Earthquake Hazards

Earthquakes can be a devastating force of nature, capable of producing a multitude of hazards.

Content:
– Primary Earthquake Hazards
— Ground Shaking
— Surface Rupture (Fissures/Cracks)
– Secondary Earthquake Hazards
— Landslides
— Liquefaction
— Tsunami
— Seiches
— Flooding
— Fires & Power Outages
— Mud Volcanoes

Primary Earthquake Hazards

Primary hazards are caused by the direct interaction of seismic wave energy with the ground.

Ground Shaking

Ground shaking is the main hazard associated with an earthquake. It can result in, beyond the panic of the population that feels the shaking, damage to infrastructure.

The severity of the shaking is dependent on the magnitude of the quake, the distance from the epicenter of the earthquake, the depth of the earthquake and the geology of the area.

Magnitude

You will feel more intense shaking from a larger magnitude earthquake compared to a small quake. Larger earthquakes release its energy over a larger area for a longer period of time.

An earthquake is not a rupture at a point but along a plane of a fault. As the rupture occurs along the fault, seismic waves are produced at every point. Hence, longer faults produce larger earthquakes that have longer durations.

For a magnitude 5 event, the actual rupturing over the fault occur in a few seconds. The duration of shaking can be much longer. Some waves, after reaching the surface, can bounce and echo within the interior of the earth.

For the M6.9 earthquake that occurred on August 21st, 2018, the rupture was complex. It lasted for approximately 27 seconds while the shaking on the surface of the earth lasted nearly 90 seconds to 2 minutes.

Distance (& Depth)

The intensity of seismic waves decreases as it travels through the ground due to its energy being dispersed. Hence, shaking is less intense farther from the fault, whether that be distance from the epicenter (location of the earthquake on the surface of the earth) or from the hypocenter (distance from the earthquake from within the earth).

In an earthquake, low and high-frequency waves are produced, with high-frequency waves (P-waves are the highest, then S-waves) diminishing rapidly with distance compared to low-frequency waves (Surface Waves – Love, Rayleigh).

If you are near the epicenter of an earthquake, you may feel a short “jolt” initially (P & S-Wave arrival), then a rolling motion (Surface Waves) versus if your further away, you may feel a rolling motion (Surface Waves).

Infrastructure damage does not solely depend on the intensity of shaking. Smaller buildings such as houses are more damaged by higher frequencies versus larger structures such as high-rises and bridges are damaged by lower frequencies.

However, large, deeper earthquakes are more likely to be felt by a larger population versus a large, shallow quake. This is because the damaging surface waves (low-frequency) waves disperse more rapidly when traveling along the surface of the earth versus in the interior of the earth.

Geology

The soil of your area can greatly amplify the shaking of an earthquake. Seismic waves slow down, but the amplitude increases as the wave move from rock to soil. Soft, loose soil or landfill may shake more intensely than hard rock at the same distance, from the same quake.

The thickness of loose soil can also play a part in ground shaking. Shaking from an earthquake in the region can be 5 or more times greater at a site in the basin than the level of shaking in the nearby mountains.

Data has shown that shaking (ground motion) is amplified at mountain tops and ridges compared to the surrounding low-lying areas.

Note: Several other factors can affect shaking. Earthquake waves do not travel evenly in all directions from the rupture surface; the orientation of the fault and the direction of movement can change the characteristics of the waves in different directions. When the earthquake rupture moves along the fault, it focuses energy in the direction it is moving so that a location in that direction will receive more shaking than a site at the same distance from the fault but in the opposite direction. This is called directivity.

Surface Rupture (Cracks/Fissures)

Surface ruptures can occur because of strong ground motion, causing the land to compress or dilate or by liquefaction, as a water-mud slurry forces its way to the surface.

Normal– and reverse– (collectively called dip-slip) faulting surface ruptures feature vertical offsets while strike-slip faulting produces lateral offsets. Many earthquake surface ruptures are combinations of both. Structures that span a surface fault are likely to suffer great damage surface ruptures.

Secondary Earthquake Hazards

Secondary hazards are caused as a consequence of that ground shaking.

Landslides

When an earthquake occurs, seismic waves cause ground shaking, which can destabilize unstable slopes and trigger landslides or rockfalls. Heavy rain and unconsolidated or fractured rock are exacerbating factors.

Liquefaction

Liquefaction occurs in waterlogged or saturated soils, in which the space between sediment is completely filled with water. Typically, the water pressure will be relatively low within the soil and loosely packed grains are held together by friction. During an earthquake, the groundwater pressure rises as the seismic waves destabilize the soil, increasing the spaces between sediment grains. With the structure of the soil lost, the soil behaves like a liquid.

