Moho Depth Estimator Calculator

Refraction seismology recovers crustal thickness from the fact that a wave striking a faster layer at the critical angle travels along the interface and continuously leaks energy back to the surface. The head wave from the crust-mantle boundary is called Pn, the wave that stays within the crust is Pg, and because Pn travels at mantle velocity it eventually overtakes Pg despite a longer path. Extrapolating the Pn travel-time line back to zero offset gives the time intercept, and rearranging the geometry produces depth equal to the intercept multiplied by the product of the two velocities divided by twice the square root of the difference of their squares. With a crustal velocity of 6.2 km/s and a mantle velocity of 8.1 km/s the critical angle is close to 50 degrees and the depth multiplier is about 4.8 km per second of intercept. The crossover distance where Pn first arrives ahead of Pg equals twice the depth times the square root of the sum of the velocities divided by their difference, which for common crustal and mantle velocities works out at roughly four to six times the interface depth. Typical results anchor interpretation: oceanic Moho lies roughly 6 to 7 km beneath the seafloor, stable continental Moho at 30 to 40 km, and collisional belts such as the Tibetan Plateau reach 70 to 80 km. Pn velocity is usually between 7.8 and 8.2 km/s. The method carries real constraints. It assumes a single homogeneous crustal layer over a planar interface, so a layered or vertically graded crust biases the answer, and a dipping Moho requires reversed shooting from both ends of the profile to separate true velocity from apparent velocity. Because the depth multiplier is near 4.8, a timing error of 0.1 second translates to about half a kilometer of depth error, and the source origin time must be known independently.

The boundary was identified by Andrija Mohorovicic, director of the meteorological observatory in Zagreb, who analyzed records of the Kupa Valley earthquake of 8 October 1909. He noticed that stations beyond a certain distance recorded two distinct P arrivals rather than one, and concluded that the earlier arrival had traveled through a faster medium below a sharp discontinuity, which he placed at a depth of several tens of kilometers. The interface has carried his name ever since. Victor Conrad proposed a second, shallower intracrustal discontinuity in 1925, though it proved far less consistent globally. The controlled-source version of the method came from industry. Ludger Mintrop patented a seismic refraction technique in 1919 and founded the company Seismos in 1921, initially to locate salt domes for petroleum exploration, which established the travel-time analysis later scaled up to whole-crust studies. Deep crustal refraction profiling expanded in the 1950s, with work at the Carnegie Institution by Merle Tuve and Howard Tatel among the early systematic efforts. Ambition briefly exceeded means with Project Mohole, which from 1957 attempted to drill through thin oceanic crust to sample the mantle directly and drilled test holes off Guadalupe in 1961 before Congress cancelled funding in 1966. Later methods supplemented refraction rather than replacing it: deep multichannel reflection profiling by consortia such as COCORP from 1975, and receiver function analysis of converted teleseismic phases, formalized by Charles Langston in 1979.