地震波形(seismogram)の読み方

What Is a Seismogram?

A 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。 is the continuous written or digital record produced by a 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。 — the device that measures ground motion at a fixed location. Reading a seismogram is fundamental to seismology because all earthquake science ultimately rests on the interpretation of these waveform records. The pattern of wiggles on a seismogram encodes the earthquake's location, depth, magnitude, fault geometry, and the properties of the Earth's interior through which the waves traveled.

The Three Components of Motion

Modern seismographs record ground motion in three orthogonal directions: vertical (Z), north-south (N), and east-west (E). The vertical component is most sensitive to 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。 arrivals and is often used for initial magnitude estimates. The horizontal components are essential for detecting 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。 arrivals and for determining the direction to the source. Analyzing all three components together allows seismologists to measure the full three-dimensional particle motion of the ground.

Understanding the Time Axis

The horizontal axis of a seismogram represents time, typically annotated with UTC timestamps. Modern digital seismograms sample ground motion at 100 or 200 samples per second, providing precise timing. The precision of arrival time measurements is critical — a timing error of even 0.1 seconds can shift hypocenter location estimates by several kilometers. GPS synchronization ensures that all stations in a 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。 share a common time reference, making cross-station arrival time comparisons valid.

Identifying the P-Wave Arrival

The 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。 arrival is the first signal to appear on the seismogram, emerging abruptly from the background noise level (called the noise floor). P-waves are compressional waves that push and pull the ground in the direction of wave propagation, creating a sharp onset that is relatively easy to identify on the vertical component. The amplitude of the P-wave phase is typically smaller than the subsequent S-wave arrival for earthquakes at teleseismic distances, making it recognizable as the first but smaller deflection on the record.

Identifying the S-Wave Arrival

The 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。 arrival appears after the P-wave, with the time gap between them — known as the S-P time — providing a direct measure of the distance to the earthquake. S-waves are shear waves that move the ground perpendicular to the direction of propagation. On the seismogram, the S-wave arrival is typically much larger in amplitude than the P-wave, producing a clearly visible increase in the 波の振幅地震波が静止位置から動く最大変位量。振幅は波が運ぶエネルギーと直接関係し、マグニチュードの算出に用いられる。 of the record. The S-wave arrival is most prominent on horizontal components.

Using the S-P Time to Estimate Distance

The relationship between S-P time and distance is well established. For earthquakes within the crust, each second of S-P time corresponds to approximately 8 kilometers of distance from the station to 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。. A 10-second S-P time indicates approximately 80 km distance. With S-P readings from three or more stations, triangulation yields the epicenter location. This technique, used routinely by seismologists, can be performed manually on printed seismograms or calculated automatically by the Earthquake Energy Calculator tool.

Surface Waves on the Seismogram

After the P- and S-wave arrivals, longer-period oscillations appear on the seismogram: the 表面波地球内部ではなく地表に沿って伝わる地震波。実体波より速度は遅いが、振幅が大きく継続時間が長いため、被害が大きくなる傾向がある。 phases. Love waves arrive first and appear prominently on the horizontal components, moving the ground sideways in a horizontal shearing motion. Rayleigh waves海の波に似た楕円運動で地面を動かす表面波。レイリー卿にちなんで名付けられ、地震時に感じる「ローリング」のような揺れの原因となることが多い。 arrive slightly later and produce an elliptical retrograde particle motion visible on both vertical and horizontal components. Surface waves have the largest 波の振幅地震波が静止位置から動く最大変位量。振幅は波が運ぶエネルギーと直接関係し、マグニチュードの算出に用いられる。 on the seismogram for shallow earthquakes and are responsible for most of the felt shaking and structural damage in populated areas.

The Coda: After the Main Phases

Following the principal wave arrivals, the seismogram shows a gradually decaying sequence of scattered energy called the コーダ波地震波の主要な到達後に続く地震記象の尾部で、不均質な地殻内での波の散乱によって生じる。コーダの継続時間は地震の規模と相関する。. Coda waves are S-wave energy scattered from heterogeneities in the crust and pile up in time as multiply scattered paths arrive at the station. The duration of the coda can be used to estimate magnitude — longer, more energetic codas indicate larger earthquakes. Coda duration magnitude was historically used in regional networks before waveform moment tensor analysis became routine.

Distinguishing Earthquakes from Noise

Reading seismograms requires distinguishing genuine seismic signals from cultural noise (traffic, machinery, ocean waves) and instrumental artifacts. Traffic noise appears as nearly continuous low-frequency tremor with regular daily patterns — peaking during commute hours and quiet at night. Ocean microseisms create persistent energy in the 0.1–0.2 Hz band. Instrumental glitches produce sudden spikes or offsets with unrealistic physical characteristics. Genuine earthquake signals have characteristic phase sequences (P then S then surface waves) and coherent appearance across multiple stations in the network.

Digital Seismogram Analysis Tools

Software packages like ObsPy (Python), SAC (Seismic Analysis Code), and SeisComp enable interactive analysis of digital seismograms. These tools support filtering, phase picking, spectrogram computation, and magnitude estimation from waveforms. The IRIS (EarthScope) DMC provides free access to waveform data for any catalogued earthquake through its web services, making professional-grade seismogram analysis accessible to researchers and advanced students. The Earthquake Energy Calculator tool provides simplified magnitude calculations that help contextualize what you observe in waveform data.

Reading Seismograms at Teleseismic Distances

For earthquakes recorded at distances greater than 1,000 km (teleseismic range), the seismogram appearance changes significantly. P-waves have passed through the deep mantle, where higher velocities produce earlier arrivals. Multiple P-wave phases appear, including PP (reflected off the surface once), PPP (reflected twice), and PcP (reflected off the core-mantle boundary). The time separations between these phases encode information about Earth's deep interior structure, and their analysis formed the basis for our understanding of mantle and core composition.

Summary

A 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。 is a window into both the earthquake source and Earth's interior. By recognizing the characteristic arrival patterns of P-waves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-waves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。, and surface waves地球内部ではなく地表に沿って伝わる地震波。実体波より速度は遅いが、振幅が大きく継続時間が長いため、被害が大きくなる傾向がある。, understanding the significance of 波の振幅地震波が静止位置から動く最大変位量。振幅は波が運ぶエネルギーと直接関係し、マグニチュードの算出に用いられる。 variations, and knowing how to use S-P times to estimate distance, you gain the ability to extract meaningful seismological information from what might otherwise appear to be a confusing sequence of wiggles. Combined with tools like the Earthquake Energy Calculator, seismogram reading transforms from a specialist skill into an accessible analytical capability.