JAPAN - EARTHQUAKE AND TSUNAMI OF 11 MARCH 2011 IN SANRIKU
The most powerful earthquake to hit Japan and one of the five most powerful earthquakes in the world occurred off the coast of Honshu, the main island of Japan and home to about 100 million people. The quake generating a devastating tsunami. This is a preliminary report. An evaluation of the tsunami source mechanism will be provided later.
Date and Origin Time - 11 March 2011, 05:46 UTC, 14:46 JST (local time)
Magnitude - The Moment Magnitude initially estimated at Mw=8.9 ( USGS) was later revised upward to 9.0. It was the fifth-strongest earthquake in the world since 1900. The largest recorded earthquake was the one which occurred in Chile on May 22, 1960 and had a moment magnitude of Mw=9.5.
Earthquake Epicenter and of subsequent aftershocks in the first two days. More than 175 aftershocks were recorded in the following days.
Epicenter - Lat:37.68N, Long:143.03E (USGS) at about 373 kilometers (231 miles) away from Tokyo and 130 kilometers (81 mi) off the east coast of the Oshika Peninsula. NASA Earth Observatory gave the epicenter was at 38.3 N., 142.4 E.
Earthquake Epicenter and location of Foreshocks and initial Aftershock distribution (Modified NASA Earth Observatory Map).
Focal depth - Shallow at a depth of about 15.2 miles (24.4 kilometers) (USGS).
Felt Reports - Strong earthquake motions from the main event on March 11, 2011 rattled buildings and toppled cars off bridges. Ground motions were felt as far away as Tokyo.
Foreshocks and Aftershocks - A magnitude 7.2 foreshock occurred on 9 March 2011 and three more foreshocks greater than 6 occurred before the main earthquake of March 11. (Foreshocks shown by dotted circles).
About fifty minutes following the main quake there was a large number of aftershocks, the largest having a magnitude of 7.1. Shortly afterwards 35 more aftershocks larger than magnitude 5.0 and 14 larger than magnitude 6 were recorded (UGSS). By mid-March 2011 more than 250 aftershocks with magnitudes of over 5.0 had occurred and 25 of these had magnitudes over 6.0. Significant aftershocks can be expected to follow in subsequent weeks and months.
The following strong aftershocks with magnitudes greater than 6 were recorded on March 27 and March 28.
March 27, 2011 - East of Honshu. Magnitude Mw=6.1, at 22:23:58 UTC;
March 28, 2011 - At 07:23:58 AM (local date and time). Epicenter 38.402°N, 142.102°E; Depth 17.1 km (10.6 miles) (USGS). Distances 109 km (67 miles) E of Sendai, Honshu, 156 km (96 miles) E of Yamagata, 161 km (100 miles) ENE of Fukushima and 368 km (228 miles) NE of TOKYO (USGS).
Crustal Movements - The earthquake was a mega-thrust event with the Pacific plate moving underneath the Eurasian plate. As a result the land mass of Honshu Island moved in an east-southeasterly direction. Based on the Global Positioning System, the Geospatial Information Authority in Tsukuba, Japan, estimated that the Oshika Peninsula near the earthquake's epicentral area, moved by a little over 17 feet eastward and that it subsided by a little over 4 feet. Additionally, the Geospatial Information Authority stated that there were land mass movements in many areas of Honshu, from the northeastern region of Tohoku to the Kanto region including Tokyo. The quake's approximate rupture length was a little more than 250 km long.
Earthquake Focal Mechanism
Death Toll and Damages - Both the earthquake and the tsunami caused great damage and deaths across sixteen prefectures in Japan. The tsunami was responsible for most of the destruction, the deaths and the large number of injured and missing. As of April 18, 2011 the death toll had risen to 13,843, with another 14,030 remained missing, according to Japan's National Police Agency. According to the agency, 136,481 had been displaced. The final death toll is estimated to rise significantly to more than 15,000. The 1896 earthquake and tsunami in the same region off the Sanriku coast resulted in about 27,000 deaths.
The combined effects of the earthquake and tsunami were severe. However, as stated, most of the destruction was caused by the tsunami. There was heavy damage to roads and railways. Fires ignited in many areas and there was a dam collapse. The disaster left about 4.4 million homes in northeastern Japan without electricity and 1.4 million without water. There were power outages for about 4 million homes in Tokyo and the surrounding areas. Many electrical generators were destroyed and there was severe destruction of nuclear reactors which required evacuation of people in the areas that were impacted. A state of emergency was declared for the region. Early estimates indicated that the total monetary losses will exceed $100 billion.
