Recent Developments in Strong Motion Measurements in Japan and the Damaging Niigata Area Earthquakes of 2007 (Abstract)

by Kazuyoshi Kudo

Kudo, Kazuyoshi (2007). Recent Developments in Strong Motion Measurements in Japan and the Damaging Niigata Area Earthquakes of 2007 (Abstract). SMIP07 Seminar on Utilization of Strong-Motion Data, p. 103 - 106.

Abstract

The strong motion observation in Japan was initiated in 1953, 20 years behind from California. In 2004, celebrating 50th anniversary of strong motion observation in Japan was held sponsored by the Strong-Motion Earthquake Observation Council in the National Institute for Earth Science and Disaster Prevention (NIED), Japan Association for Earthquake Engineering, and Earthquake Research Institute, University of Tokyo. The first part of my presentation is a summary or review of the symposium and an introduction of the strong motion observation networks by the individual organizations.

The strong motion instrumentation program in Japan has drastically been changed after the 1995 Kobe earthquake from research oriented observations to quick information of shaking level or map for emergency responses associated with the rapid developments of data telecommunication.

1. Strong Motion Observation Networks by the individual organizations

a) Ground response observation networks

The representative or massive networks are K-NET consists of 1026 stations and KiK-net of 680 surface and down-hole pairs maintained by NIED and the seismic intensity networks operated by JMA (585 stations) and the 47 prefectural governments (about 2800 stations). Intensity data assembled by JMA are quickly broadcasted by TV and radio. The other ground response strong motion observations have been conducted by universities (mostly ERI) with 109 stations including 14 down-holes and the Ports and Airports Research Institute with 60 sites including down-hole measurements.

b) Structural responses and security for facilities.

The observations in buildings have been conducted at 77 sites by the Building Research Institute. The number of the site or building is very limited, but the majority of the observations (the number is uncertain) for building responses have been operated by big consulting companies jointly with the building owners.

For keeping security mainly roads and bridges, the National Institute of Land and Infrastructure Management (NILIM) have started on-line measurements at 700 sites in Japan after the Kobe earthquake. The NILIM has conducted the observation of responses for civil engineering structures at 160 sites and dense array ground response observations at a few sites.

These observations have been operating by the governmental organizations. The large and dense strong motion observation networks have been established by private companies for emergency responses of individual facilities such as gas (Tokyo Gas: ground responses at about 3,800 sites in Tokyo Metropolitan region), electronic power companies (dams, nuclear power plants), railroad/subway, telecommunication companies and so on.

2. Results of the Symposium 2004 and some issues on strong motion instrumentation in Japan

The proceedings of the symposium is available on the URL (http://www.k-net.bosai.go.jp/KYOUKAN/symp/symptop.htm), unfortunately the papers are written in Japanese with English abstract and figure captions, except invited papers by Prof. W. D. Iwan and Prof. K-L. Wen.

The strong motion observation in Japan has been developed and improved after the Kobe earthquake, as K-NET, KiK-net, and intensity measurement network by JMA and Prefectures. Most remarkable improvements by K-NET are the homogenous installation of observation sites taking into account that the occurrence potential of moderately large earthquakes in and around Japan and the accessibility to the observed data within a very short time through the Internet. K-NET system has certainly accelerated and extended the studies on source inversion and on variations of strong motion features. However, we summarized the requests to the government, researchers, and ourselves as a form of resolution as followings (these are not formal translation):

i)	Government should provide the budget to keep stable managements of fundamental networks such as K-NET, intensity network. Request to prefectures that waveform data are stored and opened to researchers.
ii)	Improvements of strong motion dense array observation aiming to solve each specific issue in the field of strong motion seismology and earthquake engineering are required.
iii)	Recommend the instrumentation to buildings and facilities, especially to public ones. The instrumentation should also be oriented for quick decisions on the security of facilities just after the earthquake.
iv)	Recommend the quick availability of strong motion data by all organizations as K-NET.
v)	Recommend the establishment of the database or databank. This is an urgent issue due to the rapid accumulation of strong as well as weak motion records.
vi)	Recommend the data use not only for research but also for security of society, including development of new generation of strong motion instruments.

3. Some topics on strong motion records recovered during a decade.

During a decade after the Kobe earthquake, more than 10 damaging earthquakes occurred in Japan, e.g., Kagoshima (1997), Yamaguchi (1998), Tottori (2000), Geiyo (2001), Miyagi (2003), Tokachi-oki (2003), Niigata Chuetsu (2004), Fukuoka (2005), Miyagi-oki (2005), Noto (2007), Niigata Chuetsu-oki (2007). Most earthquakes occurred in land, so that near source records were retrieved by the K-NET, KiK-net, the intensity measurement networks and other networks.

Very high level accelerations were recorded. The reasons of this are confidentially near source and site amplification due to shallow soft soils. In the same time, we should note that the soft soil effects showed some nonlinear behaviors.

ii)

It was our first experience that strong motion records were obtained densely in a wide area from the M8 class and then long period (3-15 sec.) strong motion were systematically investigated. The issues of long-period ground motion are urgent and important because of rapid increase of high rise buildings with base isolation even at lowland Tokyo and vicinity. Acceleration based strong motion observations have sometimes missed to retrieve the very late arrivals that the acceleration level is low but the velocity or displacement level is high. Therefore, some research groups tend to use strong velocity meter instead of accelerometers.

4. Revision of K-NET

The K-NET renewed the instrument so as to link the seismic intensity network, to cover a high acceleration up to 4 G, to improve the resolution or quality of digitization, and to acquire waveform data with quasi-real time. I believe that the resolution 2004 might somewhat assist the renewal of K-NET.

5. Niigata Chuetsu-oki earthquake (Mw6.7) of 2007

The earthquake of July 16 was surprising for us in two reasons. The first one is that the very near source strong shaking struck the nuclear power station. The second one is that the large earthquake occurred at only 30 km apart from the Niigata Chuetsu earthquake of 2004 and with very short interval.

Prominent issues of the Niigata Chuetsu-oki earthquake of July 16 were:

·	Four nuclear power plants among 7 ones were operating at the earthquake, but they have stopped safely without serious accidents or problems as the IAEA report.
·	Nevertheless high acceleration of 1g at the basement that exceeded roughly twice of the design level, no damage was observed at least on buildings except some non-priority facilities. This issue is discussed in the field of structural engineering.
·	The fault plane has not yet determined, that is, there is no room to doubt the mechanism solution of thrust, but a dip direction has not yet determined.

Relatively dense strong motion records near source region were obtained. They are from the K-NET, JMA, Niigata Prefecture, and Tokyo Electric Power Company (TEPCO). Strong motion records at the TEPCO Kasiwazaki-Kariwa Nuclear Power Station were released recently and the English version is now preparing. It is the first case that the strong motion data at a nuclear power station are released.

A few comments on the records are that strong nonlinear behaviors are found in the vertical array records obtained near the power station. K-NET records at Kashiwazaki shows strong spike-like pulse. The data analyses and interpretation of phenomenon are still in the beginning stage and wide as well as deep discussions will be required.