The focus, or hypocentre, is the point within Earth where an earthquake’s rupture begins. The epicentre is the point on the surface directly above it. In G3 Geography, keeping these locations separate helps you read depth, distance and earthquake diagrams accurately.
The quick reasoning is three-dimensional. A map dot usually shows a surface position, while depth adds the vertical component. Two earthquakes can have the same mapped epicentre but different focal depths. A settlement’s distance from the epicentre is also different from its distance from the underground starting point.
This guide supports tectonic-process learning in the 2027 standalone G3 Geography syllabus K329. Its geometry exercises are simplified classroom models, not forecasts or current hazard assessments. Use the plate-boundary guide for the wider setting and the earthquake-risk guide for impacts and vulnerability.
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Chapters 1–6 · Locate the earthquake in three dimensions
Chapters 7–12 · Connect movement and wave travel
- Stress accumulation and sudden slip connect the locations to process
- Seismic waves travel away from the source
- Arrival time is not the same as event start time
- Several stations help constrain an earthquake location
- A plan-view map does not show depth by itself
- A cross-section reveals relationships a map can hide
Chapters 13–18 · Read maps and measurements
Chapters 19–24 · Use comparisons without overclaiming
Chapters 25–27 · Practise diagrams and evidence
CHAPTER 1 OF 27 · Locate the earthquake in three dimensions
1. The focus identifies where rupture begins
The focus is an underground starting location. It does not mean every part of the fault moves at that single point or that the whole earthquake remains confined to a dot.
A schematic uses a point to make the starting position clear. Real rupture can extend across a fault surface. Keeping this distinction avoids treating a teaching symbol as the full physical size of the event.
In a labelled cross-section, locate the point below the surface and identify its role before naming the surface point. A precise definition can be short: the focus is where rupture begins within Earth.
Do not define it as the place with the most damaged buildings. Damage is an outcome influenced by shaking, exposure and vulnerability. The location definition and the impact question belong to different parts of the explanation. Knowing the distinction makes later evidence easier to organise.
CHAPTER 2 OF 27 · Locate the earthquake in three dimensions
2. The epicentre is a surface reference point
The epicentre lies directly above the focus at the surface. It provides a convenient mapped location for an earthquake’s origin. It is not the underground point itself.
Imagine drawing a vertical line from the focus to the surface in a simple cross-section. The intersection marks the epicentre. The fault may slope at another angle; the vertical reference should not be confused with the fault trace.
A map can show the epicentre using a symbol, but that symbol alone does not provide depth, rupture extent or the full shaking pattern.
Use the term for its actual purpose. Saying a settlement is near the epicentre describes one spatial relationship. It does not automatically establish that the settlement is closest to every part of the rupture or experienced the greatest loss. The surface reference is useful precisely because its meaning is defined, not because it summarises every characteristic of the earthquake.
CHAPTER 3 OF 27 · Locate the earthquake in three dimensions
3. Focal depth adds the vertical dimension
Focal depth describes how far below the surface reference the starting point is located. A map position and a depth together provide more information than either alone.
Two invented events can share a mapped location while one starts at 10 kilometres depth and another at 100. Their map dots overlap, but the underground positions differ.
Read the unit carefully. A depth in kilometres is not a magnitude or a distance from the nearest town. A table can place these different variables beside one another without making them interchangeable.
In a short answer, state the measured relationship: Event B has a greater focal depth. If explaining consequences, additional physical and human factors are needed. Depth can influence shaking, but it is not a universal damage score. Clear variable categories help you use an unfamiliar earthquake record without turning every number into a different way of saying the event was stronger.
CHAPTER 4 OF 27 · Locate the earthquake in three dimensions
4. The fault is a surface of movement, not another name for the focus
A fault is a fracture or zone along which rock movement occurs. The focus identifies the starting point of rupture on the relevant fault. The epicentre is a surface reference above that point.
These objects have different geometries: a fault can extend across an area, while the focus and epicentre are represented as points for particular purposes.
An inclined fault in a cross-section can make the diagram confusing. Trace the fault separately from the vertical line joining the focus and epicentre. Their directions need not match.
A useful exercise is to label the objects before adding arrows. Then explain what moves and where rupture begins. If the terms are mixed at the labelling stage, a longer paragraph will usually reproduce the confusion. Correct geometry gives the process explanation a clear foundation and makes the diagram useful rather than decorative.
CHAPTER 5 OF 27 · Locate the earthquake in three dimensions
5. The fault’s surface trace need not be the epicentre
Where a fault intersects the ground surface, its trace marks that intersection. On an inclined fault, that surface trace need not lie directly above the underground starting point.
