Do "silent" tremors indicate severe tremors?
So-called silent or aseismic tremors - their crustal movements are too slow to trigger measurable seismic waves - probably cause subsequent detectable earthquake swarms. Overall, they probably also increase the pressure on the rock plates involved and could therefore be one of the reasons for severe earthquakes with a magnitude of 8 or 9 on the Richter scale, geologists working with Paul Segall from Stanford University suspect.
The researchers are therefore asking their colleagues worldwide to look for signs of these weak earthquake swarms in order to detect potential geotectonic sources of danger at an early stage. They themselves studied the phenomenon at the Hawaiian volcano Kilauea, under which four aseismic earthquakes took place between 1998 and 2002. They were detected using a dense network of GPS stations around the southern flank of the mountain, which revealed changes in the position of areas of the volcano. One of the silent tremors even reached a magnitude of 5.7 and triggered a movement of 10 centimeters of 2000 cubic kilometers of rock. However, since the movement took place over a period of 48 hours, it went unnoticed by people on site like seismometers.
But within two days, the scientists noted another sixty small tremors with a maximum magnitude of 3 - a sum that is six times as high as under normal conditions. After evaluating the GPS data, Segall and his colleagues came to the conclusion that the silent quake must have triggered these tremors. With the help of the quake swarm, they were then able to pinpoint the epicenter of the aseismic event, which was five miles below Kilauea.
With the help of these findings, the researchers now want to find out to what extent the silent tremors contribute to the build-up of pressure at plate boundaries and can therefore be a contributor to severe earthquakes. It is possible that the magnitude of silent tremors increases during an earthquake cycle between two severe tremors with the increasing tension of the rock packages, so that this could also be used to predict how far the cycle has progressed. It is also to be clarified whether silent tremors show a regularity in their strength like "normal" tremors. These follow a magnitude 6 earthquake, about ten magnitude 5, a hundred magnitude 4, and so on.
Quiet earthquakes have recently been detected along various subduction zones, for example in the north-west Pacific, off Japan and Mexico. At least off the American Pacific coast, they return at regular 14-month intervals and this periodicity raises hopes that severe earthquakes in the region can be better predicted. Silent earthquakes can sometimes last up to two weeks before the movement of the crustal parts comes to a standstill.