The microearthquake analysis method which is used consists of inversion for fault plane solutions by use of spectral amplitudes and first motion directions [48,42,43] and multievent location based on high accuracy relative timing of the phase arrivals [50] Both these algorithms are in routine use at the Icelandic seismological network within the PRENLAB-2 project. The multievent analysis aiming at inversion for the rock stress tensor is discussed in a later section.
The fault plane solutions do not consist of a unique fault plane solution for each event, but of a number of different fault plane solutions consistent with the observed spectral amplitudes and first motion directions. In addition each acceptable fault plane solution consists of two possible fault planes oriented such that the slip direction on one of the plane is normal to the other plane. Together this complicates the task to achieve a clear picture of what crustal deformation (fault movements) the microearthquakes are part of.
In a number of cases the relative locations of closely spaced similar microearthquakes have shown that such events often are situated on a plane which can be interpreted as the fault plane. This interpretation has also a sound physical basis as a slip on part of a fault will increase the instability at neighbouring areas if we have a rather similar rock stress tensor over the fault prior to the slip.
The most simple interpretation of a group of microearthquakes is to look for microearthquakes located along a plane and having acceptable fault plane solutions where one of its two possible fault planes coincides with the spatial plane. If one also requires that the slip directions on the plane are similar the process may be used to achieve a single fault slip solution for each microearthquake on the plane. The range of acceptable fault plane solutions has been reduced with no remaining ambiguity for the events of the consistent group.
In practice one will expect complications as the range of acceptable fault plane solutions may be large and random fits may occur. We will here give examples of multievent microearthquake analysis where this procedure for reducing the ambiguity of the fault plane solutions have been used.