16:00 〜 16:15
[S07-11] 中米沈み込み帯三次元熱構造とスロー地震分布
1. Introduction
Slow earthquakes are a predominant phenomenon in the Central American subduction zone (e.g., Costa Rica), which indicates that the heterogenous megathrusts there are unstable. For example, shallow earthquakes occurred with high frequency throughout Central America, and several large earthquakes (Mw>6.5) took place along the coast (Alvarado et al., 2017). GPS observations show that slow slip events occurred at an average interval of approximately 20 months below the Nicoya Peninsula (e.g., Jiang et al., 2012). However, few slow slip events or low-frequency earthquakes have been detected in the Guatemala-Nicaragua segment of the Central American subduction zone. The reason for the differential distribution of fast and slow earthquakes in Central America remains enigmatic. Therefore, we constructed a 3-D thermomechanical model to estimate the thermal state and water content distribution in Central America. We analyzed the main mechanisms that affected the occurrence of fast-slow earthquakes based on our calculation.
2. Method and Data
In this study, we adopted thermomechanical codes (Stag3d, Tackley and Xie, 2003) and the finite difference method to develop a 3-D thermal model of the Cocos Plate. The model region has dimensions of 1350 km × 750 km × 400 km (length × width × depth) and 80 × 80 × 100 grids. In our model, we applied the equations corresponding to the conservation of mass, momentum, and energy and ensured boundary conditions satisfied in the calculation (e.g., Ji et al., 2016; Yoshioka and Murakami, 2007). During the calculation, the parameters used include plate geometry (Hayes et al., 2018), subduction velocity (DeMets et al., 2010), and plate age (Müller et al., 2008). We prescribe the subduction duration to be at least ≥20 Myr to ensure that the model reaches a steady thermal state with a temperature variation of <10 °C over time with a lapse time of ≥5 Myr.
3. Results and Discussion
Compared with previous thermal structure results (e.g., Peacock et al., 2005; Wada and Wang, 2009), we found that the surface temperature of the Cocos Plate is warmer than previously estimated (Fig. 1). The cold mantle wedge (<500 °C) below the Central American subduction zone is constrained at depths of <40~60 km. In this meaning, as the Cocos Plate subducts, warm plate environment controlled slab dehydration results in the release of more fluids than mature subduction zones. The released fluids can be transported, leading to an increase in pore fluid pressure, thus contributing to the occurrence of slow earthquakes. This is the main driving mechanism promoting the occurrence of fast and slow earthquakes in Central America. Based on our calculation, we predicted that slow earthquakes have the potential to occur in the Guatemala-Nicaragua segment of Central America. However, due to a lack of near-trench geodetic monitoring gauges in Guatemala-Nicaragua, less slow earthquakes have been detected in addition to the observed coseismic slip. The occurrence of slow earthquakes beneath the Nicoya Peninsula, Costa Rica is attributable to the remarkable perturbations in slab geometry and the release of fluids at different depth ranges.
Fig. 1 Calculated thermal state of the subducting Cocos Plate. The isotherm contours (200~1100°C) are labeled. Colored spheres indicate earthquakes. The white dashed line indicates the potential slow earthquake area in Guatemala-Nicaragua predicted by this study. Red cones indicate active volcanoes.
Slow earthquakes are a predominant phenomenon in the Central American subduction zone (e.g., Costa Rica), which indicates that the heterogenous megathrusts there are unstable. For example, shallow earthquakes occurred with high frequency throughout Central America, and several large earthquakes (Mw>6.5) took place along the coast (Alvarado et al., 2017). GPS observations show that slow slip events occurred at an average interval of approximately 20 months below the Nicoya Peninsula (e.g., Jiang et al., 2012). However, few slow slip events or low-frequency earthquakes have been detected in the Guatemala-Nicaragua segment of the Central American subduction zone. The reason for the differential distribution of fast and slow earthquakes in Central America remains enigmatic. Therefore, we constructed a 3-D thermomechanical model to estimate the thermal state and water content distribution in Central America. We analyzed the main mechanisms that affected the occurrence of fast-slow earthquakes based on our calculation.
2. Method and Data
In this study, we adopted thermomechanical codes (Stag3d, Tackley and Xie, 2003) and the finite difference method to develop a 3-D thermal model of the Cocos Plate. The model region has dimensions of 1350 km × 750 km × 400 km (length × width × depth) and 80 × 80 × 100 grids. In our model, we applied the equations corresponding to the conservation of mass, momentum, and energy and ensured boundary conditions satisfied in the calculation (e.g., Ji et al., 2016; Yoshioka and Murakami, 2007). During the calculation, the parameters used include plate geometry (Hayes et al., 2018), subduction velocity (DeMets et al., 2010), and plate age (Müller et al., 2008). We prescribe the subduction duration to be at least ≥20 Myr to ensure that the model reaches a steady thermal state with a temperature variation of <10 °C over time with a lapse time of ≥5 Myr.
3. Results and Discussion
Compared with previous thermal structure results (e.g., Peacock et al., 2005; Wada and Wang, 2009), we found that the surface temperature of the Cocos Plate is warmer than previously estimated (Fig. 1). The cold mantle wedge (<500 °C) below the Central American subduction zone is constrained at depths of <40~60 km. In this meaning, as the Cocos Plate subducts, warm plate environment controlled slab dehydration results in the release of more fluids than mature subduction zones. The released fluids can be transported, leading to an increase in pore fluid pressure, thus contributing to the occurrence of slow earthquakes. This is the main driving mechanism promoting the occurrence of fast and slow earthquakes in Central America. Based on our calculation, we predicted that slow earthquakes have the potential to occur in the Guatemala-Nicaragua segment of Central America. However, due to a lack of near-trench geodetic monitoring gauges in Guatemala-Nicaragua, less slow earthquakes have been detected in addition to the observed coseismic slip. The occurrence of slow earthquakes beneath the Nicoya Peninsula, Costa Rica is attributable to the remarkable perturbations in slab geometry and the release of fluids at different depth ranges.
Fig. 1 Calculated thermal state of the subducting Cocos Plate. The isotherm contours (200~1100°C) are labeled. Colored spheres indicate earthquakes. The white dashed line indicates the potential slow earthquake area in Guatemala-Nicaragua predicted by this study. Red cones indicate active volcanoes.