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[SCG40-38] An interpretation of low residual gravity anomaly and its relationship with slow-to-fast earthquakes in the Guerrero Seismic Gap, Mexico
Keywords:tectonic tremor, residual gravity anomaly, subducting seamount, porosity
The physical characteristics of an overriding plate are likely to exert a significant influence on the occurrence of both slow and fast earthquakes. Specifically, within the shallow portion of the subducting margin, the material atop the plate interface, particularly an accretionary wedge, may experience damage from the seamount subduction beneath it. Recent numerical simulations, such as those conducted by Sun et al. (2020), have suggested that the presence of seamounts can lead to an increase in porosity within highly under-consolidated accretionary wedges. This indicates that variations in physical properties within the accretionary wedge could be crucial in controlling slip behavior along the plate interface, including phenomena such as tectonic tremors, slow slip events, tsunami earthquakes, and fast earthquakes. Therefore, gaining a comprehensive understanding of the physical properties of the plate interface is essential for elucidating the mechanisms underlying slow and fast earthquakes.
The Guerrero seismic gap represents a particularly promising area for investigating the relationship between the physical properties of the accretionary wedge and the occurrence of slow-to-fast earthquakes. Notably, no megathrust earthquakes have been recorded in the Guerrero seismic gap over the past century. However, episodic shallow tremors and possible slow slip events have been observed near the trench, along with the occurrence of a Mw 6.7 earthquake in 2002. This earthquake exhibited a prolonged source time function of over 90 seconds and relatively low seismic energy, characteristics consistent with those of tsunami earthquakes. Both the tectonic tremors and the 2002 tsunami earthquake occurred near the trench, where a positive residual gravity anomaly of approximately 50 mGal was observed. Interestingly, the slip distribution of the tsunami earthquake corresponds with this high anomaly, and tectonic tremors are clustered around it, suggesting a potential connection to the subducting seamounts as inferred from bathymetric data. This implies that the 2002 tsunami earthquake may have occurred beneath a damaged or low-rigidity accretionary wedge situated above the subducting seamount.
In contrast, the downdip portion of the high residual gravity anomaly exhibits an extremely low residual anomaly of around -50 mGal, encompassing much of the Guerrero seismic gap. Within this negative residual anomaly area, minimal seismic activity is observed in both slow and fast earthquakes, as identified using machine learning and matched filter techniques. One plausible interpretation of this negative residual gravity anomaly is that it signifies an increase in porosity within the accretionary wedge resulting from past subduction events involving seamounts. In this scenario, the presence of an under-consolidated wedge with low density or high porosity may serve to impede the occurrence of both fast and slow earthquakes.
The Guerrero seismic gap represents a particularly promising area for investigating the relationship between the physical properties of the accretionary wedge and the occurrence of slow-to-fast earthquakes. Notably, no megathrust earthquakes have been recorded in the Guerrero seismic gap over the past century. However, episodic shallow tremors and possible slow slip events have been observed near the trench, along with the occurrence of a Mw 6.7 earthquake in 2002. This earthquake exhibited a prolonged source time function of over 90 seconds and relatively low seismic energy, characteristics consistent with those of tsunami earthquakes. Both the tectonic tremors and the 2002 tsunami earthquake occurred near the trench, where a positive residual gravity anomaly of approximately 50 mGal was observed. Interestingly, the slip distribution of the tsunami earthquake corresponds with this high anomaly, and tectonic tremors are clustered around it, suggesting a potential connection to the subducting seamounts as inferred from bathymetric data. This implies that the 2002 tsunami earthquake may have occurred beneath a damaged or low-rigidity accretionary wedge situated above the subducting seamount.
In contrast, the downdip portion of the high residual gravity anomaly exhibits an extremely low residual anomaly of around -50 mGal, encompassing much of the Guerrero seismic gap. Within this negative residual anomaly area, minimal seismic activity is observed in both slow and fast earthquakes, as identified using machine learning and matched filter techniques. One plausible interpretation of this negative residual gravity anomaly is that it signifies an increase in porosity within the accretionary wedge resulting from past subduction events involving seamounts. In this scenario, the presence of an under-consolidated wedge with low density or high porosity may serve to impede the occurrence of both fast and slow earthquakes.