Strong ground motions from the 2011 off-the Pacific-Coast-of-Tohoku, Japan (Mw = 9.0) earthquake obtained from a dense nationwide seismic network
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The dense recordings of the K-NET and KiK-net nationwide strong motion network of 1,189 accelerometers show clearly the radiation and propagation properties of the strong ground motions associated with the 2011 off-the-Pacific Coast-of-Tohoku, Japan (Mw = 9.0) earthquake. The snapshots of seismic wave propagation reveal strong ground motions from this earthquake that originate from three large slips; the first two slips occurred over the plate interface of off-Miyagi at the southwest and the east of the hypocenter, and the third one just beneath the northern end of Ibaraki over the plate interface or in the crust. Such multiple shocks of this event caused large accelerations (maximum 1–2 G) and prolonged ground shaking lasting several minutes with dominant high-frequency (T < 1 s) signals over the entire area of northern Japan. On the other hand, ground motions of relatively longer–period band (T = 1–2 s), which caused significant damage to wooden-frame houses, were about 1/2–1/3 of those observed near the source area of the destructive 1995 Kobe, Japan (M = 7.3) earthquake. Also, the long-period (T = 6–8 s) ground motion in the Kanto (Tokyo) sedimentary basin was at an almost comparable level of those observed during the recent Mw = 7 inland earthquakes, but not as large as that from the former M = 8 earthquakes. Therefore, the impact of the strong ground motion from the present M = 9 earthquake was not as large as expected from the previously M = 7–8 earthquakes and caused strong motion damage only to short-scale construction and according to instruments inside the buildings, both have a shorter (T < 1 s) natural period.
KeywordsThe 2011 Tohoku Earthquake K-NET KiK-net Long-period ground motions
A destructive, Mw 9.0 earthquake occurred off the coast of Japan in the Pacific Ocean on March 11, 2011 causing extreme disasters in northeastern Japan due to high tsunami waves and strong ground motions. The toll of dead and missing persons is estimated at more than 20,000. Before the occurrence of this earthquake, Mw = 7.5 off Miyagi earthquake had been repeatedly occurred with a recurrent period of about 40 years at the plate boundary between the Pacific and the North American plates. Since no large earthquakes occurred for more than 30 years after the 1978 off-Miyagi event, it was anticipated that the next earthquake should occur within 30 years with a probability of 99% (The Headquarters for Earthquake Research Promotion 2001). However, the earthquake which occurred was a much larger, megathrust event where fault rupture spreads entirely over the plate boundary of off-Miyagi, off-Fukushima, and off-Ibaraki earthquakes with a nucleation area of 500 × 200 km in total. Due to this earthquake, the area of large ground accelerations exceeding 1–2 G and a displacement of over 1 m spread entirely over northern Japan along the Pacific Ocean side, along the source rupture area.
In spite of such destructive disasters, all the relevant features of this earthquake were well-recorded by the nationwide K-NET and KiK-net strong ground motion network of over 1,800 stations across Japan. They are spaced at an interval of 20–25 km and are operated by the National Institute for Earth Science and Disaster Research, Japan. By making full use of these observational data, though some stations were destroyed due to large tsunami waves, we could explore the source rupture process and wave propagation properties of this severe earthquake in order to make a detailed study of the cause of strong ground motion. In this study, we examine the characteristics of the high-frequency ground motions that create the large ground accelerations near the source area, and which cause significant effects on the sea surface and damage on land. These long-period ground motions developed within sedimentary basins and caused resonance in high-rise buildings. We also examine the significance of the high-frequency and long-period ground motions associated with the present earthquake by comparing those from past destructive M6-8 events, such as the 1995 Kobe earthquake (M = 7.3), the 2004 SE Off-Kii Peninsula earthquake (M = 7.4), the 2004 Mid-Niigata earthquake (M = 6.8), and the 1944 Tonankai (M = 8) earthquake, which occurred relatively recently in Japan.
Visualization of wave propagation by dense strong motion network
In the first frame of the snapshot at 60 s after source initiation, we see that large ground motions are built up from the radiation produced by a bilaterally rupturing fault from a hypocenter at off-Miyagi (marked by star in Fig. 1) from north and to south, illustrating the extent of a rectangular rupture area with increased raised ground motions. In the second (110 s) frame of the snapshot, a second large shock, almost as large as the first, spreads again over northern Japan, producing intense and long-term shaking of ground motions over northern Japan. As the strong ground motions propagate to Ibaraki, about 200 km southwest of the hypocenter, a third shock illuminates the surface area around Ibaraki. Then, the overlap of these strong ground motions extends the large, prolonged shaking area from Ibaraki to Tokyo (160 and 210 s). In the last two frames of the snapshot (260 and 310 s), we see amplified and prolonged ground shaking in populated cities, such as Sapporo, Tokyo, Nagoya, and Osaka due to the resonance of long-period ground motions within sedimentary basins. In these areas, the ground motions occur in low wave-speed sediments with a large velocity contrast between surrounding rigid (high wave-speed) bedrock. The large ground motions within the basin continued for several minutes.
Radiation of high-frequency signals from the large slips
The distribution of the peak ground acceleration (PGA) as shown in Fig. 2 shows starched PGA contours from north to south along the coast of the Pacific Ocean side due to the large fault rupture along this direction. Significant attenuation of the PGA in the Japan seaside is due to the stronger attenuation of seismic waves in the mantle structure across the volcanic front (Furumura and Kennett 2005). The PGA map confirms that the burst of the high-frequency multiple shocks from the two large slips raised ground accelerations over 500–1,000 cm/s/s around the Miyagi area. The prolonged shaking and large ground motion as illustrated in Fig. 1 is also caused by multiple shocks at interval of about 50 s with large ground accelerations from both sources.
