The 2011 Tohoku Earthquake and Tsunami Sequence. Mitchell May, EPSC 330

Transcription

1 The 2011 Tohoku Earthquake and Tsunami Sequence Mitchell May, EPSC 330

2 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 M w 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 11 th 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 M w 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

3 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

4 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 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 M w 4.5 since. The Tohoku sequence began with a M w 7.2 underthrusting foreshock which occurred at 2:45 am on March 9, 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)

5 The main shock occurred at 5:46 am (UTC) on March 11, 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 M w 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 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).

6 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 M w 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 M w 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 M w 7.1 aftershock on October 26, 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.

7 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.

8 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 M w 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 M w Meiji Sanriku, and M w Showa Sanriku events, with smaller tsunamis occurring every 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

9 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 M w 7.4 event. The Japanese government reported that a M w 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 M w 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 400km 2 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

10 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 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 earthquakegenerated 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.

11 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). References Chang, Kenneth. "Quake moves Japan closer to US and alters Earth s spin." The New York Times 13 (2011). Nettles, Meredith, Göran Ekström, and Howard C. Koss. "Centroid-moment-tensor analysis of the 2011 off the Pacific coast of Tohoku Earthquake and its larger foreshocks and aftershocks." Earth, planets and space 63.7 (2011): Ito, Yoshihiro, et al. "Frontal wedge deformation near the source region of the 2011 Tohoku Oki earthquake." Geophysical Research Letters 38.7 (2011).

12 Ide, Satoshi, Annemarie Baltay, and Gregory C. Beroza. "Shallow dynamic overshoot and energetic deep rupture in the 2011 Mw 9.0 Tohoku- Oki earthquake." Science (2011): Fujii, Yushiro, et al. "Tsunami source of the 2011 off the Pacific coast of Tohoku Earthquake." Earth, planets and space 63.7 (2011): Mori, Nobuhito, et al. "Survey of 2011 Tohoku earthquake tsunami inundation and run up." Geophysical Research Letters 38.7 (2011). Lay, Thorne, et al. "Possible large near-trench slip during the 2011 M w 9.0 off the Pacific coast of Tohoku Earthquake." Earth, planets and space63.7 (2011): Ammon, Charles J., et al. "A rupture model of the 2011 off the Pacific coast of Tohoku Earthquake." Earth, Planets and Space 63.7 (2011): Mori, Nobuhito, and Tomoyuki Takahashi. "Nationwide post event survey and analysis of the 2011 Tohoku earthquake tsunami." Coastal Engineering Journal (2012): Maeda, Takuto, et al. "Significant tsunami observed at ocean-bottom pressure gauges during the 2011 off the Pacific coast of Tohoku Earthquake." Earth, Planets and Space 63.7 (2011): Shao, Guangfu, et al. "Focal mechanism and slip history of the 2011 M w 9.1 off the Pacific coast of Tohoku Earthquake, constrained with teleseismic body and surface waves." Earth, planets and space 63.7 (2011): "Quake shifted Japan by over two metres". Deutsche Welle. 14 March Archived from the original on 14 March Retrieved14 March "New USGS number puts Japan quake at 4th largest". CBS News. Associated Press. 14 March Archived from the original on 5 April Retrieved 15 March Ammon, Charles J., et al. "A rupture model of the 2011 off the Pacific coast of Tohoku Earthquake." Earth, Planets and Space 63.7 (2011): "March 11th tsunami a record 40.5 metres high NHK"..nhk.or.jp. 13 August Archived from the original on 28 July Retrieved7 September Roland Buerk (11 March 2011). "Japan earthquake: Tsunami hits north-east". BBC. Archived from the original on 11 March Retrieved12 March "Ring of Fire Pacific Ring of Fire". Geography.about.com Retrieved Chang, Kenneth (13 March 2011). "Quake Moves Japan Closer to U.S. and Alters Earth's Spin". The New York Times. Archived from the original on 16 March Retrieved 14 March 2011