P波和S波:地震波传播方式

P-Waves: The First Arrivals

When a fault ruptures, the first seismic energy to leave the 震源地震破裂在地球内部实际发生的位置，也称焦点。震源深度对地表感受到的震动方式有重大影响。 travels as P波（纵波）速度最快的地震波，能以每秒5至8公里的速度穿过固体岩石和液体。P波沿传播方向压缩和拉伸介质，形似弹簧振动，是最先到达地震观测站的波。s — primary waves, so named because they arrive first at distant 地震仪用于探测并记录地震波引起的地面运动的仪器。现代数字地震仪可探测到小于一纳米的位移。 stations. P-waves are compressional waves: the rock alternately compresses and expands in the same direction as the wave is travelling, like sound waves in air. Because they involve compression and expansion of the rock rather than shearing, P-waves can travel through solids, liquids, and gases alike.

This ability to travel through all materials gives P-waves a crucial role in understanding Earth's interior. When P-waves pass through the liquid outer core, they slow dramatically and are refracted; through the solid inner core, they speed up again. By precisely mapping how P-wave travel times deviate from predictions, seismologists have mapped the detailed structure of Earth's layers, a technique called 地震层析成像利用地震波走时构建地球内部三维结构图像的技术，类似于医学CT扫描，可揭示地幔柱、俯冲板片等深部结构。.

P-wave speeds in crustal rocks typically range from 5 to 8 kilometres per second, varying with rock type, pressure, and temperature. In the mantle, they reach 8–13 km/s. This speed makes P-waves the first warning of an impending earthquake to arrive at distant monitoring stations — and the foundation of 地震预警（EEW）一种在强震到达前探测地震并向人员和系统发送警报的系统，可提供数秒至数十秒的预警时间，足以采取自我保护行动。 systems.

S-Waves: The Shaking Waves

The second arrival is the S波（横波）使岩石垂直于传播方向运动的地震波，晚于P波到达。S波无法穿过液体，这一特性证明了地球外核为液态。 — shear wave or secondary wave. S-waves are transverse waves: the rock moves perpendicular to the direction the wave is travelling, like the motion along a shaken rope. This shearing motion is what causes most of the ground shaking people feel during an earthquake, because the back-and-forth or up-and-down movement S-waves produce is far more disorienting and structurally damaging than the push-pull of P-waves.

S-waves travel at roughly 60 percent of P-wave speed — typically 3–5 km/s in the crust. A critically important property: S-waves cannot propagate through liquids, because liquids cannot sustain shear stress. This S-wave shadow zone beyond about 103–143 degrees from an earthquake was the original evidence that the Earth has a liquid outer core. The distinction is still exploited operationally: the absence of S-waves at distant stations is diagnostic of a liquid layer between source and receiver.

Why P-Waves Travel Faster Than S-Waves

The speed difference between P波（纵波）速度最快的地震波，能以每秒5至8公里的速度穿过固体岩石和液体。P波沿传播方向压缩和拉伸介质，形似弹簧振动，是最先到达地震观测站的波。s and S波（横波）使岩石垂直于传播方向运动的地震波，晚于P波到达。S波无法穿过液体，这一特性证明了地球外核为液态。s arises from the physics of wave propagation in elastic media. P-wave velocity depends on both the bulk modulus (resistance to compression) and the shear modulus (resistance to shearing), while S-wave velocity depends only on the shear modulus. Because the bulk modulus contribution is always positive, P-waves are always faster than S-waves in the same material.

This speed difference has a tremendously practical consequence. After an earthquake, the P-wave arrives at a monitoring station first, followed some seconds later by the S-wave. The time gap between the two arrivals — the "S minus P" time, or S-P interval — is directly proportional to the distance from the station to the 震中地震发生在地下的震源正上方对应的地表位置，新闻报道中通常将其作为地震发生的位置。. Measure the S-P interval in seconds, multiply by approximately 8 kilometres, and you have a rough distance estimate. With three or more stations, triangulation precisely locates the earthquake.

How the P-S Time Gap Locates Earthquakes

The elegant method of locating earthquakes using the P波（纵波）速度最快的地震波，能以每秒5至8公里的速度穿过固体岩石和液体。P波沿传播方向压缩和拉伸介质，形似弹簧振动，是最先到达地震观测站的波。–S波（横波）使岩石垂直于传播方向运动的地震波，晚于P波到达。S波无法穿过液体，这一特性证明了地球外核为液态。 time difference is one of the oldest tools in observational seismology. An S-P interval of 10 seconds suggests the 震源地震破裂在地球内部实际发生的位置，也称焦点。震源深度对地表感受到的震动方式有重大影响。 is roughly 80 kilometres away. An interval of 30 seconds suggests about 240 kilometres. By plotting a circle of appropriate radius around each station and finding where three or more circles intersect, the earthquake location — and with additional information, its depth — can be pinpointed.

