A field survey of the 1946 Aleutian tsunami in the far field (original) (raw)
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A seismological reassessment of the source of the 1946 Aleutian ‘tsunami’ earthquake
Geophysical Journal International, 2006
We present a re-evaluation of the seismological properties of the Aleutian 'tsunami earthquake' of 1946 April 1, characterized by a deceptively low conventional magnitude (7.4) in view of its catastrophic tsunami, both in the near and far fields. Relocation of 40 aftershocks show that the fault zone extends a minimum of 181 km along the Aleutian trench, in a geometry requiring a bilateral rupture from the original nucleation at the epicentre. Their spatial and temporal distribution are typical of the aftershock patterns of a large earthquake, and rule out the model of a landslide source exclusive of a dislocation. The analysis of the spectra of mantle waves favours the model of a large seismic source, with a static moment of 8.5 × 10 28 dyn-cm, making the event one of the ten largest earthquakes ever recorded (hence the destructive tsunami in the far field), and of a slow bilateral rupture, at an average velocity of only 1.12 km s −1 , hence the destructive interference in all azimuths for all but the longest mantle waves. The exceptionally slow character of the earthquake is confirmed by a deficiency in radiated seismic energy expressed by the lowest value measured to date of the energy-tomoment ratio. The earthquake appears as an end member in the family of 'tsunami earthquakes', resulting from the combination of anomalous, but not unprecedented, parameters, such as low stress drop and rupture velocity.
Estimation of seismic moment and slip distribution of the April 1, 1946, Aleutian tsunami earthquake
Journal of Geophysical Research: Solid Earth, 1997
The 1946 Aleutian earthquake produced a tsunami of tsunami magnitude Mr=9.3, but the surface wave magnitude is only Ms=7.4, making it a tsunami earthquake. The discrepancy between the apparent size of the earthquake based on the seismic data and the size of the tsunami has been explained by several mechanisms, including a landslide and a slow e .arthquake, but there are few seismic data available to determine the correct mechanism. We study the generating mechanism of the tsunami using tsunami waveforms recorded on tide gauges. We have modeled the source of the 1946 Aleutian tsunami as the result of an underthrusting earthquake. We performed both forward and inverse modeling of the data using a finite difference calculation to compute synthetic tsunamis. We include both vertical and horizontal deformation of the ocean bottom due to faulting in the computation. The results of the inversion of the tsunami waveforms show that the slip on the fault is mainly concentrated in the shallow section of the fault near the epicenter. There is little slip in the area of the aftershocks, indicating that the aftershocks may represent an area of afterslip or triggered seismicity. The moment estimate of the earthquake is 23x 1020 N m, or Mw=8.2. The majority of the waveforms are well-explained by this fault model and slip distribution, including the Honolulu waveform, but the Hawaiian data also suggest that other effects, such as a landslide, may be necessary for explaining the abnormal tsunami amplitudes in Hawaii. The 1946 earthquake is similar to other tsunami earthquakes, such as the 1896 Sanriku and 1992 Nicaragua earthquakes, indicating that slow, shallow rupture may explain the disproportionally large tsunamis. surface wave magnitude Ms is only 7.4 [Gutenberg and Richter, 1954]. The tsunami magnitude Mr[Abe, 1979] of the 1946 earthquake is 9.3, making it larger than two of the largest earthquakes of the 20th century (Figure 2), the 1957 Aleutian earthquake (Ms=8.1 , Mw=8.6, Mr=9.0) and the 1964 Alaska earthquake (Ms=8.4, Mw=9.2, Mr=9.1). These other earthquakes have aftershock zones of hundreds of kilometers in length, showing that rupture extended over extremely large areas. The 1946 earthquake, however, has a very small aftershock zone, about 100 km in length, as determined by Sykes [1971] and shown in Figure 1. In fact, the 1946 earthquake is the classic case of a "tsunami earthquake" [Kanamori, 1972], an earthquake with a tsunami much larger than expected from the surface wave magnitude. The tsunamirun-up exceeded 30 m in height on Unimak Island and destroyed the Scotch Cap lighthouse, killing five
