Earth and Environmental Science 11–12 · Year 12
Mapping a sinking plate: earthquake focus depths across the Java Trench and a mid-ocean ridge
Module 6: Hazards
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The idea
At a convergent boundary earthquake foci deepen with distance from the trench along the sinking slab, while at a divergent boundary they stay shallow, so a depth cross-section maps the boundary’s shape and the depth of its hazard.
What you need
- two USGS earthquake catalogue extracts (magnitude 4.5 and above, 2000 to 2025) downloaded as CSV from the USGS search page: 5 to 12° S, 108 to 112° E (across the Java Trench) and 20 to 30° N, 40 to 50° W (across the Mid-Atlantic Ridge)
- spreadsheet or graph paper, ruler, protractor
How to do it
- Download both extracts and keep the latitude, longitude, depth and magnitude columns.
- For the Java box, convert latitude to kilometres north of 12° S (1° of latitude = 110.6 km on the WGS84 ellipsoid at these latitudes) and plot depth, increasing downwards, against that distance.
- Draw a best-fit line through the events deeper than 60 km and measure its angle of dip; note where shallow events cluster.
- Plot the Mid-Atlantic Ridge events the same way against longitude.
- Compare the two cross-sections and explain the difference with the plate boundary types.
- Note that many shallow events are listed at exactly 10 km, a default depth when the true depth is poorly constrained, and state how that affects the plot.
What you should see
Computed from the USGS catalogue on 23 September 2026: the Java box holds 573 events. South of 9.5° S the median depth is 10 km (57 % of those events sit at the 10 km default), the median reaches 81 km between 8.5 and 8° S, and all 29 events deeper than 300 km lie between 6.1 and 5.0° S, the deepest at 651 km. A least-squares line through the 171 events deeper than 60 km rises 1.38 km in depth per km northward, an average slab dip of about 54°. Across the Mid-Atlantic Ridge all 489 events are shallower than 24 km (median 10 km). The learner compares a hand-drawn fit with the spreadsheet value, and the two cross-sections show deep earthquakes along the sinking slab and none beneath the ridge.
What changes
This activity lists no variables to change, measure and keep the same.
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- All earthquakes happen near the surface.
- Earthquakes at every plate boundary are the same.
- A depth of exactly 10 km means the earthquake was measured at 10 km.
Safety card
Hazards
No hazard is listed.
Controls
No control is listed.
Note
Screen-based model with no chemicals, heat or apparatus: no practical risk assessment is triggered beyond the school's normal classroom procedures.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Earth and Environmental Science Stage 6 Syllabus (2017), NESA; current, replaced by the 11–12 Syllabus (2025) from 2028EES12-13EES11/12-1EES11/12-2EES11/12-3EES11/12-4
- Earth and Environmental Science 11–12 Syllabus (2025), NESA; to be implemented from 2028, not yet taughtEES-12-02
- Australian Curriculum v9No Australian Curriculum v9 code is listed.
Sources
The pages the author read to write this activity.
- www.nsw.gov.au/education-and-training/nesa/curriculum/science/earth-and-environmental-science-stage-6-2017
- earthquake.usgs.gov/earthquakes/search
- earthquake.usgs.gov/fdsnws/event/1
- www.ga.gov.au/education/classroom-resources/analysis-of-earthquakes-using-real-map-and-data-sets
- www.ga.gov.au/education/classroom-resources/earthquakes-teacher-notes-and-student-activities