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Development of cabled seafloor seismo-geodetic network and seafloor borehole observatories in the Nankai Trough
| Content Provider | Semantic Scholar |
|---|---|
| Author | Ammon, Charles J. |
| Copyright Year | 2008 |
| Abstract | VOLUME NSF MARGINS Program, SEIZE Workshop, Mt. Hood, Oregon, 2008. Broadband Seismic Imaging of Large Earthquake Ruptures Charles J. Ammon, Penn State Broadband Seismic Imaging of Large Earthquake Ruptures During the last 15-20 years, expansion of the global seismic networks and the installation of a substantial number of regional seismic stations, including observatories with broadband seismic sensors, have yielded high-quality seismic observations. Coupled with increased computer efficiency and substantial reductions in digital data storage costs, and broad, international sharing of seismic observations, these data form a basis for an improved view of seismogenic components of large earthquake ruptures. Although only one interplate subduction event occurred near a dense strong motion network (the 2003 Tokachi Oki, Hokkaido earthquake), more than 20 large events (GCMT moment magnitude greater than or equal to 7.8) have occurred around the globe, including the extremely large 2004 Sumatra megathrust earthquake, 12 large under-thrusting events, a large under-thrusting/outer-rise doublet (2006-2007 Kuril Islands), and an intriguing rupture across a subduction plate boundary (01 April, 2007 Solomon Islands). I discuss advances in seismic methodology developed by a number of researchers active in rupture imaging, and illustrate our improved capabilities using examples from recent large earthquakes. The increased signal bandwidth (100's to several seconds period) better illuminates the slower (100's of seconds period) rupture processes, and helps define the spatial extent of seismic ruptures better than shorter-period body wave signals, traditionally used to construct seismic "finite-fault" (kinematic) models. Creative adaption of short-period seismic array signals (periods of a few seconds) to image rupture-front propagation have led to additional information on the processes associated with rupture propagation. Improved seismic rupture models for large earthquakes are important to efforts to understand the relation between seismic strain release and processes such as episodic slip and tremor along with longer-term geodetic and geologic evidence of stress transfer across subduction boundaries. Improved seismic rupture models with good resolution of critical rupture parameters spanning seismic time scales (process times from a few seconds to 10's of minutes), will provide better constraints for physical models of large earthquake ruptures. Improved seismic rupture models with good resolution of important rupture parameters spanning time scales from a few seconds to 10's of minutes (seismic time scales), will provide better constraints for physical models of large earthquakes and will contribute to our overarching efforts to understand subduction processes. NSF MARGINS Program, SEIZE Workshop, Mt. Hood, Oregon, 2008. 1 Development of cabled seafloor seismo-geodetic network and seafloor borehole observatories in the Nankai Trough Eiichiro Araki and Yoshiyuki Kaneda DONET, JAMSTEC A scientific submarine cable network called DONET (Dense Ocean-floor Network System for Earthquakes and Tsunamis) is going to be built in the rupture area of Tonankai earthquake in the south of Kii-peninsula, Japan. In this area, the Philippine Sea plate is subducting under the Honshu Island. As a result, large earthquakes occur with recurrence intervals of about 100-150 years, where the last one occurred in 1944. The objectives of the submarine cable network are precision monitoring of the seismic activity of micro to large earthquakes, slow slip of the plate boundary seafloor deformation, and Tsunamis generated due to large earthquakes. Availability of no seafloor data is large limitation for us to observe these events in the seafloor. We aim, by DONET installation, not only to improve detection ability of events, but to model these events quantitatively with better accuracy. We designed our submarine cable observation system to increase the number of observatories significantly (in an order of ten) from present submarine cable network systems for earthquake monitoring which allow only several seismometer installed in each observation node. Increased number of observatory may be installed by extending cable from each observation node [1]. Using multiple extension fiber-optic cables from observation node, our design allow maximum of 40 observatories from five observation nodes connected to backbone submarine cable. In the initial installation of DONET, by optimizing observatory density and location, we plan to install 20 seafloor observatories distributed densely, covering the area from the trench axis to the main rupture area of the last Tonankai earthquake (Fig. 1). This distribution of observatory enables us to precisely determine hypocenter of small to large earthquakes and detect relatively small ground deformation in the seafloor. We evaluated