What Is an Earthquake Swarm? Understanding Clustered Seismic Events

Imagine waking up to feel the ground shake—a magnitude 3.5 earthquake, strong enough to rattle dishes and wake you from sleep. Fifteen minutes later, another one hits. Then another. And another. Over the next few days, you feel dozens of earthquakes. Your phone buzzes constantly with earthquake alerts. But here's the strange part: none of these earthquakes is clearly larger than the others. There's no obvious "main" earthquake followed by diminishing aftershocks. Instead, you're experiencing an earthquake swarm—one of the most mysterious and unsettling patterns of seismic activity.

Earthquake swarms differ fundamentally from the mainshock-aftershock sequences most people associate with earthquakes. They can continue for days, weeks, or even months. They often occur in unexpected locations far from known fault lines. And most unnerving of all, they sometimes—though rarely—serve as warnings that something much larger is building beneath the surface.

This comprehensive guide explores earthquake swarms: what defines them, what causes them, how they differ from normal earthquake sequences, where they occur, and most importantly, whether they signal impending danger or represent a relatively harmless release of tectonic stress.

Swarms vs. Mainshock-Aftershock Sequences

To understand earthquake swarms, we must first understand how they differ from typical earthquake behavior.

Typical Earthquake Sequence (Mainshock-Aftershock)

Most earthquake sequences follow a predictable pattern described by well-established empirical laws:

The sequence:

1. Mainshock: One large earthquake releases most of the energy
2. Immediate aftershocks: Hundreds to thousands of smaller earthquakes immediately after
3. Decaying rate: Aftershock rate decreases predictably over time (Omori's Law)
4. Magnitude difference: Largest aftershock typically 1-1.2 magnitudes smaller than mainshock (Båth's Law)
5. Duration: Aftershocks continue for months to years but at ever-decreasing rates

Example: 1989 Loma Prieta Earthquake

• M6.9 mainshock on October 17, 1989
• Largest aftershock: M5.2 (following Båth's Law)
• 2,000+ aftershocks in first week
• Rate decreased predictably according to Omori's Law
• Aftershocks detectable for years afterward

Earthquake Swarm Sequence

Swarms violate these patterns in fundamental ways:

The sequence:

1. Gradual onset: Activity begins with small earthquakes
2. No dominant event: Multiple earthquakes of similar magnitude
3. Sustained rate: Earthquake frequency doesn't decay—may even increase
4. Variable magnitudes: Largest events can occur anywhere in sequence, not just at beginning
5. Unpredictable duration: May stop abruptly after days or continue for months

Example: 2020 Westmorland, California Swarm

• Began March 2020 near Salton Sea
• Over 1,000 earthquakes in first few days
• Largest events: Several M4.0-4.9 earthquakes scattered throughout sequence
• No clear mainshock; multiple similar-magnitude events
• Activity sustained for weeks without clear decay pattern
• Raised concerns about triggering larger earthquake on San Andreas Fault

Key Differences Summary

What Causes Earthquake Swarms?

Unlike mainshock-aftershock sequences, which result from sudden stress redistribution after a large rupture, swarms typically have different underlying causes. Understanding these mechanisms explains both where swarms occur and what they might signify.

1. Fluid Movement and Pore Pressure

The most common cause of earthquake swarms is the movement of fluids (water, magma, CO₂) through the Earth's crust. This is the leading explanation for most documented swarms.

The mechanism:

• Pore pressure increases: Fluids fill pore spaces in rock, increasing pressure
• Effective stress decreases: Pore pressure counteracts confining pressure, weakening rocks
• Fault strength reduced: Faults that were stable become unstable
• Sequential failure: Small earthquakes trigger adjacent small earthquakes
• Sustained activity: As fluid moves through rock, new areas become unstable

Why this produces swarms rather than mainshocks:

• Fluid pressure affects many small patches simultaneously
• No single large stressed area waiting to rupture
• Distributed weakening rather than concentrated stress
• As fluids migrate, new areas activate while old areas quiet

Example: Long Valley Caldera, California

• Volcanic caldera with magma at depth
• Experiences frequent earthquake swarms
• Swarms correlate with CO₂ and other volcanic gas release
• 1997 swarm: 2,500+ earthquakes as fluids moved through crust
• Ground deformation accompanies many swarms

2. Magmatic Intrusions

When magma pushes into the crust without erupting, it can trigger intense earthquake swarms.

