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AIR Worldwide : M6.6 earthquake strikes China’s Sichuan Province, buildings leveled in rural villages

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According to catastrophe modeling firm AIR Worldwide, A strong earthquake shook a largely rural region of China’s Sichuan province on April 20 at 8:02 AM local time (0:02 UTC). The USGS initially issued a preliminary moment magnitude estimate of 6.9, while Chinese authorities put the surface wave magnitude at 7.0 (note that the China Earthquake Administration uses the Beijing surface wave magnitude, which is different from the surface wave magnitude as determined by other international seismological agencies). The USGS has since revised the moment magnitude to 6.6, and the focal depth has been estimated at about 12.3 km (7.6 miles), making this a shallow event.

The earthquake has caused numerous landslides and significant property damage in small villages in the mountainous region near the epicenter. More than 7,000 rescuers are on the ground conducting search and rescue operations, but officials indicate that some remote areas remain unreachable because of blocked roads and disrupted communications.

According to AIR, the quake occurred about 114 km (71 miles) west-southwest of Chengdu, the provincial capital and home to more than 14 million people. It struck the same region as the powerful M7.9 Wenchuan earthquake in 2008. Residents in Chengdu felt the shaking from today’s event, but no damage has been reported there.

According to AIR, preliminary indications suggest that today’s earthquake was a dip-slip event on the Longmenshan fault, a thrust fault that runs between the eastern edge of the Tibetan Plateau and the Sichuan Basin. The seismicity in this highly active region is dominated by the convergence between the India plate against the Eurasia plate, which move relative to each other at a rate of 40 to 50 mm/year.

The Longmenshan fault was also the source of the M7.9 event in 2008, which occurred approximately 85 km northeast of today’s earthquake. At the time, AIR noted that the southernmost segment of the Longmenshan fault had not ruptured during or after the Wenchuan quake, leading to speculation that the probability of an earthquake occurring on this segment might be high resulting either from previous accumulation of high stress or from stress transfer from the Wenchuan rupture. The shallow depth of this quake and the dense, hard rock that underlies this region mean that seismic waves can travel quickly and efficiently.

According to AIR, the quake occurred in a rural, mountainous region in Lushan county. The county seat and closest city, Ya’an (estimated population of 1.5 million), which is 25 km from the epicenter, appears to have avoided major damage. This is likely a result of its distance from the event and because confined masonry and reinforced concrete are more commonly used in larger towns and cities in the region.

Small villages closer to the epicenter have not fared as well. Aerial photos taken by China’s military show extensive damage to low-rise, unreinforced masonry buildings. The village of Longmen was especially hard hit; officials reported that nearly all buildings were destroyed there.

Electrical service and telecommunications have been disrupted and water supplies compromised. Roads have been closed to all but emergency vehicles.