Overview | Earthquake | Drought | Fire | Flood & Cyclones | Landslide & Soil Erosion | Volcano
An introduction to Earthquake Geodesy : Another Effort for Earthquake Hazard Monitoring
Andreas H., Irwan M., H.Z.Abidin, D. Darmawan, D.A. Sarsito, M. Gamal Geodesy Research Group Department of Geodetic Engineering, Institute of Technology Bandung, Jl. Ganesha 10 LABTEX IX C telp/FAX +62 22 253 4286, Bandung Indonesia Abstract Earthquake is one of catastrophic event produce losses to people's life as well as infrastructure damages. Aceh Earthquake following by tsunami was one recent and biggest examples of earthquake's tragedy in the last 40 years. Almost 300,000 peoples were killed and leaving very serious damages on infrastructures. Earthquake happen when the earth's crust fails in response to accumulated deformation. Geodetic measurement document the crustal deformation leading to those failures and the deformation resulting from them, providing unique insight into the physical processes involved [ Hudnut et.al 1994 ]. Geodetic measurement (e.g. GPS and InSAR) with space and time domain (continuous or periodic) may detected ground deformation as well as the accumulation of them. Geodetic measurement may also constrains physical model of the processes that cause earthquake event. With geodetic measurement we may saw clearly inter-seismic phase of earthquake mechanism, pre-seismic signal also sometimes recorded, and well recorded co-seismic and post-seismic signals. Inter-seismic pattern is now being included as one of parameter in estimation the probability of earthquake hazard. As a conclusion, the geodetic measurements became another effort for earthquake hazard monitoring. These geodetic measurements have a title as Earthquake Geodesy. I. INTRODUCTION An earthquake is a shaking of the ground caused by the sudden breaking and shifting of large sections of the earth's crust. Why the crust could break because they fails in response to accumulated deformation (see figure 1). The accumulation of deformation result from ongoing processes such as aseismic deformation of subcrustal rock associated with relative plate motion due to convection energy from a mantle. The large energy released as a big earthquake may occurred when the earth's crust fail to response the maximum accumulation of deformation and shown us a pictures of catastrophic event produce losses to people's life as well as infrastructure damages. Many records have documented the number of big earthquakes followed by fatalities and damages ( see Table 1). Aceh Earthquake following by tsunami was one recent and biggest examples of earthquake's tragedy in the last 40 years. Almost 300,000 peoples were kill and leaving very serious damages on infrastructure. Earthquake happen almost without warning and they followed a cycle so will strike back sometimes in a future.  Figure 1. Illustration of accumulation of deformation leading to failure respond of the earth's crust when it can no longer resisted to the accumulated energy from ongoing processes such as aseismic deformation of subcrustal rock associated with relative plate motion due to convection energy from a mantle, produce an earthquake and sometimes followed by tsunami. Image with courtesy of J. Mori on KAGI lecture's note. [Mori 2004 ]
Records of such big event has proven that a single earthquake can cause thousands of deaths and not to mention thousands others became injured. Although earthquakes are uncontrollable, the losses they cause can be reduced by develop earthquake hazard monitoring and mitigation programs (e.g. conducting many researches to analyzed earthquake mechanism, matching land use to risk, building structures that resist earthquake damage, developing emergency response plans, and other means). As a result of technologies, in the early years ninety has been introduced another tools for earthquake hazard monitoring namely earthquake geodesy which will be explained more in this paper. II. SEQUENCE AND MECHANISM OF THE EARTHQUAKE Until today, the mantle convection still believed as an appropriated theory to explain the movement of the plates and the source mechanism of the earthquake. The energy from convection steering the plate to move one another. One block of plates and the others would converge, diverge, or move side by side. At the plates interface where two plates merge and locked by friction, then this earth's crust area will experienced a deformation. As the energy from the mantel will continuously forced the plates to move consistently, a consequences the deformation in the plates interface will increasingly accumulated. Within few ten years to hundred years when the accumulation of deformation reached maximum stage, this earth's crust may fails in response to those accumulated deformation and produce the sudden breaking and shifting of large sections of the earth's crust, making the ground to shake and its called earthquake. For a moments, the energy was released by the earthquake ( mainshock and aftershock), made the deformation return to zero. The earthquake has unlock the friction. But after few months to few years when this dynamic earth has return to equilibrium stage, the plates interface will experienced new deformation. Again, the merged plates could lock by the friction and the deformation will beginning to accumulated leading to failure respond of this earth's crust producing new earthquake. This sequence called earthquake cycles. In detail the earthquake cycles divided into inter-seismic, co-seismic, and post-seismic sequence. Pre-seismic is now still being analyzed before officially included as part of all sequence. inter-seismic At the inter-seismic sequence, the two plates interface is locked by friction. The upper plate is accumulating elastic deformation at a slow rate (~1cm/yr). This loading phase can last for centuries Co-seismic The earthquake releases in one moment deformation accumulated for centuries. At that stage, the upper plate "rebounds". In the subduction zone, the whole system being below sea level, this giant "kick" in sea water generates a Tsunami Post-seismic Return to equilibrium and steady loading can take years.  