Indirect stress measurement and earthquake prediction

Transcription

1 Indirect stress measurement and earthquake prediction P.Kalenda, L.Neumann, J.Kvetko Proc. Of XV INTERNATIONAL SCIENTIFIC-TECHNICAL SYMPOSIUM GEOINFORMATION MONITORING OF ENVIRONMENT: GPS and GIS TECHNOLOGIES September 13-18, 2010, Alushta (Ukraine, Crimea), Session 5 - Geodetic, geological, geophysical and ecological monitoring. 1. Introduction People have tried to predict earthquakes for a long time. Going back as far as the 6 th century (Varahamihira ), there are records of anomalous phenomena observed prior some earthquakes, such as animal behaviour or unusual clouds on the sky (Times of India 2001). However, the era of instrumental measurements of various parameters (not only seismological) started only in the 70s of the 20 th century. Many of such parameters were measured as possible precursors of big earthquakes. Keilis-Borok and Kossobokov (1990) used the parameters of seismicity itself (seismic gaps, increase of seismicity, clustering) for the estimation of seismic risk. Many researchers used radon gas measurement for the detection of the imminent occurrence of EQs (Ulomov & Mavashev 1971, Asada 1982). The measurement of electromagnetic fields, started by the Greece scientists by the VAN method 30 years ago (Varotsos & Alexopoulous 1984a, 1984b), is well known too. Many big earthquakes were preceded by thermal anomalies, as shown by Qiang et al. (1990) or Xu et al. (1991). Various methods of indirect stress measurement, like the extensometric or tilt measurements have been put into operation in recent years, mainly in China (Li et al. 2003, Shi et al. 2009). As the earthquakes result from the increase of stress in the massif overcoming the strength limit or friction on the fault, the direct stress measurement could lead to the direct earthquake prediction. However, the seismogenic depths are inaccessible for people. So we have to use indirect stress measurements close to the Earth surface for the estimation of the stress state in the depths. For example, gravimetry or the measurement of water level in wells can be used for the indirect pressure measurement. Both of these methods, however, do not enable us to estimate the stress tensor or the direction of its principal component. That is the reason why we used a completely new apparatus the vertical static pendulum (Neumann 2007, Kalenda et al. 2009), which allows measuring the microdeformation of the massif in the horizontal plane and the tilt of the plumb line in the frequency range of periods longer than 10 seconds up to the infinity. 2. Asperity model The deformations that probably precede big earthquakes could be described on the basis of the model of the earthquake focus. The asperity model (Wyss et al. 1981) appears to be the most plausible model, which shows that the movement between the wings of the fault starts at the time, when the asperity, which inhibits the movement, is going to break. The size of the asperity determines the accumulated energy and the accumulation period. The bigger the asperity the bigger the earthquakes and the longer time of its destruction (seismic cycle) will be. Several different asperities can be on one fault or in one focus area and they will decide

2 about the development of the future EQ focus in time and about the development of the release of energy. The asperity model shows (Wei 2007) that in the gradually increasing stress field (the consequence of tectonic movements), no precursors can be observed during the first phase the energy accumulation phase. This period can last tens even hundreds of years, depending on the size of the asperity and the velocity of the tectonic movement. The point a - proportional limit - (see Fig. 1); when the stress is approaching to the strength limit, the higher strength of asperity appears and the deformation rate decreases. Sun Wei called this phase deadlock. At this moment the destruction of smaller asperities starts and the main portion of stress concentrate around the main asperity. The outward behaviour of the massif is chaotic the disturbances of deformation are observed at the time of the destruction of smaller asperities. This period lasts days up to months depending on the size of the main asperity, until the point b is reached - upper yield limit (see Fig. 1), when the destruction of the the main asperity starts. The apparent stress relaxation prolongs until point c - lower yield limit, which takes a few days or weeks. Then the quickest phase of the nucleation period s.l. starts the transient stress-hardening - up to point d failure limit, when the movement on the fault plane (nucleation period s.s.) begins. The period of the transient stress-hardening lasts hours to days. Fig.1 Definition of nucleation period: a proportional limit, b upper yield limit, c lower yield limit, d failure limit, (σ y, ε y ) deformation on the proportional limit, σ d tensile strength, ε d - tensibility The whole process of the energy accumulation to the massif destruction is not observable by apparatus. The first precursors appear during the second stage (phase) around point b, when the smaller asperities are broken. The rate and meaning of the deformation changes in the massif surrounding the main asperity is changing at the same time. The nucleation phase (stage) s.l. starts. Its time length (period) with respect to the asperity size (proportial to EQ magnitude) can be described for average tectonic settings by equation log T [day] = 0.46 M 2.2. (1) We can see that the stress waves, which are generated around the main asperity before its destruction, will have periods of days or months. Such periods are not observable by standard wide-range seismic apparatuses (see Fig.2). Static apparatuses, i.e. gravimeters, deformometers or tiltmeters with the non-compensated base, should be used to observe these deformations.

