An earthquake is a phenomenon in which bedrock suddenly breaks and shifts. Understanding its causes and types helps us predict damage and prepare for disasters.
Basic Knowledge
The underground location where an earthquake occurs is called the focus, and the point on the ground surface directly above the focus is called the epicenter. The depth of the focus is called the focal depth. Shallow earthquakes (no deeper than 70 km) are called shallow-focus earthquakes, while deep earthquakes (300〜700 km) are called deep-focus earthquakes(deep-focus earthquakes・intraslab earthquakes).
Types of earthquakes:
①Plate-boundary earthquakes (trench-type earthquakes): As an oceanic plate subducts, it drags the continental plate, and accumulated strain is released all at once. M8〜9 giant earthquakes are likely to occur, and they are often accompanied by tsunamis. Examples include the 2011 Tohoku earthquake and tsunami (2011, M9.0) and the Nankai earthquake.
②Inland earthquakes (directly beneath the surface): They occur when an active fault inside a continental plate shifts. Because the focus is shallow (approximately 10〜20 km), they can cause severe local damage. An example is the Southern Hyogo Prefecture Earthquake (1995, M7.3).
③Intraslab earthquakes: Deep-focus earthquakes that occur inside a subducted oceanic plate (slab).
Types of faults: normal faults(tensional force)・reverse faults(compressional force)・strike-slip faults(horizontal movement). In Japan, reverse-fault earthquakes caused by east-west compression are common.
Focus（The underground point where an earthquake occurs）
Epicenter（The point on the ground surface directly above the focus）
Active fault（A fault that has been active within the past several hundred thousand years）
Plate-boundary earthquake（Strain is released at a trench. Accompanied by giant earthquakes and tsunamis）
Inland earthquake（A shift of an active fault. Shallow and locally destructive）
Reverse fault（The hanging wall rises because of compressional force. A common fault type in Japan）
In Depth
The elastic rebound theory (Reid) explains the occurrence of earthquakes. Strain energy accumulates in bedrock due to plate movement. When it exceeds the strength of the bedrock, the rock suddenly shifts along a fault plane and releases the energy. The energy released at this time travels as seismic waves.
There are thought to be approximately 2000 active faults in Japan, and it is extremely difficult to predict when and where they will become active. Continuous observation of plate displacement using GPS (GNSS) is used to monitor crustal movement.
The 1995 Great Hanshin-Awaji Earthquake (M7.3) was a representative inland, directly beneath-the-surface earthquake. It had a focal depth of approximately 16 km and recorded a maximum seismic intensity of 7 near the Rokko Mountains and Awaji Island directly below the epicenter.
- Focus=underground point of origin, epicenter=the point on the surface directly above it
- Plate-boundary earthquakes=trenches・giant earthquakes・frequent tsunamis
- Inland earthquakes=active faults・shallow・severe local damage
- Reverse-fault earthquakes are common in Japan (east-west compression)
- Elastic rebound theory=strain accumulation→sudden release
- Crustal movement is monitored through GPS observation
Points to Note
① Do not confuse the focus and the epicenter. The focus is underground, while the epicenter is on the surface. ② Plate-boundary earthquakes are large, at M8〜9, but inland earthquakes can also cause devastating local damage at around M7. ③ Instead of saying “active faults are dangerous,” accurately say that “areas around active faults have a high earthquake risk.”
Practice
- Explain the difference in location between plate-boundary earthquakes and inland earthquakes.
- Briefly explain the elastic rebound theory.
- State why plate-boundary earthquakes are likely to generate tsunamis.