Figure 2. Illustration of earthquake cycle in subduction zone ( Vigny et.al 2005 ) III. GEODETIC DATA AND EARTHQUAKE STUDIES ( EARTHQUAKE GEODESY ) As mention previously, earthquake happen when the earth's crust fails in response to accumulated deformation. Geodetic measurement document the crustal deformation leading to these failures and the deformation resulting from them, providing unique insight into the physical processes involved [ Hudnut et.al 1994 ]. Geodetic measurement with space and time domain (continuous or periodic) may detected ground deformation as well as the accumulation of them. Geodetic measurement may also constrains physical model of the processes that cause earthquake event. With geodetic measurement we may saw clearly inter-seismic phase of earthquake mechanism, pre-seismic signal also sometimes recorded, and well recorded co-seismic and post-seismic signals. Advances in Global Positioning System ( GPS ) measurement and data analysis technology, and the increasing number and improving quality of GPS receivers deployed in active tectonic area, plus the occurrence of another most exciting new development Interferometric Syntetic Aperture Radar (InSAR), have contributed steadily towards understanding earthquake sources and inter-seismic deformation, and hence toward improving hazard evaluation. Inter-seismic pattern is now being included as one of parameter in estimation the probability of earthquake hazard. As a conclusion, the geodetic measurements became another effort for earthquake hazard monitoring. These geodetic measurements have a title as Earthquake Geodesy. III.1 Geodetic Data and Inter-seismic Deformation Detection of slow inter-seismic strain accumulation is probably the best technique we have for identifying the location of future earthquakes in some areas, because elastic rebound requires elastic strain accumulation prior to earthquakes. With geodetic measurement we may saw clearly inter-seismic phase of earthquake mechanism and also give the better constrain to the location of predicted future earthquake. Many of the early studies of inter-seismic deformation using geodetic data ( GPS ) took place in southern California. These studies were also instrumental in developing geodetic GPS methods and in characterizing the precision and accuracy of the technique (e.g. Larson & Agnew 1991; Feigl et al 1993). Feigl et al (1993) presented a comprehensive review of GPS data collected in central and southern California between 1986 and 1992, as well as VLBI data collected between 1984 and 1991. They also discussed methods for obtaining and combining highly precise GPS solutions for the determination of station velocities in regional-scale networks. Daily solutions with phase ambiguities fixed to integers (where possible) were estimated with very loose constraints on the station coordinates.  Figure 3. Inter-seismic deformation along the coast of California. Image with courtesy of Prof Thomas Herring. [Herring 2002] III.2 Geodetic Data and Pre-seismic Deformation Just a few days before Tonangkai earthquake happen in December 1944, spirit leveling result has shown pre-seismic deformation signals (mogi 1984). The tilt start to changing four day before the mainshock. This was the amazing discovery that the earthquake tell us something before they coming. With this precursor then the scientist though the earthquake may be known before and would give the early warning. But this signal turn out to be the one and the only best signal it ever have. Many years come, the earthquakes happen with given no pre-seismic signal at all.  Figure 4. spirit leveling result has shown pre-seismic deformation signals three to four days before the mainshock exist (mogi 1984). In the GPS era, in Arequipa Peru, pre-seismic deformation signal has occurred before the mainshock come. Position in daily solutions start to give an anomalies result in few months to a few days before the mainshock happen. But again in many well recorded continuous GPS signal in a period of earthquake there are currently more negative result then positive result in pre-seismic deformation signals. Pre-seismic deformation signal is now still being analyzed whether its just an anomaly signal in some case of earthquake or we may saw well in any certain characteristics of plates interface. III.3 Geodetic Data and Co-seismic Deformation Advances in Global Positioning System ( GPS ) measurement and data analysis technology, and the increasing number and improving quality of GPS receivers deployed in active tectonic area, plus the occurrence of InSAR Technology has achieved many well recorded co-seismic deformations (e.g. Figure 5-6). GPS measurements can be related to the earthquake source process through Volterra's formula (e.g. Aki & Richards 1980) for displacement at the Earth's surface due to slip on a surface of displacement discontinuity in an elastic