78 research outputs found
Earthquakes and sea-level change in Hokkaido, north-east Japan
This thesis details the results of an investigation into the pattern of relative sea-level (RSL) changes in north-east Hokkaido, Japan. The aim of the research is to better understand the importance of seismic and non-seismic processes in controlling spatial patterns of vertical land motions over a range of timescales. The main focus is on using salt-marsh sediments as a source of data to reconstruct RSL change during the current interseismic period, since c. 300 calibrated years before present (cal. yr BP). Previous research on the Pacific coast of Hokkaido suggests that this period is characterised by subsidence caused by strain accumulation on the locked part of the Pacific/North American plates.
I apply foraminiferal-based methods of palaeoenvironmental reconstruction to develop, using transfer functions, quantitative reconstructions of RSL change at five sites in north-east Hokkaido. Contemporary foraminifera are zoned with respect to elevation and tidal inundation, and my preferred transfer function (a model that contains 87 samples and 24 taxa) has a prediction r2 of 0.75 and a root mean squared error of prediction of ± 0.32 m. I apply this transfer function to shallow fossil sediment sequences at five salt marshes and use a combination of 210Pb, 137Cs and tephra chronology to establish age models for the sequences. The reconstructions are consistent in demonstrating little net RSL change during the last 300-100 cal. yrs, with the exception of data from one site, Sarfutsu-toh, located on the northern tip of Hokkaido. Chronologies from two profiles developed on the Pacific coast record strong evidence for recent RSL rise since the mid-1980s, but during earlier periods of the 20th century reconstructed RSL was stable or falling.
I compare my reconstructions with other direct and proxy records of land and sea-level motions. Previously published GPS and repeat levelling data indicates subsidence in south-east Hokkaido during the 20th century, although the spatial patterns and rates of change have varied. An unknown amount of this subsidence at the Kushiro tide gauge likely reflects anthropogenic activities associated with sediment compaction as well as mining-induced subsidence. An analysis of the tide-gauge records from Hokkaido show a more varied pattern of land motions, although they also confirm subsidence on the Pacific coast, close to the Kuril trench. A database of Holocene sea-level index points provides insights into longer-term millennial-scale trends in RSL. Data from six regions of Hokkaido demonstrate stable RSL close to present during the mid- and late Holocene; only the northern tip of Hokkaido (around Sarubetsu) is there evidence for a small mid-Holocene highstand of 1-3 m above present.
Finally, a review of Pleistocene raised marine terrace data shows net uplift over the last c. 330 k yr, with two areas of particularly high uplift at Abashiri and on the Pacific coast near Kushiro.
The evidence presented in this research demonstrates that it is incorrect to infer that the current interseismic period is characterised by subsidence. Overall, RSL has changed little in the last 300-100 cal. yrs. The subsidence recorded in the mid- and late 20th century on the Pacific coast of Hokkaido is not typical of the full interseismic period, nor can it have been sustained over Holocene or Pleistocene timescales. Limited data from previous earthquake cycles indicate that RSL was stable, rising or falling during previous interseismic intervals. These observations suggest that a representative ‘Hokkaido earthquake deformation cycle’ may not exist. Future research should better understand the controls of Quaternary volcanic activity on regional deformation patterns, and apply microfossil-based techniques to multiple earthquake cycles at sites to help define the spatial extent of land motions associated with different events
Controls on earthquake rupture and triggering mechanisms in subduction zones
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2010Large earthquake rupture and triggering mechanisms that drive seismicity in
subduction zones are investigated in this thesis using a combination of earthquake
observations, statistical and physical modeling. A comparison of the rupture
characteristics of M ≥ 7.5 earthquakes with fore-arc geological structure suggests that
long-lived frictional heterogeneities (asperities) are primary controls on the rupture extent
of large earthquakes. To determine when and where stress is accumulating on the
megathrust that could cause one of these asperities to rupture, this thesis develops a new
method to invert earthquake catalogs to detect space-time variations in stressing rate.
This algorithm is based on observations that strain transients due to aseismic processes
such as fluid flow, slow slip, and afterslip trigger seismicity, often in the form of
earthquake swarms. These swarms are modeled with two common approaches for
investigating time-dependent driving mechanisms in earthquake catalogs: the stochastic
Epidemic Type Aftershock Sequence model [Ogata, 1988] and the physically-based rate-state
friction model [Dieterich, 1994]. These approaches are combined into a single
model that accounts for both aftershock activity and variations in background seismicity
rate due to aseismic processes, which is then implemented in a data assimilation
algorithm to invert catalogs for space-time variations in stressing rate. The technique is
evaluated with a synthetic test and applied to catalogs from the Salton Trough in southern
California and the Hokkaido corner in northeastern Japan. The results demonstrate that
the algorithm can successfully identify aseismic transients in a multi-decade earthquake
catalog, and may also ultimately be useful for mapping spatial variations in frictional
conditions on the plate interface.Funding for this research was provided by a WHOI Hollister Research
Fellowship, a National Defense Science and Engineering Graduate Fellowship, National
Science Foundation Division of Earth Sciences (EAR) grant #0738641, United States
Geological Survey National Earthquake Hazards Reduction Program Award
#G10AP00004, and the WHOI Academic Programs Office
Detection and study of a high magnitude seismic event from GPS data: Case study of the 2011 Tohoku-Oki earthquake
The advent of GPS provided a new way of measuring surface displacements due to earthquakes by deploying GPS
networks within active seismic areas. Japan is located in the confluence of several tectonic plates, hence its seismicity.
