Gerard J. Fryer

Tsunami: The Underrated Hazard

Tsunami: the Underrated Hazard, by Edward Bryant, would appear to be a welcome addition to the scholarly tsunami literature. No book on tsunamis has the broad perspective of this work. The book looks attractive, with many high-quality photographs. It looks comprehensive, with discussions of tsunami hydrodynamics, tsunami effects on coastal landscapes, and causes of tsunamis (earthquakes, landslides, volcanic eruptions, meteorite impacts). It looks practical, with a section on risk and mitigation. It also looks entertaining, with an opening chapter on tsunami legends and a closing chapter presenting fanciful descriptions of imagined events. Appearances are deceiving, though. Any initial enthusiasm for the work evaporates on even casual reading. The book is so flawed by errors, omissions, confusion, and unsupported conjecture that we cannot recommend it to anyone.

Evolution of porosity and seismic structure of upper oceanic crust: Importance of aspect ratios

Seismic properties of the uppermost igneous crust of the oceans are dominated by porosity effects, that is, the size, concentration, and shape of void spaces. Porosity is initially determined by the physics of extrusion (does an eruption form breccia, pillows, or massive flows?) but is very rapidly modified by alteration and hydrothermal deposition. Laboratory data provide insight into compressional wave velocity-porosity behavior of basalts at a hand sample scale, while well logs provide data at outcrop scale. Relating observations at all scales to porosity structure and extrapolating to seismic scale requires application of rock physics theory. Using information from ophiolites and deep ocean cores, we have defined rock physics parameters for two simple models of upper oceanic crust. The models approximate different levels of void filling by alteration products by differing in the amount of crack (low aspect ratio) porosity they contain. From the models we compute theoretical compressional wave velocity and porosity profiles. Calculated profiles agree well with both well logs and seismic data and illustrate that the increase in seismic velocities measured seismically in the upper crust need not be accompanied by large changes in total porosity.

Seismic constraints on shallow crustal emplacement processes at the fast spreading East Pacific Rise

We present the results of nine on-bottom seismic refraction experiments carried out over young East Pacific Rise crust. The experiments are unusual in that both the source and receiver are located within a few meters of the seafloor, allowing high-resolution determinations of shallow crustal structure. Three experiments were located within the axial summit caldera (ASC) over “zero-age” crust. The seismic structure at these three locations is fundamentally the same, with a thin (<60 m) surficial low-velocity (<2.5 km/s) layer, a 100 to 150-m-thick transition zone with velocities increasing by ∼2.5 km/s, and a layer with velocities of ∼5 km/s at a depth beneath the seafloor of ∼130–190 m. The surficial low-velocity layer and transition zone are defined as seismic layer 2A, and the ∼5 km/s layer is defined as the top of layer 2B. Both the surficial low-velocity layer and the transition zone double in thickness within ∼1 km of the rise axis. We model layer 2A as the extrusive sequence and transition zone and the 2A/2B boundary as the top of the sheeted dikes. The primary implication of this interpretation is that the depth to the top of the sheeted dikes deepens from ∼150 m to ∼300 m within 1 km of the ASC. The thickening of the extrusive layer is interpreted to be due to lava that either overflows the ASC walls, is emplaced through eruptions outside of the ASC, or travels laterally from the ASC through conduits. The most probable cause for the thickening of the transition zone is sill emplacement outside of the ASC, either from magma that does not reach the surface in an off-axis eruption or magma that is transported laterally during the drainage process creating the ASC. We suggest that the mechanism controlling the magnitude and rate of the dike subsidence is the mechanism that determines the thickness of the extrusive section and the total thickness of layer 2A.

Structure of young upper crust at the East Pacific Rise near 9°30'N

Eight on-bottom seismic refraction experiments are analyzed in an effort to resolve the structure of the emplacement zone of lavas and dikes at the fast-spreading East Pacific Rise. The results suggest that the volcanic section doubles in thickness within ∼1 km of the axial summit caldera (ASC) due to the emplacement of lava that either overflows the ASC or is transported through lateral tube conduits, and the depth to the top of the sheeted dike complex increases from ∼160 m within the ASC to ∼340 m a kilometer away on the flank.

Velocity‐porosity relationships in the upper oceanic crust: Theoretical considerations

We consider here the application of rock physics theories to investigate relationships between seismic velocities and porosities in the shallow oceanic crust. Classical Hashin-Shtrikman limits ignore void shapes and are too broad to provide useful constraints on velocities and porosities. Making some assumptions about the distribution of void shapes improves the constraints. Theories which ignore crack-crack interactions underestimate the effects of porosities on velocities, thus providing upper bounds on velocities and porosities. “Self-consistent” theories overestimate crackcrack interactions and so provide lower bounds. At the high porosities required to reduce basalt from a P velocity of 7km/s in massive form to the 2.2km/s observed in zero-age oceanic crust, however, the bounds are too far apart to be useful. The theories are strictly valid only for very small porosities. Using an algorithm proposed by Cheng for iteratively building up porosity to create a highly porous medium, analogous to differential computation methods traditionally used to improve upon the self-consistent approach, we have devised two hybrid theories, which we term extended Walsh and extended Kuster-Toksoz. These two theories remain approximately valid at the high porosities of oceanic crustal layer 2A to provide useful upper and lower bounds on velocity for a given porosity and pore aspect ratio distribution. We attempt the inverse problem, determining porosity from a given velocity, using on-bottom refraction data collected on the flank of the East Pacific Rise. For 120ka material with a P velocity of 2.5km/s, if our assumptions regarding the aspect ratio distribution are correct, porosity lies somewhere between 24 and 34%. Resolution on slower, zero-age crust (2.2km/s) is poorer: there we predict a porosity between 26 and 43%. Use of shear-wave information would tighten these bounds.

