This week, 4-10 April 2014, saw 106 earthquakes Magnitude 4.5+ worldwide. As usual the concentration was around the western Pacific. This included Chile with 7 of the top 14 quakes in terms of magnitude.
The largest during the week at M6.1, and the most recent was the quake that rattled Nicaragua 10 km west of Valle San Francisco.
These were not just significant tremors. They demonstrated other similarities and a significant difference.
USGS Earthquake Alerts
The two-pronged USGS alert system includes both the physical and economic disruptions caused by a tremor.
For the physical impacts such as ‘feeling’ the tremor, they use a shake map with nine categories. The map focuses on the ground shaking produced by the earthquake with colors from white (not felt) through to reds (extreme shaking).
As the USGS points out, these preliminary ground shaking maps are normally posted within several minutes of the earthquake origin time with acceleration and velocity values raw and not checked by humans.
For estimates of projected human fatalities and economic losses resulting from structural damage, colors are used along a logarithmic scale (1, 10, 100 etc.) from a low shown in green through yellow and brown to reds.
M6.1 – 10km W of Valle San Francisco, Nicaragua
This tremor occurred at a depth of 13 km (8.1 m) and close to populated areas. It was quickly followed by an aftershock about 20 miles to the south. This is a complex area of subduction of oceanic lithosphere as the Coco plate meets the Caribbean plate.
The Christian Science Monitor reported that the M6.1 earthquake shook homes and injured a reported 23 people in the town of Nagarote.
The USGS rated the quake a yellow alert level. Their assessment is that overall, the population in this region resides in structures that are vulnerable to earthquake shaking, though some resistant structures exist. The predominant vulnerable building types are adobe block and mud wall construction.
Iquique Chile Aftershocks
The main quake hit the area back on April 1 and created a tsunami. This was followed by mid sized quakes including a M6.0 on April 10. These quakes are associated with subduction along the boundary of the Nazca and South American Plates.
The USGS rated these quakes a green alert level. Their assessment is that overall, the population in this region resides in structures that are resistant to earthquake shaking, though some vulnerable structures exist.
Vulnerability to Earthquakes
The two areas are both prone to earthquakes, and in this case as a result of subduction. But the two areas are quite different in their vulnerability. Nicaragua, a poor country with a GNI /capita of approximately $1 650 is much poorer than Chile with a GNI/capita of approximately $14 310.
Compare this further to the United States with a GNI/capita of approximately $52 340. Poor countries such as Nicaragua are far less able to survive significant tremors than more affluent countries.