Damage to Nuclear Power Plant - Combined effects of the earthquake and tsunami knocked out regular and backup cooling systems at the Fukushima Daiichi Nuclear Power Plant. Four of the six reactors suffered damage to their radioactive cores. A series of explosions damaged four of the plant’s buildings housing nuclear reactors. The tsunami knocked out the backup generators and severely damaged the water pumps. Twenty four hours after the earthquake there was a hydrogen explosion that damaged the buildings housing No. 1 and No. 3 reactors. Another explosion occurred inside the building of the No. 2 reactor. There was a collapse of the No. 3 reactor's outer containment building. Subsequent reports indicated damage to the inner core-containment vessel as well. There was massive evacuation of more than 200,000 residents living within a 20 km (12 mi) radius of the Fukushima Daiichi Nuclear Power Plant. There were drastic increases in radiation levels near the damaged plant and Japan's emergency agency raised its rating of the crisis at the from a 4 to 5 -- equating the severity of the impact to that of the 1979 incident at Pennsylvania's Three Mile Island in the USA. However, the true impact may never be known with certainty.
Evaluation of Tsunami Impact on Nuclear Power Plants - The requirement for access to large amounts of water for the cooling systems of the older type of thermonuclear reactors (for a heat exchange process) limited their construction to coastal areas, rivers or lakes, or to offshore platform locations. In the U.S., the safety in the sitting and design of nuclear power plants in coastal areas was and continues to be of great concern to the U.S. Nuclear Regulatory Commission. The licensing process is strictly regulated. It takes up to seven years to prepare and evaluate the potential environmental impacts before a permit is issued for the construction of a new plant or for additional reactor units at an existing one.
Most of the older generation reactors - such as those used at the Fukushima Daiichi Nuclear Power Plant - are of the General Electric Mark I type that have performed fairly well over the years. Most of these reactors were installed in the early 1970s and survived past earthquakes, tsunamis and explosions. Presently, there are 23 GE Mark I type of such reactors operating in the U.S. and many more operating elsewhere around the world. There have been no reported accidents.
Regardless of the type of nuclear reactors used, the impact of earthquakes or tsunamis on a nuclear power plant is of great concern, particularly if the plant is located in a coastal area near a tectonic fault, or in an area that has been previously impacted by earthquakes or tsunamis generated by local or distant seismic events. The safety of nuclear power plants requires a careful evaluation of what potential effects a tsunami could have on a nuclear power plant's cooling systems - even if the cooling pumps and associated heat exchange units are located at high enough elevations. A cooling system's failure - as that which occurred at the Fukushima Daiichi Nuclear Power Plant - could result from tsunami inundation and flooding. Pumps as well as backup systems could fail. Failure could be caused not only from tsunami impact or flooding, but could also be caused by the tsunami's withdrawal of water - which could create a cave-in effect due to the loss of hydrostatic pressure. For tsunamis generated by distant earthquakes there may be enough warning to at least begin shutting down nuclear reactors. For a tsunami generated by a local earthquake, e shut down is impossible.
There is a profound inability to shut down older-type of nuclear power plants in time to avoid potential failure of their cooling system and the secondary impacts of a nuclear source melt down. A major nuclear disaster occurred in 2004 in the State of Tamil Nadu in India by the tsunami (but never revealed). A statement was made that the nuclear plant at Kalpakkan was shut down " after sea water rushed into a pump station. No radiation leak or damage to the reactor was reported". There is no way that the nuclear reactor at Kalpakkan could be shut down in such a short time, particularly after the tsunami had inundated the location of the pumps. It takes more than 24 hours to cool a reactor down, assuming that everything works well and there is no interruption of power. The same thing is now happening in Japan (and only partially revealed).
At the Fukushima Daiichi Nuclear Power Plant in Japan there is already confirmed breach of reactor's 3 containment casing. Cooling the reactor with sea water was an act of desperation, as the corrosive effect of sea water probably accelerated the failure. Such direct cooling with sea water without a contained heat exchange system in place, means that radiation will spread in the waters of the coastal area near the failed plant, but also much further out in the Pacific because of the prevailing ocean currents - the most predominant being the Kuroshio. In the North Pacific, the Kuroshio has a clockwise circulation (much like the Gulf Stream in the Atlantic Ocean). Thus, much higher levels of radiation can be expected around the plant's coastal area but also much further out to sea. Increased amounts of radioactive isotopes ( Cesium 137 and Iodine 131) can be expected, although there will be dilution of their concentrations further away from the coast. The problem in Japan is far from over. There will be long term radiation effects. Radiation in the ground water supply in the immediate area of the plant may be significantly increased, particularly in the concentration of Iodine 131.
In conclusion, many nuclear power plants have been built in vulnerable coastal areas but without proper consideration and risk assessment of potential tsunami impacts. There are plants in Japan, China, France and many other countries around the world that could face similar problems in the future - particularly plants that are old. The risks of tsunami impact to nuclear power plants have not been adequately evaluated and assessed. What happened in 2004 in India and in 2011 in Japan demonstrates the urgent need to carefully review the safety standards of nuclear power plants and to take adequate measures to avoid future, collateral to tsunami, disasters.
Earthquake's Planetary Impact – The great earthquake that struck Japan on March 11, 2011, resulted in movement of crustal material and redistribution of the Earth's mass. According to NASA, the earthquake resulted in a tiny shift in the Earth’s axis, which affected the rate of its rotation, thus shortening the length of a day by 1.8 microseconds.
