How to map
the inside
of Earth.

The deepest maps begin with something that reaches the surface. Follow a signal into a world we cannot photograph.

CARTOLOGIUM EDITORIAL · 11 OCTOBER 2026
Observation · inference · the space beneath us

I / FROM SURFACE TO CENTRE6,371 km

One planet, two kinds of image. NASA’s surface composite meets an original section based on PREM’s broad radii. Interior colors are conventions; the surface rim is emphasized.

NASA’s Earth ObservatorySource & rights +

Blue Marble 2002 — western hemisphere. NASA Science.

NASA imagery credited to NASA’s Earth Observatory; no third-party copyright notice on this source record. Public domain in the United States; NASA identifiers and endorsement are excluded. The cutaway is Cartologium’s separate illustration.

Open source record ↗

A signal is not yet a picture.

On a street map, you can point to a building and then visit it. A map of the deep Earth asks for another kind of trust. Its colors stand for properties inferred from evidence: they are not colors seen through a window in the ground.

Even the familiar blue globe above is an assembled view. NASA’s 2002 Blue Marble combines satellite observations collected over several months. That composition describes the visible surface and clouds. Joining it to a cutaway does not turn the interior into a satellite image. The seam marks a change in how the image is known.

Earthquakes release seismic waves that instruments can record at the surface. A seismogram traces the instrument’s response through time. In the USGS example below, the labels identify an earlier P arrival and a later S arrival. P waves involve compression; S waves involve shear. The two types do not travel in the same way.

USGS educational waveform example with the P and S arrivals labelled — An educational waveform example from USGS. The source does not identify an event, station or calibrated axes. Use it to recognize the labels, rather than calculate a distance or magnitude.

Look first at what this example actually provides: a trace and two labels. It does not supply a named earthquake, station, calibrated time axis or amplitude scale. Those missing particulars matter. A plausible-looking wiggle cannot, by itself, tell us where an earthquake happened or what the entire planet contains.

What was measured—and what had to be inferred?

The planet becomes a profile.

One useful simplification is to describe how properties change with depth, setting aside differences between one longitude and another. The Preliminary Reference Earth Model, or PREM, is a published global reference from 1981. Its tabulated seismic velocities make an interior readable as a profile. The figure below uses EarthScope’s version calculated for a period of one second.

III / A MODEL, IN NUMBERSPREM · 1981 · 1 s

Layer selection needs JavaScript. All layer ranges and the full numeric table are available below.

Surface layers: 0–24.4 km. PREM includes a 3 km water layer and an idealized crust. Real crustal thickness varies; this global reference is not a local crustal survey.

Mantle: 24.4–2891 km. The broad rocky shell. Both radial P and S velocities are nonzero here. Solid rock can deform over geological time; the mantle is not a global ocean of magma.

Outer core: 2891–5149.5 km. The liquid shell of the core. The reference profile sets shear-wave speed to zero here, while compressional waves still propagate.

Inner core: 5149.5–6371 km. The solid central region. PREM assigns a nonzero shear-wave speed here; that does not mean a direct S wave crosses the liquid outer core to reach it.

DEPTH BELOW SURFACE / km

RADIAL WAVE SPEED / km s⁻¹

P · compressionS · shear

READ THE SELECTED LAYER

Outer core

2,891–5,149.5 km deep

The liquid shell of the core. The reference profile sets shear-wave speed to zero here, while compressional waves still propagate.

Model values describe an averaged Earth. The thin surface layers are difficult to resolve at this whole-planet scale.

Original plot of EarthScope’s PREM 1-second table: Vpv and Vsv, the radial P and S velocities. Lines join tabulated values, preserving both sides of discontinuities. They are model values, not earthquake recordings.

Read the model values and method

Depth = 6,371 km − radius. The plot retains every source row, including repeated depths at boundaries. PREM includes anisotropy in the upper mantle; this figure uses its radial components only. The source’s ocean layer also has zero S-wave speed.

