Nafi Toksoz

Passive seismic experiment.

Seismometer operation for 21 days at Tranquillity Base revealed, among strong signals produced by the Apollo 11 lunar module descent stage, a small proportion of probable natural seismic signals. The latter are long-duration, emergent oscillations which lack the discrete phases and coherence of earthquake signals. From similarity with the impact signal of the Apollo 12 ascent stage, they are thought to be produced by meteoroid impacts or shallow moonquakes. This signal character may imply transmission with high Q and intense wave scattering, conditions which are mutually exclusive on earth. Natural background noise is very much smaller than on earth, and lunar tectonism may be very low.

Seismic imaging of the 1999 Izmit (Turkey) Rupture inferred from the near-fault recordings

We use near-fault accelerograms to infer the space-time history of rupture on the fault during the Izmit earthquake. The records show that the ground displacement and velocity near the fault were surprisingly simple. Rupture propagated toward the west at a velocity of about 3 km/s, and toward the east at a remarkably high average velocity of 4.7 km/s over a distance of about 45 km before decelerating to about 3.1 km/s on the eastern segment. Slip on the fault is particularly large down to a depth of 20 km on the central portion of the fault where it reaches about 7 m. Slip is large also below 10 km on the eastern fault segment, and this may have contributed to the loading of shear stress on the Duzce fault. On the western fault segment, large slip seems confined to shallow depths.

Seismic data from man-made impacts on the moon

Unusually long reverberations were recorded from two lunar impacts by a seismic station installed on the lunar surface by the Apollo 12 astronauts. Seismic data from these impacts suggest that the lunar mare in the region of the Apollo 12 landing site consists of material with very low seismic velocities near the surface, with velocity increasing with depth to 5 to 6 kilometers per second (for compressional waves) at a depth of 20 kilometers. Absorption of seismic waves in this structure is extremely low relative to typical continental crustal materials on earth. It is unlikely that a major boundary similar to the crustmantle interface on earth exists in the outer 20 kilometers of the moon. A combination of dispersion and scattering of surface waves probably explains the lunar seismic reverberation. Scattering of these waves implies the presence of heterogeneity within the outer zone of the mare on a scale of from several hundred meters (or less) to several kilometers. Seismic signals from 160 events of natural origin have been recorded during the first 7 months of operation of the Apollo 12 seismic station. At least 26 of the natural events are small moonquakes. Many of the natural events are thought to be meteoroid impacts.

New Seismic Data on the State of the Deep Lunar Interior

Direct shear-wave arrivals from seismtic events originating on the far side of the moon are not observed at some of the stations of the Apollo seismic network. These data suggest that the material in the lunar interior at a depth of 1000 to 1100 kilometers is more dissipative for seismic shear waves than the lithosphere above, and possibly exists in a partially molten state akin to the earths asthenosphere.

A Hybrid Method For Modeling SH Wave Scattering From Fractures In 2D Heterogeneous Medium: Coupling of Boundary Element And Finite Difference Methods

We present a hybrid method to model the SH wave scattering from 2D fractures embedded in a heterogeneous medium by coupling Boundary Element Method (BEM) and Finite Different Method (FDM) in the frequency domain. We use FDM to propagate SH wave from a source through heterogeneities to localized homogeneous domains where fractures are embedded within artificial boundaries. According to Huygens’ Principle, the boundary points can be regarded as “secondary” point sources and their values are determined by FDM. Given the incident fields from these point sources, we apply BEM to model the scattering from fractures and propagate them back to the artificial boundaries. FDM then takes the boundaries as secondary sources and continues propagating the scattered field into the heterogeneous medium. We also present a numerical iterative scheme that can account for the multiple scattering between different sets of fractures or between fractures and heterogeneities. We first compare the results calculated from this hybrid method with pure BEM method to show the accuracy of the hybrid approach and the iterative scheme. This method is then applied to calculate the wave scattered from fractures embedded in a horizontally layered medium and a more complicated Marmousi model.

Identification and quantitative characterization of igneous rocks: Method and application in the north Huimin Sag

Identification and characterization of igneous rocks have been a challenge in seismic exploration in the areas with igneous rocks. In this study, a method has been developed to accurately identify and describe igneous rocks and therefore improve oil and gas reservoir prediction in the igneous region. Based on geology, drilling, seismic, logging and other available information, forward igneous rock models are developed and igneous rock boundaries are identified and characterized using seismic attributes analysis. Thickness, area size, and volume of igneous rocks are estimated using the tuning thickness method and the top-bottom subtraction method. The obtained information of igneous rock distribution is used to characterize hydrocarbon reservoirs. In the north Huimin Sag, drilling results correlate well with the inversion results, indicating that accurate information of igneous rock distribution effectively improves the reservoir prediction.