Imagine a sloping line representing the fault and a point partway down it representing the focus. A vertical projection to the surface reaches a different position from the sloping line’s surface intersection.
This distinction is easiest to see in a cross-section. A plan-view map may not show the full geometry. Use the supplied representation for the information it contains.
Do not assume every school diagram includes a surface rupture or that every event breaks the surface. If the source omits that information, avoid adding it. The purpose of the exercise is to distinguish the objects and relationships, not invent physical features to make a familiar picture complete.
CHAPTER 6 OF 27 · Locate the earthquake in three dimensions
6. A starting point does not represent the whole rupture
An earthquake’s rupture can spread from its starting point across part of a fault. The event therefore has a spatial extent beyond the focus symbol.
This matters when comparing an epicentral dot with observations over a wider area. The dot locates the origin reference; it does not outline every part of the source.
A school explanation can recognise this without requiring advanced rupture modelling. State that the focus marks where rupture begins, while movement may extend along the fault.
Do not use the simplified point-source geometry later in this guide as a complete account of a large event. It isolates distance relationships for learning. A useful model can teach one principle while leaving other features outside its scope. Understanding that limit makes the model more useful, because you can identify which questions it can answer and which require additional information.
CHAPTER 7 OF 27 · Connect movement and wave travel
7. Stress accumulation and sudden slip connect the locations to process
Plate movement can deform rocks where parts of a fault resist slipping. Stored elastic energy is released when sudden movement occurs, producing seismic waves.
The focus identifies where that rupture begins; it is not a separate object generating an earthquake independently of the fault process.
Link the steps clearly: continuing movement, deformation and stored energy, sudden slip, and wave propagation. The precise physical conditions vary, but the basic chain helps explain why slow plate motion can be associated with sudden shaking.
A useful answer does not stop at naming a boundary type. It explains how the movement relates to the event. The plate-boundary guide develops that setting. Return to the location diagram when you need to show where the process begins and how its position is represented.
CHAPTER 8 OF 27 · Connect movement and wave travel
8. Seismic waves travel away from the source
Seismic waves carry energy through Earth and along its surface. Their travel connects the underground event to shaking recorded or experienced elsewhere.
A circular pattern around a focus in a schematic is a simplified representation of spreading waves. It does not mean every material transmits every wave in exactly the same way.
In a basic explanation, identify the source and the direction of propagation. Do not describe the epicentre as an underground starting point merely because waves are drawn beneath it.
The diagram’s arrows should support the words. If the source is below ground, place the focus there. If the map only shows the epicentre, explain what the surface point represents rather than pretending the figure is a full cross-section. Keeping representations consistent helps you connect location to movement without confusing the view from above with the view through Earth.
CHAPTER 9 OF 27 · Connect movement and wave travel
9. Arrival time is not the same as event start time
A recording station detects waves after they travel from the source. The arrival time at that station is therefore different from the earthquake’s origin time.
Stations at different locations can record arrivals at different times. The differences help locate events when interpreted using suitable wave information and models.
An introductory exercise may give a travel time directly. Use the stated relationship rather than assume the first time printed in a table is when rupture began.
This is a measurement distinction rather than a forecasting method. A record describes waves detected from an event; it does not predict the date of a future earthquake. In your answer, name the time being discussed and the station or source it belongs to. Precise time labels prevent an apparently simple subtraction from answering a different question from the one asked.
CHAPTER 10 OF 27 · Connect movement and wave travel
10. Several stations help constrain an earthquake location
One station’s evidence can constrain distance but may leave uncertainty about direction and depth. Records from multiple stations provide more information for estimating location.
A simplified school map may use intersecting distance circles. This illustrates how several observations narrow a possible surface position. Actual location methods use models and multiple measurements rather than assuming every observation is exact.
If circles fail to meet at one perfect point in a learning exercise, check plotting and data. In real records, measurement and model uncertainties can also matter.
Treat the construction as an illustration of combining evidence, not a claim that three ideal circles settle every three-dimensional earthquake parameter. The important reasoning is that independent observations reduce ambiguity. State what each representation establishes and which dimensions remain outside the simplified task.
CHAPTER 11 OF 27 · Connect movement and wave travel
11. A plan-view map does not show depth by itself
A plan view looks down on the surface. Epicentral symbols can show where events are mapped, while colour, size or labels may encode depth or another variable.
Read the legend before interpreting the symbols. A larger dot may represent magnitude rather than a physically larger epicentre. A darker colour may represent depth rather than greater damage.
