Mitchell May, 260556044, EPSC 330

The 2011 Tohoku Earthquake and Tsunami Sequence

Mitchell May, 260556044

EPSC 330

Mitchell May, 260556044, EPSC 330

The 2011 earthquake sequence east of Tohoku is classified as a megathrust

earthquake off the east coast of Japan. The earthquake had a moment magnitude of Mw 9.0,

which was calculated from CMT data with periods over 300 seconds (Nettles, 2011). The

primary rupture occurred at 05:46 am (UTC) on March 11th

2011, after several foreshock

events. The location of the earthquake epicenter was nearly 70 km east of the Oshika

Peninsula of Tohoku. With a submerged hypocenter depth of approximately 30 km, the Tohoku

event places fourth in the most powerful earthquakes recorded this century. It was also the most

powerful earthquake recorded to have hit Japan using modern data collection, but events such as

the 869 Mw 8.9 Sanriku event are similar in comparison.

The earthquake resulted in a megathrust earthquake generated tsunami with wave heights

of up to 40.5 m observed in Miyako (Okayasu, 2011). Inundation from the tsunami spanned

much of the east coast of Japan, with maximum flooding reaching nearly 10 km inland in the

Sendai region (Buerk, 2011). The earthquake shifted Honshu 2.4 m east, and moved the Earth on

its axis by estimates of between 10 cm, and 25 cm (Kenneth,

2011).

Figure 1: This figure illustrates the island of Tohoku, and its

relationship with the neighbouring fault system. Horizontal

displacements throughout the island are shown as red

arrows, observed by the GPS Earth Observations Network

of Japan. The source fault is outlined by a rectangular grid,

which encloses areas of large slip (blue and brown ellipses).

The orange star denotes the main shock epicenter, while

Mitchell May, 260556044, EPSC 330

foreshocks and aftershocks are labelled in white and orange circles, respectively (Koketsu,

2011).

Geologic Setting

The 2011 Tohoku earthquake occurred along a shallow subduction fault between the

subducting Pacific plate, and overlying North American Plate. The fault which gave rise to the

2011 Tohoku earthquake is part of a much larger fault system, commonly referred to as the “ring

of fire”. This fault system surrounds the Pacific plate, and acts as a means to accommodate

oceanic crust formation in Pacific mid ocean ridges. The ring of fire has been active for hundreds

of millions of years in some regions, with slip recurrence intervals varying with the geologic

setting.

Figure 2: The figure above shows the ring of fire over the Pacific Ocean. The region along the

east coast of Japan is referred to as the Japan Trench.

In an effort to constrain the recurrence interval of the subduction zone neighbouring

Tohoku, scientists turned to inundated sediments for answers. Due to the oceanic setting of this

subduction zone, past megathrust earthquakes can be correlated to tsunamis. The resulting

Mitchell May, 260556044, EPSC 330

tsunamis propagate vast distances inland, transporting marine sediments with them. Scientists

observed this phenomenon in some of the native Holocene sediments of Japan, and were able to

constrain 3 inundation events as a result. The sedimentation observed was believed to be formed

within the past 3000 years, which infers a recurrence interval of 800-1100 years for megathrust

ruptures in the region. Considering the most recent regional megathrust in 869, it is apparent that

the Tohoku earthquake was a recurrence of this fault mechanism. Megathrust faulting in this

region; however, is believed to be a result of a much larger tectonic sequence.

Tectonic Sequence

With a vast number of unstable faults in the region, smaller earthquakes result in energy

propagation, which influence the slip conditions of surrounding faults. In the case of the Tohoku

earthquake, the megathrust was initiated by a foreshock sequence, and has produced over 800

aftershocks of over Mw 4.5 since. The Tohoku sequence began with a Mw 7.2 underthrusting

foreshock which occurred at 2:45 am on March 9, 2011. This foreshock event was situated just

40 km north of the subsequent great earthquake epicenter, two days prior to its rupture.

Aftershocks from the March 9th event propagated radially outward, with large earthquake events

migrating towards the main shock nucleation region. Stress field alterations from the foreshock

sequence are a likely precursor to the Tohoku main shock event.