It is believed that great earthquakes have a large enough moment to affect the earth’s polar motion and that the impact is cumulative, not only on the Earth's axis of rotation and free nutation (due to non rigidity and spinning dynamics of the aspheric earth), but on the Chandler wobble (the Chandler Oscillation) of the earth's axis.
The Chandler wobble of the earth's axis (the Chandler Oscillation) is about 20 feet (9 meters) and has a period of 433 days. This wobble combines with another wobble which has a period of one year, so that the total polar axis motion varies with a period of about 7 years. It is affected by gravitational attractions of the moon and the sun as well as planetary alignment and variations in the Earth's geomagnetic field. It would be expected that the shift due to great earthquakes would also have a cumulative impact to the Earth's axis of rotation and free nutation (due to non rigidity and spinning dynamics of the aspheric earth).
Additionally, great earthquakes such as that of 2011, are known to generate self-excited, long period, toroidal and spheroidal oscillations on the Earth’s surface that tend to resonate over long periods of time, lasting many hours and days. The most important of the spheroidal oscillations have a fundamental mode estimated at 58 to 60 minutes. For example, the August 9, 1952 Kamchatka Earthquake had a fundamental frequency mode of 57 minutes (Benioff et al., 1961). Similar frequency modes were determined for the May 1960 tsunami in Chile (Bogert, 1961; Ness et al., 1961; Alsop et al., 1961; Alsop, 1964b; Bolt, 1963; Connes et al., 1962; Nowroozi and Alsop, 1968; and Dziewonski and Landisman, 1970), for the Kuril Islands earthquake of October 13, 1963 (Alsop, 1964a; Abe et al., 1970; Dziewonski and Landisman, 1970), for the great earthquake of March Alaska, 1964 (Nowroozi, 1965; Smith, 1966; and Slichter, 1967), for the Rat Islands, February 4, 1965 (Nowroozi, 1966) and reported for the December 26, 2004 great Sumatra earthquake (Pararas-Carayannis, 2005).
Since spheroidal oscillations form standing waves with vertical excursion, these could contribute to tsunami-like sea level fluctuations along certain coastal areas. For example, 11 minutes after the earthquake in Chile there were oscillations of about 5 inches observed in Lake Pontchartrain, in Louisiana. However these were caused probably by the arrival of surface seismic waves rather than from Earth spheroidal oscillations. Slow crustal deformation and displacements associated with great earthquakes - such as the Chilean 1960 event - can generate seismic waves with unusually long-periods (Kanamori & Cipar, 1974).
The earthquake generated a destructive Pacific-wide tsunami.
Tsunami Warnings Issued
A tsunami warning was issued by the Pacific Tsunami Warning Center (PTWC) in Honolulu, Hawaii. Japan's Meteorological Agency (JMA) issued an immediate tsunami warning and evacuation along the country's Pacific coast. JMA issued a warning at 14:49 JST on March 11, 2011 (local time). The earthquake occurred at 14:46 JST. Therefore there was a time interval of 3 minutes before JMA issued a warning. This is remarkably good.
Regional tsunami warnings were issued by Civil Defense authorities in more than 20 countries and Pacific islands, including Russia, Indonesia, Philippines, the Hawaiian Islands, the Pacific coasts of North America, Central America and South America.
Travel Time Chart of the March 11, 2011 Tsunami in hours (modified NOAA graphic)
Initial reports from the U.S. Pacific Tsunami Warning Center (PTWC) in Honolulu reported a wave with maximum height of 2.79 meters (9.2 feet) at an observing station at Hanasaki, Hokkaido at 3:57 p.m. local time (06:57 UTC). Subsequently PTWC reported the following tsunami wave heights and time as recorded by tide stations - which do not necessarily record the highest open coast waves since they are usually located in harbors or other sheltered areas.
1.27 meters (4.2 feet) at 10:48 UTC at Midway Island,
1.74 meters (5.7 feet) at 13:72 UTC at Kahului, Maui, Hawaii
1.41 meters (4.6 feet) at 14:09 UTC at Hilo, Hawaii
0.69 meters (2.3 feet) at 15:42 UTC in Vanuatu
1.88 meters (6.2 feet) at 16:54 UTC at Port San Luis, California
2.02 meters (6.6 feet) at 16:57 UTC at Crescent City, California
Initial reports by JMA gave the maximum tsunami height of 4.1 meters at Kamaishi at 3:21 p.m. (06:21 UTC), at 7.3 meters at 3:50 p.m. (06:50 UTC) at Soma and 4.2 meters at 4:52 p.m. (07:52 UTC) at Oarai. However much greater heights were reported at later times. The maximum tsunami runup heights in Japan are provided below.
