What is the Moho? How was it Discovered? @IRISEarthquakeScience

Uploaded October 2018 | Updated September 2026, 1 week ago

Without understanding what it is, we often hear the phrase “down to the Moho”, meaning very very deep. The Mohorovicic Discontinuity, commonly called the “Moho” is recognized as the boundary zone between Earth's crust and the mantle.
This boundary marks a change in seismic-wave velocity from the crust to the uppermost mantle within the (lithospheric) plate. This boundary was discovered by Andrija Mohorovičić, a Croatian meteorologist-turned seismologist, after an earthquake in Croatia in 1909.

Graphics and animation by Jenda Johnson, Earth Sciences Animated
Co-authors: Jenda Johnson and Vince Cronin, Baylor University
Narrated by Mladen Dordevic, IRIS
Reviewed by Robert Butler, University of Portland, and Walter Mooney, US Geological Survey

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$Earthquake Faults—3 basic types...in brief This was created using excerpts from the longer animation called, “Earthquake Faults, Plate Boundaries, & Stress” [to be released September 2019; STAY TUNED!!] A fault is a rock fracture where the two sides have been displaced relative to each other. Faults are categorized into three general groups based on the sense of slip or movement. Normal fault—the block above the inclined fault moves down relative to the block below the fault. This fault motion is caused by tensional forces and results in extension. [Other names: normal-slip fault, tensional fault or gravity fault] Examples include Basin & Range faults. Reverse fault—the block above the inclined fault moves up relative to the block below the fault. This fault motion is caused by compressional forces and results in shortening. A reverse fault is called a thrust fault if the dip of the fault plane is small. [Other names: reverse-slip fault or compressional fault.] Examples include the Rocky Mountains and the Himalayan Mountains. Strike-slip fault—movement of blocks along a fault is horizontal and the fault plane is nearly vertical. If the block on the far side of the fault moves to the left, as shown in this animation, the fault is called left-lateral (Figure 2). If it moves to the right, the fault is called right-lateral. The fault motion of a strike-slip fault is caused by shearing forces. [Other names: trans current fault, lateral fault, tear fault or wrench fault.] Examples include the San Andreas Fault, California; Anatolian Fault, Turkey. Narrated by Dr. Wendy Bohon Animated by Jenda Johnson, Earth Sciences Animated$