Without a depth encoding, overlapping dots can conceal different underground locations. The surface pattern remains useful, but it is incomplete for a three-dimensional account.
A strong map description names the represented variable and observed pattern. If asked about depth, use the relevant key or accompanying table. Do not infer depth from the map dot’s apparent importance. This is the same evidence-reading discipline used across Geography: symbols communicate a defined measure, and their visual appearance should not be assigned a different meaning because it feels intuitive.
CHAPTER 12 OF 27 · Connect movement and wave travel
12. A cross-section reveals relationships a map can hide
A cross-section shows a slice through a setting. It can display depth, a sloping boundary and the relative positions of the focus and epicentre.
Check the direction of the section line on the map if one is supplied. A slice across one direction does not show every structure in the area.
Also check vertical exaggeration. The depth scale and horizontal scale may differ to make features visible. An apparent steepness should not be treated as an exact angle unless the scales support that interpretation.
Use the section for the relationship it reveals: one event starts deeper, points align along a descending zone, or a focus lies beneath a particular surface position. Then connect that observation to the process if requested. A labelled diagram can support a concise explanation, provided the view and scales are understood.
CHAPTER 13 OF 27 · Read maps and measurements
13. Epicentral distance and source distance are different
Epicentral distance concerns the surface separation from the epicentre. Distance to the underground source includes depth. In a simplified flat cross-section, these can form the horizontal and sloping sides of a right triangle.
A place directly above the focus has zero epicentral separation in that model, but it is not zero distance from the focus. The focal depth remains.
This distinction prevents a common claim that zero distance on a map means a building occupies the underground starting point.
Actual source-to-site relationships can be more complex because Earth is curved and rupture has extent. The simple geometry below is an educational comparison, not a full seismic model. Use it to understand why a two-dimensional map distance does not automatically describe the three-dimensional path. Keep the model’s purpose and assumptions clear when interpreting its arithmetic.
| Quantity | Meaning in the classroom model |
|---|---|
| Focal depth | Vertical separation from the surface reference to the focus |
| Epicentral distance | Surface separation from the epicentre to a site |
| Straight-line source distance | Sloping separation between point source and site |
| Magnitude | Event size, not a distance |
CHAPTER 14 OF 27 · Read maps and measurements
14. A right triangle can clarify the classroom model
Use invented perpendicular distances of three kilometres horizontally and four kilometres vertically. In the flat point-source model, the straight-line distance is five kilometres by the Pythagorean relationship.
The result distinguishes three quantities: surface separation, depth and source distance. They are not three estimates of the same side.
Do not convert the five-kilometre result into a predicted shaking level or casualty figure. The calculation supplies geometry under stated assumptions, not the physical and human information required for impact assessment.
Draw the triangle and attach each label before calculating. If the diagram is mislabelled, correct arithmetic can still produce an answer to the wrong question. This is a useful connection between Mathematics and Geography: the calculation becomes meaningful when the quantities are correctly mapped to the situation.
CHAPTER 15 OF 27 · Read maps and measurements
15. Events can share an epicentre and differ in depth
Imagine two invented events mapped at the same surface position. Event A has a focus ten kilometres deep and Event B one hundred kilometres deep. Their epicentres coincide in plan view.
The depth difference is 90 kilometres. The surface map alone would not reveal it unless depth was encoded or supplied separately.
A clear comparison says that the starting locations differ vertically while the mapped surface reference is the same. It should not say the events are identical because their dots overlap.
Nor does the depth comparison establish the complete shaking or impact pattern. Magnitude, source characteristics, travel paths, ground conditions and exposed systems remain relevant. The example’s purpose is to make the location distinction visible. It provides a definite geometric finding while leaving a different class of question open.
CHAPTER 16 OF 27 · Read maps and measurements
16. Depth can matter without becoming a universal impact rule
Focal depth influences the relationship between the source and surface locations, and can affect the shaking experienced. However, depth alone does not determine damage.
A shallower event is not automatically the more damaging event in every comparison. Other physical characteristics and human conditions can differ substantially.
If a question holds relevant factors comparable, explain the role depth may play within those conditions. If it supplies different magnitudes, ground conditions or building exposure, include those differences rather than ignore them.
The strongest reasoning is conditional and specific: identify the factor, connect it through a process and assess the evidence. Avoid a universal ranking based on one column in an event table. A useful school comparison can isolate depth for learning, while a real risk explanation requires the wider system. The earthquake-risk guide develops that broader relationship.