3 Fig. 2 Spectrum of E-M waves, sound waves in the air and waves in rocks 3. The system of deformation measurement It is necessary to place the apparatuses measuring the deformations under the Earth surface to detect the anomalous stress waves. It ensures the independence of the measured values on the changes of temperature. As the changes in stress are comparable with tides, the measuring accuracy should be much better then tide amplitude. Since the stress waves spread horizontally, the horizontal deformations of empty spaces (chambers, abysses, caves) or tilt of plumb line between two points (A and B or C and D) in chamber (See Fig.3) seem to be the very sensitive to tectonic stresses. Fig. 3 Deformation of horizontal cylinder chamber in the massif (cave, gallery, tunnel) It is obvious that the relative change of position of every different couple of points will differ depending on the direction and size of the stress, and also on the geometry of the chamber. Therefore every installed apparatus has to be calibrated individually and the complete stress tensor has to be estimated on the basis of a number of apparatuses installed at different places of one chamber or at different locations. Similarly, every location should have one specific direction in which it will be more sensitive than in other directions. This direction is mostly determined by the characteristics of the rock in the location and by the prevailing tectonic situation. 4. Observation of deformation before big earthquakes The model of the asperity destruction shows that the typical deformation line during the nucleation phase s.1. should have an S-form (Wei 2007). This was observed before a number of earthquakes ( Helingen M=6.3, Tangshan M=7.8, Sumatra M=8.7 and others). We have observed these characteristic S-curves before a number of earthquakes since 2007; one in its clear form for example before the earthquake at Nicobar Island M=7.5 (see Fig. 4). Fig.4a shows the deformation measured in the gallery Ida at Malé

4 Svatoňovice in depth of 200 m under the surface. The tidal deformations especially in EW direction with the amplitude of approx. 0.1 urad are clearly visible. Fig.4b shows the deformation measured in the Prokop gallery at Příbram in depth of 1m under the surface. The distance from the Ida gallery to Prokop gallery is approx. 200 km. Deformations, caused by insolation, with the period of 24 hours with the amplitude of approx. 2-3 urad can be easily observed. The stress wave appeared in two phases in the periods of about May 28 June 2 and June 6-8. The immediate phase of enforcement before the destruction occurred between June 10-11, i.e. approximately 2-3 days before the main earthquake. Fig. 4 Deformations observed in Ida and Prokop mines before Nicobar Is. EQ A similar situation occurred before Tonga EQ M=7.9 (EMSC); both in the Prokop gallery at Příbram in depth of 96m under the surface and the active mine Lubeník in depth of 200m under the surface (see Fig.5). The S-curve before the main event was 20 days long in both cases. Aquila EQ M=6.3 was triggered by the movement of the deformation wave after the Tonga archipelago EQ in our opinion. Fig. 5 Deformations observed in Prokop and Lubeník mines before Tonga EQ

