Background: The Structural Vulnerability of Hokkaido's Power System
Hokkaido's electricity system has long faced a structural weakness: limited interconnection capacity with Honshu. The Hokuhon interconnector, operated by J-POWER, is the sole power corridor connecting Hokkaido to the mainland, with a total capacity of approximately 900 MW. When this lifeline is severed, the Hokkaido electricity market becomes instantly isolated, relying entirely on its own generation capacity.
The September 2018 Hokkaido Eastern Iburi Earthquake already demonstrated this vulnerability with an island-wide blackout affecting 2.95 million households. The December 2025 event was smaller in scale, but its impact on the electricity trading market was equally profound.
Event Timeline: 72 Hours from Earthquake to Price Peak
| Time (JST) | Event | Impact |
|---|
| Dec 8, 23:15 | M6.6 earthquake off Aomori coast (max. intensity 6+) | Triggers cascade |
| Dec 8, 23:15 | Hokuhon Pole 2 (600 MW) trips offline | Hokkaido import capacity reduced by 2/3 |
| Dec 8, 23:18 | Tomato-Atsuma Unit 2 emergency trip | Island generation capacity further reduced |
| Dec 11, 10:30–11:00 | Hokkaido Area Price peaks at ¥50/kWh | ~4× normal; isolation premium exceeds ¥30/kWh |
| Dec 14, 14:00 | Hokuhon Pole 1 restored to service | Partial restoration; prices begin to ease |
| Dec 15 | Price normalizes (¥15–22/kWh range) | Pole 2 remains offline |
JEPX Data Analysis: 384 Half-Hourly Data Points
- Period peak: ¥50.00/kWh (Dec 11, 10:30–11:00, two consecutive slots)
- Period average: ¥26.8/kWh (Hokkaido Area) vs ¥18.9/kWh (System)
- Maximum isolation premium: ¥38.2/kWh
- Slots ≥¥30: 96 slots (equivalent to 2 full days)
- Weekly average: Hokkaido ¥14.15/kWh (+¥2.62 vs prior week); all other areas nearly unchanged
Triple Shock: The Physical Mechanism of Cascade Effects
The Hokkaido price surge resulted from three overlapping factors: (1) Hokuhon Pole 2 outage reducing import capacity by 2/3; (2) Tomato-Atsuma Unit 2 trip cutting island-based generation; (3) winter cold wave pushing up heating demand while suppressing renewable output. The convergence of all three created an extreme supply-demand imbalance with an isolation premium reaching ¥38.2/kWh.
References
- Hokkaido Electric Power: Impact on Electrical Facilities from Aomori Earthquake (Dec 2025)
- J-POWER: Status of Hokkaido-Honshu Interconnection Restoration (Dec 14, 2025)
- RIM Intelligence: Japan spot power prices Dec 8-12 2025
- JEPX: Spot Market Data (spot_summary_2025.csv)
