Phil R. Cummins

Possible splay fault slip during the 1946 Nankai earthquake

This paper considers seafloor deformation associated with the 1946 Nankai earthquake, which occurred in the Nankai Trough subduction zone off the coast of southwest Japan. We note that recent high-resolution determinations of the slip distribution, assumed to be confined to the plate boundary, result in a pronounced maximum in seafloor displacement midway between the coast of Shikoku and the trough axis. We show that this feature could be due to slip on a splay fault rather than the plate boundary. Such a model for the western part of the slip distribution is able to account for the main features of the tsunamigenic seafloor deformation obtained in previous studies, with a maximum slip consistent with the plate convergent rate and history of earthquake recurrence at the Nankai Trough.

The 1946 Nankai earthquake and segmentation of the Nankai Trough

Abstract We perform a simple subevent analysis of seismic P-waveforms from historical records of the 1946 Nankai earthquake. The results of this analysis establish that the earthquake rupture consisted of two main subevents, one lying within the 1-day aftershock area off the Kii Peninsula, while the other lies much farther to the west in the area of pronounced crustal deformation of western Shikoku. These conform to two segments of the Nankai Trough which have long been referred to in studies of historical earthquakes and geology of the Nankai Trough, but to our knowledge have not previously been established as having distinct megathrust rupture behavior. We show that these two subevents are bounded by pronounced features in the geometry of the subducting Philippine Sea Plate (PSP): a tear in the plate to the east beneath the Kii Peninsula, and a subducting seamount farther to the west. We speculate that these features may be associated with low-strength parts of the upper and/or lower plates that may be susceptible to permanent, anelastic deformation which prevents the accumulation of the elastic strain energy necessary to sustain seismic rupture.

Highly accurate P‐SV complete synthetic seismograms using modified DSM operators

In previous papers [Cummins et al., 1994ab] (hereafter referred to as DSMI and DSMII respectively), we presented accurate methods for computing complete synthetic seismograms for SH and P-SV respectively in a spherical earth model. The SH calculations used computationally efficient modified matrix operators, but the P-SV synthetics were computationally intensive. Geller and Takeuchi [1995] (hereafter referred to as GT95) presented a general theory for deriving modified operators and gave the explicit form of the modified operators for the P-SV case in cylindrical or cartesian coordinates. In this paper we extend GT95s results to derive modified operators for the P-SV case in spherical coordinates. The use of the modified operators reduces the CPU time by a factor of about 5 without a loss of accuracy. 10 CPU min on a SPARC-20 workstation with one CPU are required to compute a profile of synthetic seismograms from DC to 20 sec period.

A study of the earth's free core nutation using international deployment of accelerometers gravity data

In this study we consider the influence of the Earths free core nutation (FCN) on diurnal tidal admittance estimates for 11 stations of the globally distributed International Deployment of Accelerometers network. The FCN causes a resonant enhancement of the diurnal admittances which can be used to estimate some properties of the FCN. Estimations of the parameters describing the FCN (period, Q, and resonance strength) are made using data from individual stations and many stations simultaneously. These yield a result for the period of 423–452 sidereal days, which is shorter than theory predicts but is in agreement with many previous studies and suggests that the dynamical ellipticity of the core may be greater than its hydrostatic value. The estimate of 3035–49,689 for the Q falls in between the incompatible results of previous estimates based on tidal gravity and very long baseline interferometry observations, but the formal uncertainties are large enough for this result to be compatible with all of the previous studies. More careful consideration of the four-dimensional confidence interval, however, indicates that one reason for our relatively large error bars is a strong trade-off between the real and imaginary parts of the eigenfrequency and the real and imaginary parts of the resonance strength, respectively. This trade-off should probably be considered in future work in order to facilitate comparisons between different studies. Finally, we point out that observation of the freely excited wobble in the diurnal surface gravity spectrum does not seem likely based on the most recent estimates of its nutation amplitude.

Deformation of a seamount subducting beneath an accretionary prism: Constraints from numerical simulation

We examined the process of seamount subduction via a numerical simulation using the finite element method, applying a frictional force on the plate interface that is proportional to the normal stress. We calculate the incremental stress due to infinitesimal deformation of the seamount associated with subduction, and consider the implications for stress buildup and fracturing of the seamount itself. Our results show that the maximum shear stress concentrates at both flanks of the seamount, which suggests that fracturing will start there. We can surmise that, eventually, the seaward flank may be more apt to break than the landward flank at shallow depth if the confining pressure there is sufficiently low. We consider this to be a possible scenario for the generation of a thrust fault imaged at the seaward flank of the Muroto seamount, which is subducting under the Nankai trough accretionary prism.