Figure 3: This image approximates slip of the overriding North

American plate off the east coast of Japan. Centroid moment

tensors of the foreshock, main shock, and aftershocks are labelled

in blue, red, and orange, respectively. (Ide, 2011)

Mitchell May, 260556044, EPSC 330

The main shock occurred at 5:46 am (UTC) on March 11, 2011. Centroid moment tensor

analysis was conducted to isolate the slip orientation. The large rupture area and duration of the

main shock make it necessary to limit the CMT analysis to very long periods (Nettles, 2011).

Long period bands between 300 and 500 seconds were analyzed over the 8.5 hour long

seismogram recordings. After cross referencing 100 seismic stations, a CMT with strike 203°,

dip 10°, and rake 88° was recorded. These results are consistent with the primary fault

orientation. The main rupture seismic moment varies from Mw 9.0-9.1 depending on the methods

of analysis. This uncertainty is due to the lack of constraint on hypocenter depth, and (Nettles,

2011) suggests a seismic moment range between 9.03 and 9.16.

Ocean Bottom Analysis of Main Shock

Finding vertical displacement of the primary fault is crucial for understanding water

displacement off Japan’s east coast. Using ocean-bottom pressure gauges, scientists have been

able to relate the change in water pressure to uplift. Ocean-bottom pressure gauges were used to

record water pressures near the fault scarp over the main shock event. A large negative offset of

approximately 500 ± 50 hPa appears clearly after the M9 event. Negative pressure changes

represent uplift at the observation points, as the pressure release is a result or water displacement

away from the uplift. A change in pressure of 1 hPa corresponds to roughly 10 mm of vertical

displacement, which yields a calculated uplift of 5 ± 0.5 m (Nettles, 2011).

Mitchell May, 260556044, EPSC 330

Figure 4: This figure shows data collected from

ocean bottom pressure gauges located near the

primary fault scarp along the sea floor. The drastic

drop in pressure is a result of uplifted ocean water

propagating away from pressure gauges (Nettles,

2011).

Aftershocks

High magnitude aftershocks were observed almost immediately after the main shock,

with two earthquakes generating seismic moments greater than 7. Thirty minutes after the main

shock, a Mw 7.9 earthquake occurred 250 km south of the main shock epicenter. The earthquake

had a calculated focal mechanism which coincided with interplate slip observed in the main

shock (Nettles, 2011). A second large aftershock occurred ten minutes later, of Mw 7.6. This

earthquake; however, differed from the main shock, as it was a result of normal faulting

mechanisms. The second large aftershock was located east of the main shock epicenter, within

the pacific plate.

Aftershocks of this event are continuously observed to this day, with a notable Mw 7.1

aftershock on October 26, 2013. The occurrence of aftershocks from the 2011 earthquake can be

described by Omori’s law. This law relates the occurrence interval of aftershocks to the

reciprocal of time post rupture. In megathrust earthquakes such as the 2011 Tohoku event, we

should expect to observe aftershocks in the future, but in far less abundance.

Mitchell May, 260556044, EPSC 330

Figure 5: The figure above is a cross sectional view of the fault system east of Japan. Shaded

dots represent aftershocks from the 2011 Tohoku event, which appear to follow the fault plane to

depths of 50 m.

Fault Structure

Although a vertical displacement of 5 m was calculated for the fault, five horizontal

displacements were also found, each with different values. The inconsistent nature of this

movement is a result of numerous faults interacting with one another. In an attempt to understand

this system, a fault model was developed using displacement data from (Nettles, 2011). Through

the use of a two dimensional elastic half space model, scientists have constructed a cross

sectional view of the subduction zone, assuming this zone is acting as the primary fault plane.

Mitchell May, 260556044, EPSC 330

Figure 6: The figure above follows a dislocation model in conjunction with structural

interpretations. Above the subducting thrust fault, a backstop reverse fault separates two

cretaceous sequences which differ in deformation level. This reverse fault appears to be a result

of horse structure growth, which is seen to repeat itself above the plate boundary. Other reverse

and listric faults are present, but appeared to be inactive during the Mw 9.0 event. Horizontal

displacements were found to be as high as 74 m in areas of the overlying plate (Ito, 2011).

Earthquake Preparation

Prior to the 2011 Tohoku event, earthquakes off the coast of Japan weren’t uncommon.

