Abstract Intermediate‐depth earthquakes occur at depths where temperatures and pressures exceed those at which brittle failure is expected. There are two leading candidates for the physical mechanism behind these earthquakes: dehydration embrittlement and self‐localizing thermal shear runaway. A complete energy budget for a range of earthquake sizes can help constrain whether either of these mechanisms might play a role in intermediate‐depth earthquake rupture. The combination of high stress drop and low radiation efficiency that we observe for M w 4–5 earthquakes in the Bucaramanga Nest implies a temperature increase of 600–1000°C for a centimeter‐scale layer during earthquake failure. This suggests that substantial shear heating, and possibly partial melting, occurs during intermediate‐depth earthquake failure. Our observations support thermal shear runaway as the mechanism for intermediate‐depth earthquakes, which would help explain differences in their behavior compared to shallow earthquakes.