Why are there apparent clusters of earthquakes at 10 km depth?
The location of an earthquake inside the earth (also called hypocenter), i.e. its epicenter (point on the surface above the quake) along with its depth under the surface can be determined by triangulation if at least 3 different seismometers have recorded it.
As seismic instruments are on or very near the surface, the precision of the latitude/longitude of the epicenter is much greater than the depth of the quake. In cases of smaller quakes or during quick initial calculations, the uncertainty of the depth is often too large to give a meaningful value. In such cases, most agencies will report "10 km" (or sometimes 0) as an assumed typical value.
When making lists and plots of quakes, this will therefore result in a fake cluster of quakes at 10 km depth.