Marine Geology, 2004
In a recent contribution, Fryer et al. (2004) have proposed to interpret many features of the 1946 Aleutian earthquake and tsunami as evidence for a major earthquake-triggered landslide. In particular, in support of their model, they claim that the T phase observed at Hawaiian Volcano Observatory (HVO) was generated at the time of the main shock, and dispute our earlier interpretation (Okal et al., 2003a; hereafter Paper I) that it is about 29 min late in this respect and was actually generated by the first major aftershock, occurring 27 min after and 86 km to the North of the main shock. The scope of the present Comment is strictly limited to the issue of the interpretation of the T phase at HVO. In particular, we elect, at this point, not to comment on any aspect regarding the characteristics of the far-field tsunami, which may be addressed in a later contribution; this should not, however, be taken as an endorsement of Fryer et al.'s model (Fryer et al., 2004) for the specific properties of the Ugamak slide, or more generally for the generation of the tsunami. We simply wish to address several points raised specifically in Section 8 of Fryer et al. (2004): (i) respond to their claim that our timing of the T wave train is wrong; (ii) correct the false impression left by their work that T phase amplitudes should be in direct relation to those of P waves, and (iii) emphasize that we presently know very little about the T phases of underwater landslides.
Geophysical Journal International, 2017
We extend to distances beyond 80 • the computation of the energy-to-moment slowness parameter introduced by Newman and Okal, by defining a regional empirical correction based on recordings at distant stations for events otherwise routinely studied. In turn, this procedure allows the study of earthquakes in a similar source-station geometry, but for which the only available data are located beyond the original distance threshold, notably in the case of historical earthquakes predating the development of dense networks of short-period seismometers. This methodology is applied to the twin 1947 earthquakes off the Hikurangi coast of New Zealand for which we confirm slowness parameters characteristic of tsunami earthquakes. In addition, we identify as such the large aftershock of 1934 July 21 in the Santa Cruz Islands, which took place in the immediate vicinity of the more recent 2013 shock, which also qualifies as a tsunami earthquake. In that subduction zone, the systematic compilation of for both recent and pre-digital events shows a diversity in slowness correlating with local tectonic regimes controlled by the subduction of fossil structures. Our methodology is also well adapted to the case of analogue records of large earthquakes for which short-period seismograms at conventional distances are often off-scale.
Geophysical Journal International, 2017
We extend to distances beyond 80 • the computation of the energy-to-moment slowness parameter introduced by Newman and Okal, by defining a regional empirical correction based on recordings at distant stations for events otherwise routinely studied. In turn, this procedure allows the study of earthquakes in a similar source-station geometry, but for which the only available data are located beyond the original distance threshold, notably in the case of historical earthquakes predating the development of dense networks of short-period seismometers. This methodology is applied to the twin 1947 earthquakes off the Hikurangi coast of New Zealand for which we confirm slowness parameters characteristic of tsunami earthquakes. In addition, we identify as such the large aftershock of 1934 July 21 in the Santa Cruz Islands, which took place in the immediate vicinity of the more recent 2013 shock, which also qualifies as a tsunami earthquake. In that subduction zone, the systematic compilation of for both recent and pre-digital events shows a diversity in slowness correlating with local tectonic regimes controlled by the subduction of fossil structures. Our methodology is also well adapted to the case of analogue records of large earthquakes for which short-period seismograms at conventional distances are often off-scale.
