What makes some types of ground shake more strongly during earthquakes?

Introduction

During earthquakes some places shake violently while nearby areas feel only minor trembling. The difference is not random: it depends on the earthquake source, the path seismic waves travel, and crucially the type of ground underfoot. Understanding why certain soils and rocks amplify shaking helps communities build safer structures and plan more effectively.

Quick answer: what makes ground shake more strongly?

Ground shakes more strongly when local conditions amplify seismic waves. Soft, loose sediments (like recent river deposits, reclaimed land, or thick layers of clay and sand) tend to amplify and prolong shaking compared with hard, dense bedrock. The depth and stiffness of subsurface layers, the presence of groundwater, and the match between seismic wave frequencies and the natural vibration frequency of the ground all influence how strongly a site will shake.

Basic seismic ingredients: source, path, and site

To understand why shaking varies you can think in three parts: the earthquake source (how much energy is released and which kinds of waves are produced), the path (how seismic waves change as they travel through the Earth), and the site (local ground conditions where waves arrive). The site effect — what the ground is made of near the surface — is often the biggest reason neighboring locations experience very different shaking.

Earthquake source and wave types

Earthquakes create several types of seismic waves. Body waves travel through the Earth’s interior: P-waves (compressional) arrive first and shake the ground in the direction of travel; S-waves (shear) arrive next and move the ground perpendicular to the travel direction. Surface waves (Love and Rayleigh waves) travel along the Earth’s surface and often cause the strongest shaking at distances of tens to hundreds of kilometers.

Different waves carry different frequencies and particle motions. Surface waves usually have lower frequencies and greater amplitudes, which interact strongly with soft surface layers and urban structures. The mix of wave types and their frequency content depends on the earthquake’s size, depth, fault orientation, and rupture details.

How ground properties influence shaking

Local ground conditions affect both the amplitude (how strong) and duration (how long) of shaking. Key ground properties include stiffness (or shear wave velocity), thickness of soft layers, density, damping (how quickly vibrations dissipate), and the presence of water. These properties determine how seismic waves are modified when they reach the surface.

Stiffness and shear wave velocity

Stiffness refers to how much a ground layer resists deformation. Engineers measure stiffness via shear wave velocity (Vs) — the speed of S-waves through a material. Hard rock has high Vs and transmits waves quickly with less amplification. Soft soils have low Vs, slow waves, and greater amplification. Building codes often use Vs to classify ground and set design requirements.

Impedance contrast and wave trapping

When seismic waves cross a boundary between layers with different stiffness or density, part of the wave energy is reflected and part is transmitted. A large contrast (for example, soft sediment lying on top of hard rock) can trap and amplify seismic energy within the soft layer. This trapped energy bounces between the surface and the stiff layer below, increasing the motion at the surface — similar to how a musical instrument amplifies certain frequencies.

Resonance: when ground and waves line up

Every layer of ground has a natural frequency at which it prefers to vibrate, determined mainly by the layer’s thickness and shear wave velocity. If incoming seismic waves contain energy at the same frequency, resonant amplification occurs and shaking grows much stronger. This resonance is why tall buildings (with low natural frequencies) can be especially damaged by long-period surface waves, while low-rise structures are more affected by higher-frequency shaking from shallow soils.

Thickness of soft layers

Thicker soft-sediment deposits generally produce larger amplification and longer shaking because waves travel slowly through them and take longer to die out. For example, deep river valleys, lake sediments, and human-made fills can cause stronger shaking than thin soil veneers. The classic observation is that deep sedimentary basins often experience intense, long-lasting ground motion during earthquakes.

Soil type and damping

Different soils dissipate vibration to different degrees. Cohesive clays, sands, silts, and gravels each have distinct damping properties. Some materials convert seismic energy into heat more effectively and reduce motion; others are less damping and sustain motion longer. Water content often reduces damping and can make soils behave more fluidly during strong shaking (see liquefaction below).

