For hundreds of years, people speculated, but mostly feared when the Earth began to shake.
That is, until the decade of peace and love, the 1960’s, when meteorological experts were finally able to agree upon a theory as to the causation of earthquakes.
The Theory of Plate Tectonics
This widely accepted theory postulates that the surface of the Earth is constructed with slabs of solid rock, aka ’plates’, that shift relative to each other. As these plates shift, they slide against one another. The places where plates are in contact with each other is called a ‘fault.’
Fault lines are plate boundaries with rough edges that tend to clamp together while other plates continue to shift. When plate movements occur, the force of energy that causes the plates to move is continuously retained, because the shift in plates has not happened.
Eventually, however, the built-up force overcomes the friction created by the jagged edges that define the fault. Ultimately, this built-up strain is released like the snap of an elastic band. When the waves of energy reach the surface, an earthquake occurs.
The earthquake’s epicenter can be found on the earth’s surface – directly above the hypocenter (the place where the earthquake actually occurred). To understand the mechanics of the earthquake, scientists study the nature of the waves the earthquake has produced.
After analyzing earthquakes’ waves over many decades, seismologists finally agree that earthquake wave patterns are emitted as follows:
The closer one is to the epicenter, the closer the waves will occur to one another.
Scientists use the method known as ‘triangulation’ (blending data from three seismographs) to calculate the exact location of the earthquake’s epicenter.
Throughout the 20th century, scientists have begun to create sophisticated earthquake modeling programs. However, at present, no one meteorological modeling theory brings together the massive amounts of data required to accurately predict an earthquake. Required data stems from an understanding of geological conditions at a depth where the earthquake may happen, like:
- The rock material
- The minerals
- The fluids
- The temperatures
- The pressures
Earthquakes can be created within scientific laboratories where conditions can be controlled and easily observed, however, the earthquake conditions created may not resemble the deep complicated faults that exist at the earth’s crust. As a general rule, earthquake observations typically happen at a distance, and are studied through an analysis of ground movement and seismic waves.
The art of predicting earthquakes will become more precise when scientists have access to data that includes:
- How earthquakes occur?
- What happens just before an earthquake?
- What happens during the start of an earthquake?
- Is there some signal to observe that alerts earthquake experts that a quake is imminent?
At present, scientific experts do not, unfortunately, understand any of these data points.
What is known is that an earthquake begins (technically known as nucleates) in an isolated portion of a fault line, which quickly grows in size and strength. Nucleation can happen at any point on the fault line, and even when a series of earthquakes occur, they may nucleate in different places. It is likely that the processes required to create an earthquake appear in extremely surreptitious ways, hidden by many miles upon miles of solid rock. Accurately predicting an imminent earthquake is ridiculously difficult because scientists have no idea even where they are supposed to be looking.
Earthquakes threaten nearly 75 million people in the United States, across nearly 40 states. In spite of the prediction challenges noted above, scientists at the United States Geological Survey (USGS) are vigilantly working to develop methods to offer better prediction results.
This is done by:
- Creating earthquakes in laboratory settings
- Drilling boreholes deep within the San Andreas Fault to study conditions deep within the earth
- Study ground transformation with the use of GPS sensors
Research such as this facilitates the creation of early warning signal systems that would provide people with a few moments or minutes to seek shelter and safety – away from bridge, tunnels and public transportation.
However, a reliable earthquake prediction model is, at present, just a scientist’s postulation. So if scientists can’t predict the timing or strength of an earthquake, there are other ways to apply research data to ensure our cities, towns and citizens are better protected.
Historical data regarding past earthquakes comes from several sources. However, these historical records tend to be incomplete. Other data sources include the measurements of the movements of mad-made structures that are accurately dated, such as:
- The walls of a castle, or
- The Great Wall of China
Faults that have impacted the Great Wall of China have been documented in this manner.
Prediction vs. Forecasting
Earthquake experts cannot pinpoint the time and size of an earthquake. But they do have the ability to estimate the odds of an earthquake occurring in an area over a few decades. This is accomplished by determining just how fast a fault slides over the long term. This measurement is typically a few millimeters or a few centimeters over the length of one year.
The art of prediction is easier if one knows when the last earthquake occurred. For example, the Hayward fault, just east of San Francisco has, over history, produces a significant earthquake about every 150 years or so. The latest earthquake on the Hayward Fault occurred in the year 1868. Logic dictates that the next large quake will occur in 2018 – 150 years after the last quake. However, it is important to note that this predictive estimate is somewhat general.
By combining earthquake ages with the size of the damaged areas, it is possible to understand earthquake patterns over hundreds or perhaps, thousands of years. Scientists use this information as a guideline for future behavior, but it is clear that the faults do not slip after the same period of time between earthquakes (known as The Recurrence Interval).
An earthquake does not necessarily breach the exact same place from one earthquake to the next. The reality is a quake that releases energy along one specific fault, may, by its very nature, increase the seismic stress on an adjacent area, increasing the odds of an earthquake for that specific region.
It is not prudent to take a naively optimistic position about improving the art of earthquake predictions, however, most research on the past and present behavior of active fault lines should be analyzed and shared with seismologists working towards a better prediction model.