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Earthquakes in Mexico City – why are they so strong?

Mexico City is particularly susceptible to earthquake amplification due to its geological and geographical characteristics. The city is built on an ancient lake bed, which consists of soft soils, clay, and loose sediments.

These materials have the potential to magnify the effects of seismic waves during an earthquake. Earthquakes in Mexico City happen more and more often, and 2023 recorded already over 50.

Earthquakes in Mexico City are amplified

When an earthquake occurs in a region like Acapulco, which is far away from Mexico City, the seismic waves generated by the earthquake travel through the Earth’s crust. As these waves reach the basin of Mexico City, they encounter the soft and loose sediments of the lake bed. These materials have a low density and slower seismic wave velocities compared to solid bedrock. As a result, the seismic waves slow down and are amplified as they pass through the softer sediments.

The phenomenon of amplification can result in more significant ground shaking in Mexico City compared to other areas farther from the earthquake’s epicenter. In some cases, the amplified shaking can cause more severe damage to buildings and infrastructure, leading to increased risks for the city’s inhabitants.

It’s important to clarify that this phenomenon is not unique to Mexico City; earthquake amplification can occur in other cities and regions with similar geological conditions. Scientists and engineers study these factors to better understand the potential impacts of earthquakes and to design structures that can withstand the effects of amplified ground shaking.

Earthquakes in cities with similar geological conditions

Several devastating earthquakes have occurred in cities built on soft lake beds or similar geological conditions, leading to significant damage and loss of life. Here are some examples:

Kathmandu, Nepal – April 25, 2015: The Gorkha earthquake, also known as the Nepal earthquake, struck Nepal with a magnitude of 7.8. The earthquake’s epicenter was near Gorkha district, approximately 80 kilometers northwest of Kathmandu. The city of Kathmandu is situated in the Kathmandu Valley, which contains soft lake bed sediments and alluvial soils. The amplification of seismic waves in the valley led to devastating effects, resulting in the destruction of numerous historical and modern buildings and the loss of thousands of lives.

Mexico City, Mexico – September 19, 1985: Mexico City experienced a catastrophic earthquake with a magnitude of 8.1. The earthquake’s epicenter was off the Pacific coast of Michoacán, approximately 350 kilometers from Mexico City. The city is built on an ancient lake bed, which significantly amplified the seismic waves. The soft soils and sediments in the basin contributed to the earthquake’s impact, causing the collapse of many buildings and infrastructure and resulting in thousands of fatalities.

Kobe, Japan – January 17, 1995: The Great Hanshin earthquake, also known as the Kobe earthquake, struck the port city of Kobe, Japan, with a magnitude of 6.9. While not built on a lake bed, Kobe is situated on alluvial soils near Osaka Bay. The soft soils amplified the seismic waves, leading to significant ground shaking and widespread damage. The earthquake caused the collapse of buildings, highways, and bridges, resulting in thousands of deaths and extensive destruction.

Christchurch, New Zealand – February 22, 2011: The city of Christchurch in New Zealand experienced a devastating earthquake with a magnitude of 6.3. The earthquake’s epicenter was near Lyttelton, just southeast of Christchurch. The city is built on alluvial sediments and river deposits, which amplified the shaking during the quake. The earthquake caused significant damage to buildings and infrastructure, resulting in multiple fatalities and injuries.

Wenchuan, China – 2008 Earthquake: The Wenchuan earthquake, also known as the Great Sichuan earthquake, occurred on May 12, 2008, in Sichuan province, China. With a magnitude of 7.9, it was a massive and devastating earthquake. The city of Chengdu, located on the Chengdu Basin, experienced significant seismic amplification due to its soft soils and basin-like structure. The earthquake resulted in nearly 87,000 deaths and extensive damage to buildings and infrastructure.

These examples highlight the importance of understanding the geological conditions and potential seismic hazards in cities built on soft lake beds or other vulnerable areas. Proper urban planning, construction practices, and seismic retrofitting measures can help mitigate the impact of future earthquakes in such regions.

Earthquakes in Mexico City: measures implemented

After the devastating earthquake that struck Mexico City on September 19, 1985, with a magnitude of 8.1, the city underwent significant changes in construction requirements and seismic building codes. The earthquake caused extensive damage and loss of life, highlighting the need for improved building practices and earthquake-resistant infrastructure. The changes implemented after the 1985 earthquake aimed to enhance the city’s resilience to future seismic events. Some key changes include:

1. Strengthening Building Codes: Mexico City revised and strengthened its building codes to ensure that new structures were designed and constructed to withstand seismic forces. The updated codes incorporated lessons learned from the earthquake and incorporated modern engineering practices to improve building resilience.

2. Seismic Hazard Assessments: The city conducted comprehensive seismic hazard assessments to identify areas with higher earthquake risks. This information was used to guide construction regulations and prioritize retrofitting efforts for existing buildings in vulnerable zones.

3. Retrofitting Existing Buildings: To improve the earthquake resistance of older structures, retrofitting programs were implemented. Existing buildings that did not meet updated seismic standards were retrofitted to enhance their ability to withstand ground shaking during earthquakes.

4. Zoning Regulations: Zoning regulations were updated to control the construction of new buildings in high-risk areas. The aim was to limit the development of critical infrastructure and high-rise buildings in regions with potentially higher seismic hazards.

5. Public Education and Awareness: There was an increased focus on public education and awareness regarding earthquake preparedness and safety measures. This included conducting drills, distributing educational materials, and raising awareness about earthquake risks and appropriate actions to take during and after an earthquake.

6. Urban Planning: The earthquake prompted a reevaluation of urban planning practices, with an emphasis on improving the overall city infrastructure. This included considerations for emergency response, evacuation routes, and the resilience of transportation networks.

7. Building Materials and Techniques: There was a push to use improved building materials and construction techniques that better absorb and dissipate seismic energy. Reinforced concrete and steel structures became more prevalent in new construction projects.

8. Creation of Specialized Agencies: The Mexican government established specialized agencies and bodies responsible for monitoring and enforcing the implementation of seismic building codes. These agencies played a crucial role in ensuring compliance with new construction requirements.

The changes made after the 1985 earthquake significantly contributed to enhancing Mexico City’s resilience to seismic events. While earthquakes in Mexico City cannot be prevented, these measures have helped minimize damage and protect lives during subsequent earthquakes. It’s worth noting that continuous efforts to improve and update building codes and infrastructure remain essential to ensure the city’s ongoing preparedness for seismic hazards.