The tsunami struck the shores of Sanriku a few minutes after the quake. The impact was particularly devastating along coastal areas of the northeastern coastal areas of Honshu, where the irregular coastline and numerous bays amplified the height and destructiveness of the tsunami. Hardest hit was the Miyagi Prefecture. In some areas the waves inundated as far as 10 km (6 mi) inland, carrying debris of houses and trailers. Combined, the earthquake and the tsunami caused extensive and severe damage to roads and railways and ignited fires in many areas and triggered a dam collapse. Many electrical generators were taken down, and at least two nuclear reactors partially melted down at Fukushima Daiichi Nuclear Power Plant. A fire was ignited at an oil refinery in Chiba Prefecture near Tokyo. At Ishinomaki City the tsunami washed homes away. Subsequent reports confirmed the following for other coastal cities.
Rikuzentakata - At Rikuzentakata the maximum tsunami wave was 13 meters high and overtopped the existing protective tsunami seawall which was 6.5 meters high.80% of 8,000 houses were swept away. Of the 26,000 inhabitants over 18,000 were lost. The tsunami reshaped the entire coastline and flooded the agricultural fields further north. A preexisting long and well-planted barrier beach almost disappeared.
The protective harbor gate failed to shut and 45 firemen where swept away while attempting to close them manually. ( http://www.youtube.com/watch?v=ScjHs9bClyc )
Tarou District - Along the Taru District the tsunami destroyed or overtopped the protective seawall and caused extreme destruction. Reportedly the tsunami run-up heights were 19.5m, 25.5m and 24.7 am.
Ryori Bay-Shirahama - The reported tsunami run-up height reached 23.60 meters. At the fishing village of Ryoishi the waves destroyed part of the 9 meter (30 foot) protective tsunami wall or simply overtopped it completely destroying everything in the way.
Iwate - The GPS ocean gauge located in 204 meters depth in the offshore area near Iwate measured a 6.7 meter (22 ft) tsunami height.
Koborinai - The tsunami runup reached 37.9 meters (124 feet ).
Miyako - Waves of 11.5 meter overtopped the existing tsunami barrier and seawall which was 7.6 meters high.
Run-up heights of 19.5 and 25.5 meters were reported from this area.
Natori - The tsunami height was 12 meter near Sendai airport
and 9 meters high near the fishing port.
Kesennuma - The tsunami heights ranged from 9.10 to 14.7 meters.
Miyako - The tsunami height measured by the tide gauge was 8.5 meters.
Kamaishi - Tsunami waves ranged from 7-9 meters in height.
Ofunato - The maximum tsunami height was 8.0 meters.
Arahama - The maximum Tsunami height was 9.3 meters.
Ishinomaki - The tsunami was 5 meter high in the Harbor
but runup reached 16 m at Ogachi-machi.
Kashima - 4.22 meter and 5.2 meter tsunami heights were observed at Kashima Port.
Fukushima Nuclear Power Plant - A 19 foot high protective levee was overtopped by the tsunami which submerged the lower height structures including the diesel generators. The maximum tsunami wave height was ~46 feet. A 150 foot high splash was photographed as the tsunami impacted the turbine building of the plant, passing over its roof and striking the adjacent reactor building. ( http://www.vgb.org/vgbmultimedia/News/Fukushimav15VGB.pdf ); ( http://martynwilliams.posterous.com/tsunami-damage-at-fukushima-daiichi )
Fudai - 3000 residents survived because of a 51 foot (15.5 meter) floodgate. The tsunami run-up at the towers of the floodgate, reached 66 feet (20 meters).
Far-field Tsunami Effects
Tsunami waves were recorded or observed at distant shores of the Pacific.
PHILIPPINES - Following the tsunami warning the government authorities ordered the evacuation of 19 provinces along the coast.
NOAA graphic of tsunami energy flux and deep water wave heights.
HAWAIIAN ISLANDS - Tsunami waves struck the Hawaiian Islands and maximum runup heights ranged from 2 to 3 meters (7 to 11 feet) on the islands of Maui and Hawaii (the Big Island). There was extensive damage to boats on the island of Oahu.
Kahului, on the island of Maui suffered the worst damage. There was flooding and minimal damage of a hotel lobby near Kealakekua Bay on the Big Island.
Midway Island - Four waves struck the Midway Atoll at the Northwest end of the Hawaiian Islands. The highest of the waves reached a height of nearly 5 feet and completely washed out the reef and Spit Island, the smallest in the Atoll. According to reports the waves killed hundreds of birds and swept away nests protecting seabird chicks which were unable to fly. Reportedly, 110,000 Layson and black-footed albatross chicks were killed, along with 2,000 adult birds.
OREGON - The tsunami reached the southern coast of Oregon first at 7:48 a.m. (local time). Wave heights were relatively small, ranging from 3 to four feet (90 and 120 centimeters) and the wave periods ranged from 10-15 minutes
CALIFORNIA - There was damage to docks and boats. At Crescent City, tsunami waves did extensive damage to the port docks and severely damaged 35 boats. The reported maximum wave height was estimated at 2.5 meters. The tsunami caused also extensive damage at Santa Cruz Harbor, estimated at more than 2 million dollars.
CHILE - Major damage was reported in Chile. Maximum recorded tsunami runup at Coquimbo was 2.55 meters, at Caldera 2.43 meters and at Talcahuano 2.15 meters.
