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Local Tsunamis and Distributed Slip at the Source (1999)

Geist, E. L. ; Dmowska, R.

Springer

Pure and applied geophysics 154 (1999), S. 485-512

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ISSN: 1420-9136

Keywords: Key words: Tsunami, coseismic displacement, source parameters, nonuniform slip, crack, tsunami runup.

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences , Physics

Notes: Abstract —Variations in the local tsunami wave field are examined in relation to heterogeneous slip distributions that are characteristic of many shallow subduction zone earthquakes. Assumptions inherent in calculating the coseismic vertical displacement field that defines the initial condition for tsunami propagation are examined. By comparing the seafloor displacement from uniform slip to that from an ideal static crack, we demonstrate that dip-directed slip variations significantly affect the initial cross-sectional wave profile. Because of the hydrodynamic stability of tsunami wave forms, these effects directly impact estimates of maximum runup from the local tsunami. In most cases, an assumption of uniform slip in the dip direction significantly underestimates the maximum amplitude and leading wave steepness of the local tsunami. Whereas dip-directed slip variations affect the initial wave profile, strike-directed slip variations result in wavefront-parallel changes in amplitude that are largely preserved during propagation from the source region toward shore, owing to the effects of refraction. Tests of discretizing slip distributions indicate that small fault surface elements of dimensions similar to the source depth can acceptably approximate the vertical displacement field in comparison to continuous slip distributions. Crack models for tsunamis generated by shallow subduction zone earthquakes indicate that a rupture intersecting the free surface results in approximately twice the average slip. Therefore, the observation of higher slip associated with tsunami earthquakes relative to typical subduction zone earthquakes of the same magnitude suggests that tsunami earthquakes involve rupture of the seafloor, whereas rupture of deeper subduction zone earthquakes may be imbedded and not reach the seafloor.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/s000240050241

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Estimation of submarine mass failure probability from a sequence of deposits with age dates (2013)

Geist, E. L., Chaytor, J. D., Parsons, T., Brink, U. t.

Geological Society of America (GSA)

In: Geosphere

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Publication Date: 2013-03-30

Description: The empirical probability of submarine mass failure is quantified from a sequence of dated mass-transport deposits. Several different techniques are described to estimate the parameters for a suite of candidate probability models. The techniques, previously developed for analyzing paleoseismic data, include maximum likelihood and Type II (Bayesian) maximum likelihood methods derived from renewal process theory and Monte Carlo methods. The estimated mean return time from these methods, unlike estimates from a simple arithmetic mean of the center age dates and standard likelihood methods, includes the effects of age-dating uncertainty and of open time intervals before the first and after the last event. The likelihood techniques are evaluated using Akaike’s Information Criterion (AIC) and Akaike’s Bayesian Information Criterion (ABIC) to select the optimal model. The techniques are applied to mass transport deposits recorded in two Integrated Ocean Drilling Program (IODP) drill sites located in the Ursa Basin, northern Gulf of Mexico. Dates of the deposits were constrained by regional bio- and magnetostratigraphy from a previous study. Results of the analysis indicate that submarine mass failures in this location occur primarily according to a Poisson process in which failures are independent and return times follow an exponential distribution. However, some of the model results suggest that submarine mass failures may occur quasiperiodically at one of the sites (U1324). The suite of techniques described in this study provides quantitative probability estimates of submarine mass failure occurrence, for any number of deposits and age uncertainty distributions.

Electronic ISSN: 1553-040X

Topics: Geosciences

Published by Geological Society of America (GSA)

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Great (〉=Mw8.0) megathrust earthquakes and the subduction of excess sediment and bathymetrically smooth seafloor (2015)

Scholl, D. W., Kirby, S. H., von Huene, R., Ryan, H., Wells, R. E., Geist, E. L.