Groundwater and saturation

Saturated soils—where the pore spaces between grains are filled with water—can react very differently from dry soils. Water increases the weight and lowers the effective stiffness of the soil, which can amplify shaking. In saturated, loosely packed sands, shaking can cause a temporary loss of strength known as liquefaction, which dramatically increases surface deformation and damage to buildings and infrastructure.

Common examples: where shaking is amplified

Knowing typical settings that amplify shaking helps explain many observed earthquake damage patterns.

Alluvial plains and river valleys — These areas contain unconsolidated sediments deposited by rivers. The soft sediments amplify shaking, often producing strong, long-duration motion in cities built on such plains.

Coastal lowlands and reclaimed land — Filled or reclaimed land (for ports, industrial areas, or housing) often contains loose fill or dredged material that amplifies shaking and is prone to liquefaction.

Sedimentary basins — Large basins with thick layers of sediment trap seismic energy and can produce significant ground motion over broad regions, even at some distance from the earthquake source.

Steep slopes and colluvium — Unconsolidated slope deposits can amplify shaking and, when combined with steep terrain, increase the risk of landslides during earthquakes.

Soft soils over hard rock — A thin layer of soft soil over bedrock can create resonance and amplify specific frequencies, which may target particular building heights.

Measured effects and real-world examples

Seismologists and engineers document site amplification using instruments and historical earthquake records. A well-known case is the 1985 Mexico City earthquake. The city is built on deep lake-bed sediments that amplified surface waves, causing severe shaking in areas far from the quake’s epicenter while neighboring hill areas on firmer ground fared much better.

In the 1989 Loma Prieta earthquake (California) and the 1995 Kobe earthquake (Japan), differential damage patterns reflected local soil conditions and amplification. Modern building codes and site investigations attempt to account for these effects so structures can be designed to withstand expected local ground motions.

How engineers and planners account for site amplification

Engineers use several tools to estimate and mitigate amplified shaking:

Site classification — Geotechnical investigations measure shear wave velocity, soil types, groundwater depth, and thickness of deposits to classify a site’s expected amplification and select appropriate building designs.

Ground response analysis — Numerical models simulate how seismic waves will interact with local soil profiles to predict amplification across frequencies. These models guide foundation design and building dynamic properties.

Seismic building codes — Many modern codes require design-level ground motion that considers local site effects, not just the regional earthquake hazard. This leads to stiffer foundations, base isolation, or tuned mass dampers in some structures.

Land-use planning — Authorities may restrict critical facilities (hospitals, emergency centers) from being placed on highly amplifying or liquefaction-prone sites, or require special ground improvement measures.

Ground improvement — Techniques such as deep compaction, stone columns, soil mixing, drainage improvement, and rigid foundations can reduce amplification and liquefaction risk by increasing stiffness and drainage.

Limitations and uncertainties

Predicting exact amplification at a specific location remains complex. Subsurface conditions can vary over short distances, and the earthquake source and wave path also matter. While models and site investigations reduce uncertainty, residual risk remains — especially where detailed subsurface data are scarce or for rare, very large earthquakes that produce different frequency content than expected.

Seismic hazard assessments therefore use probabilistic approaches that combine geological, geophysical, and historical data to estimate likely ranges of ground motion rather than single deterministic predictions.

Practical takeaways for the public

Being aware of local ground conditions can help individuals and communities prepare: if your home is on a river plain, reclaimed land, or near steep colluvial slopes, expect stronger shaking than nearby bedrock areas. During earthquakes, older unreinforced buildings and poorly founded structures on soft soils are more at risk. Local building codes often require more robust design in amplifying soils—check with local authorities or a qualified geotechnical engineer if you are planning construction or retrofits.

Conclusion

Some types of ground shake more strongly because seismic waves interact with local soils and geologic layers in ways that amplify and prolong motion. Soft, saturated, or thick sedimentary deposits, strong contrasts between layers, shallow water tables, and resonance between wave frequencies and ground layers are the main causes. Recognizing these effects helps engineers, planners, and residents assess risk and take practical measures to reduce earthquake damage.