EVALUATION OF THE EARTHQUAKE AND OF THE TSUNAMI
Based on data available, it is clear that the March 11, 2011 tsunami was unusually high and destructive. Its impact on Honshu's coasts was very similar or even greater than that of the tsunami caused by the Great Sanriku earthquake of 1896 and much more destructive than that of the Great Sanriku earthquake of 1933. To understand why the tsunami was so unusually high and destructive and what may be expected in the future, a preliminary examination is being undertaken of the earthquake’ focal mechanism, rupture patterns and of the spatial and temporal sequencing and clustering of major aftershocks – which define the limits of crustal displacements, the amount of energy release and the tsunami generating area. To understand the source mechanism of the great earthquake and tsunami that impacted Japan on March 11, 2011 and to evaluate potential future earthquake and tsunami activity, we must examine the seismicity and tectonics of the region aa well as past seismic and tsunami activity. The following sections provide only a brief overview of the seismicity and tectonics of the region and a preliminary analysis of the tsunami's source mechanism that contributed to its severity.
Tectonics of the Region
The islands of Japan are located on the Circum-Pacific seismic belt and are considered to be a mature island arc. Japan was originally the coastal part of the eastern Eurasia. However, oceanic crust movements caused by subduction processes - which began many hundreds of millions of years ago ( from mid-Silurian to the Pleistocene) - pulled Japan away from the Eurasian continental block and eventually opened the Sea of Japan about 15 million years ago. Subduction of the Philippine Sea Plate beneath the continental Amurian Plate, the Okinawa Plate to the south and of the Pacific Plate under the Okhotsk Plate to the north, continues to the present day and are the cause of frequent earthquakes, tsunamis and of occasional volcanic eruptions in the region.
The convergence rate between the Pacific and Eurasian tectonic plates along the east side of Honshu island is about 8 to 9 cm/year(3.1 to 3.5 in) . The Pacific plate subducts under Honshu's underlying plate and the convergence motion results in the build-up of stress. After reaching a threshold of elastic deformation, the stress is suddenly released during a sudden rupture, thus causing an earthquake. Apparently, the region where the latest Sanriku earthquake occurred had reached a very high level of stress. The foreshocks prior to the main quake and their time and directional sequencing, should have given some clues as to what was to follow.
Graphic showing the directivity of Pacific plate tectonic forces along the east coast of Japan, the Kuril Islands and Kamchtaka (Modified after UNAVCO)
Large magnitude earthquakes in the Sanriku region can have long rupture lengths. However, since subduction near Honshu does not follow a straight fault line along the tectonic boundary - as defined by the Japan Trench - the ruptures tend to be limited in length and the affected crustal blocks tend to be smaller, although their seismic energy release may be quite high.
Seismicity of the Region
Japan is one of the most seismic regions in the world, accounting for about 20 per cent of the world's earthquakes. On the western side, the Sea of Japan is a complex basin between Japan and the Korea/Okhotsk Sea Basin. It represents another sub plate with apparent rotational movement as it interacts against the Okhotsk plate, along the inland sea boundary of the Hidaka Collision Zone (HCZ). Sakhalin island, north of Hokkaido, which separates the Sea of Japan from the Sea of Okhotsk, is probably the result of transpressional tectonics along the North America-Eurasia boundary (Pararas-Carayannis, 1994)
Seismicity of Japan - Epicenters of earthquakes on the Island of Honshu and Southern Hokkaido. Epicenter of the 11 March 2011 earthquake
The high seismicity on the east coast of Japan results from compression along the Pacific-North America subduction zone, from outer rise events and from magmatic effects of plumes or super plumes which may have hydrated the subducting oceanic lithosphere. Usually, shallow normal faulting occurs in the trench-outer rise region. This is the region where most of the larger earthquakes usually occur and destructive tsunamis are generated.
Near the Island of Honshu the earthquakes are caused by the subducting Pacific plate. Small earthquakes occur frequently. Stronger earthquakes of M6.0 and large earthquake M7.0 occur less frequently and intermittently. Earthquakes of M8.0 or greater occur on the average every 50 to 100 years. Great mega-thrust events - such as that of March 11, 2011 M9.0 - occur infrequently.
Mega thrust earthquakes - including the March 11, 2011 event - have generated extremely destructive tsunamis on the shores of Honshu. All of these events were shallow (20 km) and involved a thrust mechanism of compressional stress which resulted in the uplift of the overriding tectonic plate, as well as in great horizontal movements. Large vertical displacements usually occur with such mega-thrust events. For example, the 1964 Alaska earthquake uplifted the east side of Montague Island in Prince William Sound by more than 10 meters (Pararas-Carayannis, 1972). Mega thrust earthquakes involve mainly a mechanism where the compressional stress is predominant. Earthquakes of lesser magnitude, ranging from M6.0 to M7.0 result from normal-type of faulting where shear stress is predominant - thus less likely to generate great tsunamis.
