The location of an earthquake – the geographic longitude, latitude and depth of the seismic focus – as well as the exact time of its occurrence can be modelled from the data recorded at seismic monitoring stations using what are known as inversion methods. These methods rely on the fact that pinpointing the location and time is closely linked to the propagation velocities of seismic waves – the two are interrelated. How accurately the location and time of an earthquake can be determined is therefore heavily dependent on how well the propagation velocity of the seismic waves in the subsoil is known. The geometric distribution of monitoring stations is also key to the measurement quality.
For earthquakes in Switzerland, where we have a dense network of high-quality monitoring stations, the parameter that is least easy to determine is usually the earthquake depth. This is particularly true of relatively shallow quakes, occurring in the five kilometres immediately beneath the Earth's surface. This is because the propagation velocity of seismic waves can vary substantially here and the distribution of stations around the earthquake is often not conducive to accurately determining the depth.
The depth of an earthquake is always given relative to the geographic zero-height point of the coordinate system used, with positive values below that point. Here, we use the international standard WGS 84 or the Swiss national coordinate system LV95. The geographic zero-height point in Switzerland is the sea level of the Mediterranean at Marseille (adjusted based on the sea level at the mouths of the Rhine, Inn/Danube, Ticino/Po and Rhone). For more information, see: https://www.swisstopo.admin.ch/en/faq-geodesy-ns
As the Earth's surface in Switzerland is between 193 metres (level of Lake Maggiore) and 4,634 metres above sea level, earthquakes can also occur above sea level, meaning that they have negative depth values. In most cases, however, negative depths mean that the quake occurred relatively close to the surface, usually in the top two to three kilometres. Moreover, negative depths often indicate a relatively high degree of inaccuracy of the calculated value (within approximately one to three kilometres). For example, an earthquake with a depth of '-1.4 km', such as the one above Aigle, near Montreux, on 9 July 2021, is likely to have occurred somewhere between one and three kilometres below the Earth's surface at that point.