Engineering Seismology

Summary

Engineering seismology is a scientific field focused on studying the likelihood of future earthquakes and the potential damage such seismic activity can cause to buildings and other structures. Engineering seismology utilizes computer modeling, geological surveys, existing data from historical earthquakes, and other scientific tools and concepts. Engineering seismology is particularly useful for the establishment of building codes and for land-use planning.

Definition and Basic Principles

Engineering seismology (also known as earthquake engineering) is a multidisciplinary field that assesses the effects of earthquakes on buildings, bridges, roads, and other structures. Seismology engineers work in the design and construction of structures that can withstand seismic activity. They also assess the damages and effects of seismic activity on existing structures. Engineering seismologists analyze such factors as quake duration, ground motion, and focal depth in assessing the severity of seismic events and how those events affect fabricated structures. They also consider source parameters, which help seismologists zero in on a seismic event's location and the speed and trajectory at which the quake's resulting waves are traveling.

Earthquake engineers also study theoretical concepts and models related to potential earthquakes and historical seismic events. Such knowledge can help engineers and architects design structures that can withstand as powerful an earthquake as the geographic region has produced (or possibly will produce). Mapping systems and programs and mathematical and computer-based models are essential to engineering seismologists' work. Such techniques are also useful for archeologists and paleontologists, both of whom may use engineering seismology concepts to understand how the Earth has evolved over millions of years and how ancient civilizations were affected by seismic events.

Background and History

Throughout history, humans have struggled to understand the nature of earthquakes and have faced the challenges of preparing for these seismic events. Some ancient civilizations attributed earthquakes to giant snakes, turtles, and other creatures living and moving beneath the Earth's surface. In the fourth century BCE, Aristotle was the first to speculate that earthquakes were not caused by supernatural forces but rather were natural events. However, little scientific study on earthquakes took place for hundreds of years despite many major seismic events (including the eruption of Mount Vesuvius in Italy in 79 CE, which was preceded by a series of earthquakes).

In the mid-eighteenth century, however, the British Isles experienced a series of severe earthquakes, which created a tsunami that destroyed Lisbon, Portugal, killing tens of thousands of people. Scientists quickly developed an interest in cataloging and understanding seismic events. In the early nineteenth century, Scottish physicist and glaciologist James D. Forbes invented the inverted pendulum seismometer, which gauged not only the severity of an earthquake but also its duration.

Throughout history, seismology has seen advances that immediately followed significant seismic events. Engineering seismology, which is proactive, represents a departure from reactionary approaches to the study of earthquakes. Today, engineering seismology uses seismometers, computer modeling, and other advanced technology and couples it with historical data for a given site. The resulting information helps civil engineers and architects construct durable buildings, bridges, and other structures and assess the risks to existing structures posed by an area's seismic potential.

How it Works

To understand engineering seismology, one must understand the phenomenon of earthquakes. Earthquakes may be defined as the sudden shaking of the Earth's surface as caused by the movement of subterranean rock. These massive rock formations (plates), resting on the Earth's superheated core, experience constant movement caused predominantly by gravity. While some plates move above and below one another, others come into contact with one another as they pass. The boundaries formed by these passing plates are known as faults. When passing plates lock together, stored energy builds up gradually. The plates eventually give, causing that energy to be released and sent from the quake's point of origin (the hypocenter) outward to the surface in the form of seismic (or surface) waves. Such waves occur either in a circular, rolling fashion (Rayleigh waves) or in a twisting, side-to-side motion (Love waves).

The field of seismology has developed only over the last few centuries, largely because of major, devastating seismic events. The practice of engineering seismology has grown in demand in the twenty-first century, mainly because of the modern world's dependency on major cities, infrastructure (such as bridges, roadways, and rail systems), and energy resources (including nuclear power plants and offshore oil rigs). Earthquake engineers, therefore, have two main areas of focusstudying seismology and developing structures that can withstand the force of an earthquake.

To study seismic activity and earthquakes, engineering seismologists may use surface-based detection systems, such as seismometers, to monitor and catalog tremors. They also employ equipment—including calibrators and accelerometers—that is lowered into deep holes. Such careful monitoring practices help seismologists and engineering seismologists better understand a region's potential for seismic activity.