these ability of DONET network by computer simulation. By simulating with different DONET observatory distribution, we optimized our network so that area of good hypocenter location is maximized, yet there is no obvious gap of such area in and around the DONET network. We also evaluated detection ability of ground deformation due to small precursory slip event. With precision observation of seafloor pressure change in densely distributed seafloor quartz pressure gauges, we expect to identify occurrence of small ground deformation. By such observation, slow slip event on the plate boundary may be detected. An event of moment magnitude 6 slip on the plate boundary in the seafloor was evaluated for detection by DONET network. Our simulation result suggests that such detection is possible if such event occur below the DONET network, and seafloor pressure gauge has resolution of 1 cm or better. Such detection of events in the seafloor, is impossible only with current GPS geodetic observation network which are dense, but only on land. The observation targets of DONET, such as small to large earthquakes, slop-slip events on the plate boundary, Tsunamis, require precision seismometers and pressure gauges. Small earthquakes has very small amplitude and typically observed in frequencies higher than 1 Hz. Large earthquakes show very large acceleration up to 2 G or so. On the otherhand, slow slip events may be observed between 0.01-0.1 Hz, or even lower frequencies such as days or months. In all, dynamic range and frequency band of DONET observation target are wide (smaller than 10 m/s to 2 G) and broad (from 1/years to 100 Hz ). To cover the dynamic range and frequency NSF MARGINS Program, SEIZE Workshop, Mt. Hood, Oregon, 2008. 2 band, we plan to combine two types of seismometer and three types of pressure gauges installed in each observatory. Two types of seismometer are a broadband seismometer to cover weak motion in the frequency band of 1/360 Hz to 100 Hz and a strong motion accelerometer to cover strong motion in the frequency band from DC to 100 Hz. The three types of pressure gauges are quartz pressure gauge to observe Tsunamis and seafloor deformation by absolute pressure from DC to approximately 1 Hz, a differential pressure gauge [2] to observe broadband seismic waves as a small change of seafloor pressure from 1/200 Hz to 20 Hz, and a hydrophone to observe high frequency acoustic waves. A differential pressure gauge is very sensitive and can detect pressure change of 0.1 Pa or smaller in frequencies 0.02-0.08 Hz, where very low frequency earthquakes are known to be observed. We evaluated noise characteristics of seismometers of different kind in a vault of Matsushiro Seismological Observatory, Japan Meteorogical Agency. Evaluated seismic sensors were three types of broadband seismographs (Streikeisen STS-2, Guralp CMG-3T, prototype version of Kinemetrics Cronos), three types of strong motion accerelometers (JAE JA-5 typeIII, JAE JA40G, Metrozet TSA-100S), and four types of geophones. As well to monitor Tsunamis generated in and around the network, to monitor ground deformation in vertical direction, quartz pressure gauge, that gives depth of seafloor in terms of seafloor pressure, is used. Quartz pressure gauges are stable over years typically better than 0.5 psi/year. Effect of seafloor deformation due to slip events on the plate boundary is usually very small. Therefore, the stability of quartz pressure gauge used is very important. We evaluated quartz pressure gauges for their long-term stability in environment similar to the seafloor built in a laboratory. Manufacturer of tested quartz pressure gauges were Hewlett Packard, Paroscientific, and Clark Oilfield Measurement. The laboratory test for more than continuous 150 days period was conducted to determine the type of quartz pressure gauge for the DONET observatory. To isolate seismometers from the effect of seafloor current flow, we plan to design the seismometer package to enable surfacial burial ([3], [4]) in the sediment, while the pressure gauge package will be installed in the seafloor (Fig. 2). From experience of previous seafloor deployments of Tsunami-meters [5], care will be taken to minimize temperature change of pressure gauges. These are the part of actions taken to achieve very low noise observation by the future DONET observatory. Futhermore, the technology developed for the DONET network can be applied to connect sensors installed in deep seafloor boreholes. We plan to install three borehole observatories in the NantroSEIZE project in the area of DONET. Drilling of three boreholes (NT2-11 2.5kmbsf, C0002 1kmbsf, and NT2-1 500mbsf in Fig. 1), in the area of transition from aseismic to seismically coupled plate boundary, is planned in 2009 by IODP drilling vessel Chikyu. Sensors will be installed in the following stage, after the drilling in 2009. A set of borehole instruments to monitor ground deformation in the plate boundary will be installed in each borehole. The borehole sensors consist of strainmeters, tiltmeters, seis |
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| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