It is also apparent from the snapshots of seismic wave propagation (Fig. 1) and the record section (Fig. 2) that a third shock occurred at Ibaraki at about 130 s after the earthquake rupture (marked by purple star in Fig. 2). The spread of large ground accelerations from Ibaraki (IBR003) to the north and south of the epicenter at a relatively slow apparent velocity and with a stronger curvature of the travel–time curve (blue line in Fig. 2) indicates that the third large slip most probably occurred close to Ibaraki. The hypocenter of the third large slip was found on land at the margin of Fukushima and Ibaraki prefectures. It can be inferred that the third large slip occurred either on the fault plane of the main shock or in the crust as an induced earthquake triggered by the large ground motion of the second event. Aftershock distribution of shallow earthquakes in the area beneath the Fukushima and Ibaraki prefectures supports the later. Thus, the large PGA observed in Ibaraki might be due to the superposition of strong ground motions from large slips off Miyagi or a shallow earthquake at the Fukushima and Ibaragi boundary.
PGA attenuation function
It is confirmed that most of the PGA values during the present earthquake lies within the PGA value expected from the empirical attenuation function between Mw = 7.0 and 8.0 and not exceeding the value of the Mw = 9.0 event, except some PGA values near the fault rupture area are less than D < 100 km. It is apparent that the largest PGA from the present earthquake was not as large as that we have ever observed during previous large earthquakes with magnitude less than Mw = 8.0.
Near-field strong ground motions
Large ground accelerations in excess of 1–2 G were recorded at Miyagi and Ibaraki near the source rupture area which is much larger than that expected from the empirical attenuation function of the Mw = 9.0 earthquakes in Japan.
In spite of large response in the high-frequency ground motions, a relatively weak response amplitude is found in long-period range (T > 0.6 s), which is about half of the observed strong ground motions at Fukiai and Takatori during the 1995 Kobe, Japan (M = 7.3) earthquake. It indicates that the impact of the strong ground motions during the present earthquake on Japanese wooden-frame houses (most effective resonant period is about T = 1–2 s) was not as large as that of the destructive 1995 Kobe earthquake.
Long-period ground motion developed in the Kanto basin
We calculated the velocity response spectrum of horizontal ground motions in order to compare the strength of the long-period ground motion in the center of Tokyo basin (Chiba) during the present Mw = 9.0 earthquakes. The velocity response spectrum of horizontal motion assuming a damping coefficient of h = 0.05 show that the ground motion from the present earthquake caused a larger response in a relatively wide period ranging from 0.5 to 30 s, with maximum amplitude of over 30 cm/s. The level of the velocity response spectrum for the present earthquake is roughly 1.2–1.5 times the maximum value for that of the 2004 mid-Niigata (M = 6.8) earthquake which caused a large response in the narrow-period range of about 9–12 s. The long-period ground motion from the present earthquake caused a significant impact on many low-layer to high-rise buildings, though it caused significant resonance only in high-rise buildings and in large oil storage tanks in the period range of 6–8 s during the 2004 mid-Niigata and the 2004 SE off-Kii Peninsula earthquakes.
The level of the long-period ground motions developed in central Tokyo during the present Mw 9.0 earthquake was significant, however, the level of long-period ground motion was only 1.2–1.5 times larger than those that occurred during the inland M = 6.8 earthquakes in Niigata. Also it should be noted that the peak level of the long-period ground motions (30 cm/s) in the frequency band between 0.5 and 20 s is half of that observed in Togane city, Chiba during the 1944 Tonankai (M = 8.0) earthquake which occurred in western Japan at similar distances from Chiba to that of the present earthquake (Furumura and Nakamura 2006). Such a difference in the level of the long-period ground motions between the present Mw = 9.0 earthquake and the 1944 Tonankai (M = 8.0) earthquake may be due to either the source radiation properties of these earthquakes at different subduction zones or the propagation path effects from the source to Tokyo basin or both. Different amplification properties of the long-period ground motions due to 3D structure of the Kanto basin and relation of the incidence direction of seismic waves might also be examined as a possible cause of the present observation.
Discussion and conclusion
The dense recordings of the K-NET and the KiK-net strong ground motions networks for the recent Mw = 9.0 megathrust earthquake demonstrate clearly the occurrence of anomalous strong ground motions that we have never seen previously. The visualized seismic wavefield from the dense recording data illustrates the complicated source rupture process over the large fault plane of 500 ×200 km, along with an induced earthquake on land. As such, there is a complicated source–rupture process causing large ground acceleration (>1–2 G) in a wide area of northern Japan.
Actually, the spread and the level of ground acceleration is very significant from the large earthquake, but the level of the PGA and strength of relatively long-period ground motion near the source area and the developments of the longer period ground motion in the Kanto basin was not so significant and is almost of comparable level of previous M = 7–8 earthquakes. It reveals that the level of the long-period motions depends not only on earthquake magnitude, but also on the source rupture process and/or wave propagation directions in the 3-D heterogeneous structure of the Kanto basin.
We acknowledge the National Institute for Earth Science and Disaster Prevention Research (NIED), Japan for providing the K-NET and the KiK-net strong motion data. Supplementary material comprising MPEG movies for wave propagation (Fig. 1) is available in the web: http://www.eri.u-tokyo.ac.jp/furumura/2011OffTohoku/
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