Tidal modulation of continuous nonvolcanic seismic tremor in the Chile triple junction region

Abstract

We located continuous seismic tremor with coherent amplitude wave trains in the Chile ridge subduction region (~46.5 S) in two clusters north and south of the Chonos Archipelago, between the Chile trench and the North Patagonian fore arc. Tremor persisted from December 2004 to February 2007 (the entire period of the Chile Ridge Subduction Project temporary seismic deployment), and lasted >17 h on six occasions. Tremor in the more active southern cluster reached a maximum duration of 48 h, and we observed no more than 3 continuous days without tremor activity. The cluster locations coincide with the surface projections of subducted transform faults formed at the Chile ridge. We also detected simultaneous, colocated low-frequency microearthquakes with well-defined impulsive waves within the tremor signals distributed from the surface to 40 km depth, suggesting tremors and earthquakes are part of the same process. The periodicity of tremor duration is strongly correlated with semidiurnal, diurnal, and long-period tides, M2, N2, K1, O1, P1, and Mm (12.421 h, 12.000 h, 23.934 h, 25.819 h, 24.066 h, and 27.555 days, respectively). We found a significant correlation between tremor occurrence and Earth tides when tidal stress is calculated for the slip plane of a right-lateral strike-slip fault with strike N95 E, which is near parallel to subducted transform faults (N78 E) of the Chile ridge, indicating that the very small stresses resulting from the combination of ocean loading and solid Earth tides (~1 kPa) are sufficient to facilitate or suppress tremor production; tremors occur when shear stresses are maximum and wane or are low when shear stresses are minimum.