J. Brozena (U.S.A.), D. Massonnet (France), G. Hein (Germany), K. Hudnut
(U.S.A.), J. Kahar(Indonesia), Murakami M (Japan), T. Niebauer (U.S.A.), S. Okubo(Japan), P. Paquet (Belgium), Zhao Shaorong (China)
|Applying theoretical, observational, and instrumental techniques to better understand natural hazards such as earthquake, volcanic eruption, and land slide.|
|Developing techniques to detect hidden seismic faults, premonitory signals of volcanic eruption/land slide from geodetic data. Emphasis will be placed on air-borne precise gravimetry, SAR interferometry, Satellite altimetry, dense GPS monitoring network.|
|Improving theory, which predicts changes of the geodetic bservables: baseline change, elevation change, gravity change, strain and tilts changes and so on.|
Differential use of interferometric SAR (Synthetic Aperture Radar) is one of highlights in recent geodetic works. Murakami's group successfully applied the technique to detect co-seismic and volcanic deformations of the ground surface. The 1995 Kobe earthquake (M7.2) was one of such successful examples (Murakami et al. 1995) as well as other events in Izu (Fujiwara et al. 1998a), Kyushu (Fujiwara et al. 1998b), Iwate, California (Murakami et al. 1996) and Sakhalin (Tobita et al. 1998). The SAR interferogram together with a dense GPS data enabled Murakami to estimate the fault mechanism of the Kobe earthquake (Ozawa et al. 1997).
Okubo's group evaluated the potential of the JERS-1 L-band SAR interferometry to detect subtle crustal movements even in mountainous region in Japan and its geodetic accuracy by comparing and combining other geodetic and seismological observations. They succeeded to detect crustal deformations for several cases as listed below (Kobayashi 1998 ab).
|fault motion by the Kagoshima earthquake (M6.3),|
|volcano inflation and the following earthquake (M6.1) around Mt. Iwate in 1998,|
|tensile deformation associated with earthquake swarms around the monogenic volcano area in the Izu Peninsula in 1997 and 1998, (4) volcanic deformations of Mt. Unzen during 1992 to 1993.|
Gravity changes before a coming great earthquake in a seismic risk region of Tokai District have been monitored with both absolute and relative gravimeters by Okubo and Murakami with technical assistance of Niebauer. They expect detecting secular change of gravity there. This can be associated with the subsidence at Omaezaki observed by spirit leveling, due to the subduction of the Philippine Sea Plate. They compared their results in 1995 and in 1996 with previous ones by the Geographical Survey Institute, Japan in 1987 and by the National Astronomical Observatory in 1992 (Okubo, Yoshida and Araya, 1997). They found regional gravity decrease during an interseismic period at the subduction plate boundary where the land is steadily subsiding. This paradoxical finding poses a challenging problem on the subduction dynamics at converging plate boundaries.
Gravity Change in an Earthquake Swarm Region
Okubo's group carried out both absolute and relative gravity measurements in the Izu Peninsula just before and after the March 1997 earthquake swarm occurred (Yoshida et al., 1999). The measurements revealed significant absolute gravity changes associated with the volcanic activity that caused the earthquake swarm. The gravity changes can be used to detect underground mass movement. For this purpose, they first use crustal movement observations to construct an elastic dislocation model with two tensile faults and a left lateral fault. Then they use the gravity changes to constrain the density of the material, which filled the tensile faults. They find that the density is likely to be small, and that the gravity change is reproduced well by the fault model. The smallness of the density implies that highly vesiculated magma or water would have injected into the faults.
Theory on Gravity Change and Crustal Deformation due to Seismic and Volcanic Processes
Sun Wenke and Okubo (1998) presented numerical formulation for computing elastic deformations caused by a dislocation on a finite plane in a spherically symmetric earth. It is based on their previous work for a point dislocation. The formulation enables them to compute the displacement, potential and gravity changes due to an earthquake modelled as spatially distributed dislocations. As an application of the finite-fault dislocation theory, they made a case study of the theoretical and observed gravity changes for the 1964 Alaska Earthquake. The computed results are in excellent agreement with the observed gravity changes during the earthquake. The gravity changes in the near field can reach some hundred microgals which can be easily detected by any modern gravimeter. In a far field it is still significantly large: ½ d g½ > 10 microgals within the epicentral distance q < 6°, ½ d g½ > 1 microgal within q < 16°, ½ d g½ > 0.1 microgals within q < 40°, ½ d g½ >0.01 microgals globally.