The process:

• Dike intrusion: Magma forces apart rock, creating vertical sheets
• Rock fracturing: Brittle rock breaks as magma intrudes
• Continuous earthquakes: Each increment of intrusion triggers earthquakes
• Migrating pattern: Earthquakes track magma movement

Characteristics of magmatic swarms:

• Often very intense—hundreds of earthquakes per hour at peak
• Clear spatial migration pattern as magma moves
• Associated with ground deformation (inflation)
• May precede volcanic eruption—but usually don't
• Accompanied by harmonic tremor (continuous seismic signal from fluid movement)

Example: 2014-2015 Bárðarbunga, Iceland

• One of the most intense earthquake swarms ever recorded
• Over 1,600 earthquakes M4.0+ in six months
• Largest: M5.9
• Magma dike intruded 48 km laterally through crust
• Earthquakes migrated as magma advanced
• Eventually produced effusive eruption at Holuhraun

3. Aseismic Slip and Slow Earthquakes

Sometimes faults slip without producing regular earthquakes—a process called aseismic slip or creep. This slow motion can trigger swarms of small earthquakes around the creeping patch.

The mechanism:

• Slow slip event: Fault slips over days to weeks without sudden rupture
• Stress transfer: Slow slip loads adjacent locked patches
• Small ruptures: Locked patches fail in small earthquakes
• Distributed failure: Many small patches fail rather than one large rupture

Where this occurs:

• Transition zones between locked and creeping sections of faults
• Subduction zone plate interfaces at certain depths
• Transform faults with mixed stick-slip and creeping behavior

Example: Cascadia Subduction Zone Episodic Tremor and Slip (ETS)

• Every 12-15 months, portions of subduction interface slip slowly
• Accompanied by thousands of tiny earthquakes ("tremor")
• Slow slip releases as much energy as M6.5-7.0 earthquake but over weeks
• Earthquakes cluster in tremor swarms during slip events
• No surface damage despite significant energy release

4. Tectonic Stress Redistribution

In some cases, swarms result from stress changes following distant large earthquakes or from ongoing tectonic loading reaching critical levels on complex fault networks.

Remote triggering:

• Large distant earthquakes can trigger swarms hundreds to thousands of kilometers away
• Seismic waves temporarily alter stress on faults
• If faults are near failure, this small stress change can trigger swarm
• Swarm occurs during or shortly after seismic wave passage

Example: Yellowstone Swarms

• Yellowstone experiences frequent earthquake swarms
• Located over hotspot with magma at depth
• Some swarms correlate with distant large earthquakes
• 2017 swarm: 2,400+ earthquakes over several months
• Largest: M4.4
• Combination of fluid movement, thermal effects, and stress changes

5. Induced Seismicity

Human activities can trigger earthquake swarms, particularly through fluid injection or extraction.

Wastewater injection:

• Disposal wells inject wastewater deep underground
• Increases pore pressure on pre-existing faults
• Can trigger intense earthquake swarms
• Oklahoma experienced dramatic increase in seismicity from injection

Geothermal operations:

• Injecting water into hot rock to extract heat
• Increases pore pressure and cools rock (thermal stress)
• Often produces swarms of small earthquakes
• Generally carefully monitored and controlled

Example: Oklahoma Induced Swarms

• Oklahoma went from ~2 earthquakes M3+ per year (pre-2009) to 900+ per year (2015)
• Caused by wastewater injection from oil/gas operations
• Characterized by swarm-like behavior
• Regulations introduced to reduce injection volumes
• Seismicity rate has since decreased

Famous Earthquake Swarms: Case Studies

Examining specific swarms reveals the diversity of this phenomenon and provides insight into their behavior and implications.

1. The 2000 Miyakejima Swarm, Japan

One of the most intense and scientifically significant earthquake swarms ever recorded occurred off Miyakejima Island in June-August 2000.