Modern earthquake location methods use exactly this principle, but with data from hundreds of stations processed simultaneously by computer algorithms that minimise the mismatch between observed and predicted arrival times. The 全球地震台网（GSN）由150多个宽频带地震台站组成的全球网络，提供对全球地震活动的全面监测，由美国地质调查局、美国国家科学基金会及IRIS联合运营。 and regional 地震观测网由若干地震台站协同组成、持续监测地震活动的系统。全球地震台网（GSN）拥有150多个台站，提供全球范围的观测覆盖。s provide continuous streams of 地震记录图地震仪记录的输出结果，以时间为函数显示地面运动情况。地震学家通过分析地震记录图来确定地震的震级、深度和位置。 data that feed into automated detection and location systems capable of publishing earthquake locations within minutes of the event. The 地震警报系统墨西哥的SASMEX系统，是世界上最早投入使用的公共地震预警系统之一，自1991年起运行，可为墨西哥城提供长达60秒的沿海地震预警时间。 in Japan uses the P-wave arrival at the closest stations to predict shaking intensity at distant locations before the slower, more damaging S-waves arrive.

What S-Waves Tell Us About Earth's Interior

Beyond locating earthquakes, the behaviour of S波（横波）使岩石垂直于传播方向运动的地震波，晚于P波到达。S波无法穿过液体，这一特性证明了地球外核为液态。s has been indispensable for mapping Earth's internal structure. The discovery that S-waves cannot pass through a region corresponding to depths of roughly 2,900 to 5,100 kilometres demonstrated the existence of the liquid outer core. The 体波穿过地球内部传播的地震波，包括P波和S波。传播速度快于面波，是最先到达远处地震观测站的信号。 velocity profile with depth — how fast P and S waves travel at different depths — traces the density and elastic properties of Earth's layers and can be inverted to produce three-dimensional models of mantle structure.

地震层析成像利用地震波走时构建地球内部三维结构图像的技术，类似于医学CT扫描，可揭示地幔柱、俯冲板片等深部结构。 uses small variations in P and S travel times from thousands of earthquakes recorded at thousands of stations to build three-dimensional images of the Earth's interior. Slow-velocity zones at depth often indicate hotter, less rigid material — potentially rising mantle plumes or 热点（地质学）地幔中热岩石以地幔柱形式上涌、引发与板块边界无关的火山活动的位置。夏威夷和黄石公园是典型的例子。 tracks. Fast-velocity zones often correspond to subducted oceanic slabs sinking into the mantle. This imaging has revealed that the mantle is far more heterogeneous than the simple layered models of earlier decades.

Feeling the Waves: What You Experience During a Quake

If you are close to a moderate or large earthquake, the P-wave often arrives as a sudden sharp jolt — a brief bang or bump that makes people think something has struck the building. It may be mistaken for a sonic boom or a truck collision. A second or two later, the S波（横波）使岩石垂直于传播方向运动的地震波，晚于P波到达。S波无法穿过液体，这一特性证明了地球外核为液态。 arrives with the characteristic rolling, swaying shaking that everyone recognises as an earthquake. This is the motion that can throw people off their feet, topple furniture, and damage structures.

Following the S-wave, 面波沿地球表面而非内部传播的地震波。速度慢于体波，但因振幅更大、持续时间更长，通常造成更大破坏。s — 勒夫波一种引起地面水平剪切运动的面波，以数学家A.E.H.勒夫命名，对建筑物地基破坏性尤为严重。s and 瑞利波一种使地面以类似海浪的椭圆运动方式运动的面波，以瑞利勋爵命名，通常是地震时感受到的滚动感的成因。s that travel along the Earth's surface rather than through its interior — arrive with a slower, more sustained rolling motion. For large distant earthquakes, the surface waves may arrive minutes after the body waves and can produce long-duration, low-frequency shaking that is particularly damaging to tall buildings susceptible to 结构共振当地震波频率与建筑物固有频率相匹配时发生的建筑物运动放大现象。低层建筑易与高频波产生共振，高层建筑则易与低频波产生共振。. Recognising these distinct wave types helps explain why earthquake shaking often feels like a quick jolt followed by stronger swaying — you are experiencing first the 体波穿过地球内部传播的地震波，包括P波和S波。传播速度快于面波，是最先到达远处地震观测站的信号。s, then the surface waves, in sequence.