last 100 years equal to or above magnitude 7 intensity. Yellow line indicates highest previously recorded event of 8.5 magnitude, with the red circle highlighting the 11 March 2011 event of magnitude 9.1.

Source https://earthquake.usgs.gov/earthquakes/ (Area parameters used 47.142° (N), 25.618° (S), 120.85° (W), 152.93° (E).

Hazards arising from any earthquake event can be separated in to two key areas; earthquake related hazards such as liquefaction, landslip/landslides, ground shaking, and ground rupture; and tsunami hazards from land inundation by catastrophic volumes of water displaced by seafloor movement, directly related to the earthquake (Dunn et al, 2012). The magnitude of the 2011 event has meant that legislation, preparation and mitigation of future seismic events now incorporate the expectation and possibility of megathrust earthquakes happening (Santiago-Fandino et al, 2017).

11th March 2011 saw one of the largest earthquake in recorded history to strike Japan, registering as a magnitude 9.0 event (Ozawa et al, 2011; Hooper et al, 2013). 15,896 people lost their lives, with a further 8,694 people either injured or reported missing (National Police Agency of japan, 2018). Economic losses to the Japanese economy were estimated to be up to US$235 Billion (World Bank, 2011), with the damage to infrastructure and particularly the nuclear installation at Fukushima Daiichi causing an estimated 59,000 people still unable to return to their homes as of January 2016 (Japan Times, 2016).

Occurring at 14:46 JST (Japanese standard time), the oceanic megathrust earthquake was located 70 kilometres east of the Oshika Peninsula, rupturing an extended section of the fault plane ~500Km long (Suzuki et al, 2011; United States Geological Survey, 2016). A megathrust earthquake is one of the most destructive magnitude earthquake events (usually >9 magnitude) and is differentiated from other earthquakes by its intensity and potential to generate very large tsunami waves (Biley and Lay, 2018). The elastic energy retained within the Okhotsk Plate, generated by the frictional force of the subducting Pacific Plate passing beneath it, was dramatically and explosively released. The damage from the earthquake was then followed by a tsunami as the significant movement of the seafloor (~62m movement) displaced massive volumes of seawater (Sun et al, 2017). This reached up to 40m high tsunami waves as the waters reached the Japanese shoreline and inundated the coastal regions by up to 10 Km inland (Mori, 2011).

Whilst the megathrust earthquake was expected (Davis et al, 2012), the predicted 8 to 8.6 magnitude force prediction was significantly underestimated. As seen in figure 3 though, the unprecedented magnitude of the 2011 earthquake in comparison to the previous 100-year record demonstrates the extreme nature of the scale of the event. Research since the 2011 event is now incorporating historical data such as geological evidence in the form of previous tsunami deposits within coastal stratigraphy (Wallis et al, 2018) and historical records which are extensive in Japan which detail tsunami damage and deaths dating back millennia (Ouzounov et al, 2018).

Seismic monitoring in the North West Pacific region is some of the most detailed and extensive in the world (Huang et al, 2017), but as demonstrated in the 2004 Boxing Day tsunami which affected the Indian Ocean originating off the Western coast of Northern Sumatra (Lay et al, 2005), travel distances of tsunami waves can be global with deaths and damage being inflicted ~8000 Km away in Cape Town, South Africa (Mail and Guardian, 2004). This extensive range of the impact of a megathrust earthquake in the form of a tsunami means that extensive parts of ‘the ring of fire’ perimeter of the Pacific could be the origin of a tsunami that could affect Japan (Hinga, 2015). So, although the 2011 event has dissipated some energy in this particular length of plate boundary through the earthquake, tensional increases in plate boundaries adjoining the affected area may have been increased (Toda et al, 1998) including plate margins close to Tokyo, Japan’s capital. This increase in potential fault stress raises the risk of tsunami sources originating close to the Japanese coastline, a major factor in the intensity of wave height experienced in the 2011 event. Japan also must consider the possibility of other areas within the Pacific ‘ring of fire’ generating significant seismic events that could impact their shorelines in the form of tsunami waves.

With a population of 13.8 million people, and an extended metropolitan area encompassing over 38 million people (Tokyo Metropolitan Government Bureau of Statistics Department), Tokyo is the world’s most populous metropolitan area (UN, 2017). With population density of  up to 14,883 people/km2 within the cities prefecture (Statistics Bureau, 2018), hazards and the magnitude of the casualties would be compounded and exacerbated by the population levels present (Uitto, 1998). Population density of the Ibaraki-ken prefecture (one of the worst affected areas in the 2011 event) was just 485 people/km2, and even though casualty figures would not be directly proportionate, the resultant numbers of injured or dead would be extreme in an event affecting Tokyo.