Geological Society of America (GSA)

In: Geosphere

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Publication Date: 2015-03-31

Description: Using older and in part flawed data, Ruff (1989) suggested that thick sediment entering the subduction zone (SZ) smooths and strengthens the trench-parallel distribution of interplate coupling. This circumstance was conjectured to favor rupture continuation and the generation of high-magnitude (≥Mw8.0) interplate thrust (IPT) earthquakes. Using larger and more accurate compilations of sediment thickness and instrumental (1899 to January 2013) and pre-instrumental era (1700–1898) IPTs (n = 176 and 12, respectively), we tested if a compelling relation existed between where IPT earthquakes ≥Mw7.5 occurred and where thick (≥1.0 km) versus thin (≤1.0 km) sedimentary sections entered the SZ. Based on the new compilations, a statistically supported statement (see Summary and Conclusions) can be made that high-magnitude earthquakes are most prone to nucleate at well-sedimented SZs. For example, despite the 7500 km shorter global length of thick-sediment trenches, they account for ~53% of instrumental era IPTs ≥Mw8.0, ~75% ≥Mw8.5, and 100% ≥Mw9.1. No megathrusts 〉Mw9.0 ruptured at thin-sediment trenches, whereas three occurred at thick-sediment trenches (1960 Chile Mw9.5, 1964 Alaska Mw9.2, and 2004 Sumatra Mw9.2). However, large Mw8.0–9.0 IPTs commonly (n = 23) nucleated at thin-sediment trenches. These earthquakes are associated with the subduction of low-relief ocean floor and where the debris of subduction erosion thickens the plate-separating subduction channel. The combination of low bathymetric relief and subduction erosion is inferred to also produce a smooth trench-parallel distribution of coupling posited to favor the characteristic lengthy rupturing of high-magnitude IPT earthquakes. In these areas subduction of a weak sedimentary sequence further enables rupture continuation.

Electronic ISSN: 1553-040X

Topics: Geosciences

Published by Geological Society of America (GSA)

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Non-linear resonant coupling of tsunami edge waves using stochastic earthquake source models (2015)

Geist, E. L.

Oxford University Press

In: Geophysical Journal International

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Publication Date: 2015-12-13

Description: Non-linear resonant coupling of edge waves can occur with tsunamis generated by large-magnitude subduction zone earthquakes. Earthquake rupture zones that straddle beneath the coastline of continental margins are particularly efficient at generating tsunami edge waves. Using a stochastic model for earthquake slip, it is shown that a wide range of edge-wave modes and wavenumbers can be excited, depending on the variability of slip. If two modes are present that satisfy resonance conditions, then a third mode can gradually increase in amplitude over time, even if the earthquake did not originally excite that edge-wave mode. These three edge waves form a resonant triad that can cause unexpected variations in tsunami amplitude long after the first arrival. An M ~ 9, 1100 km-long continental subduction zone earthquake is considered as a test case. For the least-variable slip examined involving a Gaussian random variable, the dominant resonant triad includes a high-amplitude fundamental mode wave with wavenumber associated with the along-strike dimension of rupture. The two other waves that make up this triad include subharmonic waves, one of fundamental mode and the other of mode 2 or 3. For the most variable slip examined involving a Cauchy-distributed random variable, the dominant triads involve higher wavenumbers and modes because subevents, rather than the overall rupture dimension, control the excitation of edge waves. Calculation of the resonant period for energy transfer determines which cases resonant coupling may be instrumentally observed. For low-mode triads, the maximum transfer of energy occurs approximately 20–30 wave periods after the first arrival and thus may be observed prior to the tsunami coda being completely attenuated. Therefore, under certain circumstances the necessary ingredients for resonant coupling of tsunami edge waves exist, indicating that resonant triads may be observable and implicated in late, large-amplitude tsunami arrivals.

Keywords: Marine Geosciences and Applied Geophysics

Print ISSN: 0956-540X

Electronic ISSN: 1365-246X

Topics: Geosciences

Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).

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Mw 8.6 Sumatran earthquake of 11 April 2012: Rare seaward expression of oblique subduction (2013)

Ishii, M., Kiser, E., Geist, E. L.

