S34C-01
Tectonic loading of active faults in central Japan revealed by dense GPS observations
We have been conducting dense GPS observations around two active faults zones in central Japan, the Atotsugawa fault (AF) system and the Itoigawa-Shizuoka Tectonic Line (ISTL) fault system. These faults constitute a part of the Niigata-Kobe Tectonic Zone, the inland strain concentration zone detected by the nationwide continuous GPS network. Our dense observations provide a key to understand detailed deformation pattern within the deformation zone and to solve the physical mechanism of the tectonic loading of active faults. AF fault sustem is a group of ENE-WSW trending right-lateral strike slip faults. Among three major faults in the fault system, AF is considered to play an important role and have the largest slip deficit of 3-5mm/year, which is consistent with the geologic slip rate. GPS observation indicates that the oblique contraction across AF may be partitioned into fault-normal and fault-parallel components, which are accommodated differently. Contraction across the fault distributes more widely. ISTL fault system is a major geologic structure in central Japan. The fault has a high probability of a large event. We have conducted a campaign GPS observation at 28 sites around ISTL every year since 2002. The campaign observation results revealed laterally heterogeneous deformation along the fault. In the northern part, WNW-ESE contraction is dominant, consistent with a reverse faulting of this portion. Concentrated contraction near the fault suggest that the stress is not accumulated but released. On the other hand, the Gofukuji fault in the central ISTL is characterized by left-lateral shear, implying strike slip faulting. The left-lateral shear strain is widely distributed over a long distance. Such a distributed shear strain implies stress relaxation in the lower crust and the fault may be in the latest stage of its interseismic period.
S34C-02
An integrated geophysical research for Atotsugawa fault system (AF), Central Japan - Relation between fault structure and surrounding crustal inhomogeneity -
An integrated geophysical observations in and around the Atotsugawa fault system (AGF), central Japan, delineated the clear relationship of the fault characteristics and the surrounding inhomogeneous crustal structure. The AGF, located within a zone of high strain rate (the Niigata-Kobe Tectonic Zone) running in the northern part of central Japan with ENE-WSW direction, is one of the prominent active faults in central Japan, and responsible for the 1858 Hietsu earthquake of M7.0. This observation project, which started from 2004, involves dense seismic observation, magnetotelluric survey, GPS measurement and refraction/wide-angle reflection experiment. Major finding so far obtained is a very low velocity anomaly (5 percents) located in the lower crustal part beneath the AGF. The upper crustal structure around the AGF is characterized by high velocity (6-6.3 km/s) patches with less seismic activity. They are 10`20 km in size and correlated with damaged area of the Hietsu event. These results strongly indicate that the high velocity patches represent asperities of this earthquake. Both edges of the AGF are bounded by low velocity areas probably representing the present volcanic activities. Probably, the anelasticity associated with the volcanism may determine the size of this fault. The low velocity body in the lower crust extends upward to a boundary part of the high velocity patches. This upwelling portion shows low resistivity, indicating the existence of fluid. The GPS measurement indicates almost the entire part of the AGF is locked although some ambiguity remains outside of our array. Present results suggest that the prominent lower crustal heterogeneity controls the stress loading process to the AGF and the stress concentration at the boundaries of asperity with aid of fluids.
S34C-03 INVITED
Imaging Rupture Asperities and Earthquake Potential of Partly Creeping Faults
The Hayward fault (HF) and Parkfield segment of the San Andreas fault are known to be source areas of moderate to large earthquakes, but also exhibit significant aseismic fault creep. Modeling of space geodetic data (GPS, InSAR) collected along the HF over a ~10-year period allows for the determination of the distribution of currently locked asperities and creeping portions of the fault zone. Sequences of repeating micro-earthquakes add further resolution to the identification of creep at depth. The inferred slip rates along the creeping portions of the HF are significantly less than the long-term slip rate, and thus a substantial slip deficit is accumulating there as well. For purposes of earthquake hazard estimation, it is important to know how much of this slip deficit will be dynamically released during the next M~7 Hayward fault rupture, and how much will catch up during an episode of accelerated, but aseismic afterslip. The same question arises for other partly coupled faults, including many subduction thrusts. At Parkfield, geodetic and seismicity data also allowed for the first-order discrimination of creeping and locked fault patches, prior to the 2004 M=6 Parkfield earthquake. Model inversions of GPS and InSAR measurements spanning and in the immediate aftermath of the earthquake show that moment release by aseismic afterslip exceeded the coseismic moment, and that afterslip occurred on portions of the fault that experienced creep during the interseismic period. Thus, dynamic rupture does not appear to break deeply into previously creeping parts of the fault, which instead recover their slip deficit by accelerated creep. Mechanical models that treat fault creep in the context of rate-state-frictional behavior produce temporal and spatial patterns of fault slip consistent with these observations. Thus, the earthquake potential of partly creeping faults may be limited to rupture of completely locked asperities, whereas adjoining, creeping portions of the fault will accommodate slip by time-dependent, velocity-strengthening creep only.
