13.4 Earthquakes and their impact on landscapes

Earthquake Generation Mechanisms

Earthquakes reshape landscapes in seconds, triggering landslides, altering rivers, and shifting coastlines. These sudden changes set off a chain of geomorphic responses that influence long-term landscape evolution and sediment dynamics. Understanding how earthquakes originate and propagate is the foundation for interpreting the surface processes they drive.

Tectonic Stress and Elastic Rebound

Tectonic forces slowly deform rocks along fault lines, storing energy as elastic strain. When the accumulated stress exceeds the frictional strength of the fault, the rock snaps back to its undeformed shape and releases that stored energy as seismic waves. This is elastic rebound theory, and it explains why earthquakes are sudden rather than gradual.

• Focal mechanism describes the orientation of the fault plane and the direction of slip. It tells you what type of faulting occurred (normal, reverse, or strike-slip) and reveals the tectonic forces at work.
• Magnitude scales quantify the energy released. The moment magnitude scale ($M_w$) is the standard for large earthquakes because it doesn't saturate at high magnitudes the way the older Richter scale does.
• Intensity scales like the Modified Mercalli Intensity (MMI) measure observed surface effects, from barely felt shaking (MMI I) to total destruction (MMI XII). Intensity depends on distance from the epicenter, local geology, and building construction.

Plate Boundary Earthquakes

Most earthquakes occur at plate boundaries, and the type of boundary controls earthquake depth, magnitude, and landscape impact.

• Convergent boundaries produce the widest range of earthquake depths. In subduction zones, earthquakes can be deep-focus (up to ~700 km), tracing the path of the descending slab along the Wadati-Benioff zone. In continental collision zones (e.g., the Himalayas), earthquakes are shallower but can still be very large.
• Divergent boundaries generate shallow earthquakes of low to moderate magnitude, driven by extensional stress during rifting and seafloor spreading. The Mid-Atlantic Ridge is a classic example.
• Transform boundaries produce shallow, high-magnitude strike-slip earthquakes. The San Andreas Fault is the textbook case, where the Pacific and North American plates slide past each other.

Intraplate Seismicity

Not all earthquakes happen at plate boundaries. Intraplate earthquakes occur within the interior of tectonic plates, often from reactivation of ancient faults or far-field stress transfer from distant plate boundaries.

These events are less frequent, but they can be highly destructive because the crust in plate interiors transmits seismic energy more efficiently over long distances. The New Madrid Seismic Zone in the central United States produced a series of $M_w$ ~7.0+ earthquakes in 1811–1812 that were felt across much of eastern North America.

Earthquake Geomorphic Effects

Mass Movements and Ground Deformation

Seismic shaking destabilizes slopes and deforms the ground surface in several ways:

• Mass movements: Shaking triggers rockfalls, debris flows, and large-scale landslides, particularly on steep or already-unstable hillslopes. The intensity of shaking and local slope conditions determine which types dominate.
• Surface rupture: When a fault breaks through to the surface, it creates fault scarps (small cliffs) and offsets features like streams, roads, and ridgelines. These offsets are direct markers of coseismic displacement.
• Regional uplift or subsidence: Large earthquakes can raise or lower entire blocks of crust by meters. This changes coastlines, shifts drainage divides, and resets local base levels for erosion. The 1964 Alaska earthquake ($M_w$ 9.2) uplifted parts of the coast by up to 10 m and dropped other areas by 2 m.
• Soil compaction and fissuring: Seismic waves compact loose soils, causing localized subsidence and opening ground cracks, especially in areas with thick, unconsolidated sediment.

Liquefaction and Hydrological Impacts

Liquefaction happens when seismic shaking causes water-saturated, loosely packed sediments to lose their strength and behave like a fluid. The grain contacts break down, pore water pressure spikes, and the ground can no longer support structures. This is why buildings on reclaimed land or river deltas are especially vulnerable.

Earthquakes also trigger major hydrological changes:

• Tsunamis generated by large undersea earthquakes (especially at subduction zones) rework coastlines through intense erosion, sediment transport, and redeposition. The 2004 Indian Ocean tsunami dramatically reshaped shorelines across multiple countries.
• Secondary effects include dam failures, changes in spring discharge, and shifts in groundwater levels. Landslide dams (discussed below) can impound rivers and later fail catastrophically, causing downstream flooding that further reshapes the landscape.

Earthquakes and Landscape Change

Immediate Geomorphic Responses

Earthquakes accomplish in seconds what normal surface processes take years or centuries to achieve.

• Coseismic landslides instantly reshape hillslopes, stripping material from ridges and filling valleys with debris. This changes local relief and disrupts drainage networks.
• Landslide dams form when debris blocks river channels, impounding water to create new lakes. These dams alter the longitudinal profile of the stream, creating a local base level upstream and starving the channel of sediment downstream.
• Coastal uplift or subsidence creates new marine terraces (where the coast rises) or submerges coastal plains (where it drops). Either scenario initiates a new cycle of erosion or deposition as the landscape adjusts to its new elevation relative to sea level.

Long-term Landscape Evolution

The effects of a single earthquake don't end when the shaking stops. Each event triggers a cascade of adjustments that ripple through the landscape system:

1. Initial disruption: Landslides, surface rupture, and elevation changes alter slopes, channels, and base levels.
2. Geomorphic response: Rivers adjust to new sediment loads and altered gradients. Hillslopes begin to stabilize or continue failing.
3. Post-seismic relaxation: Afterslip on faults and viscoelastic relaxation of the crust continue to modify surface elevations for years to decades.
4. Isostatic adjustment: Over longer timescales, the crust adjusts to redistributed mass (e.g., eroded mountain material deposited in basins).

The frequency and magnitude of earthquakes in a region control the long-term balance between tectonic uplift and erosion. In highly seismic orogens, repeated earthquakes supply enormous volumes of sediment that keep erosion rates high, maintaining steep, dynamic landscapes.

Earthquake-Induced Landslides and Landscape Evolution

Sediment Dynamics and River System Impacts

Earthquake-triggered landslides are among the most important sediment sources in tectonically active mountain belts. A single large earthquake can mobilize enough material to overwhelm a river's normal sediment transport capacity for years.

• Aggradation: The sudden influx of landslide debris causes river channels to fill with sediment, raising bed elevations and reducing channel gradients. This can shift rivers from incising to depositing, fundamentally changing their behavior.
• Landslide dams and base level: Dams created by landslide debris act as temporary local base levels. Upstream, rivers deposit sediment behind the dam. Downstream, reduced sediment supply can trigger channel incision.
• Dam breaching: When a landslide dam fails, the resulting outburst flood causes rapid incision through the dam material and catastrophic flooding downstream. This reshapes valley floors and floodplains in hours.

Long-term Geomorphic Consequences

The sediment pulse from a major earthquake can take decades to work through a river system. This has several important implications:

• Elevated denudation rates: Catchments affected by earthquake-induced landslides export sediment at rates far above background levels for 10–100+ years. Studies of the 2008 Wenchuan earthquake ($M_w$ 7.9) in China showed that landslide-derived sediment dominated river loads for well over a decade.
• Spatial variability: Not all catchments respond equally. The distribution of landslides depends on shaking intensity, slope steepness, rock type, and moisture conditions. Some valleys are buried in debris while neighboring ones are barely affected.
• Climate-tectonic interaction: The rate at which earthquake-derived sediment is flushed from hillslopes and transported through rivers depends on rainfall and runoff. In monsoon-dominated settings like the Taiwan orogen, typhoons remobilize coseismic landslide debris, creating complex, non-linear sediment pulses that shape landscape evolution over decades to centuries.