CHAPTER 17 OF 27 · Read maps and measurements
17. Magnitude is not a location coordinate
Magnitude characterises an earthquake’s size under the measurement scale used. It is different from depth, latitude, longitude or distance from a settlement.
An event table can list all these values together. Their proximity on the page does not make them interchangeable. A depth of 30 kilometres is not magnitude 30, and a larger magnitude does not identify a deeper focus.
Read the heading and unit before using each number. Then ask what question it answers: where, how deep, how large or what was experienced?
This category check is simple but powerful. It prevents numerical evidence from becoming a vague impression of severity. A clear response can compare two events on several dimensions without assuming that one value summarises all the others. The resulting explanation is easier to assess and less likely to overstate what the table establishes.
CHAPTER 18 OF 27 · Read maps and measurements
18. Intensity describes effects at a location
Intensity concerns shaking and its effects at a particular location under the scale used. It can vary across places for the same earthquake. This differs from the event’s magnitude.
The epicentre does not automatically identify the location of maximum observed loss. Ground conditions, rupture characteristics, structures and exposure can affect the spatial pattern.
Use an intensity map according to its legend. It is a different representation from a map of epicentres or a table of focal depths.
A supported answer names the measure before comparing places. If a source gives building damage rather than intensity, keep that distinction too. Damage is affected by vulnerability and should not silently become a direct measure of source size. Accurate categories allow the physical and human parts of the explanation to connect without collapsing into one undifferentiated label of strength.
CHAPTER 19 OF 27 · Use comparisons without overclaiming
19. A reported location is an estimate with a method
Earthquake locations are estimated from observations and models. Reporting a coordinate to several decimal places does not mean every dimension is known without uncertainty.
A dataset may include uncertainty information or revised values as more records are analysed. If the source provides that context, retain it in your interpretation.
For a basic classroom task, use the given values as instructed. You do not need to invent error margins. Recognising uncertainty means respecting the information supplied, not attaching an arbitrary doubt to every number.
A useful conclusion can be precise within its model: these plotted events show an increasing depth pattern in the supplied section. A separate claim about exact fault geometry may require additional evidence. This distinction keeps the analysis clear and avoids treating a teaching dataset as either infallible or unusable.
CHAPTER 20 OF 27 · Use comparisons without overclaiming
20. A depth pattern can support a tectonic interpretation
Earthquake depths plotted across a region can reveal a pattern that a surface distribution alone hides. In an appropriate subduction setting, a descending zone of events can support interpretation of a subducting slab.
Describe the observation first: event depths increase across the section in a particular direction. Then connect it to the supplied tectonic context.
Do not infer a specific boundary from any random cluster of dots without checking the map, section orientation and other evidence. A pattern suggests a relationship to explain rather than replacing the explanation.
The plate-boundary guide provides the process background. Use the depth evidence to test and refine the interpretation. Combining views is valuable because each supplies a different part of the spatial relationship.
CHAPTER 21 OF 27 · Use comparisons without overclaiming
21. Worked task: label an inclined fault diagram
Draw a sloping fault beneath a flat surface and mark a focus on the fault. From that focus, draw a vertical line to the surface. The upper endpoint is the epicentre.
Now identify the fault’s surface intersection, if the drawn line reaches the surface. It need not coincide with the epicentre because the fault is inclined.
Add one settlement away from the epicentre. Label its surface separation and the direct line to the focus as different distances. This makes the three-dimensional relationship visible.
Explain the diagram aloud using the terms rather than only pointing. If you can distinguish starting point, surface reference and fault geometry, the core concept is secure. Reversing the slope or moving the settlement should not change the definitions. The exercise tests understanding that can transfer to an unfamiliar figure.
CHAPTER 22 OF 27 · Use comparisons without overclaiming
22. Worked task: read a symbol key before comparing events
An invented map uses circle size for magnitude and colour for focal depth. Event A has a large pale symbol; Event B has a smaller dark symbol. The legend says darker colours indicate greater depth.
The supported comparison is that A has the larger represented magnitude while B has the greater represented depth. Neither symbol alone reports actual loss.
An incorrect answer might call A deeper because its dot is larger. The repair is to separate the two encodings and use each for its defined variable.
Now imagine the legend is removed. The visual differences remain, but their meaning becomes uncertain. You should not assign them a familiar interpretation without evidence. This exercise illustrates why reading the key is part of reasoning, not a decorative preliminary. It determines what the map can actually support.
CHAPTER 23 OF 27 · Use comparisons without overclaiming
23. Worked task: compare two surface positions
Use a flat point-source model with a focus four kilometres deep. Site A lies directly above it; Site B is three kilometres horizontally from the epicentre.