The 1896 and the 1933 Sanriku earthquakes were not outer rise events but occurred west of the Japan Trench - which marks the tectonic boundary. Both generated destructive tsunamis along Sanriku's coastlines. The 2011 earthquake occurred slightly to the south of the 1896 event but had many similar source characteristics. The similarities and differences are discussed in a subsequent section.
Preliminary Evaluation of the Tsunami Source Mechanism
Source Region - Preliminary analysis of the rupture and aftershock distribution indicates that the tsunami source area was relatively small compared to other tsunami sources of recent great earthquakes in Sumatra (2004) and Chile (2010). The magnitude Sumatra earthquake on 26 December 2004 had a rupture that was almost 1300 km long. By comparison, the 2011 Sanriku earthquake had a rupture that was about 300+ km long, yet packed a great deal of energy that resulted in large crustal movements. The tsunami had an approximately elliptical source region that was about 300 km long and about 150 to 175 km wide (which may be somewhat revised as more data on aftershock distribution and temporal sequencing is analyzed). Most tsunamigenic earthquakes in Japan - even the most destructive - involve relatively small crustal blocks, but proportionately large slips. In this case the slip may have been 40 meters according to a preliminary calculation by Chen Ji, (Univ. of California at Santa Barbara).
Cluster algorithm analysis of aftershocks in chronological sequence (Peter Zhol - personal communication), determined a big cluster of 260 events; a second cluster of 120 events and a third cluster of 60 events as well as 20 very small clusters - typically one or two events each. Many of the aftershocks may have occurred on unmapped, minor faults both in the intraplate region as well as on the outer rise of the subducting plate. Buckling of the crust due to subduction friction would probably have created many such minor, normal faults, which were activated giving rise to subsequent aftershocks - even out of the tsunami generating region.
Tsunami Generating Area showing the epicenter of the main earthquake on March 11, 2011, a major aftershock with magnitude 7.1 fifty minutes later and 172 aftershocks which sere recorded by the USGS by March 12, 02:04:53 UTC 2011
The March 11, 2011 quake had characteristics of severity of tsunami generation usually associated with outer rise events and slow rupture velocity, usually associated with compacted, sedimentary layers. However, the epicenter and crustal displacement region were not on the outer rise of the subducting plate but at least 150 km. West of the tectonic boundary of the two colliding tectonic plates that have created the Japan Trench. Therefore, the evaluation of tsunamigenic efficiency must include a review of what a combined rupturing impact would be on both the subducting Pacific oceanic lithosphere and on the overriding Eurasian tectonic plate, as well as on the large vertical crustal displacements that contributed to the tsunami’s severity. Additionally, the effects of the temporal elastic deformation caused by faulting and collateral impact on the sediments must be examined. Finally, a comparison must be undertaken of source characteristics of the destructive tsunamis of 1896 and 1933 in the same general area. A more complete evaluation of the tectonics and the source mechanism of the earthquake and the tsunami will be provided after more data is collected and analyzed.
It will suffice to state at this time that the March 11, 2011 quake earthquake had a complex mechanism which may have involved multiple parallel ruptures, as well as extensive vertical crustal and sediment displacements - which may account for the severity of the tsunami that was generated. Thus there is need to examine the spatial and time sequence of aftershock distribution, any clustering that may have occurred and the three-dimensional dynamics of shallow and deeper subduction processes.
Plotting the aftershock focal depths along eastern Honshu (Peter Zhol - personal communication), indicates that there was a spectacular peak at a focal depth of about 24-25 km. The significance of this to tsunami generation is being evaluated in terms of regional, spatial subduction geometry, slip, crustal movements and sediment displacements.
Histogram of Aftershock Focal Depths (modified after Peter Zhol - personal communication)
Based on the above-described cursory evaluation, it is believed that the great height of the 2011 tsunami along the Honshu’s coastlines was caused, not only by the crustal displacements due to up-thrust faulting but mainly by the displacement and excessive uplift of sediments along the leading edge of the accretionary prism of the overriding tectonic plate, as it thrusted eastward (by 17 ft.) towards the Japan Trench. To what extent and what volume of material was displaced will be examined and determined at a later time.
Past and Recent Destructive Earthquakes and Tsunamis Along the Sanriku Coast of Japan
The historic record shows that a total of 65 destructive tsunamis struck Japan between A.D. 684 and 1960 (Pararas-Carayannis, 1982). As early as 18 July 869 the Sanriku coast was hit by a tsunami resulting in loss of 1,000 lives and the destruction of hundreds of villages. On 3 August 1361, a tsunami destroyed 1,700 houses in this same area. On 20 September 1498, 1,000 houses were washed away and 500 deaths resulted from a tsunami which struck the Kii peninsula. Kyushu was struck by a destructive tsunami in September 1596. Great loss of life occurred on 31 January 1596 from a tsunami on the island of Shikoku, affecting also a number of regions in Honshu.