The sequence:

• Began June 26, 2000 with earthquakes near Miyakejima volcano
• Over 14,000 earthquakes in first week
• Earthquakes migrated westward at ~1-2 km per day
• Total migration distance: ~20 km
• Largest earthquake: M6.5 on July 1
• Total: Over 40,000 earthquakes detected

What caused it:

• Magma dike intruded laterally from beneath Miyakejima
• Earthquakes tracked magma movement precisely
• GPS showed dramatic ground deformation
• Dike reached seafloor but no submarine eruption occurred
• Miyakejima volcano later erupted (July-August 2000)

Significance:

• Demonstrated clear link between magmatic intrusion and earthquake swarms
• Allowed evacuation of Miyakejima island (3,600 residents) before eruption
• Provided unprecedented dataset on dike intrusion process
• Island remained evacuated for 4.5 years

2. The 2009 L'Aquila Swarm, Italy

This swarm gained notoriety not for its scientific characteristics but for its tragic aftermath and legal implications.

The sequence:

• Began October 2008 near L'Aquila in central Italy
• Hundreds of small earthquakes over several months
• Residents became alarmed—was a larger earthquake coming?
• Government convened expert commission (March 31, 2009)
• Commission concluded major earthquake was unlikely
• April 6, 2009: M6.3 earthquake struck, killing 309 people

The dilemma:

• Earthquake swarms sometimes precede larger earthquakes—but usually don't
• No scientific way to determine if this swarm was precursory
• Scientists faced impossible choice: warn (causing panic/disruption) or reassure (leaving people vulnerable)
• In retrospect, the M6.3 earthquake was not technically part of the swarm but a separate tectonic event

Legal aftermath:

• Seven scientists and officials prosecuted for manslaughter
• Convicted in 2012 (later overturned for most defendants on appeal)
• International scientific community outraged
• Case highlighted impossibility of earthquake prediction from swarms
• Led to new protocols for communicating earthquake risk during swarms

3. The 2012-2013 Salton Sea Swarm, California

This swarm raised significant concerns because of its proximity to the southern San Andreas Fault.

The sequence:

• Began August 26, 2012 near Brawley, California
• Over 200 earthquakes M3.0+ in first 24 hours
• Largest: M5.5 on August 26
• Activity continued for months with thousands of smaller events
• Located in Brawley Seismic Zone, ~15 km from San Andreas Fault

Why it mattered:

• Southern San Andreas hasn't ruptured since ~1690 (over 320 years)
• This section is considered ready for M7.8+ earthquake
• Concern that swarm could trigger "The Big One"
• USGS calculated probability of triggering San Andreas rupture: ~1-2% during swarm
• Swarm eventually subsided without triggering larger event

Scientific insights:

• Probably caused by combination of tectonic stress and geothermal fluids
• Swarm earthquakes occurred on small, complex fault network
• Demonstrated that swarms can occur very close to major faults without triggering them
• Similar swarms occurred in same area in 1975 and 2005

4. The 2018 Kilauea Eruption Swarm, Hawaii

The 2018 Kilauea eruption was preceded and accompanied by one of the most intense earthquake swarms in U.S. history.

The sequence:

• Began late April 2018 with increased seismicity at Kilauea summit
• May 3: Earthquakes migrated to lower East Rift Zone
• May 4: First fissure eruption in Leilani Estates subdivision
• May 4: M6.9 earthquake—largest in Hawaii since 1975
• Thousands of earthquakes over subsequent months
• Eruption continued for three months

What happened:

• Magma drained from summit, intruded into East Rift Zone
• Earthquakes marked magma movement underground
• 24 fissures opened in residential areas
• Lava destroyed 700+ homes
• Summit collapsed in dramatic fashion, triggering more earthquakes

Lessons:

• Earthquake swarms provided clear warning of impending eruption
• Allowed evacuation of Leilani Estates before main eruption
• Demonstrated value of continuous monitoring
• Showed that volcanic swarms can include very large earthquakes (M6.9)

5. The Ongoing Yellowstone Swarms

Yellowstone National Park experiences frequent earthquake swarms, generating periodic public concern about volcanic eruption.

