S. P. Horton

Source Parameters and Tectonic Implications of Aftershocks of the Mw 7.6 Bhuj Earthquake of 26 January 2001

We present and discuss the spatial distribution of more than 1000 aftershocks of the largest continental intraplate earthquake to occur during the modern seismological period. The data were recorded on a network of eight portable digital seismographs deployed for 3 weeks starting 17 days after the mainshock. We have calculated high-quality single-event locations, based on a 1D velocity model determined for the region for earthquakes with magnitudes between ∼2 and 5. Aftershock locations reveal activity concentrated on a nearly east-striking, south-dipping plane, trapezoidal in outline. The active zone tapers from about 45 km along strike at the shallow end, which is about 5 km deep, to no more than 25 km long at a depth of 35 km. The total rupture area was about 1300 km 2 . We estimate the static stress drop of the mainshock at 16 ± 2 MPa. Aftershocks extend nearly through the entire crust, with concentrations in the lower crust at about 26 km and in the upper crust at about 10 km. The fault that ruptured was not mapped at the surface and not known to have been active prior to the 2001 earthquake. The aftershock data are consistent with the Bhuj earthquake resulting from reactivation in contraction of a fault formed under extension within a failed rift.

Microtremor observations of deep sediment resonance in metropolitan Memphis, Tennessee

We measured site resonance periods throughout metropolitan Memphis, Tennessee. The site-resonance was derived from recordings of ambient ‘microtremor’ and is a direct measurement of seismic ground shaking. In the Memphis metropolitan area, the period, T0 (in s) is predicted by the empirical relation T0=−0.39899+7.8922×10−3H−3.0634×10−6H2, [350<H<1100 m], where H is the thickness (in m) of the unconsolidated sediments that overly hard Paleozoic rocks. Assuming that a quarter-wavelength relationship governs the relationship between resonant period, sediment thickness, and average shear wave speed, we estimate the average shear wave speed of unconsolidated sediments as a function of sediment thicknesses: Vs=521.15+0.37459H [350<H<1100 m]. The scatter in the observations, and discordance with earlier work suggests that caution is warranted in applying these values of wave speed indiscriminately to sites in the absence of further detailed work.

Broadband microtremor observation of basin resonance in the Mississippi Embayment, central US

We report systematic variations in the fundamental period of seismic site resonance within the Mississippi embayment. The resonance periods are revealed by peaks in horizontal to vertical (H/V) spectral ratios of microtremor measurements, and increase with the depth to the sediment/basement interface. The sediment depth varies slowly enough spatially that at any site it can be modeled as a one-dimensional problem, revealing structural details of the sedimentary column. The strong impedance contrast and gentle dip of the base of the embayment sediments may trap seismic energy within the soft sediments, increasing durations and amplitudes of strong ground motions. Broadband recordings of microtremor within the Mississippi embayment provide a simple method to study the seismic response of the basin.

Bulk Sediment Qp and Qs in the Mississippi Embayment, Central United States

We have estimated P-wave andS-wave anelastic attenuation coefficients for the thick, unconsolidated sediments of the Mississippi embayment, central United States, using the spectral distance decay of explosion P and Rayleigh waves. The sediment-trapped P wave, Psed, is observed to ranges of 80 km at 10 Hz, and 1-Hz Rayleigh waves are observed out to 130 km from a 5000-lb borehole explosion in the northern part of the embayment. Rayleigh waves of 4 Hz are seen to distances of 3 km from a smaller 50-lb explosion. Analysis of the group velocity and amplitude- distance decay of both waves yields an average Qs of 100 and Qp of 200 for embay- ment sediments that are independent of frequency. Scatter in the Q estimates comes from interference of multiple P-wave reverberations and Rayleigh-wave modes. The attenuation model is self-consistent in that it is the same as obtained by the analysis of synthetic seismograms using the inferred Q-values. Inferred Qp and Qs values are more than three times higher than previous estimates and imply that unconsolidated sediments of the embayment do not significantly attenuate small-strain earthquake ground motions. These estimates represent a lower bound to Q of the sediments since significant scattering is observed in the waveform data that contributes to the distance decay of wave amplitude. Higher Q values also imply that the unconsolidated sedi- ments of the embayment will form an efficient wave guide for surface waves radiated from shallow earthquakes or large earthquakes that rupture into the sediments, pro- ducing high-amplitude, long-duration wave trains that should be considered in earth- quake hazard assessments.

Explosion Source Strong Ground Motions in the Mississippi Embayment

Two strong-motion arrays were deployed for the October 2002 Embay- ment Seismic Excitation Experiment to study the spatial variation of strong ground motions in the deep, unconsolidated sediments of the Mississippi embayment because there are no comparable strong-motion data from natural earthquakes in the area. Each linear array consisted of eight three-component K2 accelerographs spaced 15 m apart situated 1.2 and 2.5 km from 2268-kg and 1134-kg borehole explosion sources, respectively. The array data show distinct body-wave and surface-wave arrivals that propagate within the thick, unconsolidated sedimentary column, the high-velocity basement rocks, and small-scale structure near the surface. Time-domain coherence of body-wave and surface-wave arrivals is computed for acceleration, velocity, and displacement time windows. Coherence is high for relatively low-frequency vertical- component Rayleigh waves and high-frequency P waves propagating across the ar- ray. Prominent high-frequency PS conversions seen on radial components, a proxy for the direct S wave from earthquake sources, lose coherence quickly over the 105-m length of the array. Transverse component signals are least coherent for any ground motion and appear to be highly scattered. Horizontal phase velocity is com- puted by using the ratio of particle velocity to estimates of the strain based on a plane-wave-propagation model. The resulting time-dependent phase-velocity map is a useful way to infer the propagation mechanisms of individual seismic phases and time windows of three-component waveforms. Displacement gradient analysis is a complementary technique for processing general spatial-array data to obtain hori- zontal slowness information.