In recent times, the great Meiji Sanriku tsunami of 15 June 1896 resulted in 27,122 deaths, thousands of injuries, and the loss of thousands of homes. On 3 March 1933 a tsunami in the Sanriku area reached a height of about thirty meters and killed over 3,000 people. injured hundreds more and destroyed approximately 9,000 homes and 8,000 boats. In December 1944, a tsunami in central Honshu caused almost 1,000 deaths and the destruction of over 3,000 houses. The Nankaido tsunami, on 21 December 1946, resulted in 1,500 deaths and the destruction of 1,151 houses (Pararas-Carayannis, 1982).
As previously stated, the Sanriku coast of Japan is characterized by significant seismic activity. Large earthquakes have generated destructive tsunamis in the past. Two of these destructive tsunamis were generated by local earthquakes off the coast of Sanriku, as that of March 11, 2011. Also a teleseismic tsunami from Chile in 1964 was extremely destructive on the Sanriku coast. The following is a partial listing of significant past events and of recent events that caused damage but were not as catastrophic as that of March 11, 2011.
1896 June 15 - An earthquake measuring M=8.5 (named as the 1896 Meiji-Sanriku earthquake ) occurred off the coast of Sanriku in Iwate Prefecture. It generated a tsunami of up to 25 meters (82 ft) which reached populated shorelines 35 minutes later. The waves destroyed hundreds of houses and killed over 27,000 people. Besides Japan, the tsunami caused damage in Hawaii, California and elsewhere.
1933 March 3 - A great, Mw=8.4 earthquake (also known as the Showa Earthquake) occurred on March 2, 1933 (UTC DATE). Its epicenter was at 39.14N 144.31E, off northeast Honshu along the coast of Sanriku, at about 290 kilns (180 mi) east of the city of Kamaishi, Iwate. It generated a destructive tsunami that caused extensive damage along the Sanriku coast of the Tohoku region of Honshu.
1960 May 24 - The 1960 tsunami was generated by the earthquake in Chile. It impacted the entire Japanese/Pacific coastline with waves ranging in height up to 5 meters. The tsunami was particularly destructive at Ofunato, on the southern Sanriku coast. At Ofunato and Akazaki the tsunami destroyed or washed away 432 houses and killed 52 people (Hatori et al., 1982)
1978 - The Miyagi earthquake as it was named caused its greater damage around Sendai and triggered numerous landslides.
1994 - Sanriku-Haruka-Oki earthquake. No details on the damage.
2003 May 26 - A series of earthquakes in the Miyagi Prefecture. The May event (named as the Miyagi-Oki) earthquake had a magnitude of M=7.0 event and affected the Sanriku coast. It injured 171 people and caused $97.3 million in damages.
2003 July 26 - Another earthquake in the same Miyagi area injured 676 people and damaged more than 11,000 buildings. Total damage was estimated at $195.4 million.
2005 August 16 - The Miyagi earthquake (as it was named) was a M= 7.2 event and struck about 55 kilometers (34 mi) due east of the Oshika Peninsula in Miyagi Prefecture, in eastern Honshu. A local tsunami warning was issued by the Japan Meteorological Agency (JMA). Waves ranged from 1 and 2 feet at Ofunato but no tsunami damage was reported on the east coast of Japan.
2005 November 15 - Sanriku Japan Earthquake occurred at 6:39am Japan Standard Time (UTC+9) on , 2005. The earthquake was centered in the Pacific Ocean about 330 miles east-northeast of Tokyo about 24 miles below the surface. There were no immediate reports of casualties. USGS reported it as a magnitude Mw=7.2 event, but JMA reported it as an Mw=6.9.
2008 June 14 - The Iwate-Miyagi Nairiku earthquake (as named) had a magnitude of Mw=6.9, and epicenter about 1 km (0.62 mi) east of Narusawa Onsen in the northwest Iwate Prefecture of the Tohoku region on northeastern Honshu. The earthquake caused many casualties, building collapses and power outages.
2011 March 11 - This is the great Tohoku earthquake and tsunami reported here. It is considered as the largest ever recorded in Japan and the most devastating.
Evaluation and Comparison of the 1896, 1933 and 2011 Sanriku earthquakes and tsunamis
The March 11, 2011 earthquake was one of the largest in the last century. Most recent great earthquakes struck Indonesia in 2004 and Chile in 2010. All of recent great earthquakes had rupture zones that extended for several hundred of kilometers and had slips which were 10 meters of more.
Apparently the 1896 "Meiji" earthquake in Japan occurred on a reverse fault and may have been an outer rise event. The earthquake that generated the 2011 great tsunami had characteristics of severity of tsunami generation usually associated with outer rise events and of slow rupturing velocity associated with compacted sedimentary layers. However, the epicenter and crustal displacement region of the 2011 quake was not on the outer rise of the subducting plate but at least 150 km. West of the tectonic boundary of the two colliding plates that have created the Japan Trench.The 1896 and the 2011 quakes generated extremely destructive tsunamis. By contrast the 1933 "Showa" quake occurred on a normal fault within the Pacific Plate. Although this was a great earthquake as well, it was not as strong as the other two. The tsunami waves of the 1896 and the 2011 were much higher and much more destructive. All three tsunamis reached the shores of Honshu within 30 to 40 minutes after the main shocks were felt.