Historical context:

• Yellowstone caldera formed in catastrophic eruption 640,000 years ago
• Sits atop hotspot with magma reservoir beneath surface
• Experiences 1,000-3,000 earthquakes per year
• About 50% of earthquakes occur in swarms

Notable recent swarms:

• 1985: 3,000+ earthquakes over three months, largest M4.9
• 2010: 1,300+ earthquakes over several weeks
• 2017: 2,400+ earthquakes over several months, largest M4.4
• 2020: Multiple swarms totaling 3,200+ earthquakes

What causes them:

• Combination of magmatic fluids, hydrothermal activity, and tectonic stress
• Most swarms occur in specific areas: Hebgen Lake, Norris Geyser Basin, Yellowstone Lake
• Ground deformation sometimes accompanies swarms
• No evidence swarms indicate imminent eruption

Eruption risk:

• Yellowstone has 5-15% probability of hydrothermal explosion in any given year
• Probability of lava eruption in next few thousand years: maybe 1-2%
• Probability of supereruption in next few thousand years: ~0.001%
• Swarms are normal for Yellowstone and don't change these probabilities significantly

Geographic Distribution: Where Do Swarms Occur?

Earthquake swarms are not randomly distributed—they cluster in specific tectonic and geological settings.

Common Swarm Locations

1. Volcanic regions:

• Hawaii (Kilauea, Mauna Loa)
• Iceland (numerous volcanoes)
• Cascades (Mt. St. Helens, Mt. Rainier, Mt. Hood)
• Long Valley Caldera, California
• Yellowstone Caldera, Wyoming
• Campi Flegrei, Italy

2. Geothermal areas:

• Salton Sea region, California
• The Geysers, California
• Coso Geothermal Field, California
• Iceland geothermal fields
• New Zealand geothermal areas

3. Extensional tectonic settings:

• Basin and Range Province (Nevada, Utah)
• East African Rift
• Mid-ocean ridges
• Back-arc spreading centers

4. Transform fault environments:

• San Andreas Fault system
• North Anatolian Fault, Turkey
• Dead Sea Transform

5. Subduction zones:

• Cascadia (episodic tremor and slip)
• Japan (deep slow slip events)
• New Zealand
• Mexico

Why Geography Matters

The tectonic setting influences swarm characteristics and implications:

• Volcanic swarms: May precede eruptions; require close monitoring
• Geothermal swarms: Usually harmless; related to ongoing hydrothermal activity
• Extensional swarms: Reflect ongoing rifting; rarely produce large earthquakes
• Transform swarms: Potential concern for triggering larger tectonic earthquakes
• Subduction swarms: Often related to slow slip; generally not dangerous

Do Earthquake Swarms Predict Larger Earthquakes?

This is the question that concerns most people experiencing a swarm: is a bigger earthquake coming?

The Statistical Reality

Most swarms do NOT precede larger earthquakes:

• Studies show ~90-95% of earthquake swarms end without producing larger event
• They represent normal stress release process
• Energy dissipates through many small earthquakes rather than one large one

Occasionally swarms ARE precursory:

• ~5-10% of swarms are followed by larger earthquake
• But it's impossible to predict which swarms will produce larger events
• No reliable distinguishing characteristics

Examples of Precursory Swarms

1975 Haicheng, China (M7.0):

• Preceded by swarm of foreshocks
• Authorities ordered evacuation (though most people evacuated due to foreshocks, not official warning)
• Often cited as successful prediction
• Reality: Large foreshocks made this easier case; not reproducible for most earthquakes

2009 L'Aquila, Italy (M6.3):

• Six months of swarm activity
• M6.3 earthquake killed 309
• But retrospective analysis suggests M6.3 was not part of swarm but separate tectonic event
• Timing may have been coincidental

2016 Kumamoto, Japan (M7.0):

• Preceded by M6.2 foreshock two days earlier
• After M6.2, authorities expected normal aftershock sequence
• Instead, M7.0 mainshock occurred, killing 41 additional people
• Showed difficulty of distinguishing foreshocks from mainshocks in real-time

Why Prediction Is Impossible

The fundamental problem:

• Foreshocks and swarms look identical in real-time
• Only after larger earthquake can we identify sequence as precursory
• No physical difference between "swarm earthquake" and "foreshock"
• Both result from same physics—fault failure under stress