Refined Crustal Deformation Analysis before the Kobe Earthquake
Zhao detected a high-angle reverse fault was detected in the Shikoku-Kinki region7 southwest Japan through inversion analysis of horizontal displacements observed with GPS during 1990-1994 (Zhao and Takemoto, 1998). The active blind fault is characterized by reverse dip-slip (0.7 ± 0.2 m.yr-1 within a layer 17-26 km deep) with a length of 208 ± 5 km, a (down-dip) width of 9 ± 2 km, a dip-angle of 51° ± 2° and a strike direction of 40° ± ° (NE). The fact that hardly any earthquakes (ML >2.0) occurred at depth on the inferred fault plane suggests that the fault activity was largely aseismic. Based on the parameters of the blind fault estimated in their study, they evaluated stress changes in this region. It is found that shear stress concentrated and increased by up to 2.1 bar.yr-1 at a depth of about 20 km around the epicentral area of the 1995 January 17 Kobe earthquake (ML = 7.2, Japan), and that the earthquake hypocentre received a Coulomb failure stress of about 5.6 bar.yr-1 during 1990-1994. The results suggest that the 1995 Kobe earthquake could have been induced or triggered by aseismic fault movement.
Secular Gravity Monitoring in a Volcanic Region
After the 1986 eruption of Izu-Oshima volcano, Japan, Okubo's group observed anomalous gravity variations localized at the summit (Watanabe et al., 1998). Based on a vertical cylindrical conduit model, they estimate the time variations of the head of magma in the summit conduit and clarify the magma drain-back process after the 1986 eruption.
They thus demonstrated that it is quite plausible to detect magma movements directly by microgravity observations. Even when it causes only minor deformation in the case of Basalt magma.
We find that recent advances of geodetic measurements including SAR, GPS, Absolute/Relative gravity provides us with an unprecedented opportunity toward better understanding of natural hazards such as earthquakes and volcanic eruptions. Geodetic adjustment/inversion theories also turns out to play important roles in modeling seismic and volcanic processes. We recommend to set up several test fields where those advanced geodetic techniques are applied in an integrated manner to monitor and assess potential risks of natural hazards.
Fujiwara S., Rosen PA, Tobita M, Murakami M (1998a): Crustal deformation measurements using repeat-pass JERS 1 synthetic aperture radar interferometry near Izu Peninsula, Japan, J. Geophys. Res., 103(B2): 2411-2426.
Fujiwara S, Yarai H, Ozawa S, Tobita M, Murakami M, Nakagawa H, Nitta K, P.A. Rosen, Werner CL (1998b): Surface displacement of the March 26,
1997 Kagoshima-ken-hokuseibu earthquake in Japan from synthetic aperture radar interferometry, Geophys. Res. Let., 25: 4541-4544.
Kobayashi, S. (1998a): JERS-1 Researches, EORC Annual Report 1997, No. 1, March 1998, 19-27.
Kobayashi, S. (1998b): The Earthquake Remote Sensing Frontier Research (a) Geodetical evaluation of the accuracy of D-InSAR by comparison with GPS,precise gravity measurements and leveling surveys in Omaezaki, central Japan, (b) Criteria for evaluating the accuracy of the D-InSAR technique and its applications to monitoring of volcanic deformation, EORC Annual Report 1997, No. 1, March 1998, 58-59, 1998.
Murakami M, Fujiwara S, Saito T (1995): Detection of Crustal Deformations Associated with the 1995 Hyogoken-Nanbu Earthquake
by Interferometric SAR, J. Geogr. Surv. Inst., 83: 24-27. (in Japanese)
Murakami M, Tobita M, Fujiwara S, Saito T, Masaharu H (1996): Crustal Movements from the 1994 California Earthquake Revealed by Interferometric SAR, J. Geophys. Res., 101(B4): 8605-8614.
Okubo S, Yoshida S, Araya A (1997): Interseismic Gravity Change at a
Subducting Plate Margin: a Paradoxical Observational Result at Omaezaki,
Tokai, Japan, IAG Symposia 117, "Gravity, Geoid and Marine Geodesy", Sep. 30 - Oct. 5, 1996, Tokyo, Japan, Springer-Verlag, 305-309.
Sun Wenke, Okubo S (1998): Surface potential and gravity changes due to
internal dislocations in a spherical earth -- II. Application to a finite fault, Geophys. J. Int., 132: 79-88.
Tobita, M, Fujiwara S Ozawa S, Rosen PA, Fielding EJ, Werner CL, Mas. Murakami, Nakagawa H, Nitta K, Murakami M (1998): Deformation of the 1995 North Sakhalin earthquake detected by JERS-1/SAR interferometry, Earth, Planets and Space, 50: 313-325.
Watanabe H, Okubo S, Sakashita S, Maekawa T (1998): Drain-back process of basaltic magma in the summit conduit detected by microgravity observation at Izu-Oshima volcano, Japan. Geophysical Res. Lett., 25: 2865-2868.
Yoshida S, Seta G, Okubo S, Kobayashi S (1999): Absolute gravity change associated with the March 1997 earthquake swarm in the Izu Peninsula, Japan, Earth Planets Space, 51: 3-12,
Zhao Shaorong, Takemoto S (1998): Aseismic fault movement
before the 1995 Kobe earthquake detected by a GPS survey: implication for
preseismic stress localization ?, Geophys. J. Int., 135: 595-606.