These are the real dangers associated with earthquakes

This article deals with dangerous natural phenomena such as earthquakes that can affect people, their property, and the environment.

These are the real dangers associated with earthquakes
Building in downtown Mexico City destroyed in the September 1985 earthquakes. Photo: Civil Protection

We tend to associate disasters caused by earthquakes with the collapse of buildings, and some with the opening of cracks in the ground. However, in many cases, these effects are not the main cause of death and destruction. Earthquakes can also cause tsunamis, large-scale fires, landslides, floods, and mudflows.

In many earthquakes, particularly those affecting urban areas, the greatest risk is caused by fires. About 90% of the damage that occurred during the 1906 earthquake (magnitude 8) in San Francisco, California, was caused by fire rather than building collapse. The sudden ground shaking caused by the quake ruptured electrical and gas pipes, which sparked and started a large fire that affected the entire city. Likewise, water sources were affected, preventing the fire from being extinguished for more than three days. During the 1923 earthquake in Japan (magnitude 8.3), 71% of the damage in the city of Tokyo and almost 100% in the city of Yokohama was attributed to fire.

Tsunami - a Japanese word adopted by most languages to refer to tidal waves - can be caused by subduction earthquakes such as the 1985 Michoacan earthquake. During the occurrence of a large subduction earthquake, the ocean floor undergoes sudden deformation. The ocean floor rises or sinks by several centimeters, raising or sinking a large mass of water. This mass of water propagates as a large wave that reaches a height of several meters when it reaches the coast. Tsunami waves travel at speeds of several hundred kilometers per hour but are not seen offshore, where their height does not exceed one meter and they have wavelengths greater than several hundred meters.

However, when the wave approaches the coast, due to the decrease in-depth, its speed slows down and the height of the crest increases until it exceeds several meters in height. In 1946, an earthquake off the coast of the Aleutian Islands (magnitude 7.6) caused a tsunami that reached the Hawaiian Islands, more than 3000 km away, causing 159 deaths and damages of more than 25 million dollars. The 1992 Nicaragua earthquake (magnitude 7.3) was weakly felt by coastal populations, but the tsunami that followed 50 minutes later caused severe damage and claimed more than 167 victims.

In mountainous regions, earthquakes can cause major landslides. During the 1970 earthquake in Peru (magnitude 7.8), of the 66,000 people who died, 25,000 were buried in the town of Yungay due to a landslide caused by ground movement. This danger is becoming greater every day in cities with human settlements on the slopes of hillsides. These same landslides can generate dams in rivers, which, when they break, carry large quantities of water and mud downstream, burying towns in riverbeds up to several days after the quake.

The city of San Francisco was badly affected by an earthquake in 1906.
The city of San Francisco was badly affected by an earthquake in 1906. Photo: NPS

Seismic hazard mitigation

The destructive effects of earthquakes include aspects such as ground shaking, fires, tsunamis, landslides, and interruption of vital lines (roads, water, electricity, gas, communications) in addition to causing panic and psychological shocks in a large part of the population. Property damage and loss of life depend on the time of day the earthquake occurs, its magnitude, the distance of populations from the epicenter, the geology of the area, the type of construction, the population density, and the duration of strong ground motion. In general, earthquakes that affect large cities during working hours have the greatest impact.

Strong ground motion produces the greatest damage and loss of life of all the effects associated with earthquakes. Buildings built on hard rock suffer less damage than buildings built on poorly consolidated soils, such as water-saturated sediments (like the soils of Mexico City) or artificial fill (like the soils of San Francisco Bay or Kobe).

Seismic hazards built on saturated sediments or artificial fills are subject to longer duration of strong motion and higher amplitudes of strong motion. The strength of a building decreases with time: the longer the building is subjected to strong motions, the lower its strength. Both saturated sediments and artificial fill are susceptible to liquefaction. When subjected to strong movements, individual grains lose cohesion and tend to flow like water. Some of the buildings that collapsed in Mexico City did so because the soil lost cohesion.

The type of construction material and the design of the building, in addition to the magnitude of the earthquake and the geology of the region, are important factors in estimating seismic risk. Structures of stucco, adobe, and other mud materials are the weakest and usually the first to fall during an earthquake. Brick structures without rebar reinforcement and carelessly built concrete structures are also among the first to collapse, mainly when they are close to the epicenter.

A magnitude 6.4 earthquake in India in 1993 killed 30,000 people, while a magnitude 6.7 earthquake in Northridge, California killed only 61 people. Both cities are highly populated and the earthquakes occurred a few kilometers below the populations, however, the type of construction made a big difference. In India, most of the houses are made of stone and brick without reinforcement, while in California the houses are made of reinforced concrete.