Statistical approaches fail:

• Researchers have tried to identify statistical patterns distinguishing precursory from non-precursory swarms
• No reliable indicators found
• Even sophisticated machine learning cannot predict which swarms will produce larger earthquakes

What Scientists Can Do

Probability calculations:

• During swarm, can calculate elevated probability of larger earthquake
• Example: "Probability of M6.0+ earthquake in next week increases from 0.001% to 5%"
• Still usually means larger earthquake won't happen, but risk is elevated
• Helps emergency managers make informed decisions

Operational Earthquake Forecasting:

• New Zealand and Italy operate systems that update forecasts during swarms
• Provide real-time probability assessments
• Communicate evolving risk rather than binary prediction
• Help public and officials make risk-informed decisions

Living Through an Earthquake Swarm

If you find yourself experiencing an earthquake swarm, here's what you need to know and do.

What to Expect

Physical experience:

• Frequent shaking—potentially dozens of felt earthquakes per day
• Difficult to sleep due to nighttime earthquakes
• Items falling off shelves repeatedly
• Car alarms, building alarms triggering frequently
• Cracks appearing in walls (especially drywall)
• Constant low rumbling sounds between larger events

Psychological impact:

• Anxiety and fear about larger earthquake coming
• Difficulty concentrating on work or normal activities
• Sleep disruption and fatigue
• Children may be particularly frightened
• Uncertainty about when swarm will end

Social effects:

• Constant discussion of earthquakes among neighbors
• Rumors and misinformation spreading
• Some people leaving area temporarily
• Business disruptions
• Media attention if swarm is significant

What You Should Do

Immediate actions:

• Secure your home: Remove heavy items from high shelves, secure furniture and appliances
• Emergency supplies: Prepare earthquake emergency kit (water, food, first aid, flashlight, radio)
• Family plan: Establish meeting places, communication plan
• Know safe spots: Identify safe locations in each room (under desks, doorways)
• Practice "Drop, Cover, Hold On": Review earthquake safety procedures

Stay informed:

• Monitor official sources: USGS, local emergency management, geological survey
• Ignore rumors and unofficial predictions
• Understand that scientists likely cannot predict if larger earthquake will occur
• Pay attention to official advisories or evacuations (rare but possible in volcanic swarms)

During each earthquake:

• Drop, Cover, Hold On—even for small earthquakes
• Stay inside if you're inside (don't run outside)
• If outside, move away from buildings
• If driving, pull over safely and stop
• After shaking stops, check for damage and injuries

Managing stress:

• Maintain normal routines as much as possible
• Talk about fears with family and friends
• Limit earthquake-related news consumption
• Consider temporary relocation if stress is overwhelming
• Seek counseling if anxiety becomes debilitating

What You Should NOT Do

• Don't evacuate based on predictions: Unless official evacuation ordered
• Don't spread rumors: Share only information from official sources
• Don't assume larger earthquake is coming: Most swarms end without larger event
• Don't ignore safety precautions: Even small earthquakes can cause injuries
• Don't panic: Swarms are usually harmless even if unsettling

Scientific Monitoring and Research

Understanding how scientists study earthquake swarms reveals both the progress made and challenges remaining.

Monitoring Networks

Seismic networks:

• Dense arrays of seismometers detect even tiny earthquakes
• Real-time processing identifies and locates events within seconds
• Can detect earthquakes as small as M-1.0 or smaller with dense networks
• Track spatial and temporal evolution of swarms

GPS networks:

• Measure ground deformation with millimeter precision
• Detect inflation/deflation associated with magma or fluid movement
• Can identify slow slip events that may trigger swarms

InSAR (Interferometric Synthetic Aperture Radar):

• Satellites measure ground deformation over wide areas
• Particularly useful for remote swarms
• Can detect centimeter-scale deformation

Gas monitoring:

• Measures CO₂, SO₂, and other gases in volcanic areas
• Changes in gas emission can indicate magma movement
• Helps distinguish volcanic from tectonic swarms

Analysis Techniques

Double-difference relocation:

• Precisely locates earthquakes relative to each other
• Reveals fine structure of fault networks activated during swarms
• Can track migration of swarm activity

Moment tensor inversion:

• Determines fault orientation and slip direction
• Reveals whether earthquakes occur on many faults or single fault system
• Helps understand stress field evolution during swarm

Statistical seismology:

• ETAS models (Epidemic-Type Aftershock Sequence) describe earthquake clustering
• Can distinguish swarms from mainshock-aftershock sequences statistically
• Helps forecast probability of continued activity

Open Questions

Despite advances, fundamental questions about swarms remain:

• Triggering mechanisms: What initiates a swarm? Why do some stress changes produce swarms while others produce mainshock-aftershock sequences?
• Duration predictability: Why do some swarms last days while others continue for months? Can we predict when a swarm will end?
• Precursory identification: Can we develop reliable methods to identify which swarms will produce larger earthquakes?
• Spatial migration: What controls the speed and direction of swarm migration?
• Maximum magnitude: What determines the largest earthquake a swarm can produce?

The Future of Swarm Understanding

Technological Advances

Machine learning:

• AI algorithms analyzing massive datasets from past swarms
• May identify subtle patterns humans miss
• Could improve forecasting of swarm evolution
• Unlikely to enable prediction but may refine probabilities

Distributed Acoustic Sensing (DAS):

• Uses fiber optic cables as dense seismometer arrays
• Can detect thousands of tiny earthquakes in swarms
• Reveals detailed structure of fault activation
• Already providing unprecedented views of swarm behavior

Improved modeling:

• Physics-based models of fluid-rock interaction
• Better representation of fault network complexity
• May eventually simulate swarm behavior from first principles

Practical Applications

Hazard assessment:

• Better understanding of swarms improves earthquake hazard maps
• Distinguishes areas where swarms occur from areas where mainshocks dominate
• Informs building codes and preparedness planning

Volcano monitoring:

• Swarms often precede eruptions by days to weeks
• Better swarm analysis improves eruption forecasting
• Can extend warning times for evacuations

Induced seismicity management:

• Understanding swarm triggers helps manage induced seismicity
• Can adjust injection/extraction operations based on swarm activity
• Reduces risk of triggering damaging earthquakes

The Bottom Line

Earthquake swarms represent a fundamentally different mode of seismic energy release than the mainshock-aftershock sequences most people associate with earthquakes. Rather than one large event releasing accumulated stress, swarms distribute energy across hundreds or thousands of smaller earthquakes over days to months.

Most swarms result from fluid movement through the crust—whether magma, water, or other fluids. These fluids weaken rocks, allowing many small patches to fail rather than one large rupture. This explains both why swarms cluster in volcanic and geothermal areas and why they tend to migrate through space as fluids move.

The critical question for anyone experiencing a swarm is whether it signals an impending larger earthquake. The frustrating answer is that scientists cannot predict this. About 90-95% of swarms end without producing larger events—they simply represent normal stress release. But 5-10% are precursory, and there's no reliable way to distinguish these cases in advance.

Living through a swarm is unsettling. The ground shakes repeatedly, sleep is disrupted, anxiety builds, and uncertainty dominates. The best response is preparation: secure your home, maintain emergency supplies, have a family plan, and stay informed through official sources. Ignore predictions and rumors. Most importantly, understand that while swarms are unnerving, they rarely lead to disaster.

Earthquake swarms remind us that Earth's crust is dynamic and constantly adjusting to tectonic stresses. They provide windows into processes occurring kilometers beneath our feet—fluid movement, magma intrusion, slow slip on faults. As monitoring technology improves and our understanding deepens, we may eventually develop better tools for assessing swarm hazards. For now, awareness, preparation, and perspective are our best defenses against the uncertainty swarms create.

Additional Resources

Learn about different types of earthquake phenomena including how earthquake depth affects damage and why earthquakes cannot be predicted. Explore regional earthquake threats in California, the Pacific Northwest, the central United States, and Mexico City. Discover how Tokyo became the world's most earthquake-prepared city. Find earthquake safety basics in our comprehensive FAQ, and monitor earthquake activity including active swarms on our real-time earthquake map.