地震群:小地震不停止时

What Defines an Earthquake Swarm?

An 地震群在数天至数月内发生于局部区域、且无明显主导主震的一系列地震，常与火山活动或流体注入相关。 is a sequence of earthquakes occurring in a limited geographic area over a period of days to months, with no single dominant event that clearly qualifies as a 主震一次地震序列中震级最大的地震，决定了该事件的整体震级。此前有时会出现前震，之后必然伴随余震。. In a typical mainshock-aftershock sequence, one event is dramatically larger than all the others. In a swarm, the largest events are close in magnitude to each other, and the sequence may produce hundreds or thousands of small earthquakes that wax and wane over weeks.

Swarms differ from regular aftershock sequences not only statistically but often mechanically. While aftershocks are driven by 库仑应力传递地震改变邻近断层应力状态、从而可能触发或延迟未来地震的过程，用于预测哪些断层正被推向更接近破裂的状态。 redistribution from a large rupture, swarms are more commonly associated with fluid migration (water or magma moving through the crust), slow fault creep, or volcanic activity. The diffuse, distributed nature of swarm seismicity reflects the fact that no single large fault segment is failing all at once; instead, many small patches are failing in sequence, driven by a common underlying process.

Volcanic Swarms: Magma on the Move

The most dramatic and well-studied swarms accompany volcanic activity. When magma forces its way upward through the crust, it creates new fractures and exploits existing ones, producing swarms of 火山地震与火山活动相关、由岩浆运动、气体压力或火山附近岩石破裂引起的地震。常成群发生，有时预示即将喷发。s as it goes. These swarms characteristically migrate upward over time — the hypocentres of the earthquakes track the advancing front of magma intrusion.

Volcanic swarms are intensely monitored because they are among the most reliable precursors to volcanic eruptions. Before the 2018 Kīlauea eruption in Hawai'i, thousands of small earthquakes occurred beneath the summit area, indicating magma movement within the volcanic plumbing system. Similar swarms preceded the 2010 Eyjafjallajökull eruption in Iceland and the 2021 Cumbre Vieja eruption on La Palma. Volcanologists use 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。 data to track swarm migration in real time, helping forecast eruption location and timing.

Induced Seismicity: Human-Caused Swarms

诱发地震活动由水力压裂、废水回注、采矿或水库蓄水等人类活动引发的地震。大多数震级较小（低于4级），但部分曾超过5.5级。 — earthquakes caused by human activities — overwhelmingly manifests as swarms. The injection of fluids deep into the crust (whether wastewater from oil and gas operations, carbon dioxide for sequestration, or geothermal brine) increases pore fluid pressure along pre-existing faults, reducing the effective normal stress and enabling them to slip at stress levels below their natural threshold. This produces clusters of earthquakes that track the progression of the pressure front through the subsurface.

Oklahoma experienced a dramatic example of induced seismicity between 2009 and 2015, when wastewater injection rates increased enormously following the shale oil boom. Annual earthquake rates (for events above magnitude 3.0) increased from fewer than 50 to more than 900, with several events exceeding magnitude 5.0. Once injection operations were curtailed, seismicity rates began to decline, confirming the causal connection. Similar induced swarms have been documented in Kansas, Texas, Colorado, Ohio, and internationally at enhanced geothermal sites and hydraulic fracturing operations.

Tectonic Swarms: Fluid Migration Along Faults

Not all swarms are volcanic or induced. Purely tectonic swarms occur when natural fluid migration through the crust — from high-pressure aquifers, mineral dehydration reactions in metamorphic rocks, or tectonic pumping — raises pore pressure along active fault zones. These swarms are common in geothermal areas, in zones of active mountain building, and along fault systems with high permeability.

The Salton Sea region in Southern California experiences frequent swarms related to the geothermal system and complex fault interactions at the southern end of the San Andreas system. Central Italy has a long history of swarms in the Apennine mountain range, driven by complex interactions between active normal faults and fluid-rich limestone aquifers. These tectonic swarms can occasionally culminate in a significant earthquake — the 2016 Amatrice earthquake in Italy was preceded by weeks of elevated seismicity — but most dissipate without a major event.

Should You Worry? Risk Assessment During Swarms

Earthquake swarms understandably alarm residents of affected areas, especially when they persist for days or weeks. Use the Seismic Risk Checker to evaluate your location's baseline seismic risk, which provides context for interpreting swarm activity. The 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。 agencies responsible for monitoring your region will issue statements assessing whether the swarm represents elevated hazard.

For most tectonic swarms in areas without volcanoes or recent large earthquakes, the probability of a damaging 主震一次地震序列中震级最大的地震，决定了该事件的整体震级。此前有时会出现前震，之后必然伴随余震。 following a swarm is low but not zero. Probabilistic models based on the swarm's magnitude distribution, spatial pattern, and temporal evolution can estimate this probability. The honest answer for residents is: continue normal precautions, ensure your 地震应急包为应对地震后生存需要而预先准备的物资集合，通常包括水（每人每天1加仑，供3天使用）、食物、急救用品、手电筒和收音机。 is ready, and stay informed through official channels. Swarms that show signs of acceleration, migration toward populated areas, or association with volcanic unrest deserve closer attention and may prompt precautionary measures.