Work package number: WP4
Work package title: Establishment of Borehole Observation System and High Resolution Seismic Studies in the Marmara Sea
WP Leader: IU
The main objective of this WP is to install a multi-parameter borehole system and surface array as closest as to the main Marmara Fault (MMF) in the western Marmara Sea, and measure continuously the evolution of the state of stress of the fault zone surrounding the MMF and to detect any anomaly or change which may occur before earthquakes by making use of the data from the arrays already running in the eastern part of the Marmara Sea. The data from the new borehole system is also integrated to the data set to be used in the analysis.
The key objectives of this WP are the following.
- To design and build multiparameter borehole system consisting of very wide dynamic range and stable borehole (VBB) broad band seismic sensor, and incorporate 3-D strain meter, tilt meter, and temperature and local hydrostatic pressure measuring devices,
- Determination of 1-D S-wave velocity structure beneath the borehole system by array microtremor measurements,
- Combining borehole and surface network data for earthquake location improvement,
- Determination of surface and near-surface effects on seismic waves,
- Estimation of the near-surface geology effects masking the source related information.
- Monitoring rupture nucleation and propagation using borehole and surface array data,
- Closely inspection and monitoring of the last stages of the preparation stage of a major rupture,
- To measure continuously the evolution of the state of stress of the fault zone surrounding the Main Marmara Fault (MMF), and to detect any anomaly or change which may occur before earthquakes,
- To identify the presence of repeating earthquakes along the MMF,
- To better understand the existing seismically active structures and their role in local tectonic settings,
- To understand the stress transfer mechanism from east to west,
- To obtain continuous high resolution locations of micro-seismicity including development of relative location technique from cross-correlation methods (multiplet analysis) and study the space-time evolution of the activity,
- To search for low frequency events (non-volcanic tremor) from continuous recording,
- To analyze triggering mechanisms like Coulomb transfer, aseismic slip, or fluid migration and possible declustering methods.
These activities will be performed by four well instrumented networks; one is multiparameter borehole system which will be deployed in the frame of this project, in the western part of MMF, and the other three which are already running are; 1) PIRES array having two sub arrays each consisting of five seismic stations located on the Princess Islands, very close to the MMF, 2) CINNET array located around Cinarcik Basin in the eastern part of MMF. The data from these networks will be integrated and outputted to other workpackages, especially to WP2, WP3 and WP6.
Description of work
Task 1. Deployment of surface microearthquake array and borehole seismometers and integrating of borehole and surface array data for better location of microearthquakes
Bringing face to face the seismograms of microearthquakes recorded by borehole and near-surface instruments portrays quite different contents. The shorter recording duration and nearly flat frequency spectrum up to the Nyquist frequencies of borehole records are faced with longer recording duration and rapid decay of spectral amplitudes at higher frequencies of a surface seismogram. The main causative of the observed differences are near-surface geology effects that masks most of the source related information the seismograms include, and that give rise to scattering, generating longer duration seismograms.
In view of these circumstances, studies on microearthquakes employing surface seismograms may bring on misleading results. Particularly, the works on earthquake physics and nucleation process of earthquakes requires elaborate analysis of tiny events. It is obvious from the studies on the nucleation process of the 1999 Izmit earthquake that tens of minutes before the major rupture initiate noteworthy microearthquake activity happened (Bouchon et al.,). The starting point of the 1999 rupture was a site of swarm activity noticed a few decades prior the main shock. Nowadays, analogous case is probable in western Marmara sea region, prone to a major event in near future where the seismic activity is prevailing along the impending rupture zone. Deploying a borehole seismometer e.g. in Marmara Island and/or eastern end of the Ganos fault zone may yield invaluable data to closely inspect and monitor the last stages of the preparation stage of major rupture.
The proposed Multiparameter digital Borehole seismic station will use the latest update technologies and design ideas to record “Earth tides” signals to the smallest magnitude -3 events. As the region is seismically active, strong motion sensors will also be incorporated so as to cover the widest dynamic range ever recorded in borehole systems. The Measurements of other parameters with the confines of a deep borehole will assist the study of seismology to give better understanding of seismic activities. The additional parameters are: local tilt, temperature, local hydrostatic pressure, local “3-D strain” and acoustic signals. The data collected by this system will be used in other tasks in the package, especially Task 2 and Task 3.
