Coseismic seafloor deformation in the trench region during the Mw8.8 Maule megathrust earthquake
Andrei MaksymowiczC. D. ChadwellJ. A. RuizA. M. TréhuEduardo Contreras‐ReyesWilhelm WeinrebeJuan Díaz‐NaveasJ. C. GibsonPeter LonsdaleM. D. Tryon
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Abstract The M w 8.8 megathrust earthquake that occurred on 27 February 2010 offshore the Maule region of central Chile triggered a destructive tsunami. Whether the earthquake rupture extended to the shallow part of the plate boundary near the trench remains controversial. The up-dip limit of rupture during large subduction zone earthquakes has important implications for tsunami generation and for the rheological behavior of the sedimentary prism in accretionary margins. However, in general, the slip models derived from tsunami wave modeling and seismological data are poorly constrained by direct seafloor geodetic observations. We difference swath bathymetric data acquired across the trench in 2008, 2011 and 2012 and find ~3–5 m of uplift of the seafloor landward of the deformation front, at the eastern edge of the trench. Modeling suggests this is compatible with slip extending seaward, at least, to within ~6 km of the deformation front. After the M w 9.0 Tohoku-oki earthquake, this result for the Maule earthquake represents only the second time that repeated bathymetric data has been used to detect the deformation following megathrust earthquakes, providing methodological guidelines for this relatively inexpensive way of obtaining seafloor geodetic data across subduction zone.Keywords:
Seafloor Spreading
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Abstract Marine positioning is relevant for several aspects of tsunami research, observation, and prediction. These include accurate positioning of instruments on the ocean bottom for determining the deep‐water signature of the tsunami, seismic observational setups to measure the earthquake parameters, equipment to determine the tsunami characteristics during the propagation phase, and instruments to map the vertical uplift and subsidence that occurs during a dip‐slip earthquake. In the accurate calculation of coastal tsunami run‐up through numerical models, accurate bathymetry is needed, not only near the coast (for tsunami run‐up) but also in the deep ocean (for tsunami generation and propagation). If the bathymetry is wrong in the source region, errors will accumulate and will render the numerical calculations inaccurate. Without correct and detailed run‐up values on the various coastlines, tsunami prediction for actual events will lead to false alarms and loss of public confidence.
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At the 2003 Tokachi-oki earthquake of M8, seafloor phenomena such as a generation process of tsunami, seafloor uplifts, etc., were observed using a cabled observatory installed on the seafloor. The seafloor uplifts were observed not before the main shock but continuously after the main shock. The uplifts were 0.35，0.37, and 0.12 m for epicentral distances of 25.5, 31.4, and 81.8 km, respectively. Pressure fluctuations that took place co-seismically show about 100 times in amplitude to those observed as the uplifts. The uplift of the seafloor generated not only tsunami but high amplitude acoustic waves. Both the tsunami and acoustic waves were generated by the uplift and superposed to each other. After the main shock, a continuous uplift of the seafloor is observed at the all three pressure gauge locations and the rate of uplift was about 0.004 m/day. These phenomena may imply that there was a change in the state of friction on the plate boundary interface by the main shock. In this paper, we demonstrate what was observed using ocean bottom pressure gauges installed right above the focal area of the earthquake, and then discuss these phenomena in tsunami generation and in post-seismic slip processes.
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The 1998 Papua New Guinea tsunami was greater than expected from its earthquake magnitude. The area of significant impact was small, approximately a 30 km stretch near the mouth of Sissano Lagoon, Papua New Guinea. To explain the localized nature of the event, a submarine landslide has been conjectured to be responsible. Our study indicates that offshore bathymetry is critical to predicting tsunami coastal behavior. Model runs with newly obtained bathymetric data indicate that an earthquake fault source combined with the existing seafloor geometry may also explain the concentrated tsunami. Although the definitive cause of the Papua New Guinea tsunami remains uncertain, local bathymetry had a notable effect on the wave behavior.
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Bedforms are common features in shallow marine environments, and their presence evokes questions regarding the spatial and temporal stability of the seafloor. Though observation of bedform dynamics from multibeam bathymetry and its derived products enhances understanding of seafloor stability, the ability to successfully detect bedform migration depends on (1) the survey resolution and positioning uncertainty, and (2) the establishment of an optimum survey-repetition rate.