When earthquakes occur, engineering seismologists quickly attempt to locate the hypocenter and the epicenter (the surface point that lies directly above the hypocenter). They are able to do so by monitoring two types of waves—P and S waves—that move much quicker than surface waves and, therefore, act as precursors to surface waves. These engineers also work to determine the magnitude (a measurement of an earthquake's size) of the event.

Magnitude may be based on a number of key factors (or source parameters), including duration, distance to the epicenter and hypocenter, the size and speed of the surface waves, the amount of energy (known as the stress drop) that is released from the hypocenter, P and S waves, and the directions in which surface waves move (the wave propagation path). Analyzing an earthquake's magnitude provides an accurate profile of the quake and the conditions that caused it.

In addition to developing a profile of a region's past seismic activity, earthquake engineers use such information to ascertain the type of activity a geographic region may experience in the future. For example, scientific evidence suggests that the level of stress drop is a major contributor to the severity of seismic activity that can cause massive destruction in major urban centers. Similarly, studies show that the duration of ground motion (the “shaking” effects of an earthquake) may be more of a factor in the amount of damage to buildings and other structures than stress drop.

The field of engineering seismology is less than one century old, but in the twenty-first century, it plays an important role in urban development and disaster prevention. Earthquake seismologists work with civil engineers and architects to design buildings, roads, bridges, and tunnels that may withstand the type of seismic activity that has occurred in the past.

Applications and Products

Engineering seismology applies knowledge of seismic conditions, events, and potential to the design and development of new and existing fabricated structures. Among the methods and applications employed by earthquake seismologists are the following:

Experimentation. Engineering seismologists may construct physical scale models of existing structures or proposed structures. Using data from a region's known seismic history, the engineers attempt to re-create an earthquake by placing these models on so-called shake tables, large mechanical platforms that simulate a wide range of earthquake types. After the “event,” engineering seismologists examine the simulation's effects on the model structure, including its foundations, support beams, and walls. This approach enables the engineers and architects to directly examine the pre- and postsimulation structure and determine what sort of modifications may be warranted.

Computer Models. One of the most effective tools utilized by engineering seismologists is computer modeling. Through the application of software, engineering seismologists can input a wide range of source parameters, ground motion velocities, wave types, and other key variables. They also can view how different structural components withstand (or fail to withstand) varying degrees of seismic activities without the expense and construction time of a shake table. Computer modeling has become increasingly useful when attempting to safeguard against earthquake damage to dams, nuclear power plants, and densely developed urban centers. Computer modeling is also used by engineering seismologists to predict the path of destruction that often occurs after an earthquake, destruction such as that caused by fires or flooding.

Seismic Design Software. Earthquake engineers study seismic activity in terms of how it affects structures. To this end, engineers must attempt to predict how earthquakes will strike an area. Seismic design software is used to create a map of a region's seismic activity and how those conditions will potentially cause structural damage. The software enables government officials to establish formalized building codes for buildings, bridges, power plants, and other structures. This software is easily obtained on the web and through the US Geological Survey (USGA) and other organizations.

Mathematics. Engineering seismology is an interdisciplinary field that relies heavily on an understanding of physics and mathematics. To quantify the severity of earthquakes, to calculate the scope of seismic activity, and, in general, to create a profile of a region's seismic environment, engineering seismologists utilize a number of mathematical formulae. One of the most recognized of these formulae is the Richter scale, which was developed in 1935 by American seismologist and physicist Charles Richter. The Richter scale uses a logarithm to assign a numerical value (with no theoretical limit) to establish the magnitude of an earthquake. The Richter scale takes into account the amplitude (the degree of change) between seismic waves and the distance between the equipment that detects the quake and the quake's epicenter.