S34C-04 INVITED
Mojave Compliant Zone Structure and Properties: Constraints from InSAR and Mechanical Models
Long-lived zones with significantly lower elastic strength than their surroundings are associated with active Mojave faults (e.g., Li et al., 1999; Fialko et al., 2002, 2004). In an earthquake these weak features concentrate strain, causing them to show up as anomalous, short length-scale features in SAR interferograms (Fialko et al., 2002). Fault-zone trapped wave studies indicate that the 1999 Hector Mine earthquake caused a small reduction in P- and S-wave velocities in a compliant zone along the Landers earthquake rupture (Vidale and Li, 2003). This suggests that coseismic strain concentration, and the resulting damage, in the compliant zone caused a further reduction in its elastic strength. Even a small coseismic strength drop should make a compliant zone (CZ) deform, in response to the total (not just the coseismic) stress. The strain should be in the sense which is compatible with the orientations and values of the region's principal stresses. However, as indicated by Fialko and co-workers (2002, 2004), the sense of coseismic strain of Mojave compliant zones was consistent with coseismic stress change, not the regional (background) stress. Here we use finite-element models to investigate how InSAR measurements of Mojave compliant zone coseismic strain places limits on their dimensions and on upper crustal stresses. We find that unless the CZ is shallow, narrow, and has a high Poisson's ratio (e.g., 0.4), CZ contraction under lithostatic stress overshadows deformation due to deviatoric background stress or coseismic stress change. We present ranges of CZ dimensions which are compatible with the observed surface deformation and address how these dimensions compare with new results from damage-controlled fault evolution models.
S34C-05
Seismic and aseismic processes on the Psathopyrgos normal fault, western rift of Corinth, Greece.
The western rift of Corinth (Greece) displays a fast opening rate (1.5 cm/year from GPS) and a very high, strongly fluctuating microseismic activity, monitored since 2000 by a local seismometer array. Several moderate to large earthquakes (M=6 to 6.7) have occurred historically on its segmented normal fault system, and one earthquake in this magnitude range is expected to occur in the coming decades. We focus here on the activity of the Psathopyrgos fault, the westernmost one in the rift ( 15 km away from the city of Patras), which did not rupture in historical time (more than 300 years), despite its clear morphological slip activity. This raises the question of possible creep on this fault, as a future coseismic release of more than 3 to 4 meters of slip on a 15 km long segment seems unlikely. A large seismic swarm occurred on this fault, in November-December 2002, with several events above magnitude 3, mostly located between 4 and 10 km in depth, and one reaching magnitude 3.5, at the exceptionally shallow depth of 3 km: this swarm suggests a large-scale activation of the whole fault, by creep or fluid migration. About 30 minutes before this M3.5 event, a strong compression signal is recorded oh the borehole dilatometer installed 15 km away in the Trizonia island, reaching its maximum at the time of the event, then gradually decaying. This signal can be modelled by a 10 cm aseismic creep involving the shallowest, eastern part of the fault. These observations raise the question of the possibility for mature normal faults to slip intermittently, both seismically and by creep.
S34C-06
Local stress concentration on the seismic belt along the Japan Sea coast inferred from numerous precise focal mechanisms -Implications for the brittle-ductile interaction model-
We found a rotation of the maximum horizontal compressional stress axis near the seismic belt along the Japan Sea coast in the Chugoku district, Southwest Japan. The azimuth of the axis near the seismic belt is WNW, while that in the surrounding area is EW. The maximum horizontal compressional stresses of the EW direction are widely observed in the shallow crust in the inland plate along the Nankai trough. That of the WNW direction is unusual in the Chugoku, Kinki and Shikoku districts in Japan, so it suggests the local stress concentration probably due to ductile faulting in the lower crust beneath the seismic belt. The data used are obtained by two dense seismic observations. The first was conducted by the group for the dense aftershock observations of the 2000 Western Tottori earthquake. They installed 72 seismic stations for 40 days with 4-5 km spans in the aftershock area and recorded more than 3000 aftershocks. The second was conducted by the group of the joint seismic observation in the Seinan area, Japan. They installed 40 online seismic stations for about two years and recorded more than 1000 earthquakes occurring in the Chugoku district, Southwest Japan. We estimated the stress field from inversions of numerous polarity data using the Horiuchifs method. In the aftershock area of the 2000 Tottori earthquake, we conducted the stress inversions at a bin with a 3 km width, and succeeded to estimate spatial changes in the stress field along the fault strike, which is perpendicular the strike of the seismic belt. Consequently, we found the detailed stress change across the seismic belt. The result is explained by ductile faulting in the lower crust predicted by the brittle-ductile interaction model.
S34C-07
Effects of heterogeneous structures of the seismic velocity and the stress field on earthquake generations elucidated by dense temporary seismic networks
Within recent 3 years, three large thrust-type inland earthquakes (the 2004 Mid-Niigata Prefecture Chuetsu Earthquake, the 2007 Noto Hanto Earthquake, and the 2007 Niigata Prefecture Chuetsu Offshore Earthquake) have occurred at the southern end of the eastern margin of the Japan Sea. The margin was initially formed as a rifted passive margin, mainly during the early Miocene when the Japan Sea opened. In the margin, E-W shortening has continued since the late Pliocene. Thus, it is considered that the complex structures, associated with both the crustal stretching and the folding, can potentially nucleate the mainshock and trigger a sequence of aftershocks for three large earthquakes. Therefore, it is important to image the complex seismogenic structures and stress field around the source region and clarify their relations to the rupture process, or stress concentration process. We have immediately deployed dense temporary seismic networks in the source region, after the occurrence of each mainshock. On the basis of accurate aftershock distributions, aftershocks associated with each mainshock rupture are aligned along high-dipping fault planes. Tomographic analysis clearly shows that each mainshock rupture occurred along a velocity boundary between the hanging wall and the footwall. It is therefore interpreted that each mainshock fault plane might be a reactivated reverse fault through inversion tectonics. Furthermore, it is found from stress tensor analysis that the maximum compressional stress axis tends to rotate near the mainshock hypocenter. Present results suggest that heterogeneous structures of crust and the stress field are of crucial importance to earthquake generation. In future, we will investigate heterogeneous structures in Kanto- region using new data set obtained by MeSO-net (Metropolitan Seismic Observation network in Japan), to evaluate seismic potentials.