震源地(epicenter)vs震源(hypocenter): 違いは何か?

Epicenter: The Point on the Surface

The 震央地下で地震が発生した震源の真上にあたる地表の地点。ニュース報道では地震の発生場所としてよく報じられる。 is the point on the Earth's surface directly above where an earthquake begins. It is the location reported in news headlines — "earthquake strikes 30 km south of city X" — because it tells you where on the map the seismic energy originated most directly above ground. The epicenter is the geographically relevant location for emergency responders, residents, and infrastructure managers trying to understand where they should focus their attention.

However, it is crucial to understand that the epicenter is a projected point on the surface, not where the rupture actually started. The actual rupture initiation point is always somewhere below the surface. For a shallow earthquake 10 km deep, the epicenter is a reasonable proxy for the source location. For a deep earthquake 200 km below the surface, the epicenter might be more than 100 km away from the closest surface expression of the fault system, and the area of maximum surface shaking may not even be centred on the epicenter.

Hypocenter: Where the Rupture Begins

The 震源地震の破壊が実際に始まる地球内部の地点。フォーカスとも呼ばれる。震源の深さは、地表での揺れ方に大きく影響する。 (also called the focus) is the three-dimensional point within the Earth where the earthquake rupture initiates. It is defined by three coordinates: latitude, longitude, and depth. The depth is often the most uncertain of the three, because determining depth requires particularly good data coverage — ideally stations both near the epicenter and at varying distances.

Understanding the hypocenter depth is essential for hazard assessment. Shallow earthquakes (less than 70 km deep) release their energy closer to the surface and generally cause more intense surface shaking for a given magnitude than deep earthquakes. The 2015 Nepal earthquake (Mw 7.8) had a hypocenter approximately 15 km deep, which contributed to its devastating surface shaking. By contrast, deep focus earthquakes in the 沈み込み帯1枚のプレートがもう1枚のプレートの下にもぐり込み、マントルへと沈み込む領域。沈み込み帯は世界最大級の地震（M8.5以上）を引き起こし、深い海溝や火山弧を伴う。s beneath South America and the Kuril Islands occur at depths exceeding 500–600 km; although some of these events have very large magnitudes, their great depth means the shaking at the surface is spread over a much larger area and is less intense locally.

How Scientists Triangulate Earthquake Locations

Locating an earthquake requires solving for four unknowns: latitude, longitude, depth, and time of origin. The data used are the arrival times of P波（初期微動）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。s and S波（主要動）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。s at 地震計地震波によって生じる地面の揺れを検知し記録する装置。現代のデジタル地震計は、ナノメートル未満の変位も検出できる。 stations of known location. With the travel time from a station and knowledge of wave velocities in the Earth, a circle of possible hypocenter locations can be drawn around each station. With three stations, the three circles intersect at (ideally) one point that gives the epicenter. With four or more stations, over-determined systems allow both epicenter and depth to be estimated simultaneously.

In practice, Earth's velocity structure is not perfectly known, stations have timing uncertainties, and seismic waves are affected by complex three-dimensional geology. Modern earthquake location algorithms use iterative least-squares fitting to minimise the mismatch between observed and predicted arrival times, often incorporating three-dimensional velocity models and waveform cross-correlation to achieve sub-kilometre location accuracy for well-recorded events. 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。's global station coverage has made it possible to reliably locate earthquakes anywhere on Earth to within a few tens of kilometres under routine operating conditions.

Why Depth Matters: Shallow vs Deep Earthquakes

Earthquake depth profoundly influences the distribution and character of surface shaking. Consider two earthquakes, both of magnitude 7.0: one at 10 km depth and one at 200 km depth. The shallow earthquake concentrates its energy in a small area directly above the hypocenter, producing intense, potentially devastating shaking in a limited region. The deep earthquake spreads its energy over a much larger footprint at the surface, producing moderate shaking over a wide area — potentially felt across an entire country — but with less intensity at any individual location.

Depth also affects what types of secondary hazards are produced. Shallow earthquakes are more likely to generate 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。s (if they occur under the ocean and involve significant vertical displacement of the seafloor), 液状化水を含んだ緩い土壌が強い揺れによって一時的に強度を失い、液体のように振る舞う現象。建物が地面に沈下・傾斜・崩壊することがある。 of saturated soils, and permanent 地表地震断層（地表断層）地震時に断層に沿って地表面に生じる目に見える変位。地表断層の破壊帯を横切って建設された構造物は、構造強度にかかわらず引き裂かれることがある。 visible at the surface. Very deep earthquakes rarely generate tsunamis because the seafloor deformation, transmitted through hundreds of kilometres of rock, is diffuse rather than concentrated. The geographic footprint of a deep earthquake is so large that even with a high マグニチュード地震が放出した総エネルギー量を表す単一の数値。整数値が1増えるごとに、放出エネルギーはおよそ31.6倍になる。, the local 震度人・構造物・自然環境への観測された影響から判定される、特定地点における揺れの強さの尺度。震央からの距離が離れるほど小さくなる。 at the 震央地下で地震が発生した震源の真上にあたる地表の地点。ニュース報道では地震の発生場所としてよく報じられる。 may be surprisingly modest.

The Role of Seismic Networks in Location Accuracy

The accuracy of hypocenter and epicenter determinations has improved dramatically over the past century, driven by the growth of 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。s from a handful of stations to global networks of thousands. 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。, operated by 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 partner institutions, provides high-quality broadband data from more than 150 stations worldwide, enabling reliable location of earthquakes above about magnitude 4.0 anywhere on Earth.

Regional dense networks — like those operated by CalTech in Southern California, the Japan Meteorological Agency, and seismological institutes in New Zealand and Switzerland — provide vastly denser station coverage within their regions, enabling location accuracies of a kilometre or less for local events. This precision is essential for aftershock studies, fault mapping, and verifying compliance with nuclear test ban treaties. Use the Distance from Epicenter tool to estimate how far you are from an earthquake's epicentre and how that distance affects the shaking you might experience.