11:45 AM - 12:00 PM
[SIT15-10] Introduction of slow earthquakes
★Invited Papers
Keywords:slow earthquake, slow slip event, tectonic tremor, diffusion process
The term "slow earthquakes" refers to transitional underground shear slip movements, in which both the slip rate and the extension speed of the slip region are extremely slow compared to typical earthquakes. These phenomena occur at various temporal and spatial scales, and due to the vast differences in scale, it is impossible to capture the complete picture of the phenomenon with a single geophysical observational method. Consequently, they have been referred to by different names depending on the observational method. The longest-period phenomena are called slow slip events, which are slow earthquakes captured by geodetic instruments such as GNSS and strainmeters. Seismologically, they are observed with signals at periods from 10 s to several hundred s and at high frequencies above 2 Hz. Due to the historical context of their discovery, the former are referred to as very low-frequency earthquakes, and the latter as tectonic tremors or low-frequency earthquakes. Tectonic tremors are considered to be successive occurrences of pulse-like low-frequency earthquakes. Although these phenomena are named differently, in many cases, they occur at almost the same location and at the same time, with the same deformation mechanism, i.e., shear slip consistent with regional tectonic deformation, which led to the hypothesis that they are a single phenomenon, a slow earthquake, emitting signals across an extremely broad frequency range.
Slow earthquakes have been observed worldwide. Regions such as the Nankai, Cascadia, and Mexican subduction zones, which were among the relatively early discoveries, suggested that the phenomenon might be related to the subduction of relatively young plates. However, subsequent discoveries in regions with older subducting plates, like New Zealand and the Japan Trench, as well as outside of subduction zones, such as the San Andreas Fault and the Alpine Fault, revealed that the phenomenon is not limited to a specific tectonic environment. It might be observable in most places with some tectonic deformations, if very sensitive observational instruments were available. Nevertheless, the regions of occurrence are slightly shifted from those of regular earthquakes. In western Japan, it is observed in a band-like zone on the edges of the locked plate interface, which is expected to generate giant earthquakes in the future. In contrast, in eastern Japan, there is a greater regional variation. Particularly in the source area of the M9 Tohoku-Oki earthquake in 2011, slow earthquakes were scarcely observed, suggesting that slow and regular earthquakes occur complementarily.
The occurrence of slow earthquakes is likely to be related to fluids. Seismic tomography studies showed a high Vp/Vs ratio around the slow earthquake regions. Additionally, the small stress changes during slow earthquakes are suggested by direct observations of slow slip events and the high sensitivity of tectonic tremors to tidal and external stress disturbances. One mechanism that enables shear slip with a small stress change under a high pressure is the reduction of effective normal stress due to pore fluids. The diffusion of fluid or fluid-related stress changes might control slow earthquakes. Indeed, various characteristics observed in slow earthquakes are often explained by diffusion equations, contrasting with regular earthquakes, which are explained by wave equations. Diffusion is a very fundamental physical process, and various movements governed by diffusion equations are likely occurring inside the Earth. Slow earthquakes might represent just the tip of the iceberg of these verious phenomena that are occurring inside the Earth but elude our observation.
Slow earthquakes have been observed worldwide. Regions such as the Nankai, Cascadia, and Mexican subduction zones, which were among the relatively early discoveries, suggested that the phenomenon might be related to the subduction of relatively young plates. However, subsequent discoveries in regions with older subducting plates, like New Zealand and the Japan Trench, as well as outside of subduction zones, such as the San Andreas Fault and the Alpine Fault, revealed that the phenomenon is not limited to a specific tectonic environment. It might be observable in most places with some tectonic deformations, if very sensitive observational instruments were available. Nevertheless, the regions of occurrence are slightly shifted from those of regular earthquakes. In western Japan, it is observed in a band-like zone on the edges of the locked plate interface, which is expected to generate giant earthquakes in the future. In contrast, in eastern Japan, there is a greater regional variation. Particularly in the source area of the M9 Tohoku-Oki earthquake in 2011, slow earthquakes were scarcely observed, suggesting that slow and regular earthquakes occur complementarily.
The occurrence of slow earthquakes is likely to be related to fluids. Seismic tomography studies showed a high Vp/Vs ratio around the slow earthquake regions. Additionally, the small stress changes during slow earthquakes are suggested by direct observations of slow slip events and the high sensitivity of tectonic tremors to tidal and external stress disturbances. One mechanism that enables shear slip with a small stress change under a high pressure is the reduction of effective normal stress due to pore fluids. The diffusion of fluid or fluid-related stress changes might control slow earthquakes. Indeed, various characteristics observed in slow earthquakes are often explained by diffusion equations, contrasting with regular earthquakes, which are explained by wave equations. Diffusion is a very fundamental physical process, and various movements governed by diffusion equations are likely occurring inside the Earth. Slow earthquakes might represent just the tip of the iceberg of these verious phenomena that are occurring inside the Earth but elude our observation.