Geological Society of America (GSA)

In: Geology

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Publication Date: 2013-02-20

Description: The magnitude 8.6 and 8.2 earthquakes off northwestern Sumatra on 11 April 2012 generated small tsunami waves that were recorded by stations around the Indian Ocean. Combining differential travel-time modeling of tsunami waves with results from back projection of seismic data reveals a complex source with a significant trench-parallel component. The oblique plate convergence indicates that ~20–50 m of trench-parallel displacement could have accumulated since the last megathrust earthquake, only part of which has been taken up by the Great Sumatran fault. This suggests that the remaining trench-parallel motion was released during the magnitude 8.6 earthquake on 11 April 2012 within the subducting plate. The magnitude 8.6 earthquake is interpreted to be a result of oblique subduction as well as a reduction in normal stress due to the occurrence of the Sumatra-Andaman earthquake in 2004.

Print ISSN: 0091-7613

Electronic ISSN: 1943-2682

Topics: Geosciences

Published by Geological Society of America (GSA)

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Explanation of Temporal Clustering of Tsunami Sources Using the Epidemic-Type Aftershock Sequence Model (2014)

Geist, E. L.

Seismological Society of America (SSA)

In: Bulletin of the Seismological Society of America (BSSA)

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Publication Date: 2014-08-08

Description: Temporal clustering of tsunami sources is examined in terms of a branching process model. It previously was observed that there are more short interevent times between consecutive tsunami sources than expected from a stationary Poisson process. The epidemic-type aftershock sequence (ETAS) branching process model is fitted to tsunami catalog events, using the earthquake magnitude of the causative event from the Centennial and Global Centroid Moment Tensor (CMT) catalogs and tsunami sizes above a completeness level as a mark to indicate that a tsunami was generated. The ETAS parameters are estimated using the maximum-likelihood method. The interevent distribution associated with the ETAS model provides a better fit to the data than the Poisson model or other temporal clustering models. When tsunamigenic conditions (magnitude threshold, submarine location, dip-slip mechanism) are applied to the Global CMT catalog, ETAS parameters are obtained that are consistent with those estimated from the tsunami catalog. In particular, the dip-slip condition appears to result in a near zero magnitude effect for triggered tsunami sources. The overall consistency between results from the tsunami catalog and that from the earthquake catalog under tsunamigenic conditions indicates that ETAS models based on seismicity can provide the structure for understanding patterns of tsunami source occurrence. The fractional rate of triggered tsunami sources on a global basis is approximately 14%. Online Material: Figures of residual analysis for tsunami and earthquake catalogs.

Print ISSN: 0037-1106

Electronic ISSN: 1943-3573

Topics: Geosciences , Physics

Published by Seismological Society of America (SSA)

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Were Global M〉=8.3 Earthquake Time Intervals Random between 1900 and 2011? (2012)

Parsons, T., Geist, E. L.

Seismological Society of America (SSA)

In: Bulletin of the Seismological Society of America (BSSA)

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Publication Date: 2012-08-01

Description: The pattern of great earthquakes during the past ~100 yr raises questions whether large earthquake occurrence is linked across global distances, or whether temporal clustering can be attributed to random chance. Great-earthquake frequency during the past decade in particular has engendered media speculation of heightened global hazard. We therefore examine interevent distributions of Earth’s largest earthquakes at one-year resolution, and calculate how compatible they are with a random-in-time Poisson process. We show, using synthetic catalogs, that the probability of any specific global interevent distribution happening is low, and that short-term clusters are the least repeatable features of a Poisson process. We examine the real catalog and find, just as expected from synthetic catalogs, that the least probable M ≥8.3 earthquake intervals during the past 111 yr were the shortest ( t 〈1 yr) if a Poisson process is active (mean rate of 3.2%). When we study an M ≥8.3 catalog with locally triggered events removed, we find a higher mean rate of 9.5% for 0–1 yr intervals, comparable to the value (11.1%) obtained for simulated catalogs drawn from random-in-time exponential distributions. We emphasize short interevent times here because they are the most obvious and have led to speculation about physical links among global earthquakes. We also find that comparison of the whole 111-yr observed M ≥8.3 interevent distribution (including long quiescent periods) to a Poisson process is not significantly different than the same comparison made with synthetic catalogs. We therefore find no evidence that global great-earthquake occurrence is not a random-in-time Poisson process.

Print ISSN: 0037-1106

Electronic ISSN: 1943-3573

Topics: Geosciences , Physics

Published by Seismological Society of America (SSA)

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