The results of PRENLAB research during 1996-1998 are listed task by task in
Chapter 3, related to the projects and tasks as these were described in its workprogramme.
- The multidisciplinary cooperation in PRENLAB is a significant progress to be reported. It is not so usual that seismologists, other geophysicists and geologists have cooperated on such a wide basis in projects aiming at seismic risk. In future this cooperation will be still deepened, and gradually more disciplines taken in.
- It has been demonstrated in several studies involving work of
seismologists, geologists and geophysicists that it is possible on basis of
microearthquakes to
map subsurface faults with a great accuracy. A good agreement is between such
studies within the project based on microearthquakes and the results
of studying
the faults on the surface when these are exposed. But new faults have been found and their tilts and sense of motion found in many cases. There are indications that arrangements of individual fault planes of microearthquakes along faults express fluid activity in the faulting process.
- It has been demonstrated that stresses inferred from microearthquakes coincide generally
very well with what can be expected, based on paleostress studies of the geologists
in this project, and in agreement with results gained from the borehole experiment. One of the problems
of interpretation of double-couple focal mechanism solutions of earthquakes
is to distinguish between the fault plane and the auxiliary plane of the solution. Methods
for doing this have been developed with three different approaches,
by comparison with geological fault data, by basing the selection on groups of
solutions on the same fault and by applying stability criteria
on the possible fault plane solutions for each earthquake.
- Among significant new results that can be reported is that changes of
shear-wave splitting at one of the SIL stations in the South
Iceland seismic zone (SISZ) indicate changes of deviatoric stress with time. It has been proposed that this may be attributed to the preparatory stage of the eruption in Vatnajökull which started on
September 30, 1996. An increase of deviatoric stresses was indicated, starting at the seismic station SAU inside the SISZ about 5-6 months before the eruption, at 160 km distance from the eruptive site. The increase in stress continued until around the start of the eruption. It was suggested that intrusive activity at great depth under the eruption site caused the stress changes. Volumetric borehole strainmeters at two sites in the same area also indicated change in strain rate during the same period of time. The more general interpretation of these findings is that these changes indicate a strain wave going through Iceland during these 5-6 months. These indications may be of enormous significance for understanding strain waves and how these are transmitted. Strain waves have been defined in Iceland on basis of historical observation of clustering in time of large earth activity in an area spanning several hundreds of kilometers. The main problems with explaining interaction between stress release events over long distances are that in the most used earth models, i.e. elastic half space models and similar models, the stresses will attenuate so strongly with distance that the daily changes caused by earth tides and changes in atmospheric pressure would be more effective in triggering earth events than stress changes from earthquakes at a large distance. Therefore it has been proposed that strain waves should be attributed to common source of large intrusive activity deep in the brittle crust or below it which trigger events at sites which are ripe for release. To understand strain waves has relevance both for understanding their sources as well as how they are transmitted. The observations above may be a key to this understanding.
To be able to explain strain waves physically opens the possibility to predict increased probability
for triggering of earthquakes based on monitoring of stress changes from outside of the seismic zones. It is to be pointed out that one of the main pillars for
using Iceland as a test area for earthquake prediction was that it would be
possible to monitor stress or strain changes caused by measurable
pulsations of the Iceland plume. It was pointed out that such understanding would have a consequence in general
for understanding how stresses are transmitted in the crust anywhere.
- The geological field studies and interpretations have
revealed significant variations in the direction of stress fields
in the same area. Such changes have also been indicated in evaluating
microearthquakes. Former modelling of historical activity also shows
the significance of such changes in time and space.
- A result of a great significance is that it has been
shown that it is possible
to use satellite radar interferometry for monitoring stable plate motion during
a period of a couple of years in the favourable conditions that prevail
in Iceland. The relative average plate motion being only of the order of 2 cm per year. This is of enormous significance for constructing a dynamical model
of stress build-up in an earthquake area.
A semicontinuous GPS station in the middle of the SISZ shows significant relative plate motion, compared to a station outside the zone during the first year of the experiment, of 11.5 mm. The motion appears to be in the same direction during the one year of observation, averaging to 1 mm per month. No significant earthquakes have occurred during this time, i.e. the motion is stable for the time being. The result shows how a powerful and significant tool continuous GPS monitoring is in the area. Because of noise with period of the order of days, continuous measurements allow us to see much smaller changes than measurements which are made only on a long-term basis. The results obtained highlight how significant it is to increase the number of GPS with continuous monitoring in the area, both for modelling of the motion around the plate boundary as well as a for alerting about changes that may occur in the stable motion, and to look for concentration in time and space of the observed motion.
- It has been demonstrated on two occasions by observations and
modelling how fluid intrusion may trigger earthquakes. In one case this
was a magnitude 5.8 earthquake, in the other it was an intensive earthquake
sequence. This may be
a key to explain or understand foreshocks which are frequently reported before
large earthquakes in Iceland.
- The high ongoing seismic activity in the Hengill area indicates many kinds of patterns that may characterize source areas of large earthquakes, like foreshock-mainshock relations and migration of activity. Study and modelling of such patterns in this area may be a key to understand premonitory activity and nucleation patterns in large earthquakes.
- The multidisciplinary observations within the frame of the PRENLAB project are focussed in modelling work of many kinds. Modelling work has been carried
out with the purpose of interpreting observations in various fields. An
example of this is the modelling of observed tension gashes on the
surface following an earthquake, based on knowledge of the upper crustal
structure. By this modelling it was concluded that it is possible to
draw conclusions about the underlying stress field from open fissures
striking a few degrees away from the fault strike.
Modelling work has
been carried out also with the aim of adapting and developing existing
methods for the special conditions in Iceland, with its rifting, time
dependency of stress field, etc. A model has been created which describes magma intrusion into two layered crust, where the lower layer is of much lower rigidity than the shallower layer. This model which is an earth realistic model for some areas favours the ideas that stress changes from faulting or rifting may cause more significant stress changes at distance than previously assumed.
