**4.1 Design concept of DONET toward forecasting the Tonankai earthquake**

In Fig. 10, DONET has a submarine cabled real-time seafloor observatory network for the precise earthquake and tsunami monitoring. For the purpose of understanding and forecasting the earthquake and related activities underneath the seafloor, the twenty sets of state-of-arts submarine cabled sub-sea measurement instrument will be deployed in seafloor at the interval of 15-20km. All of the twenty sets of preliminary interface have been installed just on July 31, 2011 and are to be prepared in consideration of the improvement of observation capability in the future.

As described by Kawaguchi et al. (2011), operating large-scale subsea infrastructure over a long period of time (20-30 years) is one of a challenge of underwater technology. The increase of measurement instruments has a big influence on the total system reliability, because of the state-of-arts instrument is a bottleneck to maintain long-term reliability. A novel system design concept is necessary for the observatory network development to make two demands such as 'high reliability system design' and 'state-of-arts measurement' united. The observatory network should be able to replace, maintenance and extend while operating, and should be have a redundancy for the internal or external observatory network component failure. To achieve these requirements, DONET adapt a strategy to combining the following three major components with different system reliability: (i) high

Fig. 10. An overview of Dense Oceanfloor Network System for Earthquakes and Tsunamis

In Fig. 10, DONET has a submarine cabled real-time seafloor observatory network for the precise earthquake and tsunami monitoring. For the purpose of understanding and forecasting the earthquake and related activities underneath the seafloor, the twenty sets of state-of-arts submarine cabled sub-sea measurement instrument will be deployed in seafloor at the interval of 15-20km. All of the twenty sets of preliminary interface have been installed just on July 31, 2011 and are to be prepared in consideration of the improvement of

As described by Kawaguchi et al. (2011), operating large-scale subsea infrastructure over a long period of time (20-30 years) is one of a challenge of underwater technology. The increase of measurement instruments has a big influence on the total system reliability, because of the state-of-arts instrument is a bottleneck to maintain long-term reliability. A novel system design concept is necessary for the observatory network development to make two demands such as 'high reliability system design' and 'state-of-arts measurement' united. The observatory network should be able to replace, maintenance and extend while operating, and should be have a redundancy for the internal or external observatory network component failure. To achieve these requirements, DONET adapt a strategy to combining the following three major components with different system reliability: (i) high

**4.1 Design concept of DONET toward forecasting the Tonankai earthquake** 

(DONET) in Tonankai region.

observation capability in the future.

reliability backbone cable system, (ii) replaceable science node, and (iii) extendable measurement instruments.

### **4.2 Expectation of preseismic monitoring by DONET**

Fig. 11 shows a map projection of 3-D subduction plate boundary model in Tonankai region. In this study, we introduce a plate interface bended by spline curve along dip direction, taking it into account the structural survey published by Nakanishi et al. (2008). hypocenter of shallower part of slow earthquakes is mainly based on the recent studies (e.g., Obara & Shiomi, 2009).

Megathrust earthquake (Mw 8.2) occurring in LA has periodic recurrence time of 113 years. Fig. 12 shows snapshot of slip velocity field in the interseismic and preseismic stage of the megathrust earthquake. Comparing shallower part of slow earthquake activity with deeper part, we found that the shallower part of slow earthquakes is less active than the deeper part in the interseismic stage of the megathrust earthquake (in the top of Fig. 12), because stress shadow from LA has more effective on the shallower part. In the preseismic stage (in the bottom of Fig. 12), the shallower part of slow earthquake comes to be similar to the deeper part and to be more active especially around the center of LA, because locked region is only around the center of LA and slip deficit in the preseismic stage is more than deeper part. These simulation results suggest that monitoring the shallower part of slow earthquakes may be effective on the ground that it is more sensitive to the preseismic change of the megathrust earthquake because of free surface condition.

In order to detect the preseismic slip of the next Tonankai earthquake in the near future, DONET would play an important role in monitoring shallower part of slow earthquake migration from the view of shortening recurrence interval and increasing migration speed as pointed out in the section of 3.2. Considering the location of 20 observation points as shown in Fig. 11 and numerical simulation results in Figs. 11 and 12, we expect DONET to do precise detection of preseismic change of the Tonankai earthquake as listed in Table 1.


Table 1. DONET's major roles in monitoring seismic & crustal change due to the Tonankai earthquake.

Fig. 11. A 3-D map projection of a subduction plate boundary model for Tonankai earthquake. Frictional parameter for Shallow Stable Zone, Deep Stable Zone, Large & Small Asperity, and Transition Zone is the same as Fig. 3 but (*κ*1, *κ*2) = (0.3, 0.1) on the basis of afterslip propagation speed investigated by Ariyoshi et al. (2007) and the value of *d*c2 for SA in shallower part is 0.3 mm. Note that we use shade effect on the color scale by applying the command "grdgradient" (Wessel & Smith, 1998) in order to show the bending shape visually. Twenty open circles with five nodes represent observation points of DONET as shown in Fig. 10.

 "Long" & "Short" represent long-term & short-term forecast based on recurrence interval and migration speed of slow earthquakes, respectively. "Coverage" represents significant effect on estimating hypocenters of slow earthquakes with high precision due to the coverage from trench side. "Dislocation" represents key observation points for estimating slip amount of continental plate relative to oceanic plate. "Main" represents main part of seismic slip process generated from LA. "Afterslip" represents aseismic slip along trench which may triggers nearby Nankai earthquake.

On the basis of numerical simulation results as shown in Fig. 12 and Table 1, DONET is expected to play important role in monitoring preseismic change of the next Tonankai earthquake from the view of various points including shallower part of slow earthquake activity and in judging the possibility of triggering the nearby Nankai earthquake, which had occurred in 1946 two years after the 1944 Tonankai earthquake.

Fig. 11. A 3-D map projection of a subduction plate boundary model for Tonankai

shown in Fig. 10.

which may triggers nearby Nankai earthquake.

had occurred in 1946 two years after the 1944 Tonankai earthquake.

earthquake. Frictional parameter for Shallow Stable Zone, Deep Stable Zone, Large & Small Asperity, and Transition Zone is the same as Fig. 3 but (*κ*1, *κ*2) = (0.3, 0.1) on the basis of afterslip propagation speed investigated by Ariyoshi et al. (2007) and the value of *d*c2 for SA in shallower part is 0.3 mm. Note that we use shade effect on the color scale by applying the command "grdgradient" (Wessel & Smith, 1998) in order to show the bending shape visually. Twenty open circles with five nodes represent observation points of DONET as

 "Long" & "Short" represent long-term & short-term forecast based on recurrence interval and migration speed of slow earthquakes, respectively. "Coverage" represents significant effect on estimating hypocenters of slow earthquakes with high precision due to the coverage from trench side. "Dislocation" represents key observation points for estimating slip amount of continental plate relative to oceanic plate. "Main" represents main part of seismic slip process generated from LA. "Afterslip" represents aseismic slip along trench

On the basis of numerical simulation results as shown in Fig. 12 and Table 1, DONET is expected to play important role in monitoring preseismic change of the next Tonankai earthquake from the view of various points including shallower part of slow earthquake activity and in judging the possibility of triggering the nearby Nankai earthquake, which

Fig. 12. Snapshots of slip velocity on the plate boundary about 20 years after the megathrust earthquake (top; interseismic period) and 2.5 years before (bottom; preseismic period). Color scale is the same as Fig. 4 but shade effect by applying the command "grdgradient" (Wessel & Smith, 1998) in order to show the bending shape visually. Twenty open circles with five nodes represent observation points of DONET as shown in Fig. 10.