EarthScope model record ↗ · Original numeric table ↗

PREM, period 1 second
Depth (km)	P (km/s)	S (km/s)
0	1.45	0
1	1.45	0
2	1.45	0
3	1.45	0
3	5.8	3.2
4	5.8	3.2
5	5.8	3.2
6	5.8	3.2
7	5.8	3.2
8	5.8	3.2
9	5.8	3.2
10	5.8	3.2
11	5.8	3.2
12	5.8	3.2
13	5.8	3.2
14	5.8	3.2
15	5.8	3.2
15	6.8	3.9
16	6.8	3.9
17	6.8	3.9
18	6.8	3.9
19	6.8	3.9
20	6.8	3.9
21	6.8	3.9
22	6.8	3.9
23	6.8	3.9
24.4	6.8	3.9
24.4	8.02206	4.39602
25	8.02138	4.39616
26	8.02024	4.39639
27	8.01911	4.39662
28	8.01798	4.39685
29	8.01684	4.39708
30	8.01571	4.39731
31	8.01458	4.39754
32	8.01345	4.39777
33	8.01231	4.398
34	8.01118	4.39823
35	8.01005	4.39846
40	8.00438	4.39962
45	7.99872	4.40077
50	7.99305	4.40192
60	7.98172	4.40422
70	7.97039	4.40653
80	7.95906	4.40883
80	7.95906	4.40883
90	7.94773	4.41113
100	7.93641	4.41344
110	7.92508	4.41574
120	7.91375	4.41805
130	7.90242	4.42035
140	7.89109	4.42265
150	7.87976	4.42496
160	7.86843	4.42726
170	7.8571	4.42957
180	7.84577	4.43187
190	7.83444	4.43417
200	7.82311	4.43648
210	7.81178	4.43878
220	7.80045	4.44109
220	8.55895	4.6439
230	8.57819	4.6509
240	8.59743	4.6579
250	8.61666	4.6649
260	8.6359	4.6719
270	8.65514	4.6789
280	8.67438	4.6859
290	8.69362	4.6929
300	8.71286	4.6999
310	8.7321	4.7069
320	8.75133	4.7139
330	8.77057	4.7209
340	8.78981	4.7279
350	8.80905	4.7349
360	8.82829	4.7419
370	8.84753	4.7489
380	8.86677	4.7559
390	8.886	4.7629
400	8.90524	4.7699
400	9.13392	4.93249
410	9.18511	4.96166
420	9.23631	4.99083
430	9.2875	5.02
440	9.3387	5.04918
450	9.38989	5.07835
460	9.44109	5.10752
470	9.49228	5.13669
480	9.54348	5.16586
490	9.59468	5.19504
500	9.64587	5.22421
510	9.69707	5.25338
520	9.74826	5.28255
530	9.79946	5.31173
540	9.85065	5.3409
550	9.90185	5.37007
560	9.95304	5.39924
570	10.00424	5.42841
580	10.05543	5.45759
590	10.10663	5.48676
600	10.15783	5.51593
600	10.15776	5.51602
610	10.17325	5.52376
620	10.18874	5.5315
630	10.20422	5.53924
640	10.21971	5.54699
650	10.2352	5.55473
660	10.25069	5.56247
670	10.26617	5.57021
670	10.75132	5.94513
685.3	10.79897	5.98986
735.3	10.95459	6.13609
771	11.0656	6.24054
771	11.06559	6.24039
785.3	11.09179	6.25067
835.3	11.18192	6.28608
885.3	11.26984	6.32065
935.3	11.35563	6.35442
985.3	11.43936	6.38742
1035.3	11.5211	6.41967
1085.3	11.60095	6.4512
1135.3	11.67896	6.48204
1185.3	11.75523	6.5122
1235.3	11.82983	6.54173
1285.3	11.90283	6.57064
1335.3	11.97431	6.59897
1385.3	12.04435	6.62674
1435.3	12.11304	6.65398
1485.3	12.18043	6.68071
1535.3	12.24662	6.70696
1585.3	12.31167	6.73276
1635.3	12.37567	6.75813
1685.3	12.4387	6.78311
1735.3	12.50083	6.80771
1785.3	12.56213	6.83197
1835.3	12.62269	6.85592
1885.3	12.68258	6.87957
1935.3	12.74188	6.90296
1985.3	12.80067	6.92611
2035.3	12.85902	6.94906
2085.3	12.91702	6.97182
2135.3	12.97473	6.99442
2185.3	13.03224	7.0169
2235.3	13.08963	7.03927
2285.3	13.14696	7.06157
2335.3	13.20432	7.08382
2385.3	13.26179	7.10605
2435.3	13.31944	7.12828
2485.3	13.37735	7.15055
2535.3	13.4356	7.17287
2585.3	13.49426	7.19529
2635.3	13.55341	7.21781
2685.3	13.61313	7.24048
2735.3	13.6735	7.26331
2741	13.68042	7.26593
2741	13.68044	7.26597
2785.3	13.69098	7.26563
2835.3	13.70302	7.2652
2885.3	13.71522	7.26471
2891	13.71662	7.26465
2891	8.06479	0
2971	8.19943	0
3071	8.36024	0
3171	8.51302	0
3271	8.65809	0
3371	8.79577	0
3471	8.92636	0
3571	9.05019	0
3671	9.16756	0
3771	9.2788	0
3871	9.38422	0
3971	9.48413	0
4071	9.57885	0
4149.5	9.64977	0
4171	9.66869	0
4271	9.75397	0
4371	9.835	0
4471	9.9121	0
4571	9.98558	0
4671	10.05576	0
4771	10.12295	0
4871	10.18747	0
4971	10.24963	0
5071	10.30975	0
5149.5	10.35572	0
5149.5	11.02826	3.50431
5153.9	11.02994	3.50549
5171	11.03642	3.51002
5271	11.07249	3.53522
5371	11.10541	3.55823
5471	11.1352	3.57905
5571	11.16186	3.59767
5671	11.18537	3.61411
5771	11.20576	3.62835
5871	11.223	3.64041
5971	11.23711	3.65027
6071	11.24809	3.65794
6171	11.25593	3.66342
6271	11.26063	3.6667
6371	11.2622	3.6678