Liquefied soil will force open ground cracks in order to escape to the surface. The ejected water-soil mixture can result in flooding.

Whether and where liquefaction will take place depends on many factors. These include the degree of saturation, the grain size distribution and consistency at a site, the strength, duration, and frequency content of the shaking and even the grain shape and depth of soil. There is much active research into the mechanisms of liquefaction because its effects can be so severe, yet its process remains imperfectly understood.

During the strong M6.9 earthquake of August 21^st, 2018, liquefaction did occur in Felicity, Trinidad – which verified the liquefaction probability map generated by the USGS seen below.

Tsunami

Tsunamis are multiple, long-wavelength, long-period sea waves produced by the sudden displacement of a large volume of water. Tsunamis generated by an earthquake are usually due to vertical (and sometimes) horizontal changes along the seafloor that disrupt the water column. Earthquakes can also generate submarine or subaerial landslides that can also trigger a tsunami.

A tsunami is one of the most powerful and destructive natural forces. Tsunamis radiate outward in all directions from the point of origin and can move across entire ocean basins. When they reach the coast, they can cause dangerous coastal flooding and powerful currents that can last for several hours.

While earthquakes account for most tsunamis across the globe, not all earthquakes can generate a tsunami. The University of the West Indies Seismic Research Center (UWI SRC) outlines the following criteria for tsunamigenesis:

1. The earthquake must occur beneath the ocean or cause material to slide in the ocean.
2. The earthquake must be strong, at least magnitude 6.5.
3. The quake must rupture the Earth’s surface and it must occur at shallow depth – less than 70km below the surface of the Earth.
4. The earthquake must cause vertical movement of the sea floor (up to several meters).

You can read more about tsunamis, and earthquake-generated tsunamis here.

Tsunamis can devastate coastal communities and cities. It has the potential to cause significant loss of life and property damage. Tsunamis can also reshape coastlines and destroy marine and coastal habitats.

Thankfully, across the Eastern Caribbean, tsunamis are relatively rare, but not unheard of. Typically, tsunamis in the Eastern Caribbean have a recurrence period of once every 50 years. The Caribbean has observed with 105 tsunamis over the last 500 years according to the NOAA National Centers for Environmental Information.

Seiches

A seiche is a standing wave in an enclosed, or partially enclosed, body of water. If you have observed water sloshing back and forth in a swimming pool, bathtub, or cup of water, you may have witnessed a small-scale seiche.

Seiches can be caused by earthquakes when seismic (surface) waves cause the body of water to slosh back and forth in a closed (pool, lake, dam) or partially closed (bays, Gulf of Paria) bodies of water. It can also be caused by tsunamis entering partially closed bodies of water such as bays or gulfs (Gulf of Paria, Maracas Bay, etc.).

Small scale seiches, such as those occurring within a pond, lake or dam, there may be minor, unusual flooding near the water’s edge. In very rare cases, there may be some property or infrastructure damage. In the case of large scale seiches, such as those that would occur in the Gulf of Paria or within bays of Trinidad, Tobago or any other Eastern Caribbean island, these can become particularly destructive and produce tsunami-like effects.

Flooding

Flooding is not the first hazard you’d think of in the event of an earthquake, but it can and does occur.

Earthquakes can uplift the ground, causing rivers to flow upstream and flood low-lying areas. Quakes can also block stream and river flow, either due to incredible surface faulting or earthquake-triggered landslides.

Man-made drainage can also be destroyed in urban areas as well as broken high-pressure pipes, both contributing or directly causing flooding.

Liquefaction events, as groundwater ruptures the surface, can also flood unsuspecting areas.

In catastrophic events, water barriers such as levees or dams can be breached or fail completely, causing a torrent of water to flood surrounding areas.

Fires & Power Outages

Large scale fires associated with earthquakes are typically the result of broken gas lines or compromised electrical lines or equipment. This was the case in the 1906 San Francisco Earthquake which resulted in 90% of the damage resulting from the M7.9 quake that devastated 80% of the city. 

Earthquakes can also lead to a compromised electrical grid. Downed power lines, utility posts and even issues at power generation plants and substations can all contribute to outages post-earthquake.

Mud Volcanoes

There is an established link between large seismic events triggering mud volcano eruptions across the world. In Trinidad, and Eastern Venezuela, one of the many earthquake hazards are the eruption, or formation of new mud volcanoes.

However, research has shown than in Trinidad, mud volcanoes have erupted during relatively seismically quiet periods (Deville & Guerlias, 2009)

Following the M6.9 earthquake on August 21st 2018, both the Devil’s Woodyard and Piparo Mud Volcano erupted.