In order to surveille this activity, one of wider GPS network in the world was deployed, i.e., GEONET. By processing
data from 93 GEONET reference stations, we analyze the 2011 Tohoku-Oki earthquake using PPP strategy. We stu-
died the time series during the event setting up a threshold value at we consider the time series are being altered by
the earthquake. We also identified the time after the occurrence when the maximum displacements happen. With the
study of these two parameters, we aim to show their different behavior as the main shock propagates along the Japan
islands, with a focus on a better understanding of the earthquake and its propagation. To achieving this, a least square
adjustment method was used to relate epicentral distance to topocentric displacements and the time of detection to
epicentral distance. The results show an exponential behavior of the distance-displacement regression versus a linear
behavior of the distance-time regression. Besides, we use the former linear regression to calculate and approximation
of the velocity of the shock wave
Proceedings of the Fourth Workshop on the Okhotsk Sea and Adjacent Areas
This report is the outcome of the fourth PICES Workshop on “The Okhotsk Sea and Adjacent Waters” held August 27–29, 2008, in Abashiri, Japan. (PDF contains 319 pages
THE RUPTURE PROCESS OF RECENT TSUNAMIGENIC EARTHQUAKES BY GEOPHYSICAL DATA INVERSION
Subduction zones are the favorite places to generate tsunamigenic earthquakes, where
friction between oceanic and continental plates causes the occurrence of a strong
seismicity. The topics and the methodologies discussed in this thesis are focussed to the
understanding of the rupture process of the seismic sources of great earthquakes that
generate tsunamis.
The tsunamigenesis is controlled by several kinematical characteristic of the parent
earthquake, as the focal mechanism, the depth of the rupture, the slip distribution along
the fault area and by the mechanical properties of the source zone. Each of these factors
plays a fundamental role in the tsunami generation. Therefore, inferring the source
parameters of tsunamigenic earthquakes is crucial to understand the generation of the
consequent tsunami and so to mitigate the risk along the coasts.
The typical way to proceed when we want to gather information regarding the source
process is to have recourse to the inversion of geophysical data that are available.
Tsunami data, moreover, are useful to constrain the portion of the fault area that extends
offshore, generally close to the trench that, on the contrary, other kinds of data are not
able to constrain.
In this thesis I have discussed the rupture process of some recent tsunamigenic events, as
inferred by means of an inverse method.
I have presented the 2003 Tokachi-Oki (Japan) earthquake (Mw 8.1). In this study the
slip distribution on the fault has been inferred by inverting tsunami waveform, GPS, and
bottom-pressure data. The joint inversion of tsunami and geodetic data has revealed a
much better constrain for the slip distribution on the fault rather than the separate
inversions of single datasets.
Then we have studied the earthquake occurred on 2007 in southern Sumatra (Mw 8.4).
By inverting several tsunami waveforms, both in the near and in the far field, we have determined the slip distribution and the mean rupture velocity along the causative fault.
Since the largest patch of slip was concentrated on the deepest part of the fault, this is the
likely reason for the small tsunami waves that followed the earthquake, pointing out how
much the depth of the rupture plays a crucial role in controlling the tsunamigenesis.
Finally, we have presented a new rupture model for the great 2004 Sumatra earthquake
(Mw 9.2). We have performed the joint inversion of tsunami waveform, GPS and satellite
altimetry data, to infer the slip distribution, the slip direction, and the rupture velocity on
the fault. Furthermore, in this work we have presented a novel method to estimate, in a
self-consistent way, the average rigidity of the source zone. The estimation of the source
zone rigidity is important since it may play a significant role in the tsunami generation
and, particularly for slow earthquakes, a low rigidity value is sometimes necessary to
explain how a relatively low seismic moment earthquake may generate significant
tsunamis; this latter point may be relevant for explaining the mechanics of the tsunami
earthquakes, one of the open issues in present day seismology.
The investigation of these tsunamigenic earthquakes has underlined the importance to use
a joint inversion of different geophysical data to determine the rupture characteristics.
