Bam Iran 2003 magnitude

Iran sits in the zone of convergence between the Arabian and Asian plates and has numerous mountain ranges that formed by folding and faulting of the rocks in the collision zone. There are many earthquakes in Iran, some of which are extremely destructive and have killed many people. For instance, in 893 an earthquake in Ardabil Iran killed an estimated 150,000 people, and other deadly earthquakes have stricken most regions of Iran, including the capital, Tehran. On December 26, 2003, the ancient Silk Road walled city and citadel of Bam, Iran, was leveled by a magnitude 6.7 earthquake that struck at 5:27 a.m., killing an estimated 27,000-50,000 people and injuring more than 20,000. Bam was in a region characterized by high seismicity and many earthquakes. In fact Bam had survived larger earthquakes in the past during its 2,000-year history without being destroyed, so many scientists were puzzled why a moderate-sized earthquake would totally destroy the city.

The earthquake was preceded by foreshocks on the afternoon of December 25, but since this area is characterized by high seismicity, residents were not alarmed and did not prepare for what was to come.

Many of Bam's residents returned home on Thursday evening, December 25, for the Friday holiday, and were woken at 4:00 a.m. on Friday morning by a strong foreshock that sent many residents into the streets. All seemed calm after a short while, so the residents returned indoors, and most were sleeping at 5:27 a.m., when the main earthquake hit, releasing most of its energy directly below Bam, leveling most of the ancient city that had withstood many earthquakes, drought, and seizure by roving warriors including Genghis Khan. Bam was the oldest walled city, originally founded during the Sassanian period (250 b.c.e.), but much was built in the 12th century, and more in the Safavid period between 1502 and 1722. The walled city included about 4 square miles (6 km2), including more than 10,000 buildings, and was surrounded by 38 towers. Most of the buildings were made of mud bricks, clay, and straw and were not reinforced; hence when the shaking was most intense, these buildings collapsed, burying the residents inside.

Why was Bam destroyed by a magnitude 6.7 earthquake, when it had survived larger earthquakes in the past? This example illustrates that every earthquake is different in terms of where and how its energy is released, and Bam had been fortunate in the past. This event was relatively shallow, and the earthquake focus (the point where the energy was first released) was below Bam. The energy was focused, or directed, by surrounding rock structure directly at the old city, much like sound can be focused or directed by cupping one's hand around the mouth, or by walls in a city or canyons in the wilderness. This earthquake released most of its energy directly toward Bam, destroying the city.

Continue reading here: Kobe Japan 1995 magnitude

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