Earthquake-prediction research in a natural laboratory
PRENLAB 
EC project, Environment and Climate 1994-1998
Contract EENV4-CT96-0252
Proposal PL950257, 2.3.2 Seismic risk

Subproject 1

Real-time evaluation of earthquake-related-processes and development of database, and coordination of the project:

Ragnar Stefánsson, coordinator/contractor

Department of Geophysics, Icelandic Meteorological Office
150 Reykjavík, Iceland

e-mail: ragnar@vedur.is
Tel 354 5600600, Fax 354 5528121

Status report no 1, November 29, 1996

Milestones

Here I will describe the status the project referred to the revised workprogramme.

March 1 Dr. Einar Kjartansson was hired to the project and he has been working on it full time since that. We expect that 4 staff members of Department of Geophysics have been working full time since March 1 on the project.

Task 1 Database, development and service

Since the start of the PRENLAB project in March the number of SIL stations in operation has increased from 18 to 27. The number will increase to 32 during the next weeks, if weather conditions will allow. This is an increase in the size of the SIL network which in many ways is comparable to what was applied for in the proposal to EC, the Environment program. According to our own evaluation the cutting of our Evironment application did not permit any new stations. The new stations are funded by Icelandic communities, hydrothermal and hydroelectrical power companies, civil security organizations, the Icelandic research council, and indirectly by research groups carrying out tomographic studies, which can make use of the powerful SIL acquisition system. The largest supporter of this build up project of the SIL system is The Icelandic Meteorological Office/The State of Iceland, which besides contributions to the initial costs garantees the operation of the system.

All the data collected by this large extended network will of course be open to the project.

At the moment, and so it will be during the next two years there will be more than 30 extra broad band stations operated continuously and remote at places not covered by the SIL system, mainly for collecting teleseismic data. This is a part of the Hot Spot project, lead by Gillian Foulger, Durham. Among other participants are Princeton University, with Jason Morgan and Guus Nolet and Bruce Julian of the Geological Survey, besides the Meteorological Office. Plans are in preparation for including the wave form information from these stations into the SIL evaluation processes, especially as concerns the local activity. This is a very significant addition to the data that we can approach for the PRENLAB project and can provide us a much more general overview about the stress condition in the country as a whole as the Hot spot station are distributed where we have no or few SIL stations. A refined and easily accessable database for SIL data is under construction. The new database contains currently all seismogram data from 1995 and events and manually verified phase arrival data since january 1. 1995.

All data will be kept online on hard disks. Seismogram data is stored using packed binary format where only the number of bits that is required to store sample to sample variation is stored.

Other data is stored in relational database tables. Station parameters such as coordinates, instrument characteristics and time corrections are stored in seperate tables. This information is incorporated into headers when data is extracted from the database.

Data from the SIL system have been provided for the PRENLAB cooperative projects in Uppsala (Böðvarsson), Edinburgh (Crampin) and Paris ( Angelier, Bergerat).

Work has been carried out for a new, reevaluated and refined catalogue of earthquakes in Iceland since 1926. This catalogue is now awailable in digital form since.....

Spatial changes in seismicity have been studied in an area along the Reykjanes Peninsula, the South Iceland Lowland and into the eastern volcanic zone.

Work has been carried out for refined estimation of magnitudes and locations of historcial earthquakes.

Much work has been carried out in intepreting data from volumetric borehole strainmeters. Premonitory changes and coseismic changes, volumetric strain and earthquakes (foreshocks), of the 5.8 magnitude earthquake at Vatnafjöll, near the eastern end of the South Iceland Seismic zone have been studied with results that indicate fluid intrusion coinciding with foreshocks and the main shock.

A long term oweview (since 1979) of the 7 Icelandic volumetric strainmeters is being worked out. Methods have been developed for cleaning the strainmeter record of weather influences.

The seismicity of volcano Katla which is beneath the Mýrdalsjökull glacier has been studied. Eruptions in Katla pose a considerable danger because of enormous water- and mudflows which accompany the eruptions. It is one of the objectives of the SIL network to help to warn for the eruptions.