Uplift of Oahu, Hawaii, during the past 500 k.y. as recorded by elevated reef deposits

Studies of paleo–sea level and past climate have focused upon proxy methods in ice and deep-sea cores and more direct information provided by past shorelines, in some cases preserved as raised or submerged reef deposits in tropical areas. Paleo-shorelines need to be constrained by accurate tectonic history because these environments and their marine deposits can be confused with past tsunami deposits and vice versa. A maximum 21-m-high extensive emerged reef on Oahu, Hawaii, U-series dated to 334 ± 17 ka, together with a mean U-series age of 335 ± 22 ka (n = 5) for slightly higher, energetic shoreline deposits nearby, suggest a marine isotope stage 9 (MIS 9) highstand, and extend the earlier work indicating a linear uplift for Oahu of 0.060 ± 0.001 mm/yr over the past 500 k.y. Five of the past six major emerged interglacial highstand reefs on Oahu have been identified, and these data provide little evidence for past maximum sea levels significantly greater than 2 m above the sea level datum at that time. There is currently no evidence for a MIS 11 highstand on Oahu.

Reflectivity of the ocean bottom at low frequency

The theoretical reflectivity of the ocean bottom estimated from a model composed of fluid sediment layers is misleading because the effects of shear propagation are ignored. Reflectivity computations which include the effects of shear may be made using Thomson–Haskell matrix theory. The delta‐matrix extension of this theory provides a method for computation of plane‐wave reflectivity of a viscoelastic layered ocean bottom at the frequencies of interest in acoustics. The results from a hypothetical turbidite section show that when elastic parameters vary continuously with depth, conversion of compressional to shear energy is unimportant at frequencies above 20 Hz. However, at discontinuities, conversion to shear does occur. This strongly affects the reflectivity at all frequencies, except at small grazing angles and near‐normal incidence.

Compressional‐shear wave coupling induced by velocity gradients in marine sediments

The very high compressional and shear velocity gradients of marine sediments may result in continuous interconversion between P (compressional) and S (shear) types of motion at low frequencies. Since the ray theories commonly used in modeling acoustic interaction with the ocean bottom implicitly assume that P and S are decoupled, the importance of such phenomena must be assessed. The problem is investigated here through theoretical studies of the reflectivity function for representative models of the ocean bottom. The only practical approach for including all such wave phenomena in a determination of reflectivities involves numerical solution of the wave equation. For a depth‐varying structure, the most efficient numerical scheme is the classical approximation by homogeneous layers. This procedure can be readily modified to isolate the effects of gradient‐induced coupling by forcing the P‐ and S‐wave potentials to be independent and studying the effects of this on the reflectivity. Unfortunately, the resu...

Effects and Consequences of Transverse Isotropy in the Seafloor

Transverse isotropy, a form of anisotropy with a single vertical axis of symmetry, is extremely widespread in marine sediments, but its effects on seismic interpretations have been inadequately quantified. This anisotropy increases with depth, just as propagation velocities themselves increase. Indeed, it is impossible to separate the effects of velocity gradients and anisotropy. Using elastic parameters appropriate for a carbonate sequence, we have used ray-tracing and wavefield modeling codes to investigate the consequences of ignoring anisotropy. If isotropy is erroneously assumed, sound-speed gradients will be underestimated, the thickness of the sedimentary sequence overestimated (often seriously) and the shear velocity overestimated (hence estimates of Poisson’s ratio will also be incorrect). Transverse isotropy may also affect the bottom reflection loss, but from our preliminary studies such effects appear to be minimal. From compressional-wave traveltime data alone the identification of transverse isotropy appears almost impossible. Shear-wave information is vital if the phenomenon is to be adequately characterized.

The Hawaiian megatsunami of 110+10 ka : the use of microfossils in detection

INTRODUCTION McMurtry et al. (2004) described a thin (c. 20–50 cm) bioclastic, carbonate gravel from the NW coast of Hawaii, on the flanks of the extinct Kohala volcano (Fig. 1). This unit is found between modern altitudes of c. 1.5–61 m above sea-level. The deposit is sandwiched between a fossil soil below and a modern soil above in Keawe’ula Bay (Fig. 2). Dating of coral fragments from within the deposit indicate an age of 110±10 ka (McMurtry et al., 2004). Given rates of subsidence on Hawaii, this would place the deposit at an original palaeo-altitude up to 491 m. The deposit contains a range of bioclasts including bivalves, gastropods, corals, bryozoans and foraminifera, largely representing assemblages from a reef flat. The geological setting of the unit, coupled with the evidence from the contained marine fossils, indicate a megatsunami genesis, probably linked to the collapse of the submarine Alika Slide at about 120 ka and with a run-up in excess of 400 m and at least 6 km inland (McMurtry et al., 2004). FOSSILS PRESERVED IN THE TSUNAMI DEPOSIT Fossils of the tsunami, and adjacent deposits, have been collected at 14 sites (material is deposited in the collections of the British Geological Survey, registered as MPA51883–51891, 51893–51897). Further information is available in BGS archives (see report IR/02/197R, available through the BGS library at: http://www.bgs.ac.uk and http://geolib.bgs.ac.uk). The tsunami deposit on the flank of Kohala volcano contains a range of macrofossil debris and prolific microfauna (Fig. 3). The …