Southern California Geotechnical Services and Consulting

In most cases a manometer survey is taken to further investigate potential foundation problems but in this case it was done to determine the slabs flatness before it was tensioned.

The Manometer survey was taken on the slab two weeks after it had been poured in place.

Comments on the 03 11 2011 Japan (Sendai – Honshu) Earthquake Disaster

Like other areas on the Pacific Ring of Fire, Japan is an area which is a focus of change on the face of the earth, and as such, a focus of earth movement, and earthquakes.  On March 11, 2011, one of the largest earthquakes in recorded history occurred off the east coast of northern Honshu, near the city of Sendai, a M8.9 to M9 earthquake struck – and has spawned one of the world’s greatest disasters.

Japan, from a structural geologic and tectonic standpoint, is a volcanic back-arc created by subduction of two major earth tectonic plates under the eastern leading edge of the Eurasian and North American tectonic plates.  It is important to note this is not a typo, the northerly half of Japan actually rests on the easterly leading edge limits of the North American plate which is believed to be an arcing wrap-around continuation of the same NA plate portion the Aleutian Islands of Alaska are situated.  The underthrusting of the ocean plates induces both the formation of earthquakes as the plates incrementally slip, as well as volcanic activity as a result of the melting and ascension of the byproducts of melting.   The geology of Japan is comprised of a crystalline basement of both intrusive and shallow extrusive igneous rocks, and accumulations of sedimentary rocks of both an indigenous nature and sourced from scraping from and fragmental remnants of ancient subduction events.  local and regional metamorphism has altered the rocks into metamorphic belts and zones.  The Pacific plate subduction zone occurs in an ocean trench off the east coast of the northerly half of the main island of Japan known as Honshu, as well as the northern major island known as Hokkaido.  This ocean trench is known as the Japan Trench.  The other major plate is the Philippine plate to the southwest of the Pacific plate.  This plate slips under the Eurasian plate and the south / east coast of the south half of Honshu and southern Japan (including the two southern major islands known as Shikoku and Kyushu)  and continues south and west into Indonesia.  In looking at maps of the tectonic plates, structural geology, and distributions of volcanoes, there is to be noted a profound correlation between the distribution and geometry between the earthquakes, volcanoes, and structural geologic features of Japan.

The tectonic activity interacting with the curvature of the earth has introduced several major fault and seismic zones in Japan.  Japan, the world’s third largest economy, is also a country with a very dense population, approximately 127-million, in a land area that is on the scale of the size of California.  As such, urbanization is very dense, and industrialization is very heavy.   Japan is also a world leader in civil engineering.

These tectonic plates, in particular, the Pacific Plate, move at an average velocity of up to 70 to 83-mm per year in the immediate area of Japan.  This is a very high rate, over 50-percent to over twice that of many other seismic areas on earth, including California and New Zealand (about 25 to 50-mm, and 40 to 45-mm, respectively).   As no surprise, Japan is an area long known for frequent and large earthquakes and numerous volcanoes, including multiple episodes of historic activity.

The March 11, 2011 earthquake event was measured as M8.9 – one of the largest earthquakes ever – and included many aftershocks of M6 to over M7, which continue.  This event included several large foreshocks, the largest being an M7 event on March 9 situated approximately 40-km away from the epicenter of the March 11 event.  Since 1973, the Japan Trench has had over 9 major earthquake events equal or exceeding M7.  In January 1995, the Kobe Earthquake occurred,  December 1994,a 7.8 event occurred about 260-km to the north of the March 11, 2011 event.  In 1978,a M7.7 event occurred about 35-km to the southwest.

In the case of the March 11, 2011 earthquake, a large section of the Pacific Plate mobilized and slid under Japan.  This M8.9 to M9 event was huge, and caused the entire earth to shift 8 to 10-cm on its “figure” axis (a center of mass axis close to but not exactly aligned with the earth’s rotational axis), according to some experts – as well as caused parts of Japan in the area of the epicenter to move westward up to 13-feet.  The earthquakes helped define this subduction zone, and it apparently slips under Japan at an angle of about 14-degrees.

Since this event occurred on a subducting ocean plate, its epicenter was relatively deep – approximately 25 to 32-km beneath the ground surface.  The epicenter was also about 80-miles (130-km) offshore of Sendai.  Epicenters on strike slip faults, for example, such as those on the Laguna Salada fault zones of California and bordering areas of Mexico, and the Christchurch, New Zealand earthquakes had epicenters of only about 5-km by comparison.  Review of the USGS SHAKE maps and other ground motion data indicates that this focal depth combined with the distance offshore helped attenuate ground motions, which were diffused and relatively moderate considering the exceptional magnitude of this earthquake.  Although there are many areas of local focusing and amplification of waveforms especially in coastal cities near Sendai, and nearby inland cities in alluviated terrain (including Yamagata and Fukushima located to the south of Sendai), general groundmotions were generally less than 0.5g and typically 0.3g or less away from the vicinity of the epicenter.  Although these ground motions may have been attenuated somewhat, the duration of shaking was still very long – reportedly in excess of 5-minutes!  Numerous aftershocks representative of the overall plate adjusting to the deformation associated with the foreshocks have been occurring since the Friday March 11 event, and illustrate plate adjustments extending over 200-miles (ie. to south of Tokyo and north of Honshu) to the north and south of Sendai.  A total of over 450 M4.5 or greater earthquakes have been reported associated with that in the period Friday March 11 through Monday March 14.

