How Do Horizontal and Vertical Movements Differ in Subducted Plates?

[neongoats]

For my homework I was given a velocity diagram of the Juan de Fuca plate similar to this:

The first question is as follows:
Measurements indicate that the Juan de Fuca plate is moving at a speed of ~40 mm/year.
Calculate, and draw diagrams that show, the horizontal and vertical components of this
motion at two locations in Figure 2: at x=40 km (ie, well before the Juan de Fuca plate
encounters the North American plate), and at x=340 km (ie, after the Juan de Fuca plate has
subducted below the North American plate). I'm just super confused on what exactly this question is looking for and where I should even start? I know from the next question that I'm supposed to be finding velocity, but I don't know how to use this diagram to do that. I don't understand how the speed is relevant in comparison to the velocity on the graph and how the velocity would be used if it depends on depth etc. etc. Can someone decipher this question for me and gives me hints on where to go?

 

[gneill]

Hi neongoats, Welcome to Physics Forums.

In future, please retain and use the Posting Template provided when a new thread is started here in the Homework area of Physics Forums.

Seismology is not my area, but it appears to me that your Figure A provides a cross-sectional view of the plate showing how it approaches horizontally and then bends downwards at an angle which you can estimate/calculate using the distance and depth axes. I suppose the dotted red line represents the interface of the plate with the continental material, so its curve follows the "surface" of the moving plate.

Presuming that the plate's bulk moves uniformly with the given speed then you can sketch velocity vectors along that curve. Use the calculated angles to extract the vertical and horizontal components of your vectors.

That would be my take on the question.