Tetsuo Takanami

Precise aftershock distribution of the 2007 Chuetsu-oki Earthquake obtained by using an ocean bottom seismometer network

The Chuetsu-Oki Earthquake occurred on July 16, 2007. To understand the mechanism of earthquake generation, it is important to obtain a detailed seismic activity. Since the source region of the 2007 Chuetsu-oki Earthquake lies mainly offshore of Chuetsu region, a central part of Niigata Prefecture, it is difficult to estimate the geometry of faults using only the land seismic network data. A precise aftershock distribution is essential to determine the fault geometry of the mainshock. To obtain the detailed aftershock distribution of the 2007 Chuetsu-oki Earthquake, 32 Ocean Bottom Seismometers (OBSs) were deployed from July 25 to August 28 in and around the source region of the mainshock. In addition, a seismic survey using airguns and OBSs was carried out during the observation to obtain a seismic velocity structure below the observation area for precise hypocenter determination. Seven hundred and four aftershocks were recorded with high spatial resolution during the observation period using OBSs, temporally installed land seismic stations, and telemetered seismic land stations and were located using the double-difference method. Most of the aftershocks occurred in a depth range of 6–15 km, which corresponds to the 6-km/s layer. From the depth distribution of the hypocenters, the aftershocks occurred along a plane dipping to the southeast in the whole aftershock region. The dip angle of this plane is approximately 40°. This single plane with a dip to the southeast is considered to represent the fault plane of the mainshock. The regions where few aftershocks occurred are related to the asperities where large slip is estimated from the data of the mainshock. The OBS observation is indispensable to determine the precise depths of events which occur in offshore regions even close to a coast.

Delamination structure imaged in the source area of the 1982 Urakawa-oki earthquake

[1] The Kuril arc collides with the northeast Japan arc in the southern part of Hokkaido, Japan. 3-D tomographic inversion of data from a dense network of sensitive ocean-bottom seismographs and land stations has allowed imaging of previously unseen details of the arc-arc collision structure. A low velocity body dips gently southwestward, at depths of 35 to 45 km, from east of the Hidaka Mountains to the source area of the 1982 Urakawa-oki destructive earthquake (Ms 6.8). The low velocity body is the lower half of the lower crust of the Kuril arc, which must have been delaminated by the collision. We believe that the continuing collision of the delaminated lower crust with the northeast Japan arc resulted as an episode of aseismic slow slip prior to the 1982 Urakawa-oki earthquake as well being the reason for the high seismic activity in this region.

Crustal structure of the Vøring Margin, NE Atlantic: a review of geological implications based on recent OBS data

Modelling of extensive seismic datasets recorded on Ocean Bottom Seismographs (OBS) on the outer Voring Margin, NE Atlantic, has provided significant new insights into deeper sedimentary structures, distribution of sill-intrusions in the sedimentary section, top of the crystalline crust, the lower crust and Moho. Primarily based on the modelling of S-waves, it is concluded that the high-velocity lower crust most likely consists of a mixture of plume-related Late Cretaceous/Early Tertiary mafic intrusions mixed with older continental blocks. Northeastwards in the Voring Basin, the landward limit of the lower crustal high-velocity layer steps gradually seawards, closely related to five crustal scale lineaments. Evidence for an interplay between active and passive rifting components is found on regional and local scales on the margin. The active component is evident through the decrease in magmatism with increased distance from the Iceland plume, and the passive component is illustrated by the fact that all resolved crustal lineaments to a certain degree acted as barriers to magma emplacement. A lithospheric delamination model is invoked to explain the observed variations in crustal velocities and thickness. The location of six Tertiary domal structures in the Voring Basin is between, or in the vicinity of, pre-breakup high-velocity structures, which may act as rigid blocks during compression. It is proposed that the existence and trend of these high-velocity structures, subject to mild NW–SE compression, is the most important factor controlling the formation, spatial distribution and trend of the domes. Structures in the high-velocity lower crust may be the single most important element in controlling the formation of the domes; all modelled highs in the lower crustal Early Tertiary intrusive layer seem to be related to the formation of domes.

Aftershock observation of the 2003 Tokachi-oki earthquake by using dense ocean bottom seismometer network

The Tokachi-Oki earthquake occurred on September 26, 2003. Precise aftershock distribution is important to understand the mechanism of this earthquake generation. To study the aftershock activity, we deployed forty-seven ocean bottom seismometers (OBSs) and two ocean bottom pressure meters (OBPs) at thirty-eight sites in the source region. We started the OBS observation four days after the mainshock for an observation period of approximately two months. In the middle of the observation period, nine OBSs near the epicenter of the mainshock were recovered to clarify the depth distribution of aftershocks near the mainshock. From the data overall OBS, seventy-four aftershocks were located with high spatial resolution. Most of the aftershocks were located in a depth range of 15–20 km and occurred within the subducting oceanic crust, the 5.5-km/s layer of the landward plate and the plate boundary. No aftershocks were found in the mantle of the subducting plate. The low seismic activity beneath the trench area where the water depth is greater than about 2000 m suggests a weak coupling between the two plates. The depth of the mainshock is inferred to be 15–20 km from the aftershock distribution.