Tokyo, Japan’s capital and largest urban conurbation, has additional unique factors which mean that seismic hazard events are incredibly difficult to predict, with the historic record showing three major earthquakes affecting Tokyo in 1703, 1855 and 1923 (Bozhurt et al, 2007). Immediately following the 2011 seismic event to the North of Tokyo, off the Oshika peninsular, a magnitude 7.9 event was recorded close to Japan just 30 minutes after the main earthquake (Simons et al, 2011). The scale of the 2011 event, and the proximity to the countries capital has meant that city impacting hazard models have had to be revised their potential maximum magnitude (Imamura et al, 2018).

Located at the juncture of three tectonic plates, Tokyo is located close to the Sagami trench, a zone in which the complex subduction may well limit the maximum magnitude of earthquake to hit (Toda et al, 2008). Below Tokyo, due to the interaction of two separately subducting plates, a complicated layering is potentially underway known as the Kanto fragment (Stein et al, 2006), causing frictional tensions at two separate boundaries and thus increasing the complexity of earthquake prediction (Figure 4).

4

Figure 4. Stylised representation of the tectonic plate configuration caused by the subduction of both the Pacific and Philippine plates beneath the Okhotsk plate, and the layered configuration of the lithosphere. (Insert shows Japan in relation to the Western Pacific. Source: Modified from the Guardian Newspaper  https://www.theguardian.com/world/2015/may/31/japan-alert-powerful-earthquake

 

Topography considerations are also a major factor to consider for Tokyo’s tsunami risk, as the city sits at the North Western edge of Tokyo bay, a large shallow bay with a narrow 9 Km opening (Figure 5). Modelling of tsunami risks affecting Tokyo itself produce conflicting results with the narrow entrance to Tokyo Bay potentially protecting the city or amplifying tsunami damage (Sasaki et al, 2012) depending upon the magnitude and location of source used within models.

5

Figure 5. Stylised 3D map of Tokyo Bay and the surrounding bathymetry showing the constrained entrance to the bay. Source: Google maps.

Liquefaction hazards in Tokyo have been identified following the 2011 event by using Lidar surveying pre and post event (Konagai et al, 2012). This process has generated high resolution results to determine areas affected, and subsequently at future risk of soil subsidence (Figure 6) (Yasuda et al, 2012). Identification of risk allows remediation and prevention works to be undertaken to prevent future earthquake damage due to liquefaction (Bhattacharya et al, 2011).

6

 

 

Figure 6. Example of areas within Tokyo mapped after the 2011 event, demonstrating liquefaction predominantly on reclaimed land. Source: Yasuda et al, 2012

 

 

Building codes (dictating earthquake resilience) were updated extensively in 1981 throughout Japan, dictating that buildings should be built to remain standing when affected by an earthquake of 7 magnitude or higher (Aoyama, 1981). This is a requirement of new buildings constructed after 1981, therefore buildings constructed prior to this date (known as Kyu-Taishin) have a potentially lower resilience to high magnitude earthquakes compared to those constructed post 1981 (known as Shin-Taishin) (Sharma and Louzado, 2015). Kyu-Taishin (older, less resilient buildings) constitute ~20 to 30% of buildings throughout Japan, and therefore continue to be a potential hazard source as demonstrated in the 1995 Hanshin earthquake where 8.4% of older buildings were seriously damaged, compared to 0.3% of Shin-Taishin (building constructed to the newer more stringent building regulations).

To answer the original question, as to whether Tokyo is at greater risk of destruction following the 2011 event, we need to define ’risk’. Disaster prediction and prevention defines risk as the hazard multiplied by vulnerability, divided by the resilience or ‘capacity to cope’ (Smith, 2003). In Tokyo’s current situation, it can be argued that the hazard of a megathrust event has been increased due to the changes in tension regime along the faults near to Tokyo (Stein, 1999). Awareness of this risk, and specifically a realisation of the scale of the potential magnitude of events, which may well exceed the maxima encountered within recorded history, enables relevant bodies and individuals to be prepared. This ability to prepare and engineer for events far exceeding previously estimated maximum magnitudes can diminish the vulnerability of those within the affected area, as well as increasing the resilience and ability to cope with a major event (such as the 2011 event).

After considering all the evidence examined within this discussion, I feel even though the hazard from a megathrust event and subsequent tsunami may have increased since the 2011 event, raised awareness of the potential hazard may have diminished vulnerability and increased resilience, thus meaning risk has remained at the level prior to the 2011 event.

 

 

 

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