Dramatic changes in atmospheric pressure, a storm surge, or a seismic wave causes rocking water called a seiche . This causes seismic waves and tsunamis. Restoring Forces Forces that try to return the water surface to its original flatness after a wave has formed on it are called restoring forces. The ocean surface does have waves and ripples, so the restoring forces are not quickly nor fully successful, rather they cause waves to oscillate and eventually become smaller as they return to their once flat surface. In larger waves, called gravity waves, the restoring force is weaker because the wave is bigger and gravity has a larger effect on it.
Gravity pulls the crest down, but the water's mometum overcomes the gravity and causes a trough.
This is why gravity waves last much long than capillary waves and can travel for much long distances until they finally break on a shoreline.