medium. GPS measurements of surface displacement can thus be inverted to determine the geometry of earthquake rupture(s). This determination is particularly important for earthquakes that do not rupture the ground surface, or when seismic data, including aftershock distributions, do not clearly determine the rupture geometry. In estimating the source geometry, an earthquake is typically represented by one or more rectangular dislocations with spatially uniform slip. Once the fault geometry is known, it is possible to determine the distribution of slip on the fault surface, and further on it help us for better understanding the co-seismic mechanism and the stress transfer mechanism concerning the evaluation to next earthquake potential occurrence and the constrain of predicted area of future earthquake. Below given some examples of co-seismic captures in some areas suffering the earthquake (e.g. Kurile island 1994, Aceh 2004, and Chi-chi Earthquake 1999 )  Figure 5. Horizontal displacement vectors from the M8.1 Kurile Islands (Hokkaido-Toho-Oki), Japan, earthquake, October 4, 1994. Displacements are relative to a station 1100 km away. Error ellipses show 99% confidence regions. After Tsuji et al (1995).  Figure 6. Co-Seismic from the M9.0 Mega thrust earthquake Aceh, December 26, 2004. More then 2 meter displacements occurred in city of Banda Aceh. Irwan et al (2005).  Figure 7. Average co-seismic deformation interferogram from Chi-chi earthquake 1999 (Liu.et.al 2004) III.4 Geodetic Data and Post-seismic Deformation GPS will play an increasingly important role in improving our knowledge of post-seismic processes. Traditional seismic instruments are insensitive to post-seismic processes, with the exception of aftershocks. Strainmeters and tiltmeters record short-term transients following earthquakes, but only geodetic survey measurements are likely to have the spatial coverage and long-term stability needed to resolve post-seismic strains with characteristic times of years to decades. With given the precision of the measurements and the relative ease of GPS surveying, we can expect a revolution in our understanding of post-seismic processes in the upcoming decade (Segall & David, 1997) The 1989 Loma Prieta and 1992 Landers earthquakes are the first to yield significant post-seismic strain signals. Following the Loma Prieta earthquake, Savage et al (1994) found evidence for a decaying transient with a characteristic time of 1.4 years, from the first 3.3 years of data. Advanced various data sets of GPS and others geodetic measurements, including interferometric SAR images, could help clarify competing interpretations of post-seismic deformation. The fact has shown that the energy of an earthquake would never reached a hundred percent by the mainshock, some cased shown only fifty percent released by the mainshock while post-seismic hold the rest which can be released up to several years and even more. With continuous GPS networks we can also look forward to the ability to resolve rapid post-seismic signals, which were previously only measurable with strain and tilt meters.  Figure 8. Example of post-seismic deformation from Hector Mine Earthquake. The signals still continue to change exponentially after more than four years since the mainshock, to return to equilibrium and steady loading at the plates interface. graphic with courtesy of Prof Thomas Herring. [Herring 2002] IV. CONCLUSION Detection of slow inter-seismic strain accumulation is probably the best technique we have for identifying the location of future earthquakes in some areas, because elastic rebound requires elastic strain accumulation prior to earthquakes. With geodetic measurement we may saw clearly inter-seismic phase of earthquake mechanism and also give the better constrain to the location of predicted future earthquake. Inter-seismic pattern is now being included as one of parameter in estimation the probability of earthquake hazard. Pre-seismic signal also sometimes recorded among others mostly negative result. Pre-seismic deformation signal is now still being analyzed whether its just an anomaly signal in some cased of earthquakes or we may saw well in any certain characteristics of plates interface. Well recorded co-seismic deformations can be related to the earthquake source process. Further on this records can thus be inverted to determine the geometry of earthquake rupture(s). This determination is particularly important for earthquakes that do not rupture the ground surface, or when seismic data, including aftershock distributions, do not clearly determine the rupture geometry. Once we found the geometry of the rupture it help us for better understanding the co-seismic mechanism and the stress transfer mechanism concerning the evaluation to the next earthquake potential occurrence and the constrain of predicted area. Geodetic measurements will play an increasingly important role in improving our knowledge of post-seismic processes. Advanced various data sets of GPS and others geodetic measurements, including interferometric SAR images, could help clarify competing interpretations of post-seismic deformation. The fact has shown that the energy of an earthquake would never reached a hundred percent by the mainshock, some cased shown only fifty percent released by the mainshock while post-seismic hold the rest. As a last sentence of conclusion, the geodetic measurements became another effort for earthquake hazard monitoring. REFERENCES
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