11. Fault diagnosis: Understanding faults. The present perfect tense. Fault diagnosis questions and answers. A fault is a fracture or zone of fractures between two blocks of rock. Faults allow the blocks to move relative to each other. This movement may occur rapidly, in the form of an earthquake - or may occur slowly, in the form of creep. Faults may range in length from a few millimeters to thousands of kilometers. Most faults produce repeated displacements over geologic time. During an earthquake, the rock on one side of the fault suddenly slips with respect to the other. The fault surface can be horizontal or vertical or some arbitrary angle in between.
Earth scientists use the angle of the fault with respect to the surface (known as the dip) and the direction of slip along the fault to classify faults. Faults which move along the direction of the dip plane are dip-slip faults and described as either normal or reverse (thrust), depending on their motion. Faults which move horizontally are known as strike-slip faults and are classified as either right-lateral or left-lateral. Faults which show both dip-slip and strike-slip motion are known as oblique-slip faults.
Learn about the present perfect and do the exercises to practise using it.
The present perfect is formed from the present tense of the verb have and the past participle of a verb.
We use the present perfect:
for something that started in the past and continues in the present:
They've been married for nearly fifty years.
She has lived in Liverpool all her life.
when we are talking about our experience up to the present:
I've seen that film before.
I've played the guitar ever since I was a teenager.
He has written three books and he is working on another one.
We often use the adverb ever to talk about experience up to the present:
My last birthday was the worst day I have ever had.
and we use never for the negative form:
Have you ever met George?
Yes, but I've never met his wife.
