Focus; hypocenter

Japanese: 震源 - しんげん（英語表記）focus; hypocenter

The location where an earthquake occurs. In other words, it is the source of seismic waves, and refers to the point where the fault first begins to rupture (→ slippage). The source fault has a spatial extent, and the area corresponding to its size is called the seismic region. Since the rupture of a fault often propagates in one direction from the rupture starting point, the epicenter is often not in the center of the source region, but on the periphery. The point on the ground surface directly above the source is called the epicenter, and the distance from the source to the earthquake observation point is called the epicenter distance. The epicenter distance can be calculated using the Omori formula proposed by seismologist Fusakichi Omori, where d is the epicenter distance (km), t is the duration of the initial tremor (seconds), which is the time difference between the S wave that arrives later and the P wave that arrives earlier at the observation point, Vp _km is the speed of the P wave (per second), and Vs _km is the speed of the S wave (per second). It is also possible to replace the above with k and make d = kt . In 1918, Omori calculated the value of k to be 7.42, and d = 7.42 t . If the duration of the initial tremor (PS time) from three or more observation points is known, the location (depth) of the epicenter and the location of the epicenter can be determined. The larger the earthquake, the larger the seismic source area and the larger the wavelength of the seismic waves emitted. In particular, plate boundary earthquakes on the ocean floor have a high possibility of developing into huge earthquakes, and as the seismic source area expands, the aftershock area and tsunami source area also become larger. In addition to the destruction of faults, seismic waves are also emitted from volcanic eruptions, underground magma movement, rock collapses, nuclear tests, etc., and the source of such seismic waves is also called the seismic source. In addition, when seismic waves are generated artificially using explosives, they are sometimes called artificial seismic sources to distinguish them from natural phenomena.

Source: Encyclopaedia Britannica Concise Encyclopedia About Encyclopaedia Britannica Concise Encyclopedia Information

Japanese:

地震の発生した場所。すなわち，地震波の放射源であり，最初に断層の破壊（→ずれ）が始まった点をさす。震源断層には空間的な広がりがあり，その大きさに対応する領域を震源域という。断層の破壊は破壊開始点から一方向に伝わることが多いため，震源は震源域の中央ではなく，周縁にある場合も多い。震源の真上にある地表の点を震央といい，震源から地震の観測点までの距離を震源距離という。震源距離は，地震学者の大森房吉が提唱した，震源距離（km）を d ，観測点に遅く到着する S波と早く到着する P波の時間差である初期微動継続時間（秒）を t ，P波の速さ（秒速）を V_pkm，S波の速さ（秒速）を V_skm とした場合の大森公式で求められる。また，上記のを k に置き換えて d ＝k t とすることもできる。1918年，大森は k の値 7.42を算出し，d ＝7.42t とした。なお，三つ以上の観測点の初期微動継続時間（PS時間）がわかれば，震源の位置（深さ）や震央の位置がわかる。震源域は大きな地震ほど大きくなり，放射される地震波の波長も大きくなる。特に海底でのプレート境界地震は巨大地震に発達する可能性が大きく，その震源域の広がりに伴って余震域や津波の波源域も大きくなる。断層の破壊以外，たとえば火山噴火や地下のマグマの移動，岩盤の崩落，核実験などによっても地震波は放射され，そのような地震波の発生源も震源という。また，爆薬等を人為的に用いて地震波を発生させる場合は自然現象と区別して人工震源ということがある。

出典　ブリタニカ国際大百科事典小項目事典ブリタニカ国際大百科事典小項目事典について　情報

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