Select the outer core and follow the dashed S curve. Its zero value represents a liquid region that does not transmit shear waves. The P curve continues. At the inner-core boundary, the model gives shear speed a nonzero value again. This is a property of the modeled material, not a claim that an ordinary S wave can pass straight through the intervening liquid.

The large rocky mantle is predominantly solid, even though it can deform and flow over geological time. The core has liquid and solid regions. A cutaway that colors all three in warm tones can accidentally make the planet look like a bowl of molten rock. Here the colors separate regions; they do not measure temperature.

At this scale, another choice becomes visible: the crust almost disappears. Enlarging it would make it easier to discuss, but would change the scale of the drawing. The globe emphasizes its surface rim; the numeric profile retains the source’s thin surface layers. Neither view should be mistaken for the crustal thickness beneath a particular city.

From a layered Earth to a mapped Earth.

A radial profile makes the same description available at every surface location. To investigate variation from place to place, seismic tomography combines signals that have sampled different paths. A proposed velocity model predicts travel times; comparing predictions with recordings helps researchers revise it. The result is an inferred distribution of wave speeds.

IV / MANY PATHS, DIFFERENT CONSTRAINTSSCHEMATIC

A single path leaves many possible interiors consistent with it.

Crossing paths interrogate a region from several directions.

Original diagram of sampling geometry. These are illustrative paths, not computed seismic rays or a recovered tomogram. More paths do not automatically guarantee a correct model.

The arrangement of the measurements matters as much as the colors in the result. Sampling geometry, noise, assumptions and the methods used to stabilize an inversion affect what can be resolved. Research on geophysical tomograms shows why a sharply drawn boundary or a fine output grid does not automatically establish equally fine knowledge.

Before reading an interior map as a landscape, find its depth, the property being plotted, the units, the reference model and the evidence for resolution. A velocity difference is not itself a photograph of a hot plume, a mineral or a reservoir of melt. Such interpretations need further physical evidence.

Read one boundary, twice.

Return to the profile and select the outer core. Compare what happens to P and S at its upper boundary. Then return to the globe. The color change and the change in the plotted properties describe the same broad boundary through different visual languages.

Return to the model ↑

Mapping the inside out begins with this discipline: keep the observation, the model and the interpretation close enough to compare. It lets a reader move from a mark on a record to a structure inside the planet without losing the reasoning between them.

Explore the landscape beneath the water →Continue to A Moon Made of Maps →Explore the Seismic Atlas →Return to surface relief →Follow the unseen-world inquiry →

Sources and method.

The profile plots the published table’s depth, Vpv and Vsv columns in kilometres and kilometres per second. All rows are retained, including repeated depths at discontinuities. The broad cutaway uses a 6,371 km Earth radius, a 3,480 km core radius and a 1,221.5 km inner-core radius. It omits the model’s finer subdivisions.

The NASA composite and USGS example have individual source and rights controls. Cartologium’s cutaway, plot and sampling diagram are original visual explanations. No computed ray tracing, local tomographic model or new earthquake observation is presented.

Inspect the EarthScope model record · Read the source table. The table was retrieved on 11 October 2026. Its one-second period is a model parameter, not a record-update interval.

NASA. (2018, April 4). Blue Marble 2002 [Satellite-data composite]. https://science.nasa.gov/resource/blue-marble-2002/
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Wald, L. (n.d.). The science of earthquakes. U.S. Geological Survey. https://www.usgs.gov/programs/earthquake-hazards/science-earthquakes
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U.S. Geological Survey. (n.d.). Inside the Earth. This dynamic Earth. https://pubs.usgs.gov/gip/dynamic/inside.html
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Dziewonski, A. M., & Anderson, D. L. (1981). Preliminary reference Earth model. Physics of the Earth and Planetary Interiors, 25(4), 297–356. https://doi.org/10.1016/0031-9201(81)90046-7
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EarthScope Consortium. (n.d.). Seismic tomography. https://www.earthscope.org/what-is/seismology/seismic-tomography/
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Day-Lewis, F. D., & Lane, J. W. (2004). Assessing the resolution-dependent utility of tomograms for geostatistics. Geophysical Research Letters, 31, L07503. https://doi.org/10.1029/2004GL019617
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U.S. Geological Survey. (n.d.). Example of seismic waveform [Educational illustration]. https://www.usgs.gov/media/images/example-seismic-waveform
Inspect the image record