全球地震計ネットワークの説明

What Is the Global Seismographic Network?

The 世界地震観測網（GSN）世界の地震活動を包括的に監視する、150以上の広帯域地震観測点からなる世界規模のネットワーク。USGS・NSF・IRISが共同で運用している。 (GSN) is an international collaborative network of standardized, high-quality broadband seismograph0.001〜50Hzという広い周波数帯域にわたって地震波を記録できる地震計。現代の世界的な地震観測網における主要な観測機器。 stations distributed around the world to provide comprehensive coverage of seismic activity. Established in the late 1980s through a partnership between the US Geological Survey地震の監視、National Earthquake Information Centerの運用、世界の地震データのリアルタイム公開を担う、アメリカ政府の主要機関。 and the Incorporated Research Institutions for Seismology (IRIS, now part of EarthScope), the GSN currently operates approximately 150 stations in over 90 countries. Each station is equipped with a 広帯域地震計0.001〜50Hzという広い周波数帯域にわたって地震波を記録できる地震計。現代の世界的な地震観測網における主要な観測機器。 capable of recording ground motions across a broad frequency range — from tidal deformations to high-frequency body waves — and transmits data in real time to data management centers. The GSN serves as the backbone of global seismic monitoring, providing the data stream that underpins 地震観測網地震活動を継続的に監視する、連携した地震観測点の集合。世界地震観測網（GSN）は150以上の観測点から構成され、世界規模の観測範囲を提供する。 operations worldwide.

The Data Repository

All GSN data are archived at the IRIS Data Management Center in Seattle (now part of EarthScope Consortium) and made freely available to researchers around the world. This open-data philosophy has been transformative for seismology: scientists anywhere can download waveform data from any GSN station for any time period, enabling studies of earthquake sources, Earth structure, and geodynamics that would be impossible with limited proprietary data. The GSN archives contain hundreds of terabytes of continuous seismic waveform data stretching back to the network's inception and growing at approximately 1 terabyte per month.

Station Distribution and Coverage Gaps

The ideal 世界地震観測網（GSN）世界の地震活動を包括的に監視する、150以上の広帯域地震観測点からなる世界規模のネットワーク。USGS・NSF・IRISが共同で運用している。 would have stations evenly distributed across the globe, but the reality of geography, politics, and logistics produces a distribution that is far from uniform. Stations are densely concentrated in the United States, Europe, and Japan, while the southern hemisphere — particularly the open oceans, Antarctica, and parts of Africa — has much sparser coverage. These coverage gaps affect the precision with which earthquake hypocenters in undersampled regions can be located and the quality of seismic tomography models of mantle structure beneath remote areas. Considerable effort has been invested in deploying stations in difficult environments: on ocean island chains, in the Antarctic interior, and in remote areas of Africa and the Pacific. Recent technological advances have enabled the deployment of seismometers on the seafloor — ocean bottom seismometers (OBS) — which, when operating for months or years, can fill some of the most significant gaps in the oceanic coverage.

Station Siting and Noise Requirements

A GSN station is only as useful as the quality of its data, and data quality depends critically on siting. Stations must be located far from sources of cultural noise — highways, factories, ocean surf — and ideally installed in a vault drilled into bedrock or in a mine or borehole. Thermal stability is essential: temperature fluctuations as small as a few hundredths of a degree Celsius can produce spurious signals in ultra-sensitive 広帯域地震計0.001〜50Hzという広い周波数帯域にわたって地震波を記録できる地震計。現代の世界的な地震観測網における主要な観測機器。s. Achieving the stringent low-noise requirements of a GSN station in a remote location without reliable power infrastructure is a significant engineering and logistical challenge, requiring solar or wind power, battery backup, satellite communications, and periodic maintenance by trained field technicians.

Real-Time Data Transmission

One of the defining features of the modern 世界地震観測網（GSN）世界の地震活動を包括的に監視する、150以上の広帯域地震観測点からなる世界規模のネットワーク。USGS・NSF・IRISが共同で運用している。 is real-time or near-real-time data transmission. Each station sends its data continuously to regional or global data centers via satellite link or internet connection, typically with a latency of a few seconds to a few minutes. This real-time capability transforms the GSN from a research archive into an operational monitoring tool. When a large earthquake occurs, data from dozens of GSN stations around the world are received within minutes, allowing automated algorithms to compute a preliminary location, depth, and magnitude. For the largest events — potential 津波海底地震時の海底の急激な変位によって発生する一連の海の波。津波はジェット機並みの速度（時速700km以上）で海洋全域を伝わることがある。-generating megathrust earthquakes — this rapid analysis is directly fed into tsunami warning centers1991年から運用されている、世界初の公共向け緊急地震速報システムの一つであるメキシコのSASMEX。沿岸部の地震から、メキシコシティに最大60秒の警報時間を提供する。 that issue alerts to coastal populations. The combination of real-time data and automated processing allows the アメリカ地質調査所（USGS）地震の監視、National Earthquake Information Centerの運用、世界の地震データのリアルタイム公開を担う、アメリカ政府の主要機関。 Earthquake Hazards Program to publish a preliminary solution for most globally significant earthquakes within 15–20 minutes of their occurrence.

