Sedimentary basins can amplify earthquake shaking because seismic waves behave differently in soft, deep sediments than in surrounding hard rock. Waves slow as they enter lower-velocity material, change direction at boundaries and can become trapped within the basin. The resulting motion may be stronger at particular frequencies and can continue after direct waves have passed. Basin edges can generate additional surface waves, while three-dimensional shape influences where energy concentrates. Amplification is not a single multiplier applied to every earthquake or building. It depends on the source, direction, wave frequency, sediment thickness, stiffness and nonlinear soil behaviour. Understanding basin effects helps explain why neighbourhoods at similar distances from a fault may experience different shaking and why local geological models are important for building codes, emergency scenarios and urban risk reduction.<\/p>\n
A basin is a depression in Earth’s crust filled with sediment or sedimentary rock. Some form through tectonic subsidence; others occupy valleys or coastal regions. Urban areas such as Los Angeles, Mexico City, Seattle and parts of Tokyo overlie deep basins. The basin may be hidden beneath a flat surface, so its three-dimensional geometry must be inferred from boreholes, gravity, seismic surveys and earthquake records.<\/p>\n
Shear waves usually travel more slowly through soft sediments than competent bedrock. When waves cross a boundary, part of their energy reflects and part refracts. Conservation of energy and impedance contrast can increase motion in the softer layer. The basic earthquake source is described in the earthquake causes guide<\/a>.<\/p>\n Waves can reflect between the ground surface and deeper basin boundary, producing reverberation. A basin may \u201cring\u201d after the strongest direct arrival, extending duration. USGS teaching simulations compare this sloshing with waves in a bathtub. Longer duration can increase the number of damaging cycles experienced by structures, especially after initial shaking has weakened them.<\/p>\n A sediment layer has preferred vibration periods related to thickness and wave speed. Thick basins tend to influence longer-period motion than shallow soft soil. Buildings also have natural periods. Risk can increase when ground motion contains strong energy near a structure’s period, although damping, design and nonlinear response determine the actual outcome.<\/p>\n At a steep basin edge, incoming waves convert into surface waves that travel across the sediments. Interference between these and direct waves can create bands of stronger or weaker motion. The pattern depends on which direction waves approach, so the same district may respond differently to earthquakes on different faults.<\/p>\n Curved boundaries can concentrate seismic energy in some places and spread it in others. Three-dimensional simulations reveal complex hotspots that one-dimensional soil columns cannot reproduce. However, detailed models require accurate geometry and velocity structure. A colourful simulation should be interpreted with its assumptions and validated against recordings.<\/p>\n During weak shaking, soil behaviour can be approximated as linear. Strong motion can reduce stiffness and increase damping, changing the amplification and shifting energy between frequencies. Pore pressure may rise in saturated loose sediment, contributing to liquefaction. Strong-motion observations are essential because behaviour inferred from small events may not transfer directly to a major earthquake.<\/p>\n Fault rupture length, direction and speed shape the waves entering a basin. Rupture moving towards a city can create directivity pulses. Waves also travel through mountains and crustal structures before reaching local sediments. Basin amplification is therefore one part of total ground motion, not a replacement for source and regional path modelling.<\/p>\n Strong-motion networks compare instruments inside and outside basins. Spectral ratios show amplification at particular periods, while duration measures reveal extended shaking. The earthquake location guide<\/a> explains how networks first determine the event’s origin; local ground-motion analysis then examines what happened at each station.<\/p>\n Cities constantly vibrate because of traffic, industry, wind and ocean waves. Arrays can analyse this ambient noise to estimate resonance and basin structure without waiting for a damaging earthquake. The methods provide valuable constraints but do not duplicate strong shaking, so results are combined with boreholes, active surveys and earthquake data.<\/p>\n A one-dimensional model treats layers as horizontal beneath a site and is useful for local soil response. Two- and three-dimensional models represent basin edges, variable depth and wave propagation across the structure. Greater complexity is justified only when data constrain it. Model comparisons and blind tests help distinguish improved physics from overfitting.<\/p>\n USGS research using recordings and three-dimensional simulations found that Seattle and Tacoma basin structures can increase shaking intensity and duration for some local earthquakes. Basin-edge-generated surface waves were observed for particular paths. The result does not mean every location or earthquake receives the same amplification; it demonstrates the importance of direction, period and basin geometry.<\/p>\n Parts of Mexico City are built on soft former lake sediments that strongly amplify selected frequencies and prolong motion. Damage patterns in major earthquakes show the interaction between soil response and building period. The example is often taught, but it should not be reduced to \u201csoft soil is always worse\u201d: thickness, frequency and structural design determine which buildings are most affected.<\/p>\n After an earthquake, instruments and models produce maps of shaking. The earthquake intensity-map guide<\/a> explains how sparse observations are combined. Basin models improve estimates between stations, while incoming reports and recordings update the product. Rapid maps guide response but are not final building-by-building damage surveys.<\/p>\nTrapping and reverberation<\/h2>\n
Natural periods<\/h2>\n
Basin-edge effects<\/h2>\n
Focusing and defocusing<\/h2>\n
Nonlinear soil response<\/h2>\n
Source and path effects<\/h2>\n
Recording the effect<\/h2>\n
Ambient-noise methods<\/h2>\n
One-dimensional and three-dimensional models<\/h2>\n
Puget Sound example<\/h2>\n
Mexico City as a wider example<\/h2>\n
Shake maps and rapid response<\/h2>\n
GIS and urban planning<\/h2>\n