Sendai and Sumatra earthquakes and tsunamis.

The 8.9 magnitude Honshu earthquake of March 11, 2011,   occurred due to thrust faulting within the Japan trench. The tectonic configuration of Japan and its surroundings is very complex with 4 plates meeting just below Japanese islands. The islands lie over four plates: the Pacific, North American, Eurasian and Philippine sea plates. The Pacific plate  subducts into the Eurasian plate, the junction of these two plates lie just beneath Hokkaido and Honshu. This is the main Japanese trench and the rate of subduction of the Pacific plate is about 83 mm/year. The length of the Pacific plate is about 2000 km. Thus the geological and tectonic settings of the Japanese islands is very complex and at any given point of time one of these four plates move/thrust giving rise to major earthquakes. The depth of focus of the earthquakes varies from 700 km to 25 km or less.  The eastern margin of the Japanese islands, along the subduction zones is the loci of several active volcanoes.  The 1400 m high Shinmoedake volcano, located over this foci in Kagoshima Prefecture, Kyushu (southwest Japan) has become active after the Honshu earthquake throwing ash and rocks to a height of 4  km.

Earthquakes of this magnitude is not uncommon in and around Honshu. This is not a rare event that occurred at this site.  Over nine earthquake of magnitude >7 occurred in this area since 1973.  An earthquake of magnitude 7.8 struck in an area 260 km north of the 3/11 earthquake in the year 1994. This earthquake caused injuries to 700 people.  Similarly in 1978, an earthquake of magnitude 7.7 struck 35 km south west of the current 3/11 event. This caused injuries to 400 people. Besides this, 8.4 magnitude earthquake of Sanriku in 1933, 8.3 magnitude earthquake of Tokachi in 2003 are note worthy earthquakes in this region. All these earthquakes occurred due to thrust faulting below the Japanese Islands.  Between 9th March and 11th  March, 2011, Honshu experienced several foreshocks of magnitudes of  7.2 to 4.2. before the 3/11 earthquake. The main shock was followed by > 100 aftershocks and the after shocks are still rocking the region.

We have  witnessed two devastating earthquakes of magnitudes 9.1 ( Sumatra)  in 2004 and the recent 8.9 magnitude earthquake of Honshu accompanied by tsunamis. The Honshu tsunami occurred even before Sumatra tsunami faded out of our memory. The Sumatra earthquake occurred due to thrust faulting, similar to the one occurred in Honshu, where 1600 km long ocean plate fractured with a slip of 15 mts. The land near Banda Ache was lifted to a height of about 30 m. The tsunami generated due to this major event caused over 250000 deaths in Indonesia, Sri Lanka and India and causing dislocation of population in several coastal regions bordering the Indian Ocean. In the case of Honshu earthquake, the lateral shfit currently estimated is about 8 m. More data on the slip amount is being calculated. Both the earthquakes are shallow with the focus depth placed between 25-30 km. But the dimension of Sumatra earthquake and tsunami is larger by several factors compared to the Honshu earthquake. The Sumatra tsunami could travel 3500  km from the source caused devastating damage. A similar features was expected from Hnashu tsunami but the energy of the waves attenuated even before they could reach the nearby islands. The wave height measured at Hawaii islands, located at a distance of 6300 km from the epicenter was about 0.7 m.  

Majority of the earthquakes over Honshu occurred due to slip at shallow levels. The buildings in Japan are built strictly according to the codes. This is the reason one could see pictures in the television where tall structures in Tokyo were swinging at the time of the earthquake and returned to its normal poison. The death toll due to  earthquake in Sendai is far less compared to that due to the tsunami. We have to learn a lot from the Japanese civil engineers about making tall earthquake resistant structures. We do have codes on papers. Only an earthquake of magnitude half of that of Honshu will be able to prove how strong these structures are!!

The recent events all along the Pacific rim only demonstrates the dynamic changes that are taking place within the internal Earth system. We have a long way to go to  understand the dynamics of this system. Earth Sciences need to be given priority at school level itself, like other countries, to generate state-of-art younger generation of earth scientists to tackle such natural disasters in future. As far as Indian coast is concerned, the east coast is more vulnerable to tsunami related disasters as the coast is in line of sight of the grate Andaman-Nicobar-Sumatra – Sunda arc system. The west coast is not facing such arc system and hence chances of tsunami related disasters occurring are minimum. Only normal faulting, as evident from several earlier faults events, is a cause of concern, both on shore and off shore of the west coast, as reported in 1985 based on an integrated geophysical and geological analysis.