The results shown here have important implications for the implementation of new
tsunami warning systems – particularly in the near-field – the improvement of the current
ones, and furthermore for the planning of the inundation maps for tsunami-hazard
assessment along the coastal area
A review on slow earthquakes in the Japan Trench
Slow earthquakes are episodic slow fault slips. They form a fundamental component of interplate deformation processes, along with fast, regular earthquakes. Recent seismological and geodetic observations have revealed detailed slow earthquake activity along the Japan Trench—the subduction zone where the March 11, 2011, moment magnitude (Mw) 9.0 Tohoku-Oki earthquake occurred. In this paper, we review observational, experimental, and simulation studies on slow earthquakes along the Japan Trench and their research history. By compiling the observations of slow earthquakes (e.g., tectonic tremors, very-low-frequency earthquakes, and slow slip events) and related fault slip phenomena (e.g., small repeating earthquakes, earthquake swarms, and foreshocks of large interplate earthquakes), we present an integrated slow earthquake distribution along the Japan Trench. Slow and megathrust earthquakes are spatially complementary in distribution, and slow earthquakes sometimes trigger fast earthquakes in their vicinities. An approximately 200-km-long along-strike gap of seismic slow earthquakes (i.e., tectonic tremors and very-low-frequency earthquakes) corresponds with the huge interplate locked zone of the central Japan Trench. The Mw 9.0 Tohoku-Oki earthquake ruptured this locked zone, but the rupture terminated without propagating deep into the slow-earthquake-genic regions in the northern and southern Japan Trench. Slow earthquakes are involved in both the rupture initiation and termination processes of megathrust earthquakes in the Japan Trench. We then compared the integrated slow earthquake distribution with the crustal structure of the Japan Trench (e.g., interplate sedimentary units, subducting seamounts, petit-spot volcanoes, horst and graben structures, residual gravity, seismic velocity structure, and plate boundary reflection intensity) and described the geological environment of the slow-earthquake-genic regions (e.g., water sources, pressure–temperature conditions, and metamorphism). The integrated slow earthquake distribution enabled us to comprehensively discuss the role of slow earthquakes in the occurrence process of the Tohoku-Oki earthquake. The correspondences of the slow earthquake distribution with the crustal structure and geological environment provide insights into the slow-earthquake-genesis in the Japan Trench and imply that highly overpressured fluids are key to understanding the complex slow earthquake distribution. Furthermore, we propose that detailed monitoring of slow earthquake activity can improve the forecasts of interplate seismicity along the Japan Trench
Source model of the 2007 M_w 8.0 Pisco, Peru earthquake: Implications for seismogenic behavior of subduction megathrusts
We use Interferometric Synthetic Aperture Radar, teleseismic body waves, tsunami waveforms recorded by tsunameters, field observations of coastal uplift, subsidence, and runup to develop and test a refined model of the spatiotemporal history of slip during the M_w 8.0 Pisco earthquake of 15 August 2007. Our preferred solution shows two distinct patches of high slip. One patch is located near the epicenter while another larger patch ruptured 60 km further south, at the latitude of the Paracas peninsula. Slip on the second patch started 60 s after slip initiated on the first patch. We observed a remarkable anticorrelation between the coseismic slip distribution and the aftershock distribution determined from the Peruvian seismic network. The proposed source model is compatible with regional runup measurements and open ocean tsunami records. From the latter data set, we identified the 12 min timing error of the tsunami forecast system as being due to a mislocation of the source, caused by the use of only one tsunameter located in a nonoptimal azimuth. The comparison of our source model with the tsunami observations validate that the rupture did not extend to the trench and confirms that the Pisco event is not a tsunami earthquake despite its low apparent rupture velocity (<1.5 km/s). We favor the interpretation that the earthquake consists of two subevents, each with a conventional rupture velocity (2–4 km/s). The delay between the two subevents might reflect the time for the second shock to nucleate or, alternatively, the time it took for afterslip to increase the stress level on the second asperity to a level necessary for static triggering. The source model predicts uplift offshore and subsidence on land with the pivot line following closely the coastline. This pattern is consistent with our observation of very small vertical displacement along the shoreline when we visited the epicentral area in the days following the event. This earthquake represents, to our knowledge, one of the best examples of a link between the geomorphology of the coastline and the pattern of surface deformation induced by large interplate ruptures
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The seismic coupling of subduction zones revisited
The nature of seismic coupling for many of the world's subduction zones has been reevaluated. Geodetic estimates of seismic coupling obtained from GPS measurements of upper plate deformation during the interseismic period are summarized. We compared those with new estimates of seismic coupling obtained from seismological data. The results show that with a few notable exceptions the two methods agree to within about 10%. The seismological estimates have been greatly improved over those made 20-30 years ago because of an abundance of paleoseismological data that greatly extend the temporal record of great subduction earthquakes and by the occurrence, in the intervening years, of an unusual number of great and giant earthquakes that have filled in some of the most critical holes in the seismic record. The data also, again with a few notable exceptions, support the frictional instability theory of seismic coupling, and in particular, the test of that theory made by Scholz and Campos (1995). Overall, the results support their prediction that high coupling occurs for subduction zones subjected to high normal forces with a switch to low coupling occurring fairly abruptly as the normal force decreases below a critical value. There is also considerable variation of coupling within individual subduction zones. Earthquake asperities correlate with areas of high coupling and hence have a semblance of permanence, but the rupture zones and asperity distributions of great earthquakes may differ greatly between seismic cycles because of differences in the phase of seismic flux accumulation
GPS音響観測に基づく2011年東北沖地震の地震時・地震後変動に関する研究
Tohoku University木戸元之課
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