Some incredible ground motions were recorded in the Tohoku region, particularly in the Fukushima (ie. cities of Fukushima and Tsukidate) and Miyagi (ie. coastal cities of Sendai and Natori) Prefectures  of Japan.  In the current information presented by the CESMD Engineering Strong Motion Data Center of the USGS (reference: “http://www.strongmotioncenter.org/cgi-bin/ncesmd/apktable.pl?iqrid=Japan_11Mar2011″) the highest reported ground motion was 2.993gin the town of Tsukidate, not far to the east of the reactors of Fukushima.  Shiogama, located on the coast just to the northeast of Sendai, had reported peak ground motions of 1.883g, and the city of Sendai had reported ground motions of 1.798g.

The earthquakes caused widespread damage by direct shaking and secondary seismic effects such as liquefaction.  These effects severely damaged refineries, industrial complexes, and of high interest in the world news – nuclear power plants.  At least three nuclear plants in area of Fukushima had issues with respect to cooling and containment – although these may have included partial meltdowns, have not resulted in the 1986 Chernobyl disaster type leakage where reactor materials escaped.  Rather, the problems thus far have been  more on the level of Three Mile Island, with some escape of relatively low level radiation in steam – as there is no core breach.  An excellent layman summary article is available here: http://cosmiclog.msnbc.msn.com/_news/2011/03/14/6268351-clearing-up-nuclear-questions.

It is important to note that the reactor problems are, according to some preliminary accounts, partially a result of bad luck with respect to timing in that the cooling was shut off just prior to the earthquake, the diesel powered backup generators were disabled by the tsunami, and backup battery power for the pumps lasted only about 8-hours.  The remainder and  vast majority of Japan’s 58 nuclear powerplants, however, were undamaged.

The most horrific damage was not due to the direct action of earthquake, but rather due to the generation of tsunamis of up to 10-meters and locally more.  These waves were produced when large sections of the associated sea floor suddenly deformed.  This deformation caused the overlying ocean water to move suddenly with the sea floor, inducing a sloshing type wave which radiates away from the source of deformation at speeds of 500 or more miles per hour.   Within several minutes to about half an hour, these waves spread into the east coast of Japan, and within 24-hours had spread around the majority of the Pacific and parts of the Indian and Antarctic Oceans.  The tsunamis hitting Japan, in particular, the low-lying area of Sendai, flooded into and destroyed low elevation level areas along the coast.  These waves came in at very high speed – and in the case of Sendai, within minutes, had completely annihilated a large portion of the city as 25 to 40-foot high walls of water saturated with debris in the form of anything from clay and sand to construction rubble/debris to multi-story buildings, trucks, busses, freighters and other ships and even aircraft inundated and scoured the land.  This suspended mixture of debris and water was stopped only by elevation.  As the water receded, scouring in the reverse direction occurred as the wave moved inextolerably back to the sea, leaving a layer of debris in the wake.  This movement of water created giant whirlpools off the coast.

Entire villages and towns were essentially wiped off the face of the map by these waves, leaving nothing but carpets of devastation, or just literally barren ground that was once a populated area.  These areas offer little to any survivors currently – there is literally no drinkable water, no food, no electricity and little access, as virtually anything touched by these waves was contaminated or destroyed.  An interesting article by AP is here: http://news.yahoo.com/s/ap/20110314/ap_on_re_as/as_japan_tsunami_vanished_village.  For perspective on loss of life and limb, consider the disaster took place in the mid afternoon (2:45pm local time) on a work day.  It only took several minutes to less than half an hour for the tsunamis to reach the shoreline following the earthquake  The Sendai area has over 1-million residents.  The Fukushima area has a population of over 2-million.  The loss of life is likely much higher than the current 3000.  These effects – leftover toxic soup from sewage, debris, hazardous materials like fuel oils, household chemicals, rotting food; dead bodies of livestock and people (including the continual washing up of additional dead); dense rubble comprised of literally “everything including the kitchen sink”; and the ongoing lack of sanitation accommodation for survivors – makes for an acutely hazardous and widespread condition that in this writer’s opinion is much more concerning than the nuclear threats from reactor damages – which appear to have been a focus of media hyping and sensationalize alarmism.

Use the embedded map below to navigate the the Fukushima area:

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