The 1896 Maximum Tsunami Height Distribution North of Sendai (modified after Hatori et al., 1982?)
The number of fatalities for the 1896 earthquake and tsunami was 26,360. The 1933 earthquake and tsunami death toll was 3,064. The final death toll of the 2011 tsunami is not yet known but it could be as high as that of the 1896 event and perhaps even higher. Collateral effects from the explosions of the nuclear plant may have a long term impact on the long-term death toll.
The maximum height for the 1896 and the 1833 tsunamis occurred at Ryori Bay in Sanriku, Iwate Prefecture. The height of the 1896 tsunami reached 38.2 m, the highest ever in Japan since the tsunami of 1868. The 1933 tsunami in the same area reached a height of 23.0 m. It remains to be determined what the maximum height of the 2011 has been. There are reports that it may have been 30 meters or more but this needs to be confirmed.
The ground motions of the 1896 earthquake were not substantial and the estimated seismic intensity of 4 was assigned to this event (JMA scale). The quake's rupture velocity was relatively slow, indicating the presence of compacted sedimentary layers in the source region. Its tsunami was extremely high, however, leading observers to believe this was a tsunami earthquake with a slower fault slip than that which occurs during normal earthquakes. The ground motions of the 1933 earthquake were stronger and were assigned an intensity of 5 (Japanese scale). Yet the tsunami it generated was not as high as the 1896.
The earthquake of March 11, 2011 was clustered in a 6.2-year time period between 2004 and 2011 during which three great earthquakes occurred. The 2011 earthquake was the third of the great earthquakes to strike the planet in such a short period of time. This appears to be a statistical anomaly. Additionally, the 2011 Japan earthquake was followed by a great number of strong aftershocks within the first two weeks, which indicated that great stress had accumulated along the Japan Trench tectonic boundary - off the Island of Honshu. This too is a statistical anomaly. It would have been expected that the strength of aftershocks would diminish in strength and would be spaced further apart in time. This did not happen. The strong aftershocks have continued indicating that the stress in the region may not have been totally released. By mid-March 2011, more than 250 aftershocks with magnitudes over 5.0 had occurred and 25 of these had magnitudes over 6.0.
The recent high magnitude aftershocks which continue in this region are extremely worrisome. There have been at least ten major aftershocks closely clustered in both time and space. The time sequence of events is also alarming. There was a significant magnitude 7.3 foreshock on March 9, followed by thirteen more events. The main Mw=9.1 earthquake of March 11 was followed by aftershocks with magnitudes 6.4, 6.4, 6.8 and 7.1, and 360 more
including two with Mw=6.6 and a Mw=6.4 on March 22. Clearly,
something unusual is happening which is not fully understood at this time.
Based on these observations it can be concluded that significant aftershocks can be expected to follow in subsequent days, weeks and months. Also, stress transference may result in yet another tsunamigenic earthquake with magnitude up to Mw=7.8 in the near future. Such an earthquake could occur further North or more likely further South and closer to Tokyo. Deeper earthquakes could probably occur to the west of the March 11, event, along the Benioff-Wadati zone. Thus, It is very possible that some volcano in the region may become active, particularly if deeper earthquakes continue to occur. Also, subsequent earthquakes in the region may be shallower and could have magnitudes up to 7.8. Events of such magnitude should not be considered as aftershocks. Usually, aftershocks have diminishing magnitudes. This is not happening presently. There was subsequent event of 7.1. Some of the latest events which characterized as aftershocks, may be actually foreshocks of another pending major earthquake - particularly if they occur outside the region affected directly by the March 11 quake. The more recent quake on April 12, 2011 (Tuesday at 8:08 a.m. local time; 7:08 p.m. Monday ET), was rather shallow (13.1 kilometers - 8 miles), had a magnitude of 6.4 and epicenter at 35.406N, 140.542E (about 77 miles east-southeast of Tokyo, according to the U.S. Geological Survey. Thus, if a similar large earthquake occurs again in the area - and this far south of the March 11 quake - it should be considered as a separate event and not as an aftershock. It is a separate earthquake and may indeed be a foreshock of another major or even great earthquake. The question arises: can another major or great earthquake occur again in the area, so close in time and space? Indeed it can. Such was the case with the December 26, 2004 Sumatra earthquake. Another great earthquake occurred on March 28, 2005 further south, closer to Padang and Nias Island.
Other regions of the Pacific are statistically overdue for a large tsunamigenic earthquake. Kamchatka and the Kuril Islands may have large tsunamigenic earthquakes in the near future. Seismic regions along the Aleutian Islands appear to have been dormant and are likely tsunamigenic sources in the near future. The Cascadia Subduction zone has not produced any significant tsunamigenic earthquakes in more than 400 years. A large earthquake is statistically overdue for California. the central Peru region has also been unusually quiet. Another destructive tsunami in the Pacific is very possible. The northern Caribbean Margin is also statistically overdue for the generation of a major tsunami.
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Japan Meteorological Agency Shindo Database Search Retrieved August 11, 2009
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