Earthquake Depths

The geographic pattern of earthquakes is well known and correlates quite strongly with plate-boundaries. The deepest earthquakes occurr within the subducting oceanic lithosphere and are limited to the outer 700 km of the planet. We can explore the first order patterns in earthquake depths using an earthquake catalog (a tabulated list of earthquakes). I used the GCMT Catalog for the following plates, so I included moderate and large earthquakes are earthquakes with Mw greater than about 4.5 and the catalog is complete for events larger than about Mw 5.5.

First, lets look at numbers of events. The number of shallow earthquakes is much larger than those at depth, but the pattern with depth shows a small increase for depths between 500 and 650 km.

 

Distribution of the number of earthquakes as a function of depth (using 50-km wide bins and the Global CMT Catalog) and a linear (left) and logarithmic (right) scales. Numbers to the right of the bars indicate the event count for each bin. Figure by C. J. Ammon, Penn State.

The number of events is informative, but not directly reflective of the energy release in earthquakes. For a better approximation to the energy release, here's the cumulative seismic moment in 50-km bins.

Distribution of seismic moment in the GCMT catalog as a function of depth using 50-km wide bins. The number to the right of each bar represent the total moment in the bin with units of 10^20 N-m. Figure by C. J. Ammon, Penn State.

 

Depending on how you look at it, the minimum between 200 and 550 km, or the maximum from 550 to 650 km is shifted down by one bin and more pronounced than in simple event-number counts. But again, the shallow activity dominates.

We can be more creative and communicate the depth distribution on a planetary scale if we plot the earthquakes on a cross section of Earth. First, I'll spread the events out using event longitude, which requires a little thought to see the geographic correlations. But the relatively sparse geographic distribution of deep earthquakes (in the deep slabs) is illuminated.

 GCMT catalog earthquake distribution shown on a cross section of the planet (surface, 660km discontinuity, and the outer and inner core boundaries are shown). The earthquakes are grouped by longitude, which starts and zero on the right ("east") and then increases counterclockwise around the circle. Figure by C. J. Ammon, Penn State.

 

One more view, showing earthquake depth distribution as a function of time. Think of the cross-section as a clock with 1975 at the top and increasing to the end of 2015 in a clockwise direction. Note that the GCMT catalog does not include all the deep events for a year and that you can see a hint of improvement in catalog completeness with time (as more and more seismometers were deployed). In general, the pattern is relatively stationary. The large earthquakes show an interesting drought in large, deep events from 1977 to 1994, when deep events occurred beneath Fiji and Bolivia.

GCMT catalog earthquake distribution shown on a cross section of the planet (surface, 660km discontinuity, and the outer and inner core boundaries are shown). The earthquakes are grouped by time. The top of the clock is 1975, and time increases clockwise, as labeled. Figure by C. J. Ammon, Penn State.