Chapter 2
The Science of Inland Earthquakes
Field of expertise: Earthquake Geology

Shinji Toda
Translated by Catherine Sachi Kikuchi

Summary
The 2011 Tohoku earthquake released the strain that had accumulated underground in easternJapan, but it also triggered many subsequent earthquakes as part of the reaction. Seismic activity
also increased on land, and earthquakes were caused by active faults, such as the one that
occurred one month later in Hamadori, Fukushima Prefecture. Although individual active faults only
produce major earthquakes once every several thousand to several tens of thousands of years, therisk of inland earthquakes changes over time because they may become temporarily likelier tomove due to huge subduction-zone earthquakes. Prediction evaluation that takes into account theearthquake chain is essential.
Keywords: inland earthquake, local earthquake, active fault, induced earthquakes, aftershocks

Introduction
Land earthquakes, or inland earthquakes, are earthquakes shallower than 20 kilometers in
depth that occur inside the landward plate. Although they seem to be completely unrelated to theTohoku earthquake, they are strongly influenced by huge earthquakes that occur in the ocean.

1: Problems Revealed by the Great East Japan Earthquake
What happened?
The plate boundary fault that caused the Tohoku earthquake (magnitude M9) was about
500 kilometers long and 200 kilometers wide. Its displacement reached a maximum of about 50meters, moving the seafloor along the Japan Trench and generating a massive tsunami. Theground also moved significantly on land some distance from the epicenter, and the Oshika
Peninsula in Miyagi Prefecture moved about 5 meters southeast. In just 3 minutes, the inland
Tohoku region, which had been slowly compressed in the east-west direction before the mainquake, was pulled in the opposite direction, reversing the location and mechanism of theearthquake. As a result, seismic activity increased in southern Akita Prefecture, off the coast ofAkita, northern Aizu Basin in Fukushima Prefecture, around Nikko, northern Nagano Prefecture,1

near the Fukushima-Ibaraki prefectural border, and around Choshi (Toda, 2013). At a fair distancefrom this area, a magnitude 6.4 earthquake occurred in eastern Shizuoka Prefecture. A large
inland earthquake that caused damage was also triggered (Figure 2-1a).
Tohoku Offshore Earthquake Slip (meters)
Akita Pref. Offshore

Fukushima Hamadori

North Nagano Pref.

Depth of Epicenter

South Shizuoka Pref

Figure 2-1 a). Outside of the epicenter of the M9 earthquake, a wide area experienced many
aftershocks for about 6 months due to the 2011 Tohoku earthquake.
One month later, a M7.0 earthquake occurred on April 11 in Hamadori, Fukushima Prefecture,caused the Yunotake Fault and the Idosawa Fault in Iwaki City to shift, and a 30 kilometer long
fault appeared (Fig. 2-1b).

Figure 2-1 b). The fault that appeared due to the April 11, 2011, Hamadori earthquake (M7) inFukushima Prefecture.

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Tokyo Metropolitan area
2011 Tohoku Earthquake

Number of earthquakes with magnitude 3 or above

2016 Fukushima
Pref. Offshore
Earthquake (M7.4)

Fukushima Hamadori2011 Tohoku Earthquake

Number of earthquakes with magnitude 3 or above

Seismic activity directly beneath the Tokyo metropolitan area also increased, and Tokyo observed
frequent tremors not only from aftershocks of the M9 earthquake, but also from seismic activity
directly beneath it. Although seismic activity in the Tokyo metropolitan area has since waned, it isstill higher than before the Tohoku earthquake (Fig. 2-2)

Figure 2-2. Time-series of seismic activity in Hamadori, Fukushima Prefecture (A) and the Tokyo
Metropolitan Area (B). The blue line: cumulative curve, green vertical lines: individual earthquakes
and their magnitudes. Seismic activity is still active in both regions.
The reality of the damage
The damage caused by the inland earthquake immediately after the Tohoku earthquake
was very minor compared to the damage caused by the main shock itself. However, the Northern
Nagano Earthquake (M6.7), which occurred about 13 hours later, caused a seismic intensity of 6+
in Sakaemura, resulting in partial damage to several houses, schools, and community centers, aswell as road cave-ins. In addition, the earthquake in Hamadori, Fukushima Prefecture (M7.0),
recorded a seismic intensity of 6-lower on the Japanese seismic intensity scale in Iwaki City, killing
4 people, injuring 10, and causing large-scale landslides in several places.

