Subduction-Zone Mega-Thrust Earthquakes and Tsunami Formation

Introduction

• Subduction-zone mega-thrust earthquakes are the most powerful earthquakes.
• They can produce tsunamis through various mechanisms often missed by simple models.
• Basic model: Rock above convergent plate boundary stores elastic energy, which is released when friction is overcome, causing the overriding plate to slide abruptly, heaving seawater and generating a tsunami.
• Thrust Fault: Overriding plate thrusts up over the subducting plate.

Case Studies of Tsunami-Producing Mechanisms

2011 Japan Earthquake

• Magnitude 9 earthquake.
• Ruptured a fault 500 km long and 200 km wide over nearly 3 minutes.
• Fault displacement of up to 50 meters near the Japan Trench.
• Overriding plate slid up the fault plane, uplifting the sea floor by 10 meters.
• Produced both local and distant tsunamis.
• 96% of citizens evacuated due to effective emergency education.

2010 Chile Earthquake

• Magnitude 8.8 earthquake.
• Rupture zone 600 km long by 130 km wide.
• Little to no displacement at the trench similar to Japan earthquake.
• Fault displacement ranged from 5 to 10 meters, causing westward displacement of the continental shelf.
• Broad uplift of seafloor by 2-3 meters generated local tsunami.
• Evacuation was a key survival strategy, though not all were aware of the danger.

1964 Great Alaska Earthquake

• Magnitude 9.2 earthquake.
• Ruptured a segment 800 km long over 4 minutes.
• Continental shelf uplift of over 9 meters, while inboard areas subsided by 2 meters.
• Tsunami generated by splay faults like Patton Bay Fault.
• Displacement caused larger uplift, producing bigger and closer tsunamis.
• Ground shaking caused underwater landslides, leading to surges up to 50 meters high.
• Led to 77 deaths; importance of immediate evacuation highlighted.

Key Lessons and Insights

• Tsunami-producing mechanisms include fault displacement, deformation of the overriding plate, and splay faulting.
• Rapid evacuation of tsunami inundation zones is crucial for survival.
• Examples like Alaska 1964, Chile 2010, Japan 2011, and Sumatra 2004 emphasize the importance of understanding subduction zone processes and emergency response strategies.