Detailed analysis of tsunami waveforms generated by the 1946 Aleutian tsunami earthquake
Natural Hazards and Earth System Science, 2001
The 1946 Aleutian earthquake was a typical tsunami earthquake which generated abnormally larger tsunami than expected from its seismic waves. Previously, Johnson and Satake (1997) estimated the fault model of this earthquake using the tsunami waveforms observed at tide gauges. However, they did not model the second pulse of the tsunami at Honolulu although that was much larger than the first pulse. In this paper, we numerically computed the tsunami waveforms using the linear Boussinesq equation to determine the fault model which explains the observed tsunami waveforms including the large second pulse observed at Honolulu. The estimated fault width is 40-60 km which is much narrower than the fault widths of the typical great underthrust earthquakes, the 1957 Aleutian and the 1964 Alasuka earthquakes. A previous study of the 1896 Sanriku earthquake, another typical tsunami earthquake, suggested that the additional uplift of the sediments near the Japan Trench had a large effect on the tsunami generation. In this study, we also show that the additional uplift of the sediments near the trench, due to a large coseismic horizontal movement of the backstop, had a significant effect on the tsunami generation of the 1946 Aleutian earthquake. The estimated seismic moment of the 1946 Aleutian earthquake is
Evidence for frequent, large tsunamis spanning locked and creeping parts of the Aleutian megathrust
GSA Bulletin, 2018
Elevations at Driftwood Bay were mapped with a Magellan PM500 real-time kinematic Global Navigation Satellite System (GNSS) survey instrument with ±1.5 cm horizontal and ±3 cm vertical accuracy. Tidal benchmarks at Nikolski were surveyed to provide comparison to tidal datums calculated by NOAA for the Nikolski, Alaska tide gage (NOAA station ID: 9462450). We also obtained a tidal curve for Driftwood Bay from a pressure sensor deployed near the study site 31 July-8 August, 2013 that was coupled to a barometric pressure sensor to correct for atmospheric pressure changes. We tied the pressure sensor deployed at Driftwood Bay to the NOAA tide gage at Nikolski during the GNSS elevation survey. The tidal curve from Driftwood Bay was converted to a tidal datum referenced to the National Tidal Datum Epoch using the Online Tidal Datum Computation Tool developed by JOA Surveys (ref to https://www.tidaldatumtool.com). All elevations we report are referenced to mean tide level (MTL) as defined by the local tidal datum tool at Driftwood Bay, which incorporates a-0.137 m deviation from MTL at Nikolski due to variation in the geoid. 2 1.2 Dating The timing of sand sheet deposition was estimated using multiple methods, including 137 Cesium activity and radiocarbon dating. Gamma spectroscopy, using low-background, high-efficiency, high-purity Germanium detectors, yielded 137 Cs activities in 1-cm-thick sediment intervals sampled within 17 cm of the surface in core 201 (Table S1). We used peak 137 Cs activity to identify the chronostratigraphic interval of AD 1963 (Pennington et al., 1973) and its relationship to the depth of the youngest sand sheet. Samples were initially dried, homogenized by grinding, packed into standardized vessels, and sealed for at least 24 h before counting. Activities were corrected for self-absorption using a direct transmission method (Cutshall et al., 1983; Cable et al., 2001). Radiocarbon dates obtained through the National Ocean Sciences Accelerator Mass Spectrometry facility in Woods Hole, Massachusetts. We sampled plant macrofossils from above and below the sand sheets and tephras for dating, including delicate peat moss (Sphagnum spp) stems and leaves, black stems of sedges (Carex spp), and woody twigs. We computed calibrated radiocarbon ages from lab-reported ages using OxCal (version 4.2.4) (Bronk Ramsey, 2009a) with the IntCal13 data set of Reimer et al. (2013). Table 2 reports one-sigma, analytic ages in radiocarbon years before 1950 (14 C yr BP), and two-sigma, calibrated ages in solar years before 1950 (cal yr BP). Posterior ages that estimate the times of sand sheet and tephra deposition are listed in Table 3 based on a Bayesian age-depth model constructed with OxCal (Bronk Ramsey, 2008). A Bayesian age-depth model incorporated radiocarbon analyses (Table 2) of samples from multiple cores and the activity of 137 Cs in the upper 25 cm of sediment in core 201 (Table S1). We used OxCal (Bronk Ramsey, 2009a), to construct the model, which computed probability