By contrast, the fault that ruptured on Friday was a so-called strike-slip fault, in which the earth movement is largely horizontal. That kind of movement would not ordinarily create a tsunami.

But under certain circumstances it can, Dr. Patton said.

A strike-slip fault might have some amount of vertical motion that could displace seawater. Or the fault’s rupture zone, which in this case was estimated to be about 70 miles long, may pass through an area where the seafloor rises or drops off, so that when the fault moves during the quake, it pushes seawater in front of it.

Another possibility is that the tsunami was created indirectly. The violent shaking during the quake may have caused an undersea landslide that would have displaced water and created waves. Such events are not uncommon; several occurred during the 1964 9.2-magnitude Alaska earthquake, for example.

Dr. Patton said a combination of factors may have contributed to the tsunami. Studies of the seafloor will be crucial to understanding the event. “We won’t know what caused it until that’s done,” he said.