A’s distance from the focus is four kilometres. B’s straight-line distance is five kilometres in the model. Their epicentral distances are zero and three respectively.
The result establishes the geometric comparison. It does not show that B experiences a specified fraction of A’s shaking or damage. That would require a physical model and relevant site information.
Write the conclusion in two parts: the calculated distances, then the limit of the exercise. This is not weakening the answer. It is showing exactly what the calculation means. Good quantitative Geography keeps the mathematical result connected to its assumptions and avoids adding a more ambitious prediction than the model can supply.
CHAPTER 24 OF 27 · Use comparisons without overclaiming
24. Revise by moving between map and section
Start with a map dot and add a stated depth. Draw a corresponding cross-section and label the focus and epicentre. Then reverse the exercise: use a section to explain what a plan-view map would show and what it would omit.
Next, add a sloping fault and a settlement. Distinguish the fault trace, vertical depth and surface separation before doing any calculation.
Check the first inaccurate connection rather than rewriting every tectonics note. A student may understand the process but confuse one label; another may label correctly while missing the rupture mechanism.
A parent or tutor can ask where the event starts and what the surface dot represents. These simple questions reveal understanding. Later, try a differently oriented figure without looking at a model answer. Successful transfer provides stronger evidence than recognising one diagram that has just been copied.
CHAPTER 25 OF 27 · Practise diagrams and evidence
25. A nearby town name is a reporting reference
An earthquake report may describe an epicentre as a stated distance from a named town. The town helps readers locate the event geographically; it is not automatically the epicentre or the place most affected.
Imagine an invented report saying an event is 40 kilometres east of Town M and 20 kilometres deep. The first statement locates the surface reference relative to the town. The second gives a vertical dimension. They should not be combined into a single 60-kilometre distance by addition.
Use a map to establish direction and a section to establish depth. If the task asks for a straight-line source distance in a simplified geometry, a different calculation is required.
This example is useful because familiar place names can distract from the spatial relationship. Read the prepositions and units carefully. A concise report contains several distinct pieces of information, and each should retain its meaning when translated into a diagram.
CHAPTER 26 OF 27 · Practise diagrams and evidence
26. Latitude and longitude need depth for a three-dimensional point
Latitude and longitude locate a surface position. Adding depth helps specify an underground starting location. A pair of coordinates alone does not communicate the full three-dimensional position of a focus.
If a dataset lists the same latitude and longitude twice, check the depth and time before assuming it duplicates the same event. Different events or starting depths can share similar surface coordinates.
Likewise, the number of printed decimal places is not a complete description of location accuracy. Use uncertainty information when supplied and avoid inventing it when absent.
The practical reading sequence is position, depth, time and the other event properties relevant to the question. This keeps an earthquake catalogue from becoming an undifferentiated list of numbers. You can then compare the requested dimension accurately and identify what additional information is needed for a broader tectonic or impact explanation.
CHAPTER 27 OF 27 · Practise diagrams and evidence
27. Questions students and parents often ask
Are focus and hypocentre different things here? They refer to the underground starting location of rupture.
Is the epicentre underground? No. It is the surface point directly above the focus.
Is the fault trace always the epicentre? No. An inclined fault can intersect the surface elsewhere.
Can two events share an epicentre but have different depths? Yes. A surface map can hide that vertical difference.
Does depth alone predict damage? No. Other physical and human factors matter.
Does the largest map symbol show the greatest depth? Only if the legend defines it that way.
What is the first useful check? Identify whether the figure is a map or a cross-section, then read its labels, units and key.
Continue with the relevant eduKate guide
Plate boundaries: Connect the location evidence to tectonic processes.
Earthquake risk: Move from source characteristics to consequences.
Tsunami processes: Identify the additional displacement link needed for tsunami generation.
Unfamiliar evidence: Read maps, sections and measurements within their scope.
Compare Geography learning routes. Use the guide matching your subject and current task; a G3 extension does not imply identical G2 assessment requirements.
Sources and next steps
SEAB: 2027 G3 Geography K329 · USGS: the science of earthquakes · USGS: locating earthquakes in three dimensions · USGS: earthquake locations and depth.
Worked examples and invented figures are learning illustrations. Check your school’s instructions and the official syllabus for your examination year.
Bring one current task and its feedback when seeking help. Ask eduKate about appropriate support and current availability. The Clementi Mathematics guide illustrates finding a learning gap before adding practice. Check Geography support separately.