DSE Geography | Plate Tectonics and Earthquakes: A Tutor Guide

DSE Geography | Plate Tectonics and Earthquakes: A Tutor Guide | Distance From Wave Lag

2026-11-03

Tutor lesson-prep guides · Secondary Geography

Students can recite three plate boundaries, then freeze at a seismogram because nobody told them the lag between the two arrivals is a distance. This lesson reduces the method to one line: lag divided by the speed difference. A 40-second lag gives 343.6 kilometres, and the practice set is 12 questions, 24 marks, 36 minutes.

Objectives — Students can: (1) convert a 40-second lag, with P waves at 6.5 and S waves at 3.7 kilometres per second, into 343.6 kilometres; (2) compute a lag of 23.3 seconds at 200 kilometres; (3) describe the landforms and hazards of convergent, divergent and conservative boundaries; (4) compute 3 minutes per question and 0.67 marks per minute for 12 questions worth 24 marks in 36 minutes.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Wave speed	Which travels faster, a P wave or an S wave?	P waves, about 6.5 kilometres per second against 3.7
Boundary type	What is a boundary where two plates move towards each other called?	Convergent, forming a subduction zone and volcanic arc
Unit conversion	How many metres are in a kilometre?	One thousand
Time budget	12 questions worth 24 marks in 36 minutes means how long each?	3 minutes

02 Lesson flow (45 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Read the seismogram	Show three station records and ask which arrival is first	Identify P first, S later
8-20 min	Derive the lag rule	Work out the gap over one kilometre, 0.116 seconds	Write lag divided by speed difference
20-30 min	Substitute	Turn 40 seconds into distance, then reverse for 200 kilometres	Get 343.6 kilometres and 23.3 seconds
30-38 min	Three boundaries	Attach one landform and one hazard to each	Explain why subduction gives deep quakes
38-43 min	Timed practice	Three questions at 3 minutes each	Units and decimals complete
43-45 min	Exit check	Four oral answers, notes closed	Rule and direction both correct

03 Teaching script (read it out)

Scene 1: The lag is a clock for distance

P waves cover 6.5 kilometres a second and S waves 3.7, so over one kilometre the gap is 1 over 3.7 minus 1 over 6.5, which is 0.116 seconds. A lag of 40 seconds therefore means 40 divided by 0.116, about 343.6 kilometres. Students who can rebuild this in a minute do not need a formula sheet, and cannot lose the method under pressure.

Ask the student why the rule divides by the difference instead of multiplying.

Scene 2: Run it backwards to check

At 200 kilometres the lag is 200 times 0.116, about 23.3 seconds. Because the speed difference is nearly constant in the shallow crust, distance and lag rise together, so a claim that 300 kilometres gives a 5-second lag is visibly wrong. This reverse check catches most arithmetic slips in ten seconds.

Ask the student which lag is larger, at 343.6 or at 200 kilometres, and why.

Scene 3: Three boundaries need a landform and a hazard

Convergent: an oceanic plate subducts, producing a trench, a volcanic arc and deep earthquakes, as in Japan. Divergent: plates separate, magma rises to build a mid-ocean ridge or rift, and earthquakes are shallow, as at the Mid-Atlantic Ridge. Conservative: plates slide past each other, producing a fault and shallow earthquakes, as at San Andreas. One place name, one landform and one focal depth per type prevents the usual mix-up.

Ask the student why subduction produces deep earthquakes but a ridge does not.

Scene 4: Plate speed as a sense of time

At 5 centimetres a year, a plate moves 0.05 metres a year, so one metre takes 20 years and a hundred metres takes 2,000. That conversion shows why an earthquake is not sudden in cause but sudden in release: stress accumulates for decades and then exceeds rock strength. Writing that sentence beats writing that plates collide.

Ask the student to convert 5 centimetres a year into the time needed for one metre.

04 Question types and worked steps

Type	Example	Steps	Answer
Epicentral distance	Wave speeds and a lag given	Invert the speed difference, divide the lag, answer in kilometres	One decimal place
Lag from distance	Distance and speeds given	Multiply by the speed difference	Rises with distance
Boundary identification	A landform or focal depth given	Name the boundary, the motion, the landform	All three stated
Hazard explanation	Why is this region seismic	Location and boundary, stress accumulation, release	A mechanism, not just plate movement

Sanity check (teach this): Order check: 1 over 3.7 is about 0.270 and 1 over 6.5 about 0.154, a difference of 0.116 seconds per kilometre; 40 divided by 0.116 is about 345 kilometres, so 34.5 or 3,450 means a misplaced decimal. Direction check: the lag grows with distance, so 200 kilometres cannot show a larger lag than 343.6. Speed check: 5 centimetres a year is 0.05 metres a year, so one metre takes 20 years, not 2.

05 Independent practice (3 questions)

Question	Answer	What to watch
Distance for a 40-second lag	About 343.6 kilometres	Decimal point in place
Lag at 200 kilometres	About 23.3 seconds	Multiplied, not added
Time for one metre at 5 centimetres a year	20 years	Centimetres converted to metres

06 Common mistakes and what to say

Mistake	Student writes	Say this
Multiplying the speeds	“6.5 minus 3.7 is 2.8, times 40 is 112 kilometres.”	“Divide, do not multiply. Over one kilometre the gap is 0.116 seconds, so 40 seconds is 40 divided by 0.116, about 344 kilometres.”
Skipping the unit conversion	“5 centimetres a year is 5 metres a year, so one metre takes 0.2 years.”	“5 centimetres is 0.05 metres, so one metre takes 20 years. Centimetres and metres differ by a hundred.”
Mixing the boundary types	“The Mid-Atlantic Ridge is convergent, so it has deep earthquakes.”	“A ridge is divergent, with rising magma and shallow earthquakes. Deep earthquakes belong to subduction zones.”
Stopping at plate movement	“There are earthquakes because plates move.”	“Give the mechanism: movement builds stress on a fault, and when it exceeds rock strength the stress is released as an earthquake.”

07 Homework (paper, section and question numbers)

Geography chapter index — Revise river management, then do practice questions 1 and 2
S4 Mass movement and slope safety — Question 3 contrasts gravity hazards with seismic hazards
S5 Coastal landforms and sea level — Questions 4 and 5 compare processes beyond plate movement

08 Exit check (last 5 minutes)

Question	Answer	Decision
Distance for a 40-second lag	About 343.6 kilometres	Error: multiplied instead of divided
Lag over one kilometre	About 0.116 seconds	Error: added the speeds
Which boundary is a mid-ocean ridge	Divergent, shallow earthquakes	Error: said convergent
Where deep earthquakes occur	Convergent subduction zones	Error: said conservative

09 Tutor FAQ

Must the formula be memorised?

No. Rebuild it once from one kilometre and it takes thirty seconds on the day, which is safer than a half-remembered formula.

How many examples per boundary?

One each: Japan, the Mid-Atlantic Ridge and San Andreas. Examples exist to support mechanism, not to be listed.

Are calculations worth much?

Two or three marks, but they gate the longer answer: without the distance, the comparison and explanation cannot start.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.