Task 2. Analyzing response of near-surface geology to earthquake ground motion and its effects masking the source related information through borehole data
This task will take advantage of the data from multi-parameter borehole system and surface array (Task 1). Strong motion data recorded by vertical arrays of accelerometers offer the opportunity to study the propagation of the waves in the subsoil. However, boreholes recording, that due to the lower signal-to-noise ratio and a smaller effect of the shallow layer attenuation might seems to be more appropriated to study source parameters and to determine the input ground motion, are affected by the contamination of down-going waves (Parolai et al., 2009) that have therefore to be removed before any estimation of source parameters can be carried out. On the other hand, the surface recordings might be strongly biased by the site response that has to be accounted for. Within this task, different methods for correcting the borehole seismograms, both based on the knowledge of the subsoil structure or independent from it (Bindi et al., 2010), will be tested, improved, and developed. The influence of the amplitude of ground motion recorded in the results of the different correction schemes (and therefore in turn on the source parameters, and in the estimation of the input ground motion) will be estimated. A close cooperation is envisaged with Task 1 of WP9, where recordings from data installed in nearby buildings (SOSEWIN) will also be examined.
Task 3. Monitoring the fault zone and source process in the near field
Continuous evolution of fault zone properties:
The zones surrounding major faults are highly fractured and their physical properties (density of open fractures, fluid presence and circulation, velocity of seismic waves) evolve with their state of stress. Recent advances based on seismic noise correlation techniques (Campillo and Paul, 2003), allow very fine measurements of possible changes in seismic wave velocities over time. We propose to use these techniques to monitor the evolution of seismic wave velocities in the zone surrounding the Main Marmara Fault (MMF).
We will proceed in two major stages. First, we will test the application of seismic noise correlation to the Marmara Sea environment and to the configuration of its permanent seismic stations. This will help define the stations and the paths of the seismic waves, which best sample the fault zone.
In a second stage, we will try to set-up and organize a semi-automatic system, which, from the Kandilli Observatory, will correlate the seismic signals and make a daily monitoring of the seismic wave velocities in the fault zone.
The objective of this work is to measure continuously the evolution of the state of stress of the fault zone surrounding the MMF and to detect any anomaly or change which may occur before earthquakes.
Search for repeating earthquakes on the Main Marmara Fault:
Recent advances in measuring deformation at the earth surface have changed considerably our view and understanding of how tectonic plates move relative to one another. With the discoveries of non-volcanic tremors, low-frequency earthquakes, slow slip events, and silent earthquakes over the last decade, our knowledge of the dynamics of plate motion has changed dramatically and has been greatly enriched. We now know that plate boundaries can slip in a variety of ways between the continuous slip that takes place at depth and the catastrophic slip of the large earthquakes. Because all the slip modes of a plate interface, like the North Anatolian Fault, necessarily interact with one another, understanding where and when large earthquakes will occur requires accurate measurements of the various modes of slip of the interface. In this respect, repeating earthquakes have proven to be a powerful tool to investigate if a fault is slowly slipping (Vidale et al., 1994). Repeating earthquakes are small seismic events, typically a few hundred meters in size, which occur repetitively from time to time at exactly the same place. Each time they rupture the same patch of the fault, which requires a continuous reloading of stress. This reloading is produced by the slow slip of the fault area surrounding the patch. Thus, the simple presence of repeating earthquakes on a fault segment implies that this segment is slowly creeping. The amplitude and timing of these repeating events gives an estimate of the amount of slow slip that is occurring.
We will try to identify the presence of repeating earthquakes along the Main Marmara Fault. In a first stage this will require the fine relocation of events that occur along the various segments of the MMF and, in a second stage, the development of a computer code able to identify recurrent seismic waveforms. If we find these repeating events, we will model their source and study their timing to infer the presence and the rate of slow slip occurring. The observation that the Izmit earthquake began 44 minutes before the catastrophic rupture with the slow slip of the fault at depth, accelerating as the time to the earthquake was approaching (Bouchon et al., 2011), emphasizes the close connection that we think exists between slow slip and the nucleation of large earthquakes.
