Introduction — Depth of focus (tectonics)

Focal depth, or depth of focus, denotes the vertical distance beneath Earth’s surface where seismic rupture begins. Earthquakes are commonly classified by focal depth: shallow-focus events occur <70 km (43 mi), intermediate‑depth events between 70 and 300 km (43–190 mi), and deep‑focus events from 300 to 700 km (190–430 mi). Because shallow ruptures lie close to populated areas, they typically produce the strongest surface shaking for a given magnitude. Intermediate earthquakes commonly originate within the lithospheric or upper‑mantle portions of subducting slabs and occupy a transitional depth range between near‑surface seismicity and the much deeper seismicity recorded in some subduction zones. Deep‑focus earthquakes occur within the mantle and are principally linked to the descent of older, colder oceanic lithosphere beneath another plate; they mark the deepest limit of observed seismicity. The planar, downdip distribution of seismicity that outlines a subducting slab—the Wadati–Benioff zone—records earthquakes across these depth classes and is the principal tectonic setting for intermediate and deep events at convergent margins. The physical mechanism for seismic rupture at several hundred kilometres depth remains unresolved because the high pressures and temperatures there should suppress conventional brittle failure; this paradox has focused laboratory, petrological and seismological inquiry on alternative rupture processes. A prominent hypothesis invokes mineral‑physics driven instability, notably transformation of olivine to a denser spinel‑structured phase within the slab, which may nucleate or facilitate faulting and is spatially and temporally correlated with many deep‑focus earthquakes.

In 1922 H. H. Turner (Oxford) produced the first compelling evidence that seismic ruptures can originate well below the shallow crust, overturning the prevailing view that earthquake foci were restricted to near‑surface levels. His findings introduced the concept of a distinct class of deeper seismic sources, later termed deep‑focus earthquakes. Empirical confirmation arrived in 1931 when detailed analysis of multiple seismograms unambiguously demonstrated the existence of intermediate and deep events, providing reproducible observational support beyond Turner’s initial work. These 1931 analyses also enabled construction of travel‑time curves for intermediate and deep earthquakes, furnishing a systematic means to predict seismic wave arrival times with distance and depth and thereby greatly improving methods for locating deeper foci. Together, Turner’s 1922 study, the 1931 seismogram confirmations, and the development of deep‑event travel‑time models mark a pivotal shift in seismology’s understanding of the vertical distribution of earthquake sources within the Earth.

Fixed depth

When instrumental records are insufficient to determine an earthquake’s hypocentral depth, seismologists assign a predetermined “fixed depth” value as a surrogate. This practice is especially common for offshore events, where sparse local seismic coverage often prevents reliable depth determination.

The United States Geological Survey currently adopts 10 km as the standard fixed depth for most shallow earthquakes, reflecting the observation that many shallow hypocenters cluster near that depth. As a result, numerous catalogue entries for events with unconstrained depths are reported at 10 km, producing an artificial concentration of shallow-depth records that must be considered when using catalogue data for statistical or tectonic analyses.

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Historically, the USGS used 33 km as its fixed-depth default; the switch to 10 km represents a substantive change in how unconstrained focal depths are represented in earthquake catalogues and has implications for temporal consistency and comparisons across datasets.