How the GSN Locates Earthquakes in Minutes

Locating an earthquake requires measuring the arrival times of 地震波地震や爆発によって発生し、地球内部を伝播する弾性波。地震波は、震源で放出されたエネルギーを遠方の地点まで運ぶ。s at multiple stations. The most basic approach uses the differential arrival times of P and S waves at a single station: because the P wave岩盤中も液体中も伝わる、秒速5〜8kmの最も速い地震波。P波はスリンキーのように進行方向に対して物質を圧縮・伸長させながら伝わり、地震観測点に最初に到達する。 travels faster than the S wave進行方向に対して垂直に岩盤を揺らしながら伝わる地震波で、P波の後に到達する。S波は液体中を伝わることができず、この性質から地球の外核が液体であることが証明された。, the gap between their arrivals grows with distance. With three or more stations, the hypocenter can be triangulated. Modern automated algorithms use dozens or hundreds of stations and phase arrivals to compute the best-fitting location that minimizes the misfit between observed and predicted arrival times, given a model of Earth's velocity structure. The depth of the hypocenter — whether an earthquake is shallow, intermediate-depth, or deep — is determined by the pattern of wave arrivals and the shape of the 地震記象（地震波形記録）地震計が記録した出力で、地面の揺れを時間の関数として示したもの。地震学者はこの記録を解析し、地震の規模・深さ・位置を求める。 at nearby stations. For large events, additional information comes from the long-period waves recorded by 広帯域地震計0.001〜50Hzという広い周波数帯域にわたって地震波を記録できる地震計。現代の世界的な地震観測網における主要な観測機器。s, which constrain the earthquake's source mechanism and 地震モーメント断層面積・平均変位量・岩石のせん断弾性率の積として算出される、地震が放出した総エネルギーの尺度。モーメントマグニチュードの算出基盤となる。.

Uncertainty and Error in Earthquake Locations

Even with the GSN, earthquake locations carry significant uncertainty. Errors in the assumed velocity model of Earth's interior, uneven station coverage, and phase identification errors all contribute. Typical location uncertainties for well-recorded teleseismic events are on the order of 10–20 kilometers horizontally and somewhat larger in depth. For events in poorly sampled regions — particularly in the southern hemisphere oceans — uncertainties can be much larger. Regional seismic networks, which have stations closer to earthquake-prone areas, provide dramatically better location precision for local events, complementing the global coverage of the GSN.

The Role of IRIS and USGS

The GSN was established through a formal partnership between the アメリカ地質調査所（USGS）地震の監視、National Earthquake Information Centerの運用、世界の地震データのリアルタイム公開を担う、アメリカ政府の主要機関。 and IRIS (now EarthScope Consortium). The アメリカ地質調査所（USGS）地震の監視、National Earthquake Information Centerの運用、世界の地震データのリアルタイム公開を担う、アメリカ政府の主要機関。 Earthquake Hazards Program provides scientific leadership, funds a large fraction of station operations, and integrates GSN data into its earthquake monitoring and シェイクマップ（ShakeMap）地震発生後の揺れの強さの分布を示すUSGSの成果物。地震計データ、地震動モデル、「揺れを感じましたか？」の報告を組み合わせて作成される。 products. IRIS/EarthScope manages the data infrastructure — the Data Management Center, the real-time data streaming system (SEED format), and the station operations at many remote sites. International partners from dozens of countries operate their own national networks that share data with the GSN. This collaborative model — combining US federal funding, university research capacity, and international scientific cooperation — has produced a global monitoring capability far beyond what any single agency or country could achieve alone, exemplifying how the 地震観測網地震活動を継続的に監視する、連携した地震観測点の集合。世界地震観測網（GSN）は150以上の観測点から構成され、世界規模の観測範囲を提供する。 community has organized around shared infrastructure for the benefit of earthquake science and public safety worldwide.