Nonlinear site response evidence of K-NET and KiK-net records from the 2011 off the Pacific coast of Tohoku Earthquake
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2011
Received: 7 April 2011
Accepted: 8 June 2011
Published: 27 September 2011
We analyzed the acceleration time histories recorded by the K-NET and KiK-net stations of the Mw 9 Tohoku Earthquake in order to investigate site response issues related to near-source effects. Time-frequency analysis of K-NET stations in the Miyagi prefecture, closest to the rupture area, show that sites having a Vs30 < 400 m/s present a combination of deamplification at frequencies higher than 5 to 10 Hz and cyclic mobility (high acceleration peaks riding over a low frequency carrier). This suggests strong nonlinear site response at these stations. Furthermore, using KiK-net data we are able to compute borehole transfer functions from the mainshock and events having small PGA values from the local dataset. The ratio between weak-motion and strong-motion borehole transfer functions constitutes an indicator of nonlinear site response. This ratio reveals strong dependence on Vs30 and shows that widespread nonlinear behavior took place during this large earthquake.
Key wordsSite response cyclic mobility Tohoku Earthquake K-NET KiK-net
On the afternoon of March 11th, 2011, an earthquake of magnitude Mw 9 occurred off the Pacific coast of To-hoku, Japan (Japan Meteorological Agency, JMA hereafter, 2011). This event is one of the largest earthquakes in the world that has been well recorded in the near vicinity of the source (NIED, 2011). The estimated fault plane is 500 km along strike in the northeastern part of Japanese mainland and 200 km along dip (Simons et al., 2011). This event brought devastating damages especially by the tsunami that took place after the mainshock. Yet, the recorded ground motions were also very large. One site recorded a JMA intensity value of 7 (maximum value of this scale) and 28 sites recorded values of 6+ (JMA, 2011). The observed PGA values from K-NET and KiK-net also showed large accelerations for many sites (19 sites showed PGA larger than 1 g) and strong motion duration longer than 80 s (time interval between 5% and 95% of the Arias intensity). This event produced a wealth of data showing different types of nonlinear soil behavior, going from traditional high frequency deamplification to liquefaction. These phenomena could be seen on the records of sites at soft soils over a large area along the fault’s rupture including Miyagi, Chiba and Tokyo prefectures. For example, Fujikawa (2011) reported that boil sand or liquefaction could be seen for some sites located on soft soil close to Tokyo.
This study shows a preliminary analysis of the recorded data from the Mw 9 Tohoku Earthquake by the K-NET and KiK-net stations to infer nonlinear effects. We first perform time-frequency analysis of the K-NET Miyagi stations, which are the closest to the rupture area, to assess the effect of strong nonlinear effects (cyclic mobility and liquefaction) on the recorded ground motion. Secondly, we assess the mean soil behavior using the KiK-net stations by computing borehole transfer functions (Fourier spectral ratios between the surface and downhole receivers) using weak-motion and strong-motion data, respectively.
2. Data and Analysis
In addition, some K-NET stations, in the vicinity of the Miyagi prefecture and located on soft to relatively stiff soil (200 < Vs30 < 400 m/s) according to NEHRP site classification (1994), showed large acceleration peak values riding over a low frequency carrier. This has already been shown by Iai et al. (1995), Archuleta (1998), and Bonilla et al. (2005) as an indicator of soil nonlinearity known as cyclic mobility. Laboratory studies show that the physical mechanism that produces such phenomenon is the dilatant nature of cohesionless soils, which introduces the partial recovery of the shear strength under cyclic loads. This recovery translates into the ability to produce large deformations followed by large and spiky shear stresses. The spikes observed in the acceleration records are directly related to these periods of dilatancy and generation of pore pressure. In this case, the large values of recorded ground motion are a combination of near-fault and nonlinear site response effects involving fluid mobilization.
A preliminary analysis of K-NET and KiK-net records from the Mw 9 Tohoku Earthquake has been performed. The results show that widespread nonlinear site response took place in several K-NET and KiK-net stations. Some of the K-NET stations show signs of pore pressure effects on dilatant soils. These nonlinear effects produce high acceleration peaks (between 0.2 to 1 g) riding a low frequency carrier that lasted several seconds. The analysis of KiK-net stations shows that nonlinear effects correlates well with Vs30. The most striking observation is that even sites having Vs30 > 800 m/s were affected at frequencies higher than 11 Hz. They went probably nonlinear due to the presence of a thin layer of about 10 m thick with V s between 200 and 400 m/s. This event, together with its foreshocks and aftershocks, produced an impressive amount of data that still need to be analyzed. Thus, future studies should include foreshock and aftershock data to complete nonlinear soil behavior at KiK-net stations. In addition, we need to study the relation between lithology and velocity profiles beyond 30 m depth to have a better understanding where nonlinear effects took place. Indeed, this work shows that for sites having Vs30 > 800 m/s, nonlinear effects might be located at shallow depths; but this is not clear for softer soils. Finally, this event shows the need to take into account source, wave propagation and dynamic site characterization in order to better estimate the expected ground motion.
We are indebted to the National Research Institute for Earth Science and Disaster Prevention (NIED) in Japan for providing the data for this analysis in spite of all human and technical difficulties they are facing during the earthquake’s aftermath. We are also grateful to John Douglas and an anonymous reviewer for their comments that helped to improve this manuscript.
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