Although the SIL system is a seismic data acquisition system that is primarily designed for automatic acquisition and evaluation of data from local microearthquakes it can also be used for collecting teleseismic and regional data for deep structure studies. It broadens the scientific use of the network and has made it easier to extend the network to a large part of the plate boundary in Iceland. The SIL station software has now been modified allowing for selection of waveform data additionally at 20 and 4 samples per second. This makes it economically possible to save long time periods of seismological data from the SIL stations. We have developed an automatic procedure to select and store teleseismic data in the SIL system based on USGS/NEIC information on teleseismic events in the whole world which are measureable in Iceland. From USGS/NEIC we receive E-mail messages with a single-line information on earthquakes they have determined, the so called "E" type messages. A selection program reads the messages and selects events that fulfill certain criteria of magnitude and epicentral distance. The program uses the iaspei91 model to compute the first arrival time at each station. The teleseismic body wave data are fetched with a sampling rate of 20 samples ( in some cases 100 samples) per seconds and the surface wave data with sampling rate of 4 samples per second from the 1-3 days long ringbuffer of the SIL site stations

Work has been carried out for studying and refining the alert thresholds for the SIL related alert system in Iceland.

An alert detector monitoring large amplitudes, background noise (tremor) and directivity of the microseisms has been tuned for the 1Hz sensors of the system, using Hekla tremor from 1991 as test set. Implementation and tuning of a bandpass filter is needed for broad-band sensors.

A real time filter has recently been introduced into the on line process of the system, to be tuned for detecting signals and harmonic tremor, which cannot be detected through the automatic event detection of the SIL system. The continuous seismic signal at the SIL site stations is bandpass filtered at 0.5-1Hz, 1-2 Hz and 2-4 Hz and the 1 minute mean amplitude is scanned and sent to the SIL center, where the interpretation of characteristics of the tremor is carried out and linked to the alert system. Visual presentation of this data gives a useful indication of the multiplicity of activity in real time.

The extension of the SIL system into the highlands of Iceland has lead to many problems in the automatic detection and analysis. The SIL system was developed in seismic zones. Monitoring in the highlands reveals in many ways new problems. Much work has been carried out to lower the detection threshold for earthquakes in the volcanic central Iceland. The new real time filter mentioned above will be used for this purpose together with tuning of parameters.

Work has been carried out on recent high seismic activity near the Hengill tripple junction in SW Iceland, spatial and temporal varations of activity have been studied and migration within the area. Fault plane solutions of more than 20.000 earthquakes have been studied in the area. Work on detailed mapping of seismically active faults in the Tjörnes fracture zone based on multievent hypocenter locations and on fault plane solutions. This has been done in cooperation with Nordic Volcanological Institute.

Work has been carried out to find 3-D crustal Velocity structure in SW Iceland from local earthquake tomography in cooperation with Uppsala University

Work has started on a method to use cross-correlation of waveforms to accurately and automatically determine onsets and classify earthquakes, in cooperation with Uppsala University. There has been close cooperation is with Uppsala in various other fields, stress tensor inversion procedures, mapping of faults etc.

References

Angelier, J., Rögnvaldsson, S. Th., Bergerat, F., Guðmundsson, Á., Jakobsdóttir, S. and Stefánsson, R., 1996: Earthquake focal mechanisms and recent faulting: A seismotectonic analysis in the Vordufell area, South Iceland seismic zone. In: Thorkelsson, B. (editor) Seismology in Europe. Papers presented at the XXV ESC General Assembly. ISBN 9979-60-235-X. Reykjavík, pp. 199-204

Ágústsson, K. ...1996. The 1987 Vatnafjöll earthquake south Iceland viewed by strainmeters: information on source processes from associated deformation. Abstracts for the 25th ESC General Assembly. Kristjan:

Ágústsson, K., 1996. Continuous strain measurements in SW Iceland with focus on the 1987 Vatnafjöll earthquake and the 1991 Hekla eruption. Thesis for Fil. lic. degree Uppsala, 1966, pp 1-34.

Ágústsson, K., Alan Linde, Ragnar Stefánsson, 1996. The 1987 Vatnafjöll earthquake south Iceland viewed by strainmeters: Information on source processes from associated deformation. In: Thorkelsson, B. (editor) Seismology in Europe. Papers presented at the XXV ESC General Assembly. ISBN 9979-60-235-X. Reykjavík, pp 175-180.

Ágústsson, Kristján, Ragnar Stefánsson, Alan T. Linde, Selwyn Sacks, 1996. Long-term strain variations in SW-Iceland.Abstracts for the 25th ESC General Assembly.

Ágústsson, Kristján, 1996. Þyngdarmælingar á Suðurlandi (A report on gravity measurements in the south-west part of Iceland). Rit Veðurstofu Íslands

Ágústsson, Kristján, 1996. Alvakinn. ( A report on testing and tuning of the seismic alert system). Rit Veðurstofu Íslands

Böðvarsson, R., Rögnvaldsson, S. Th., Jakobsdóttir, S., and Slunga, R., Stefánsson, R. 1996. The SIL data acquisition and monitoring system. Seismological Research Letters. pp 35-46.

