Seismic energy release behavior of subduction zones around Taiwan

In an active seismic zone, the total amount of energy released is of interest. On the other hand, how this total amount of energy is released, e.g. whether in many small earthquakes or in a single, strong earthquake, is of great significance for human society and for engineering design and corresponding codes as well. The behavior of seismic energy releasing for oceanic subduction and continental collision or subduction zones were explored by a proposed observational approach based on contour maps of seismic energy release on both horizontal and vertical planes, magnitude-energy curve and magnitude-energy-time relationship. In Taiwan and its vicinity, more than 200,000 earthquake events have been recorded. At certain areas in Taiwan, the mean annual seismic energy released could exceed 2 x 10¹² Joules/km^2. An observational approach is adopted in this work to identify the energy releasing behavior of different geological zones in Taiwan, including zones of oceanic subduction and continental collision or subduction. The proposed observational approach includes: 1. Construction of an energy contour map - in conjunction with the geological map and the tectonic framework of Taiwan, three zones requiring observation can then be selected; 2. Time variation of energy release - This aspect helps to determine whether the accumulated energy is occasionally relieved. The short-term pattern of seismic energy release can be compared to the long-term mean "energy release rate", and it is helpful in identifying possible quiescence, precursory clustering or even foreshocks; 3. Magnitude-Energy relationship - This aspect provides information about what portion of energy is released in terms of strong earthquakes; 4. Magnitude-Energy-Time relationship - It provides the time variation and the associated magnitudes of energy release. Background noises and aftershocks as well as their magnitudes of energy can be clearly seen from this type of plot; 5. Energy-Depth relationship - This map is helpful in identifying the seismic pattern of different geological structures, e.g., the detailed structures within a subduction zone; 6. Magnitude-Energy-Depth relationship - This provides the time variation of energy release and the associated magnitudes in relation to depth. It shows that strong earthquake is less possible to happen in a deeply subducted plate (with depths greater than 50 km). 7. Several seismic gaps were identified, which specifies the areas where further study and evaluation are essentially needed to access the possibility of occurrence of strong earthquakes. Accordingly, it was found that distinct features of energy-releasing behavior exist for the two types of subduction. The oceanic subduction tends to have a more active background noise by continuously emitting small magnitude earthquakes and to release energy from a wider range of depth owing to the existence of subducted plate. On the other hand, surficial strata of a continental subduction tend to release almost all of the seismic energy through a major earthquake with relatively much less long-term noise. Meanwhile, several seismic gaps have been found based on the proposed release energy contour. Together with the tectonic framework and the regional geologic setting, these gaps potentially may involve aseismic zones (e.g., accretionary deposit) or zones awaiting to release its accumulated energy (e.g., the upper crust). The quite zones prone to yielding major earthquakes are accordingly highlighted for a further study. Keywords: Seismic energy release; Seismic behavior of subduction zone; Seismicity of Taiwan; Seismic gap