5 The biggest deformations in the whole period were observed before Chile EQ M=8.8 (see Fig. 6). The beginning of the nucleation phase s.1. and the stress wave were possible to observe at the end of It was observed Argentina EQ (M=6.3) at January 1, which triggered the main phase of nucleation. Exactly 12 hours before the main event in Chile the island of Ryukyu EQ (M=7.3) on the opposite side of the Earth was observed, which became the main trigger of the main event in Chile. Fig. 6 Deformations observed in Ida mine before and after Chile EQ on After that the stress waves trigger earthquakes in Afghanistan (March 27, 2010, M=5.8), Turkey (March 8, 2010, M=6.0). The biggest aftershock in Chile (M=7.2) was observed in March 11, Even the volcano eruption of Eyjafjallajökull could be connected with the transfer of deformation from Chile, as can be seen from the anomalous peaks of the noise in the EW direction from March 10 to March 17, 2010 (see Fig. 6). 5. Conclusion The measurements of microdeformations by means of the vertical static pendula in the mines and caves in central Europe proved that before the biggest earthquakes it was possible to observe anomalous deformations corresponding with the asperity model. The lengths of anomalous periods were directly related to the magnitude of the main event. The nucleation phase of the earthquakes with M 7 on the Euro-Asian lithosphere plate lasted 2-3 weeks, and the one of Chile EQ (M=8.8) lasted 65 days. The attenuation of deformations depends only on the characteristics of the contacts of the lithosphere plates, due to the wavelength of the stress waves, which is longer than 10 5 km. The transfer of deformations within one lithosphere plate is almost perfect. Static deformometry apparatuses make possible to predict earthquakes, especially the time of occurrence and its magnitudo. Localisation is more difficult, but possible (Kalenda and Neumann 2010). References Asada, T. (1982): Earthquake Prediction Techniques: Their Application in Japan (Japan: University of Tokyo Press.

6 EMSC (2010): Centre Sismologique Euro-Méditerranéen. European-Mediterranean Seismological Centre. Kalenda, P., Neumann, L. & Wandrol, I. (2009): Indirect stress measurement by static vertical pendulum. Proceedings of 47th Int. Sci. Conf. Experimentální analýza napětí 2009, TU Liberec. Kalenda, P. & Neumann, L. (2010): Static vertical pendulum observations of anomalous tilt before earthquakes (case study). The 5th International Symposium on In-situ Rock Stress August 25-27, 2010, Beijing, P.R.China. Keilis-Borok, V.I. & Kossobokov, V.G. (1990): Premonitory activation of earthquake flow: algorithm M8. PEPI, Vol. 61, 1-2, Li, J.Z., Z. Q. Bai, W. S. Chen, Y. Q. Xia, Y. R. Liu, & Z. Q. Ren (2003): Strong earthquakes can be predicted: a multidisciplinary method for strong earthquake prediction. Natural Hazards and Earth System Sciences (2003) 3: Neumann, L. (2007): Static Pendulum with Contactless 2d Sensor Measurements Open the Question of Gravity Dynamic and Gravity Noise on the Earth Surface. Physics Essays (Vol. 20 No. 4). Qiang Zuji, Xu Xiudeng & Lin Changgong (1990): Thermal anomaly precursor of impending earthquake. Chinese Science Bulletin, 35, pp Shi, Y. & Zhang, H., Liu, Ch., Cao, J. & Sun, Y. (2009): How far are we from numerical earthquake prediction? Proceedings of International Symposium on Earthquake Seismology and Earthquake Predictability July 5 to 9, 2009, Beijing, China. Times of India (2001): A temblor from ancient Indian treasure trove? The Times of India News Service 28 April Ulomov, V. I. and Mavashev, B. Z. (1971): The Tashkent Earthquake of 26 April, Acad. Nauk. Uzbek SSR FAN, Varotsos, P. et al. (1984a): Physical Properties of the Variations of the Electric Field of the Earth Preceding Earthquakes, I, Tectonophysics, 110 (1984) Varotsos, P. et al. (1984b): Physical Properties of the Variations of the Electric Field of the Earth Preceding Earthquakes. II. Determination of Epicenter and Magnitude, Tectronophysics, 110 (1984) Wei Sun (2007): Damaging Earthquakes Can be Predicted The Earthquake Gestation Physical Model and Earthquake Imminent Precursor. Engineering Science, 9 (7): p 7-17 (in Chinese) Wyss, M., Johnston, A.C. & Klein, F.W. (1981): Multiple asperity model for earthquake prediction. Nature 289, (22 January 1981); doi: /289231a0. Xu Xiudeng, Qiang Zuji & Lin Changgong (1991): Thermal anomaly and temperature increase before impending earthquake. Chinese Science Bulletin, 6, pp