Task 4. High Resolution Seismology in Marmara Sea with Arrays
The Çınarcık Basin (CB) is the complex transition zone between the unruptured main Marmara Fault and the recently ruptured zone of Izmit-Duzce earthquake sequence. The details of the actual deformation in the basin are poorly understood despite the fact that it is critical in understanding the stress transfer mechanism from east to west. The microseismic activity provides the most effective keys for developing a realistic model of the on-going deformation (Bohnhoff et all, 2006). The seismic events on the inferred Northern Boundary Fault on the Cinarcik Basin are rare but provide the essential clues for the prominent rupture in Marmara Sea (Örgülü & Aktar, 2002; Karabulut et al, 2002; Özalaybey et al, 2002). Since the seismogenic zone is entirely offshore and the number of permanently operating OBS is very limited, a permanent seismic array (PIRES) was installed on the Prince Islands, at few kilometres distances to the fault. The array consists of two subarrays installed on the two outermost islands (Yassiada and Sivriada) of the Princes Islands group offshore Istanbul. Each PIRES subarrays consist of five seismic stations at the surface spaced in a cross-shape layout. The use of the array data improved by an order of magnitude both the detection and the resolution capabilities of the seismic monitoring on the northern boundary of CB (Bulut et al, 2009). The network has recently been enlarged towards the other Princes Islands in order to improve the azimuthal control of the focal area. We additionally integrate data from local permanent stations; the ARNET seismic network on the Armutlu peninsula and CINNET array (Task 5). Combined data allowed obtaining a well-resolved scale for the hypocentral map to better understand the existing seismically active structures and their role in local tectonic settings.
Second order source properties of the local activity will also be monitored in view of the observations published recently by Bouchon et al. (2011) related to the precursory phenomena of Izmit Earthquake (1999). PIRES arrays were already used to reveal the fine details of time dependent spectral properties of seismic swarms in Çınarcık Basin (Bulut et al, 2011). We plan to monitor whether the well-known scaling of corner frequency and moments holds for repeating events of various order of magnitude or if an intermediate behaviour similar to the Izmit Precursors can be observed.
The PIRES arrays will also be used to study the structural properties of the fault zone. Since the Marmara Fault is entirely offshore, the PIRES arrays are the only on-land stations which are located closest the fault zone in the whole of Marmara Sea. They are therefore best positioned to monitor the properties of the fault zone at close distance. The high accuracy of array processing will allow the monitoring of any possible structural transformation likely to occur inside the fault zone. In particular, a modification of pore pressure is expected to have a signature in the S-wave propagation characteristics. A systematic application of cross-correlation to the array data from active quarries will reveal minor details of the wave propagation at the upper crustal level. A similar on-line monitoring procedure will also be applied to the receiver functions of teleseismic events for monitoring of the total crust.
Task 5. Monitoring structural characteristics on Çınarcık Fault (CINNET)
The analysis of the seismicity in the Çınarcık basin appears of central importance for addressing the question on the transition from the last Izmit, 1999, Earthquake to the next major event in the Marmara region. A fine monitoring of the seismicity in the East Marmara Sea region might help to address several important questions: how the westward migration of large earthquakes since 1939 will enter the Marmara Sea in a complex transition zone between the Izmit fault and the Main Marmara Fault (Dewey, 1976; Stein et al., 1997). Twelve years after Izmit earthquake, are we back to a background activity and how is this background activity related to the extension deformation of the region (Karabulut et al., 2002) What is the recent evolution of seismic clusters observed in the Çınarcık basin (Karabulut et al, 2011). What are the interaction mechanisms between the North Anatolian Fault and the regional seismic cluster? Are the seismic clusters related to the nucleation of the major strike-slip events?
The recent analysis of the seismicity history in the Çınarcık Basin (Karabulut et al, 2011) provides interesting information on two types of aftershocks activity: the first type of enhancement is on strike-slip fault segments (Izmit Fault, Princes Island section of the Main Marmara Fault, Gemlik Fault) immediately following the main shock and related to Coulomb stress transfer; the second type of enhancement is attached to extensional clusters (Yalova, Tuzla) with a few days delay in the onset of strong activation, probably related to pore pressure increase. We observe a fast decay of the activity on strike-slip segments and slower evolution of seismic clusters with extensional features. Two years after the Izmit earthquake, seismic activity returned to the pre-earthquake pattern with most of the activity occurring within extensional clusters. It appears that the influence of the last large strike-slip event on the spatial seismicity distribution in the eastern Marmara Sea is less significant than the effect of the long term regional extension.
The CINNET network was deployed in 2008 around the Çınarcık basin to obtain a fine monitoring of the activity in the region (Karabulut et al, 2011). We propose to maintain and develop this network in connection with KOERI regional network. One objective is to update the network with telemetry systems and to implement up to date software for data management (Seiscomp). Scientifically, objectives are threefold: 1) obtain continuous high resolution locations of micro-seismicity including development of relative location technique from cross-correlation methods (multiplet analysis) and study the space-time evolution of the activity (Got et al, 1994); 2) search for low frequency events (non-volcanic tremor) from continuous recording (Shelly and Hardebeck, 2010); 3) analyze triggering mechanisms like Coulomb transfer, aseismic slip, or fluid migration and possible declustering methods.