Magnitude 6.5 earthquake hit Iguala, Mexico

A very strong earthquake magnitude 6.5 occurred near Iguala Mexico at 07:47 PM local time (01:47:26 UTC). Luckily the hypocenter was at a depth of 64.9 km (40.3 miles) according to USGS with epicenter 42 km (26 miles) SW of Iguala, Guerrero, 56 km (34 miles) ESE of Arcelia, Guerrero, 62 km (38 miles) NNW of Chilpancingo, Guerrero and 166 km (103 miles) SSW of Mexico City, Mexico (18.038°N, 99.796°W). USGS has revised the magnitude from 6.7 to 6.5.  The Mexican seismological service reports 6.8 magnitude with depth of 61 km. UNAM recorded 6.5 magnitude with depth of 58km and they said that earthquake is 30km far from USGS estimate – 17.84N, 99.98W. EMSC reports  about 6.7 magnitude with depth of 61 km.

The epicenter has been located in  Zumpango del Río following the Mexican seismological agency. Zumpagno del Rio is only 15 km from Chilpancingo, the city where the readers report comes from. The seismological information shows a huge difference between where the Mexican Seismological Service think the epicenter is, and where the USGS think the hypocenter is. Over 30km!

6.5 magnitude from UNAM (SSN), with 58km depth
6.5 magnitude from USGS, with 64km depth

The seismological service said that the shaking lasted 47 seconds. WAPMERR, the theoretical damage engine has calculated that fatalities are possible and that several thousand people might have been injured. USGS PAGER issued yellow alert for estimated fatalities (43%) and economical losses (41%). Latest reports talk about 3 victims so far. In Chilpancingo, several houses and businesses were damaged, while the Cathedral of the Assumption of Mary has a serious crack in one of the towers. Civil Protection has continued monitoring of damage, mainly in Zumpango del Rio, where the epicenter occurred. Helicopters are flying over for more reports, as people gradually have returned inside their buildings in a city that is ready to receive thousands of pilgrims for the annual celebration of the feast of Our Lady of Guadalupe.

This is a subduction earthquake where an Oceanic plate glides below the North American plate.  The depth of 64 km is perfectly normal for this kind of earthquakes.

From such an earthquake, the shaking in the soft soil (clay basin) of Mexico City, can sometimes be amplified many times, and could be the reason why we have seen damaged buildings in Mexico City despite expected low ground accelerations due to the distance from the epicenter. Mexico City was built on a lake originally, and hence the soft alluvial soils that magnify ground motion.

The epicentral region has been hit by many earthquakes in the past 110 years according to CATDAT – here are just some of the damaging earthquakes within 100km of the epicenter. Chilpancingo has had damage on average every 30 years.

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