|MadSci Network: Earth Sciences|
Greetings. For the sake of others reading this question, let me first describe what a tsunami is and how one can be formed. Then I'll be able to say why sometimes subaquatic quakes don't generate them.
The word tsunami comes from the Japanese words meaning harbor ("tsu") and wave ("nami") and describes a series of waves created when a body of water (usually the ocean) is rapidly displaced on a massive scale. Earthquakes, volcanic eruptions, underwater explosions, landslides, and large meteorite impacts all have the potential to generate a tsunami, and the effects of the tsunami can range from unnoticeable to devastating.
The key factor necessary for generating a tsunami is the rapid displacement of water. While a sudden landslide into water might cause a tsunami, a gradual landslide won't. This explains why some subaquatic earthquakes cause tsunamis and others don't.
Earthquakes can be categorized into three basic groups based on the geometry of their underlying fault structure. An earthquake where the main direction of movement on the fault plane is vertical is known as a dip-slip fault. An earthquake where the main direction of movement is horizontal is known as a strike-slip, or sometimes transform, fault. Oblique-slip earthquakes have significant components of both strike-slip and dip-slip.
Since they cause the ground to move vertically, dip-slip faults have the ability to rapidly displace large amounts of water. Hence, dip-slip faults are able to generate tsunamis. Strike-slip faults, on the other hand, move the ocean floor horizontally. Thus, they do not displace any water and are therefore unable to produce tsunamis. Oblique-slip faults may or may not be able to generate tsunamis based on the amount of vertical movement.
I hope this has helped.
James R Holliday
Try the links in the MadSci Library for more information on Earth Sciences.