Earthquake engineers use such mathematical data as part of their analyses when working with civil engineers on construction projects. Earthquake engineers also are increasingly called upon by government officials to use this data to assess individual structure and citywide structural deficiencies that resulted in earthquake destruction. Forensic engineering was called into service in 2009 when Australian emergency officials intervened in Padang, Indonesia, after a magnitude 7.6 quake devastated that city. Engineers used mathematical formulae and statistical data to assess system-wide structural deficiencies in Padang rather than analyzing damage on a structure-by-structure basis. In light of the countless variables involved with studying earthquakes and their effects on fabricated structures, the use of established logarithms, data sets, and mathematical formulae is a time-honored practice of engineering seismologists.

Sensors. Not all earthquakes cause immediate and significant damage to affected structures. The Greater San Francisco area in California experienced more than eighty earthquakes in 2015 alone, with the majority of those quakes registering between 2.0 and 4.0 on the Richter scale. However, seismic activity on a small but frequent basis can cause long-term damage to structures. For example, seismic events can shift soil pressure on underground structures (such as pipes and foundations). Earthquake engineers are, therefore, highly reliant on sensor equipment, which enables them to gauge the effects of frequent seismic activity not only on above-ground structures but also on the ground itself.

To examine shifts in soil pressure caused by seismic activity, seismology engineers used an array of tactile pressure sensors, which were originally designed for artificial intelligence systems but were later utilized for the purposes of designing car seats and brake pad systems. The use of such equipment helps engineers study the long-term effects of seismic activity on water pipes, underground cables, and underground storage tanks.

One of the best-known types of seismic detector systems is the seismograph. The seismograph uses a pendulum-based system to detect ground motion from seismic activity. Originally, the modern seismograph was designed to detect only significant earthquakes and tremors, and it could be found only in stable environments (such as a laboratory). Modern seismographs are available in many types. Some may be placed underground, others can be used in the field, while others are so sensitive that they can detect distant explosions or minute tremors.

Global Navigation Satellite System–Acoustic (GNSS-A) Positioning Technology. GNSS-Acoustic uses seafloor geodetic information and satellite radio wave data to determine slip deficit rate (SDR) in a region and for the detection of earthquakes. The GNSS-A technique has shortcomings in temporal resolutions in terrestrial observation.

Earthquake Early Warning (EEW) Systems Technology. The EEW detects and offers warnings before an earthquake and minimizes the damage and loss of life. It also provides benefits of psychological preparedness and activation of emergency plans and situational assessment. It uses real-time seismic instruments, data processing software, and seismic-source information for providing real-time warnings to the public.

Seismic-Cloaking Protection of Critical Infrastructure (SCPCI). The SCPCI involves the boring of tilted holes around a structure to prevent it from the earthquake by reflecting and diverting seismic waves and reducing the ground motion.

Social Context and Future Prospects

The study of earthquakes dates back hundreds of years. Earthquake engineering, however, represents an evolution toward a practical application of the study of seismic activity to the design and construction of large buildings, power plants, and other structures.

Engineering seismologists work closely with seismologists and civil engineers. On the former front, these engineers help design and operate detection equipment and systems to help explain seismic activity. This collaboration is critical, as improved knowledge of seismic activity can save lives and property.

For example, Japan has long utilized engineering seismology practices in its urban centers. The magnitude 8.9 earthquake in that country in March 2011 did not devastate Tokyo because of strict building codes that, among other things, cause skyscrapers to sway with the region's seismic waves rather than stand in a rigid fashion. Comparatively, the magnitude 7.0 Haiti earthquake in 2010 virtually flattened the country's capital, Port-au-Prince, and outlying areas, mainly because Haiti did not have earthquake-safe building codes. Its buildings were built using insufficient steel and on slopes with no reinforcing foundation or support systems. One observer in Haiti reported that Port-au-Prince would likely not have survived even a magnitude 2.0, much less the 7.0 quake it did have.

The significance of the 2011 Japan disaster, the rare 5.8 Virginia earthquake that struck the East Coast of the United States in August 2011, the magnitude 7.8 to 8.1 Nepal earthquake in 2015 that killed almost nine thousand people, and the 7.8 magnitude 2023 Turkey-Syria earthquakes that killed over 53,500 people continues to cast light on the need for earthquake engineering in structural design and construction. As regions with a history of major seismic activity (and regions with the potential for such activity) continue to grow in size and population, engineering seismologists are likely to remain in high demand.

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