Why do so many earthquakes occur at a depth of 10km?

Ten kilometers is a "fixed depth". Sometimes data are too poor to compute a reliable depth for an earthquake. In such cases, the depth is assigned to be 10 km. Why that number? In many areas around the world, reliable depths tend to average 10 km or close to it. For example, if we made a histogram of the reliable depths in such an area, we'd expect to see a peak around 10 km. So if we don't know the depth, 10 km is a reasonable guess. The USGS used to use 33 km, but increased understanding indicates that 10 km is more likely.

Some areas, like subduction zones, are known to have many earthquakes much deeper than 10 km. In those areas, a deeper fixed depth would probably be appropriate. The most common reason for having to fix the depth is that the earthquake occurred too far from the nearest seismic station. A useful rule of thumb is that a reliable depth requires that the distance from the epicenter to the nearest station must be less than the depth of the earthquake.

Modern computational and theoretical advances can now produce reliable depths at greater distances from the nearest station, so the rule of thumb does not always apply. However, the rule of thumb does illustrate one conclusion: fixed depths are more common for shallow earthquakes than for deep ones.American Geophysical Union, Fall Meeting 2010, abstract #S21A-2015 The characteristic earthquake model predicts that faults or fault segments tend to generate essentially same size earthquakes having a relatively narrow range of magnitudes near the maximum. The model makes no assumptions about recurrence. Some important implications: (1) on individual faults and fault segments the earthquake occurrence does not follow a log linear G-R relationship that is dominated by the characteristic event, (b) G-R has a low b-value in the moderate-magnitude range, (c) estimates of the likelihood of large earthquake occurrence based on extrapolation of the frequency of occurrence of small earthquakes may be subject to considerable error. Observations in favour of the model were made in several seismotectonic settings around the globe involving shallow and subduction earthquakes. However, other studies on earthquake statistics and observations about the fault behavior indicated patterns of rather uncharacteristic earthquakes. Characteristic earthquakes of intermediate depth (CEID) were not reported so far. Such a type of earthquakes were investigated in the Hellenic Arc, South Aegean Sea, with two independent sets of evidence. In the first, couples of individual large earthquakes occurring in the same intermediate-depth nest were examined, while the second is based on the G-R relation for the entire descending lithospheric slab. On 11 August 1903 and on 8 January 2006 two large earthquakes occurred at the same intermediate-depth (h~70 km) source near Kythira Is, SW Aegean Sea, sharing nearly the same size with Mw of 6.7±0.2 and 6.9, respectively, which are the maximum measured for instrumental intermediate-depth earthquakes in the SW Aegean Sea. The pattern of macroseismic intensity was impressively identical for the two earthquakes, with the maximum intensity observed in the Mitata village of Kythira For equal epicentral distances high intensities were felt towards the Hellenic Arc and the Mediterranean Sea, while low intensities were felt towards the Aegean marginal sea. This may reflect same pattern of energy radiation and same mode of faulting. From the examination of historical documentary sources and instrumental records it results that candidate earthquake couples of the CEID type were the large earthquakes of 28 March 1846 and 24 June 1870 as well as the large earthquakes of 12 October 1856 and 26 June 1926 in the SE Aegean Sea. However, such a conclusion is unstable since the magnitude determination of the three historical events is quite uncertain. GR relationships for several segments of the Hellenic Arc showed that deviations from linearity for shallow seismicity could be explained by the characteristic earthquake model (Papadopoulos et al., JGR, 1993, EPS, 2003). CIED-type earthquakes were investigated in the Hellenic Arc by constructing G-R (magnitude-frequency) diagrams for instrumental seismicity data sets with earthquakes of intermediate-depth. Preliminary results showed deviation from the log linear relation that is dominated by the characteristic event in the upper magnitude range, and that the G-R relationship has a low b-value in the moderate-magnitude range. I conclude that the occurrence of characteristic earthquakes at intermediate depths is very likely. Keywords: 4564 OCEANOGRAPHY: PHYSICAL / Tsunamis and storm surges, 7223 SEISMOLOGY / Earthquake interaction, forecasting, and prediction, 7230 SEISMOLOGY / Seismicity and tectonics, 7240 SEISMOLOGY / Subduction zones The ADS is Operated by the Smithsonian Astrophysical Observatory under NASA Grant NNX09AB39G