Human beings and many other living things inhabit Earth's outer crust. The crust is a brittle shell broken into major tectonic plates. These major plates are so large that they include continents as well as parts of the floor of the surrounding oceans. One important scientific observation for these major plates is their continuous movement. These gigantic plates move due to the convection currents induced from the heat dissipation from the interior parts of the Earth. Experts predict that every year these plates move approximately 1 to 10 centimeters. This continuous motion plays a significant role in the existence of life on Earth. It sustains the global carbon cycle from Earth's interior to the atmosphere. However, there is an undesired consequence of this beneficial system, especially for those of us living near plate boundaries earthquakes! And as we have seen recently in Japan and in Turkey, earthquakes and a possible ensuing tsunami can cause great damage and casualties.
The theory of plate tectonics explains what happens at plate boundaries. According to this theory, there are three primary plate boundary conditions; divergent, convergent, and transform boundaries (Figure 1). Divergent plate boundaries are characterized by ocean ridges and sea floor spreading; volcanoes are the most obvious setting. Here, a new crust is generated because the plates pull away from each other. Convergent plate boundaries are characterized by trenches and island arcs. In this setting, the crust is consumed in the Earth's interior as one of the plates dives under another. In the case of transform plate boundaries, the crust is neither produced nor destroyed, as plates horizontally slide past each other. Significant earthquakes can occur under all of these boundary conditions.
Figure 1: Illustration for main types of plate boundaries (Cross section by José F. Vigil from This Dynamic Planet a wall map produced jointly by the U.S. Geological Survey, the Smithsonian Institution, and the U.S. Naval Research Laboratory) (1)
We typically associate Japan with earthquakes because we know that it is a very earthquake prone island. The state of Alaska in the United States is also earthquake prone. The total number of earthquakes in Alaska per year is greater than the total number of earthquakes in the rest of the United States. The examples of Japan and Alaska reveal that more earthquakes occur at locations close to the plate boundaries. On a global scale, Japan, the Philippines, Indonesia, Chile, and western United States, are located along the so-called "Pacific Ring of Fire," where about 90% of the world's earthquakes and 80% of the world's largest earthquakes occur (Figure 2).
Figure 2: Pacific ring of fire a zone of frequent earthquakes and volcanic eruptions
The magnitude of an earthquake is a representation of the total amount of energy released by the event. Typically, it is measured using the recorded ground oscillations from a seismogram. However, the interpretation of the magnitude is not straightforward because the magnitude scale is logarithmic. For instance, a magnitude 7.0 earthquake produces approximately 10 times more ground motion and releases about 32 times more energy compared to a magnitude 6.0 earthquake (2).
According to the statistics published by the US Geological Survey, every year on average 134 earthquakes with magnitudes 6.0 to 6.9 occur worldwide, 17 earthquakes with magnitudes 7.0 to 7.9, and at least one large earthquake with a magnitude greater than 8 (Figure 3). Further, the number of earthquakes of magnitude 7.0 or greater has remained fairly constant but the number of moderate earthquakes (i.e., 6.0 or less) appears to be increasing. According to experts at the US Geological Survey, a partial explanation may lie in the fact that there is a tremendous increase in the number of seismograph stations in the world over the last twenty year. Thus, the actual number of earthquakes has not increased, but our ability to detect them. In scientific terms, this is referred to as reporting bias (3). When it comes to myths about earthquake activity related to weather and time, scientists rejects any connection. Earthquakes occur whether it is warm or windy, early in the morning or late at night.
Figure 3: Earthquake facts and statistics (a) 1980 1989, (b) for 1900 - 1999, and (b) for 2000 - 2009 (Retrieved from USGS National Earthquake Information Center (2))
In the United States, earthquakes are one of the most significant natural hazard for around 75 million Americans living in 39 states, including the state of California where the majority of the state's population lives within 32 km of active faults. Historically, the region has been very active (Figure 4). To help predict earthquakes in California, a multidisciplinary group of scientists and engineers from various disciplines established a team entitled Working Group on California Earthquake Probabilities (WGCEP). The team had a very ambitious objective to develop a comprehensive earthquake rupture forecast model for the state of California using the best available science. The details of the sophisticated model are beyond the scope of this essay, but the recently released report (USGS Open File Report 2007-1437) is available for public access (5). In her essay entitled, "The big one is evitable. Catastrophe is not," Cathleen Decker, an editor of the Los Angeles Times, refers to the future predictions presented in the report as a "Chilling look into the future" (7). Based on historical evidence and scientific data, it is almost certain (with a 99% chance) that there will be at least one earthquake with magnitude 6.7 or greater in the state of California within the next thirty years. The likelihood of a more significant earthquake (magnitude 7.5 or greater) within the next thirty years in California is 46%. In the Greater Bay Area specifically (area includes large cities such as San Francisco, San Jose, Oakland), the probability of at least one earthquake with magnitude 6.7 or greater within the next thirty years is about 67% (6).
Figure 4: Epicenters of historic earthquakes greater than magnitude 5 recorded in the state of California since the 19th century (4)
The current state of science considerably reduces the risk of death and damage by making resources available to individuals, teachers, policy makers, and engineers, but unfortunately, science at this time can neither prevent nor predict the exact time when an earthquake will occur. Casualties, financial losses, and mental trauma are sometimes inevitable for earthquake victims. Social and emotional suffering are often not limited to actual victims, but to everyone who has access to the news. Unlike financial and material losses, the psychological consequences of an earthquake exposure are long lasting. To address these consequences, earthquake preparedness should include mental and social aspects of the disaster as well.
(1) This dynamic Earth The Story of Plate Tectonics by W. Jasquelyne Kious and Robert I. Tilling (Online edition) U.S. Department of the Interior, U.S. Geological Survey http://pubs.usgs.gov/publications/text/Vigil.html
(2) USGS Earthquake Hazard Program - Earthquake Facts and Statistics
(3) USGS Earthquake Hazard Program - Are Earthquakes Really on the Increase?
(4) California Geological Survey - Probabilistic Seismic Hazards Assessment - Historic Earthquakes
(5) The Uniform California Earthquake Rupture Forecast, Version 2 (UCERF 2) By 2007 Working Group on California Earthquake Probabilities, 2008
(6) USGS Earthquake Hazard Program - 2008 Bay Area Earthquake Probabilities
(7) Los Angeles Times (January 17, 2010)