Böðvarsson, R., Rögnvaldsson, S. Th. and Slunga, R., 1996: Waveform correlation as a tool for automatic phase picking. Abstracts of the XXV General Assembly of ESC, Reykjavík. p. 32.

Guðmundsson, G., 1996. Seismicity in the central volcanoes beneath Mýrdals and Eyjafjallajökull. Abstracts for the 25th ESC General Assembly, S1.14, p. 101.

Guðmundsson, G. Steinunn Jakobsdóttir, Reynir Böðvarsson, 1996. Automatic selection of teleseismic data in the SIL system. Abstracts for the 25th ESC General Assembly. B2.09, p. 34.

Halldórsson, P., Skaftadóttir, Th., and Guðmundsson, G. 1996. A new catalogue of earthquakes in Iceland, 1926-1974. Abstracts for the 25th ESC General Assembly. W6.11, p. 156.

Halldórsson, P. 1996. Estimations of magnitudes of historical earthquakes. Abstracts for the 25th ESC General Assembly. W1.07. p. 138.

Halldórsson, P. 1996 Spatial changes in seismicity on the Reykjanes peninsula and South Iceland lowland. Abstracts for the 25th ESC General Assembly. A1.07. p. 4.

Jakobsdóttir, S., 1996. The SIL-network: The need of automatic processing in seismically active areas. Abstracts for the XXV General Assembly of ESC.

Jakobsdóttir, S., Scherbaum, F., 1996. Effects of the acausal response of zero phase FIR filters on the onset time determination of P waves for intermediate and big earthquakes. Abstracts for the XXV General Assembly of ESC.

Jakobsdóttir, J. 1996. Alert-detector in the SIL-network. Abstracts for the XXV General Assembly of ESC.

Kjartansson, E. 1996. Database for SIL earthquake data. Abstracts for the 25th ESC General Assembly, SC-B2. p. 32.

Linde, A.T., Sacks, I.S., Stefánsson, R., Agustsson, K., Kamigaichi, O., Kanjo, K. Strain measurements and volcanic eruptions. Abstracts for the XXV General Assembly of ESC. p. 100.

Mochizuki, M., Shiobara, H., Shimamura, H., Stefansson, R. and Gudmundsson, G.1996. Crustal structure beneath Reykjanes ridge obtained by OBS refraction study. Abstracts for the 25th ESC General Assembly. S3.02, p. 111.

Mochizuki, M., Shimamura, H., Stefansson, R., Shiobara, H., Gudmundsson, G. and Brandsdottir,B. 1996. Microseismicity and crustal structure offshore north of Iceland. Abstracts for the 25th ESC General Assembly. S3.03, p. 111.

Rögnvaldsson, S. Th., Guðmundsson, G., Ágústsson, K., Jakobsdóttir, S. and Stefánsson, R., 1996: Recent seismicity near the Hengill triple-junction, SW Iceland. In: Thorkelsson, B. (editor) Seismology in Europe. Papers presented at the XXV ESC General Assembly. ISBN 9979-60-235-X. Reykjavík, pp. 461-466.

Rögnvaldsson, S. Th. and Flóvenz, 1996: 3-D P- and S-wave velocity structure beneath South-west Iceland derived from local earthquake tomography. Abstracts of the XXV General Assembly of ESC, Reykjavík. p. 59

Rögnvaldsson, S. Th., Guðmundsson, G., Ágústsson, K., Stefánsson, R. Jakobsdóttir, S. 1996. Recent seismicity near the Hengill triple-junction, SW Iceland. Abstracts for the 25th ESC General Assembly. S1.33, p. 106.

Rögnvaldsson, S. Th., Guðmundsson, Á. and Slunga, R., 1996: Seismicity and style of faulting in the Tjörnes fracture zone, North Iceland. Abstracts of the XXV General Assembly of ESC, Reykjavík. p. 74.

Shimamura, H., Stefansson, R., Mochizuki, M., Watanabe, T., Shiobara, H., Gudmundsson, G. and Einarsson, P. 1996. Northern Reykjanes ridge microseismicity revealed by dense OBS arrays. Abstracts for the 25th ESC General Assembly. S3.01, p. 111.

Stefánsson, Ragnar, 1996. Towards Earthquake prediction in Iceland. In: Thorkelsson, B. (editor) Seismology in Europe. Papers presented at the XXV ESC General Assembly. ISBN 9979-60-235-X. Reykjavík, pp. 3-8.

