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Mapping Magnitude: The Evolution of Earthquake Maps

This is a guest post by Sonia Kahn, Processing Technician in the Geography and Map Division.

Did you know that worldwide, roughly 55 earthquakes are recorded per day? Of course, the vast majority of these seismic events are minor, making it all the more impressive that we are able to detect them. The technology used to gather data on earthquakes and seismic movements has vastly improved over time, and with it the mapping of earthquakes has also advanced. Today the United States Geological Survey (USGS) provides an interactive map which displays the precise coordinates, time, and magnitude of the latest earthquakes, not just in the US, but worldwide. Modern day GIS technology has helped revolutionize the way in which, and speed with which, we can map seismic events, but humans have been recording tremors and attempting to map them for centuries.

The earliest known records of earthquakes are from China, dating as far back as 1831 BC. It was nearly 3,600 years later that John Michell, a British engineer, managed to identify the cause of earthquakes as rock moving deep below the surface of the Earth. Early maps showing areas of seismic activity use a gradated effect to show the intensity and frequency of quakes in different regions. The location of volcanoes was also often featured on early seismic maps as it was originally thought that earthquakes and volcanic eruptions were successive phenomena. Further research would eventually reveal that while earthquakes have the potential to cause volcanic eruptions, and similarly, the movement of magma during an eruption can trigger an earthquake, they are independent events. The commonality between these disasters is that both often occur in similar areas, on the borders of the Earth’s tectonic plates.

This mid-19th century map of the world shows seismic intensity with varied degrees of shading. It also includes the location of volcanoes. Seismographic map of the world, showing the surface distribution in space of earthquakes as discussed from the British Association catalogue by Robert Mallet. Made by G. Falkner, 1857. Geography and Map Division.

In a similar fashion, this undated map of the Philippines uses shading to show the regions with the most frequent number of earthquakes. Distribucion de Temblores. Made by Observatorio de Manila, Islas Filipinas, no date. Geography and Map Division.

Featured here are several maps from a Report on Earthquake Observations in Japan produced by the Central Meteorological Observatory of Japan in 1892. Volcanoes continued to be included on earthquake maps until the turn of the century when the scientific community came to a general consensus that volcanic events occurred independent of earthquakes and vice versa.

From Reports on Earthquake Observations in Japan. Central Meteorological Observatory of Japan, 1892. Geography and Map Division.

The advent of the modern seismograph in 1890 revolutionized our understanding of earthquakes once again as it allowed humans to record ground motion during an earthquake. Seismographs are placed in the ground, and when an earthquake strikes, the device moves along with the land. The resulting recording is known as a seismogram, and depicts earthquake intensity based on the amplitude of the waves generated while the seismograph was in motion.

This notated example of a seismogram was published in a 1906 book following the massive earthquake which struck San Francisco earlier that year. After earthquake and fire. San Francisco, Mining and scientific press, 1906. Library of Congress.

In measuring ground displacement, seismographs offered a more quantitative measure of intensity than previously available. However, the Richter scale that many are familiar with today, and the concept of earthquake magnitude was not developed for another 45 years. While a seismograph measures ground movement, magnitude is a measure assigned to the size of the earthquake. In 1935 seismologist Charles Richter postulated that if one knew the location of a seismograph from the epicenter of an earthquake, that information could be used to calculate the relative strength of the tremor based on the amplitude of the waves output by the seismograph. Today the idea of magnitude has evolved from Richter’s initial conception and now measures not just the size of the waves generated by a seismograph but also takes into consideration physical features of the quake such as the distance of the fault displacement and the quality of the ground (soft or hard) that shifted.

This map of northern California depicting the earthquake intensity of San Francisco’s ground breaking 1906 earthquake uses an early seismic intensity scale known as the Rossi-Forel scale. The Rossi-Forel scale incorporates seismographic readings, noting that lower levels of intensity (I and II) are liable to be recorded by seismographs but felt by very few people. Map of California and Nevada showing the distribution of apparent intensity in the region affected by the earthquake of April 18, 1906. Made by the California State Earthquake Investigation Commission, 1906. Geography and Map Division.

In comparison, this much more modern map of California indicates the exact location of earthquake epicenters and also includes their precise magnitudes. Earthquakes and faults in the San Francisco Bay Area (1970-2003). United States Geological Survey, 2003. Geography and Map Division.

Twenty-first century technology now allows us to map seismic events and their relative intensity nearly instantly. Modern GIS data enables us to determine the precise coordinates and depth of an earthquake epicenter. Gone are the days of shaded maps of relative earthquake frequencies. Still, what has remained true throughout history, since well before earthquakes were ever mapped, is our inability to predict when, where, and how strong the next earthquake will be. What we do know for certain is that when the next one strikes, we’ll be able to map it.

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