Shallow slow earthquakes to decipher future catastrophic earthquakes in the Guerrero seismic gap

Abstract

The Guerrero seismic gap is presumed to be a major source of seismic and tsunami hazard along the Mexican subduction zone. Until recently, there were limited observations at the shallow portion of the plate interface offshore Guerrero, so we deployed instruments there to better characterize the extent of the seismogenic zone. Here we report the discovery of episodic shallow tremors and potential slow slip events in Guerrero offshore. Their distribution, together with that of repeating earthquakes, seismicity, residual gravity and bathymetry, suggest that a portion of the shallow plate interface in the gap undergoes stable slip. This mechanical condition may not only explain the long return period of large earthquakes inside the gap, but also reveals why the rupture from past M < 8 earthquakes on adjacent megathrust segments did not propagate into the gap to result in much larger events. However, dynamic rupture effects could drive one of these nearby earthquakes to break through the entire Guerrero seismic gap.

海底地震計記録で読み解く地震空白域の将来 -メキシコ・ゲレロ州沖合の地震空白域のスロー地震活動の発見-
図: 本研究で明らかにしたゲレロ地震空白域の浅部スロー地震域(テクトニック微動発生域)、サイレントゾーン(非地震発生域)、小地震・スロー地震域。サイレントゾーンはプレート間の固着が弱く、常にゆっくりとずれ動いている可能性が高い。また、サイレントゾーンは、プレート境界形状の凹部に対応する可能性が本研究から見出されている。今回の研究では、ゲレロ地震空白域に海溝から海岸線付近まで固着が弱い領域が広がる可能性を示した。