Complete synthetic seismograms for 3-D heterogeneous Earth models computed using modified DSM operators and their applicability to inversion for Earth structure

Abstract We compute complete (including both body and surface waves) synthetic seismograms for laterally and vertically heterogeneous Earth models using the Direct Solution Method (DSM). We use the optimally accurate modified operators derived by Geller and Takeuchi [Geller, R.J., Takeuchi, N., 1995. A new method for computing highly accurate DSM synthetic seismograms. Geophys. J. Int. 123, 449–470] and extended to spherical coordinates by Takeuchi et al. [Takeuchi, N., Geller, R.J., Cummins, P.R., 1996. Highly accurate P-SV complete synthetic seismograms using modified DSM operators. Geophys. Res. Lett. 23, 1175–1178] and Cummins et al. [Cummins, P.R., Takeuchi, N., Geller, R.J., 1997. Computation of complete synthetic seismograms for laterally heterogenous models using the Direct Solution Method. Geophys. J. Int. 130, 1–16] for 1- and 3-D models, respectively. In this study we greatly reduce the CPU time by treating the laterally heterogeneous structure as a perturbation to a spherically symmetric model (i.e., using the Born approximation). Note, however, that (1) our methods do not require the use of the Born approximation and (2) the reference model for the Born approximation is not required to be spherically symmetric. The synthetic seismograms in this paper are computed using the first-order Born approximation. However, accuracy can be greatly improved by using higher order terms of the Born series; theoretical results are presented in this paper, and some preliminary numerical examples are presented in this volume by Igel et al. [Igel, H., Takeuchi, N., Geller, R.J., Megnin, C., Bunge, H.P., Clevede, E., Dalkolmo, J., Romanowicz, B., 1998. The COSY project: verification of global seismic modeling algorithms, Phys. Earth Planet. Inter., this issue].

Tsunami waveform inversion for sea surface displacement following the 2011 Tohoku earthquake: Importance of dispersion and source kinematics

This paper considers the importance of model parameterization, including dispersion, source kinematics, and source discretization, in tsunami source inversion. We implement single and multiple time window methods for dispersive and nondispersive wave propagation to estimate source models for the tsunami generated by the 2011 Tohoku-Oki earthquake. Our source model is described by sea surface displacement instead of fault slip, since sea surface displacement accounts for various tsunami generation mechanisms in addition to fault slip. The results show that tsunami source models can strongly depend on such model choices, particularly when high-quality, open-ocean tsunami waveform data are available. We carry out several synthetic inversion tests to validate the method and assess the impact of parameterization including dispersion and variable rupture velocity in data predictions on the inversion results. Although each of these effects has been considered separately in previous studies, we show that it is important to consider them together in order to obtain more meaningful inversion results. Our results suggest that the discretization of the source, the use of dispersive waves, and accounting for source kinematics are all important factors in tsunami source inversion of large events such as the Tohoku-Oki earthquake, particularly when an extensive set of high-quality tsunami waveform recordings are available. For the Tohoku event, a dispersive model with variable rupture velocity results in a profound improvement in waveform fits that justify the higher source complexity and provide a more realistic source model.

Splay fault and megathrust earthquake slip in the Nankai Trough

We consider whether splay fault slip may be a factor influencing the behavior of megathrust earthquakes in the Nankai Trough. Consideration of tsunami inversion results from other studies indicates that slip on one or more splay faults may be particularly important for the segment of the Nankai Trough offshore western Shikoku. These results suggest that during at least two megathrust earthquakes substantial slip may have occurred on one or more splay faults which cut the island arc crust over 100 km landward of the trench axis. In contrast to smaller subsidiary thrusts in the semi-consolidated and unconsolidated sediments very near the trough axis, the crustal faults considered here could accumulate and release considerable tectonic stress. Through a simple finite element calculation we demonstrate that slip on such faults can release some of the shear stress on the megathrust accumulated through plate subduction, and therefore may have some influence on the behavior of megathrust earthquakes.

Refined coseismic displacement modeling for the 1994 Shikotan and Sanriku‐oki earthquakes

We consider refined modeling of GPS measurements of coseismic displacement for two large earthquakes occurring in the Japan and Kurile Trenches. Calculations are performed for a layered sphere and the homogeneous half-space more commonly used to model such data, using fault geometry and slip distributions from previously published studies. We show that the difference between the layered sphere and homogeneous half-space calculations can be significant, as expected from previous theoretical studies. However, it does not appear large enough to seriously affect results from previously published studies of these earthquakes. Nevertheless, the fact that the effects of sphericity and especially layering are measurable in coseismic observations of great earthquakes at moderate distances indicates that these effects should be taken into account in order to fully exploit the precision of modern GPS measurements.

Tsunami source uncertainty estimation: The 2011 Japan tsunami

This article was published in the Journal of Geophysical Research: Solid Earth [©2016 American Geophysical Union] and the definite version is available at : http://dx.doi.org/10.1002/2015JB012764 The Journals website is at: http://onlinelibrary.wiley.com/doi/10.1002/2015JB012764/abstract;jsessionid=BF5813B61924FB094F31BC8F213840A1.f02t04