To explain an earthquake pattern from data, first describe it with figures, then give the process that produces it. Near a subduction zone, depth rises as you move away from the trench.
This lesson is part of plate movement and tectonic processes. It builds on linking a plate process to a landform.
What does the table show?
An original table lists six earthquakes near a trench. Distance is measured from the trench along the surface.
| Earthquake | Distance from trench (km) | Depth (km) |
|---|---|---|
| 1 | 20 | 10 |
| 2 | 60 | 25 |
| 3 | 100 | 40 |
| 4 | 150 | 70 |
| 5 | 200 | 110 |
| 6 | 250 | 140 |
Worked example: describe, then explain
Describe. Depth increases as distance from the trench increases, from 10 km at 20 km away to 140 km at 250 km away. The increase is steady, with no earthquake deeper than one that lies further from the trench.
Put a number on it. From earthquake 3 to earthquake 6, distance rises by 250 − 100 = 150 km and depth rises by 140 − 40 = 100 km. The plate descends about 100 ÷ 150 ≈ 0.67 km for every 1 km inland.
Explain. The ocean plate is sinking beneath the other plate. Earthquakes occur along the boundary of the sinking plate, which slopes downward away from the trench, so the further inland, the deeper the earthquake.
The mistake: explanation without description
A typical slip is to write “earthquakes happen at plate boundaries because plates move”. The sentence is true, but it says nothing about the table, and the marks are for using it.
| Answer | Problem |
|---|---|
| “Earthquakes happen at plate boundaries” | No figure, no pattern |
| “Earthquakes are random” | The table shows a clear trend |
| “Depth rises from 10 km to 140 km as distance rises from 20 km to 250 km, because the plate sinks” | Describes, quotes and explains |
Check yourself
A second original table near another trench gives (distance, depth): (30 km, 15 km), (90 km, 45 km), (150 km, 75 km) and (210 km, 105 km). Describe the pattern with figures, then estimate the depth at 120 km and state your assumption.
Answer
Describe: depth increases with distance from the trench, from 15 km at 30 km to 105 km at 210 km. Each 60 km inland adds 30 km of depth.
Estimate: 120 km lies between 90 km (45 km deep) and 150 km (75 km deep), so the depth is about 60 km.
Assumption: the sinking plate keeps the same slope between the two readings, so the depth changes steadily.
What to study next
Next, see why a pattern like this cannot give a warning in separating a classroom hazard model from a real-time safety forecast. Then try the tectonic practice set.
If you want a teacher to go through your table answers with you, see online one-to-one Geography tuition.