Stefánsson, R., Bodvarsson, R. and Guðmundsson, G. 1996. et al 1996. Iceland plume tectonics. Some speculations and facts. In: Thorkelsson, B. (editor) Seismology in Europe. Papers presented at the XXV ESC General Assembly. ISBN 9979-60-235-X. Reykjavík, pp. 505-511.

Stefánsson, R., Bodvarsson, R. and Guðmundsson, G. 1996. Iceland plume tectonics, some speculations and facts. Abstracts for the 25th ESC General Assembly. S4.03, p. 117.Tryggvason, A.,

Stefansson, R., Palmason, G., Palsson, H., Einarsson, P., Sigbjornsson, R., Halldorsson, P., 1996 Tillögur um aðgerðir til að draga úr hættu af völdum jarðskjálfta (In Icelandic. proposals for actions for mitigating earthquae risk). Reykjavik, 63 p.

Tryggvason, A., Rögnvaldsson, S. Th. and Flóvenz, 1996: 3-D P- and S-wave velocity structure beneath South-west Iceland derived from local earthquake tomography. Abstracts of the XXV General Assembly of ESC, Reykjavík. p. 59

New proposal

Task 1: Database development and service for other scientists. This task continues through the second phase of the project. New data have to be incorporated because of the geographical extension of the SIL monitoring system, as well as because of the extension of data included in the monitoring system. Based on ongoing research new interpretations of the multiplicity of awaileable data will continuously be incorporated in the database information. The other participants will be served with information fro the ever growing database.

Task 2: Enhancing the basis for alerts and warnings Bring together research results from the project partners and from other scientist for identifying risk areas within the seismic zones, and to describe their properties. The questions to try to answer are:

a) Where in the seismic zones will the next large earthquake occur.

b) How large motions are to be expected, What fault plane(s) and what type of time function is to be expected.

c) When can an earthquake be expected? In the terms of classical hazard assessment procedure this is done in terms of probability of occurrence during a period of time, based on stocastic models. Here we go a step further to evaluate the possibility use information obtained on changes in the confining stress or strain changes to tell about increasing probability of earthquake triggering. The changes of the confining stress conditions are indicated by historcal evidence, and have been named strain waves or strain epizodes. There are strong indications from studies of microearthquakes and from borehole strainmeter monitoring in Iceland that such changes can be monitored and have the potential medium term warnings based on increased probability of triggering..

d) To evaluate the possibility of short term prediction? It is known from history that many large earthquakes in Iceland are preceded by foreshocks or large precursory swarms or earthquakes. What is the mechanism of this precursory activity in various earthquake sites and how is it related to a following large shocks. Is it possible to transfer achieved knowledge stress/strain changes preceding a medium size earthquake to reveal precursors of large earthquakes. Testing of proposed algorithms.

Task 3: To predict the detailed characteristics of earthquake motion in the near field of catastrophic earthquakes in Iceland. This involves in first hand to modell the destruction that was caused in some of the historical earthquakes. Detailed information which is available about destruction caused by the magnitude approximately 7 earthquakes in the South Iceland seismic zone 1784, 1896 and 1912 are a base for such a modelling. But the knowledge about the general character of the SISZ faulting processes which are evolving in the PRENLAB project will also be a significant input in this modelling.

Task 4: Extending the alert system functions by real time research. The SIL system technology and the advances in real time monitoring and analysis during the PRENLAB period make it possible to evaluate in real time changes of stresses and development of faults, this together with knowledge of the physics of earthquake release and faulting in the region will enhance our possibilities for mitigating seismic risks. Monitoring of a large seimic activity can enormously increase our undestanding of the earthquake processes and its destructive effects. A sceme will be set up for utilizing in real time the new information achieved during high seismic activity. The purpose spans everything from helping in rescuing and in evacuation during destructive activiy by mapping the areas most likely to be severely struck to predict the progbable site and effects of a probable following earthquake.

Task 5: To prepare the SIL system and the alert system for use elsewhere. Significant parts of the SIL system algorithms are written long time ago and developed on basis of experience gained in Sweden and Iceland during at least 20 years. Many seismologists groups are interested to obtain the SIL system as a whole or at least significant parts of the software. This is on the other hand difficult to use by those who have not participated in this long term development. Many parts of the SIL system software has to be rewritten in a form which makes it more easily useful for other seismologists. It is significant for the success of our project that other seismologists go along the same path in prediction research and it is significant for the process of the invironment program to export the development of SIL and of PRENLAB.