SEISMOLOGICAL CONSTRAINTS ON THE MOTION OF THE CENTRAL AMERICA FOREARC SLIVER

The Central America forearc sliver is located between the Central America volcanic arc and the Middle America trench. It is well known that the volcanic arc is the site of medium- to large-sized (maximum magnitudes of 6.9) shallow earthquakes, with strike-slip mechanism. This has led many authors to suggest that the forearc sliver is being detached from the Caribbean plate and transported to the northwest. They invoke oblique plate convergence as the driving mechanism.

We have examined earthquakes along the Middle America trench to see whether this hypothetical translation is reflected in the earthquake activity along this convergent margin.

We found that, for thrust-faulting earthquakes (i.e., those which reflect convergence between the Cocos and Caribbean plates):

• Earthquakes mechanisms are similar from the gulf of Tehuantepec to Nicaragua.
• They show no strike-slip component.
• Earthquake slip vectors almost coincide with direction of plate convergence
• Trend of P-axes also coincide with plate convergence direction

Additionally, oblique plate convergence is small and not uniform.

These results lead us to suggest that:

• Oblique plate convergence is not the mechanism for earthquakes along the volcanic arc.
• The forearc sliver is probably not being detached from the Caribbean plate.
• It is likely that the forearc sliver continues into southeastern Mexico.