2: Paradigms Destroyed by the Earthquake
Conventional wisdom and necessary responses
Compared to ocean trench earthquakes, inland earthquakes have smaller magnitudes, but
their epicenters are shallower, causing strong localized tremors with an intensity of 7 or 6 on theJapanese seismic scale. Therefore, when they occur in densely populated areas, they can cause
tremendous damage. The earthquake in Hamadori, Fukushima Prefecture, was caused by anactive fault. Active faults move at intervals of several thousand to several tens of thousands ofyears, and each time they move, they cause a localized earthquake of magnitude 7 or 8. Largeearthquakes caused by individual active faults rarely occur, but since there are at least 2,000
active faults in the Japanese archipelago, the entire archipelago is subject to these kinds ofdamaging earthquakes every few years (Toda, 2016). On the other hand, as reaffirmed by the3

Tohoku earthquake, the risk of a major inland earthquake temporarily rises sharply, immediately
after an ocean trench earthquake. For example, on January 13, 1945, 37 days after the ShowaTonankai earthquake of December 7, 1944 (M7.9), the Mikawa earthquake (M6.8) occurred insouthern Aichi Prefecture. More than 2,000 people were killed and more than 50,000 houses weredestroyed. The epicenter of the earthquake was the Fukamizo fault, an active fault. In 1855, abouta year after the 1854 Ansei Tokai Earthquake (M8.4) and the Ansei Nankai Earthquake (M8.4), theAnsei Edo Earthquake (M7.0) occurred, killing an estimated 10,000 people in Edo. The Ansei-Edo
earthquake may not have been an active fault earthquake, but it is considered to be triggered by ahuge ocean trench earthquake.
The Great East Japan Earthquake and lessons learned from the past suggest that ocean
trench megathrust earthquakes may cause seismic activity to increase over a wide area even after
their occurrence, resulting in damage extending inland. For example, if a huge Nankai Troughearthquake occurs in the future, the short- and medium-term effects may spread to the Kanto
region and southeast Japan.

3: Achievements and New Approaches
After the Hyogo Prefecture Nanbu Earthquake (M7.3) that caused the 1995 Great HanshinAwaji Earthquake, the Headquarters for Earthquake Research Promotion was established in theMinistry of Education, Culture, Sports, Science and Technology (then Cabinet Office). Under thedirection of the Earthquake Research Headquarters, active faults across the country werevigorously surveyed (Figure 2-3), and the results were reflected in the probabilistic seismic motion
prediction map (The Headquarters for Earthquake Research Promotion, 2018). However, sinceindividual active faults cause large earthquakes only rarely, the degree of danger for several years
to several decades is not always accurately indicated. Seismic activity tends to be cascading, andthe effects of huge earthquakes are particularly large in both time and space. Because of this, it isimportant to take into account the most recent activity in addition to the long-term evaluation.
How does a single huge earthquake change the seismic activity in the surrounding area?
Since around the 1990s, there has been a lot of research on stress propagation to the surroundingarea as a result of fault movement and the response of seismic activity. The Tohoku earthquake
boosted this situation. In addition, the 2016 Kumamoto earthquake became a chain of major
earthquakes (M7.3 earthquake 28 hours after M6.5 earthquake), increasing the importance of timedependent seismic hazard prediction, in which the probability of earthquake occurrence changes
depending on the situation. At present, however, our knowledge is limited to understanding thesituation after the fact and explaining the mechanism in broad terms.
On the other hand, since the Tohoku earthquake, several models have been proposed tomodel the source fault motion, calculate stress changes, and predict the seismic activity responsein the surrounding area using immediate observation data. There have also been attempts toautomate these models to predict changes in hazard levels in real time. Furthermore, the risk ofdamage caused by inland active faults that may be triggered by the future Nankai Troughearthquake is also being considered. In the future, these earthquake links will be appropriatelyreflected in earthquake damage scenarios and disaster mitigation measures. In particular, more
realistic prediction and evaluation of inland earthquakes is essential to reduce the direct damage
caused by seismic tremors.

4Figure 2-3. Cross-section of strata recording past fault activity exposed by trench excavation. The
gravel layers A and A' were originally the same stratum, but they have changed due to past fault
movement. Ichinoseki City, Iwate Prefecture.

Conclusion - From the author
When people become ill, they take immediate measures to alleviate the symptoms andsearch for the cause of the symptoms through medical examination and tests. Until we learn thename of the disease, we cannot relax or give the best treatment. The same applies to earthquakes.
When a major earthquake occurs, the first priority is to take immediate action such as rescues,relief, and evacuation, but it is also important to investigate the cause and understand the ongoing
phenomena of the earth (although in the case of earthquakes, there are no drugs or surgicalprocedures to control them). As with medical checkups, research to determine where the causes ofearthquakes lie is essential even in normal times, and I believe that the correct understanding ofhazards (disaster triggers) will lead to more effective disaster mitigation and prevention in thefuture.

References
The Headquarters for Earthquake Research Promotion. (2018). National Earthquake Motion
Prediction Map 2018. https://www.jishin.go.jp/evaluation/seismic_hazard_map/ (In Japanese)
Toda, S. (2013). Chain of Great Earthquakes. Iwanami Shoten. (In Japanese)
Toda, S. (2016). How far can we predict active fault earthquakes?: What’s happening in theJapanese archipelago. Kodansha Blue Books. (In Japanese)56