In terms of construction materials and design techniques, sufficient progress has been made to avoid the total collapse of a house due to a magnitude 7 earthquake occurring a few kilometers away. However, every day there are more and more victims due to earthquakes, why? The fundamental problem lies in the planning of cities. Cities, especially in developing countries, are expanding without planning.

Large Latin American cities subject to frequent earthquakes have zoning maps that theoretically regulate the type of construction and the resistance it must withstand to avoid collapse during a strong earthquake. There are also codes specifying the type of material and design appropriate for the different regions. However, in most cases, these zone maps and building codes are not respected by builders or the authorities in charge of monitoring compliance with the standards.

As long as we continue to allow inadequate construction, settlements on soft ground, on steep slopes, and coastlines without protection against tsunamis, we will never be able to reduce the loss of human lives. We must remember that earthquakes, like many other natural phenomena, cannot be predicted, but disasters can be prevented by implementing appropriate measures before the next tremor occurs. Earthquakes are an indirect consequence of the release of heat energy from the Earth's interior, and this process has existed for more than 3 billion years and will continue for billions of years until the Earth cools down.

The seismic danger

The people of Mexico painfully remember September 19 and 21, 1985, when an earthquake of magnitude 8, and its major aftershock, caused several buildings in downtown Mexico City to collapse, burying more than 10,000 victims under the rubble. The quake did not come as a surprise to the city, which has experienced an onslaught of tremors since its inception. What did come as a novelty was the magnitude of the damage, which is estimated at more than US$4 billion, and the number of victims already mentioned.

The 1985 earthquake occurred off the coast of the state of Michoacán, more than 300 km away from Mexico City. At this distance, seismic waves are largely attenuated due to the dissipation of energy as they propagate through the Earth's interior. However, in Mexico City, having been built on the bed of an ancient lake -Lake Texcoco-, the composition of its sediments and its thickness cause the trapping of seismic energy with vibrations of periods between 1 and 3 seconds.

This phenomenon is called resonance and can be easily explained by performing a small experiment: we tie a mass to a rope to form a pendulum. If we set the pendulum to swing, it will make one oscillation every T = 2π√(l/g) where l is the length of the string and g is the acceleration of gravity. If we try to swing the pendulum at intervals other than T, more energy must be supplied to set it to swing, while at intervals of T, the pendulum will swing easily, absorbing the supplied energy very efficiently.

The resonance of sediments with seismic waves is not as simple to explain as the pendulum, however, the implications are similar, to the soil of Mexico City and other regions of the world such as San Francisco Bay in the United States and Kobe Bay in Japan are sedimentary fills that absorb seismic energy and amplify it for certain periods. Like pendulums, buildings have their natural period of oscillation. When these periods coincide with the natural or resonance periods of the subsoil, a double resonance is produced, causing buildings and structures to fall.

This phenomenon has been recorded in many earthquakes around the world: Mexico City, Mexico (1957 and 1985); Spitak, Armenia (1986); Loma Prieta, California, USA (1989); Kobe, Japan (1995). In the case of Mexico City, buildings with 8 to 16 stories high, with natural periods between 1 and 3 seconds, located in the area of the old lake, suffered more intensely from ground accelerations than other buildings.

With similar intensity to the 1985 earthquake, other earthquakes have been felt in Mexico City, particularly those that occurred on April 7, 1845, and June 19, 1958. So, what was the difference between the previous earthquakes and that of 1985? The population. Only in the last three decades has it increased by a factor of 6, and with it, the number of buildings with more than 5 floors in high seismic risk zones.

Already in 1957, with the occurrence of a 7.7 magnitude earthquake, located in front of Costa Chica in Guerrero, scientists and engineers warned about the danger of building on the old bed of Lake Texcoco. On that occasion, 160 victims and losses of more than 25 million dollars were recorded. This event did not cause appreciable damage in the epicentral zone, however, more than 270 km away, in Mexico City, the damage was considerable.

It is revealing to look at the statistics of earthquake casualties. After the invention of reinforced concrete for construction and its implementation worldwide in the 1930s, the number of earthquake casualties decreased considerably. However, since the 1960s, with the accelerated increase in population, mainly in developing countries, casualties have been on the rise. The large urban centers of these countries have expanded without planning. Urban sprawl spreads over hillsides, former lakebeds, and poorly constructed housing.

Although the greatest number of earthquake casualties occur in developing countries, the greatest earthquake losses occur in developed countries. The Northridge earthquake (magnitude 6.7) near Los Angeles, California, with 61 casualties caused damages estimated at US$15-30 billion; the Kobe earthquake (magnitude 7.2) in Osaka Bay, Japan, with 5,000 casualties caused damages estimated at US$30-80 billion. Cities in these countries are also expanding on land with high seismic risk.

By Javier Francisco Pacheco, Source: Correro del Maestro No.31, p.9-12.