Cities in Earthquake Danger

Many of the world’s largest cities are built on active earthquake faults and remain in constant danger of being devastated by a major quake.

            Among cities exceeding 10 million in population located on or near a dangerous fault line are Los Angeles, San Francisco, Istanbul, Tokyo, Delhi, Manila, Karachi, Tehran, Mexico City, and Jakarta.

            Our shaking world experiences an average of 30,000 recorded earthquakes of all magnitudes each year.  Most are low-level events that may cause shaking but little or no damage.  But in the average year, 150 quakes measure 7.0 magnitude or higher, and it is these stronger ones that do major damage when the epicenter is close to a big city.  A case in point is the recent 7.3 magnitude earthquake near Port au Prince, Haiti, that killed over 200,000 people, left millions homeless, and caused billions of dollars in property loss.

            Istanbul and Manila are typical of the big cities at risk.  Both are situated on highly active fault lines, and the buildings in both were largely constructed without earthquake-safe building codes and subject to collapse under the stress of a major quake.

            Istanbul sits on the North Anatolian Fault, which runs east-west through the top of Turkey and marks the boundary between the Anatolian and Eurasian Tectonic Plates, with additional pressure from the motion of the African and Arabian Plates on the southern boundary of the Anatolian Plate.  In 1999, a 6.7 magnitude earthquake on the North Anatolian Fault 100 miles east of Istanbul killed 2,000 people in small towns along the Marmara Sea, and an additional 200 died in Istanbul, mainly in apartment buildings that collapsed because of construction below earthquake-safe standards.

            Manila is built on the Marikina Fault, part of the Philippine Islands fault system produced by the pressure from two opposing subduction zones.  The Pacific Plate slides under the Philippine Plate from the east, and the Eurasian Plate dips under the Philippine Plate from the west.  The compression has produced a chain of volcanoes that are among the world’s most active and most dangerous, and a web of complex and unstable fault lines.  A 7.0 or greater magnitude earthquake at a shallow depth within a few miles of Manila could result in hundreds of thousands of deaths and extensive property loss.

            Delhi, Karachi, Tehran, Mexico City, and Jakarta are in the same situation, located on active fault lines with many buildings subject to failure under the stress of a major earthquake.  Although a large earthquake in the immediate vicinity of Los Angeles, San Francisco, or Tokyo would produce extensive damage and some loss of life, these cities are probably better able to withstand a 7.0 or higher magnitude event than the others because of more stringent building codes.  They have had earthquake-safe building codes in place for new construction for many years, and thousands of older buildings have been retrofitted to earthquake safety standards.

            Fault lines are hundreds of miles long.  The North Anatolian is 1,000 miles long.  The San Andreas, a little over 800 miles long.  Most major fault lines have offshoots and associated fault lines in the same area caused by the compression of converging tectonic plates.  It is impossible to tell which exact point along any given fault line will be the epicenter of the next great earthquake, or when it will happen.  Hundreds of years of stored-up fault line stress can release suddenly at any point along any fault line at any time, causing slippage and earth movement.  Living on one is a gamble.