Multivariate time-series model to estimate the arrival times of S-waves

Abstract Some computationally efficient procedures, which had been developed for the estimation of the onset time of seismic waves, are examined for their ability in determining the onset time of the S-wave in an online system. The three methods we compare are the univariate locally stationary autoregressive model (FUNIMAR or LSAR), multivariate locally stationary autoregressive model (MLSAR), and the summing AIC procedure. The summing AICs procedure is introduced by adding AIC values at each time point for the three univariate autoregressive models. Although this simple summing can provide the same estimation as that by MLSAR at substantial computational savings, its implicit assumption of independence of each component sheds some doubt upon its results. The procedure of multivariate autoregressive model (2-V MLSAR) for horizontal components is most useful for the precise estimation of the arrival time of the S-wave. FUNIMAR is sufficient to determine the arrival time of the P-wave, but not appropriate to determine the arrival time of the S-wave.

Extraction of signal by a time series model and screening out micro earthquakes

Abstract A model for the decomposition of time series into several components is shown. In the model each component is expressed by an autoregressive model. The minimum AIC procedure for the estimation of these models is shown. The crucial problem of estimating changing variance of the model was solved by the techniques of piecewise modeling and local modeling. The extraction of micro earthquake signal was shown to exemplify the proposed procedures. The stability of the procedure and a possible simplication of the procedure were also considered by using the same data set.

High Precision Estimation of Seismic Wave Arrival Times

When an earthquake occurs, vibrations propagate from the source in all directions. For large earthquakes, vibrations are soon observed even at sites far from the source. Generally, such a vibration is called a seismic wave. The seismic waves which propagate through the interior of the earth are comprised of two types: longitudinal P waves and transverse S waves. Additionally, surface waves (Rayleigh waves, Love waves, etc.) which propagate close to the surface of the earth also exist. These body waves and surface waves propagate with velocities which depend on the physical properties of the earth interior. Generally the velocity of the seismic wave increases with depth in the earth. Furthermore, the velocity of P waves is approximately 1.73 times than that of S waves, while the velocity of surface waves is 0.92 times that of the S waves. Therefore, depending on the distance of the observation point from the hypocenter and the depth of the hypocenter, there are differences in the arrival times of the individual waves. At present a table (travel time table) showing the relation between the distance, depth, and arrival times is available, and may be used to estimate the hypocenter. Observation of the arrival times at many observation points together with the use of the table allows the time and location of the earthquake to be estimated. Also, in contrast, a detailed velocity structures of the earth can be estimated by using many of the observation data of accurate arrival times. This results in the generation of a more accurate travel time table, thus more accurate hypocenter locations.

Intensive observations using pop-up type ocean bottom seismometers in the first decade of the 21st century

High dense marine seismic observation is a key to understand earthquake system beneath oceans. We have been developing long-term ocean bottom seismometers (OBSs), which are able to store more than a year continuous seismic records on sea bottoms, since end of the 20th century, and they enable us to acquire many earthquake data when we have a high dense seismic network in marine region. In the last decade, we have done intensive observations in the Japan Trench and the Nankai Trough subduction zones. These experiments reveal seismic activities there precisely, and the results are useful for further studies such as structures of the earths interior and relations between earthquakes and structural heterogeneity. Otherwise, it remains some problems based on the system. We should use it and other system jointly depending on situations as well as improve our system for future.

State Space Approach to Signal Extraction Problems in Seismology

State space methods for extracting signal from noisy seismic data are shown. The method is based on the general state space model, recursive filtering and smoothing algorithms. The self-organizing state space model is used for the estimation of time-varying parameter of the model. In this paper, we show five specific examples of time series modeling for signal extraction problems related to seismology. Namely, we consider 1) the estimation of the arrival time of a seismic signal, 2) the extraction of small seismic signal from noisy data, 3) the detection of the coseismic effect in groundwater level data contaminated by various effects from air pressure etc., 4) the estimation of changing spectral characteristic of seismic record, and 5) spatial-temporal smoothing of OBS data.

Extraction of Signal from High Dimensional Time Series: Analysis of Ocean Bottom Seismograph Data

A signal extraction method is developed based on a prior knowledge on the propagation of seismic signal. To explore underground velocity structure based on OBS (Ocean Bottom Seismogram), it is necessary to detect reflection and refraction waves from the data contaminated with relatively large direct wave and its multiples. In this paper, we consider methods based on the time series and spatial-temporal decompositions of the data. In spatial-temporal decomposition, the difference of the travel time (moveout) corresponding to underground layer structure is utilized. The proposed methods are exemplified with a real OBS data.