Japan had experienced several disastrous earthquakes in recent time, consisting primarily of the

Mw 8.2-8.5 1896 Meiji Sanriku, and Mw 8.1 1933 Showa Sanriku events, with smaller tsunamis

occurring every 10-50 years (Nobuhito, 2011). Knowledge of past earthquakes had allowed

Japan to invoke several countermeasures, aimed at mitigating future tsunami events. Barriers

both on and off shore were built along Japan’s east coast to reduce or prevent inundation from

Mitchell May, 260556044, EPSC 330

tsunamis. Other natural barriers, such as tree planting, were implemented as well to reduce the

effects of erosion n and liquefaction. In the 2011 paper by Nobuhito, it is emphasized that the

Tohoku region was highly prepared for tsunamis. This area, like many regions of Japan,

routinely practiced earthquake evacuation.

The above countermeasures were believed to be sufficient in the event of the predicted

Tohoku earthquake, but this was considering a modelled Mw 7.4 event. The Japanese

government reported that a Mw 7.4 event along a 200 km fault off shore of Sendai was expected

to occur with 99% probability within 30 years (Nobuhito, 2011). The poor forecasting during

the Mw 9.0 rupture left tsunami barriers severely damaged, and resulted in underestimated levels

of inundation in several regions of Tohoku.

Earthquake Response

The Japan Meteorological society acted as first responders to the megathrust event,

issuing tsunami warnings 3 minutes after the primary rupture (Nobuhito, 2011). It wasn’t until 17

minutes later that the resulting tsunami made landfall along a 2000 km stretch of Japanese coast.

Various coastal cities experienced flooding, with Sendai being the largest city affected. Japan’s

topography influence flooding a great deal, leaving the total tsunami inundation of 400km2

unevenly distributed (Nobuhito, 2011).

Of the numerous coastal regions affected by the tsunami, the southern region of Tohoku

was particularly vulnerable to inundation. This is due to the relatively flat topography of the

Sendai plain, which has shown evidence of large scale inundation in past events, such as the 869

Sanriku earthquake. Coastal areas in the Northeast of Japan, such as the Sanriku region, exhibit

different geomorphology. Sanriku is host to ria coastal environments, which naturally mitigate

Mitchell May, 260556044, EPSC 330

tsunamis with steep and narrow coastal bays. This mitigation was one of the reasons why

southbound inundation was observed to be so large.

Repercussions

The 2011 Tohoku earthquake and tsunami left a wake of destruction in its path, which is

not uncommon for megathrust earthquakes. With large scale damage associated with loss of life

and infrastructure, Japan is still in the process of recovery. Official reports from early 20111

suggest fatalities were 15,844 with an additional 3,394 missing (Nobuhito, 2011). The majority

of loss of life was attributed to the tsunami, and most fatalities occurred in regions of Tohoku.

Specifically, 58% in Miyagi Prefecture, 33% in Iwate Prefecture, and 9% in Fukushima

Prefecture (Nobuhito, 2011). Damage to infrastructure (roads, bridges, and buildings) occurred

in over 300,000 structures. This damage; however, doesn’t account for the Fukushima nuclear

meltdowns which occurred shortly after inundation.

Response

A large surveying project was proposed in the aftermath of the Tohoku event, with hopes

of improving earthquake forecasting methods. Because of the rarity of mega earthquake-

generated tsunamis, few surveys have been conducted. The Tohoku earthquake has given

researchers the opportunity to study inundation levels, which is the primary source of damage to

humans. After the 2011 earthquake and tsunami, a large research survey was conducted over a

2000 km stretch of Japanese coast. More than 5300 sites have been surveyed in an effort to

record run up heights and inundation levels. Results show a massive variation in inundation

levels, which is due to the complex interaction between tsunami wave propagation and

topography.

Mitchell May, 260556044, EPSC 330

Inundation and run-up heights were the primary focus of the survey, as they could be

accurately recorded to within a few centimeters. Height markers consisted of watermarks present

on buildings, trees, and walls. Other markers used for run-up height include landward debris

accompanied by seawater marks. The highly populated Sendai plane received a maximum

inundation level of 19.5 m, with mean heights closer to 10 m. Results from this survey support

the recurrence theory of mega earthquake-generated tsunamis along the subducting Pacific plate

boundary.

Figure 7: This image quantifies results

obtained from the 2011 Tohoku earthquake

survey group. Inundation distances are shown

by blue colouring of the island, while heights

are represented by vertical lines (Mori, 2012).

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Mitchell May, 260556044, EPSC 330

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