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Multibeam bathymetry data could represent nearly continuous coverage depth measurements of the seafloor and reveal geomorphological regions. Recent studies have utilized multibeam bathymetry data to provide geological maps, but their delineations were done manually. Manual classification and delineation are inherently subjective and therefore can be inaccurate. In this paper, we try to develop one strategy to explore seafloor stretching in Mariana trench arc via squeeze and excitation network, combining data clustering, slope and gradient. In our experiments, we use the high-resolution multibeam bathymetric data collected by NOAA Office of Ocean Exploration and Research (OER). The geomorphological seabed in the Mariana region is automatically classified into different classes. The experimental results demonstrate that geomorphological seabed classification strategy achieves a robust, automated delineation approach.
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For decades, sidescan sonars have been the primary tool to obtain acoustic images of the seafloor. Such images provide qualitative information on the seafloor surveyed based on amplitude variations of the backscattered acoustic signals received. In the 1980s, bathymetric sidescan sonar systems, capable of simultaneously producing acoustic images and measuring depth at numerous points across the swath, added a quantitative description of the seafloor in the form of a depth contour map. Similar claims can be made with multibeam echo sounders well known for their high-resolution swath bathymetry capabilities. Taking advantage of this high bathymetric resolution, the beamformed acoustic backscatter data can also be displayed as a geometrically correct acoustic image of the seafloor and provide textural information not available in the contoured bathymetry of the same area. Likewise, knowledge of the bathymetry, particularly bottom slopes, is needed to correct for the angular dependence of seafloor acoustic backscatter and construct a map of acoustic backscattering strength over the area. Such a map will give clues to regional variations in lithelogies.
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A newly compiled bathymetric map including parts of the Kuril, Japan, and northern Izu–Ogasawara trenches in the northwestern Pacific Ocean demonstrates that most bending-related topographic structures are limited to less than 80 km from the trench axis. This observation contrasts with one that bending-related structures of eastern Pacific trenches are limited to less than 50 km from the trench axis. The discrepancy may be due to differences in the ages of subducting oceanic plates. Bending-related topographic structures of the western Kuril and southern Japan trenches are not parallel to the trench axis, but instead are parallel to magnetic anomaly lineations. Those of the northern Izu–Ogasawara Trench are parallel to fracture zones. These observations indicate the rule that the inherited seafloor spreading fabric is reactivated instead of forming new faults when the degree of obliquity between inherited seafloor spreading fabric and trench axis reaches about 30°. This rule is applicable to most trenches around the Pacific Ocean, except for some parts of curved trenches and trenches near seamounts or other volcanic edifices constructed by off-ridge volcanism. Most bending-related topographic structures near off-ridge volcanic edifices are parallel to the trench axis. This observation suggests that inherited seafloor spreading fabric around the volcanic edifies was disrupted by volcanism.
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We propose a method of tsunami waveform inversion to accurately estimate a tsunami source by incorporating the effect of permanent seafloor deformation recorded by ocean‐bottom pressure gauges (OBPGs) within the source region. We developed a general expression of water‐depth fluctuation recorded at an OBPG following seafloor deformation of arbitrary spatiotemporal distribution. By assuming that coseismic rupture propagates with infinite velocity, the general expression can be reduced to an equation relating observed OBPG waveforms to initial sea‐surface displacement at the source by using a Green's function consisting of two terms: the Green's function used in regular tsunami inversion and a correction term to account for water‐depth change in response to permanent seafloor deformation. By using the two‐term Green's functions, the effect of seafloor deformation can be taken into account in tsunami source estimation. We applied the revised inversion method to observations of coseismic seafloor deformation and tsunami during the 2003 Tokachi‐oki earthquake ( M w 8.3) at two OBPG stations near the Kuril Trench. The tsunami source model we estimated is consistent with models previously derived using various other geophysical data sets. Furthermore, the coastal tsunami waveforms we modeled match the observed tsunami well. Forecasts of tsunami arrival times and first peak amplitudes by our method can be obtained 20 min after an earthquake, and can be provided to the coastal communities nearest to the source with a lead time of ∼10 min.
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