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In this lesson we’ll discuss the use of geological hazard maps in preventing future crises from natural disasters. We’ll go over different types of geological hazard maps and how they are used.

Natural Disasters in California

It starts as a jolt coming up from the floor. A few seconds later, the ground shakes and rumbles. Books and trinkets spill off of the shelves in your home.

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Furniture rattles across the floor. What feels like hours in actuality goes on for only seconds, rarely longer than a minute. The ceiling lights swing and the walls sway with the vibrations. This might sound terrifying, and for most it is. This is the experience of living through an earthquake.Earthquakes happen when two large pieces of the Earth’s crust, called tectonic plates, slip past each other, releasing enormous amounts of energy in vibrations called seismic waves.

Earthquakes occur along fault lines, or boundaries between two plates. Although the Earth’s crust might seem solid to us, these large plates are always moving. Imagine sliding two pieces of jagged material against each other. The boundaries aren’t smooth and they get caught together.

Pressure builds up until the plates slip past each other, causing an earthquake.California has some of the worst earthquakes in the United States, thanks to its positioning on the San Andreas fault line. Earthquakes are measured using the Richter scale, which is a logarithmic scale to measure vibrations caused by an earthquake. Although the Richter scale theoretically goes up indefinitely, the largest earthquake to date only measured 9.

5. Many earthquakes in California have broken a value of 7 on the Richter scale, with one of the the largest being 7.8 in San Francisco in 1906.

Analyzing Geological Hazard Maps

An earthquake of this magnitude can be catastrophic. Buildings are shredded to pieces, water pipes burst, power lines fall, and roads literally rip in half. So, it’s no surprise that engineers and government officials want to know how to protect their citizens from such a disaster.

Scientists need to understand past earthquakes to prevent catastrophic damage during future ones
earthquake damage

Scientists use past data, current plate movement, and other seismic data to create a prediction of earthquakes in the future in a geological hazard map. These maps are constructed to help engineers, government officials, and business owners make decisions based on future earthquakes.

Some geological hazard maps will show the type of shaking that might occur in the ground. Others show the frequency of ground shaking, probability of shaking, and even the estimated impact on different types of buildings. The maps are created according to the needs of engineers and government officials based on the data collected. Let’s look at how to analyze a few types.

Predicted Shaking Maps

The map shown here shows the likelihood of horizontal shaking as a percentage of acceleration due to gravity, or how fast objects fall to the Earth. The value for gravity is 9.8m/s2, so in parts of Southern California, the shaking experienced would be 64% of that value, or approximately 6.2m/s2.

A geological hazard map of California showing the intensity of predicted shaking as a percentage of acceleration due to gravity
Probability Maps

Another type of map is a probabilistic seismic hazard map, which shows the probability of a certain amount of shaking being exceeded in an amount of time, such as 50 years. For example, an area with 10% probability would have only a 10% chance of exceeding a certain amount of shaking in the next 50 years, meaning there is relatively low risk. However, areas shown in red on the map have a high probability of exceeding that amount of shaking.

Probabilistic seismic hazard map for 50 years in California
Probabilistic Seismic Hazard Map

These maps don’t necessarily inform people about the risk associated with an earthquake, just the probability of it happening or the potential shaking that might occur.

The damage associated with an earthquake also has to do with how populated the area is, what type of structures are present, and how prepared the area is. Even though two areas that are both red on the map are likely to have a similar amount of earthquakes, the level of damage in the two areas will be different based on these other factors.

Applying Geological Hazard Maps

So once these maps are created, what do engineers do with them? Although it might seem useful to predict when to evacuate based on these maps, geological hazard maps are actually usually used to design infrastructure in areas prone to earthquakes. Even if everyone would evacuate for a large earthquake the damage would be catastrophic without correctly designed structures.In order to prevent a catastrophe, buildings and infrastructure must be designed to withstand earthquakes in regions that have previously experienced large earthquakes and have a high risk on geological hazard maps.It turns out that not all earthquakes affect structures the same.

Earthquakes with high frequency shaking are more likely to affect small structures, where as earthquakes with long, low frequency shaking are more likely to raze high rise buildings. Geological hazard maps can be used to predict the frequency of shaking and thus be used to create buildings to withstand that particular frequency.

Lesson Summary

Earthquakes are shaking of the Earth due to tectonic plate movement. California is especially prone to earthquakes because it lies on the San Andreas fault. Scientists use past earthquake data as well as data about current plate movement to construct geological hazard maps that can be used to estimate the intensity of future shaking, frequency, probability and impact on local structures.

Engineers can use this information to design earthquake withstanding structures and infrastructure.

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