pre-Tertiary Bedrock Research Articles

Simulation of strong motions in Fukuoka City during the 2005 West Off Fukuoka Prefecture Earthquake with special reference to thick Quaternary sediments around the Kego fault

AbstractWe simulate velocity records at two stations in the building damaged area near the Kego fault in Fukuoka City by 1-D wave propagation theory during the 2005 West Off Fukuoka Prefecture Earthquake, Japan. Here we use the pre-Tertiary bedrock wave deconvolved from the record observed at a K-NET station outside of the damaged area. The observed velocity waveforms and pseudo velocity response spectra in the period range from 0.7 to 10 seconds in the predominant direction (N20°E) components are reasonably reproduced. Velocity pulses with a period of about 1 to 2 seconds in observed velocity waveforms at two stations are found to be amplified by the Quaternary sediments above the pre-Tertiary bedrock. After we confirmed the validity of our approach through the simulation, we predict the peak ground velocity (PGV) distribution around the Kego fault by considering 1-D response of Quaternary sediments at each site. The predicted PGV distribution of N20?E components is largest (about 80 cm/s) in the northeast side of the Kego fault and agrees well with the building damage distribution. Our result shows that the 1-D site responses of Quaternary sediments mainly contributed to the difference of PGVs inside and outside of the damaged area in Fukuoka City.

S-Wave Velocity Structure of the Taichung Basin, Taiwan, Estimated from Array and Single-Station Records of Microtremors

The objective of this study is to estimate the S-wave velocity structure of the Taichung basin in a near-fault region, which is needed for strong-motion evaluation for the 1999 Chi-Chi earthquake. We have conducted array measurements of microtremors with a total of 12 arrays at four sites and single-station measurements of microtremors at 48 sites in and around the Taichung basin. Based on the Rayleigh- wave inversion technique using phase velocities estimated from array records of microtremors, we find that a thick layer (the thickness of about 1000 to 1400 m) with an S-wave velocity of VS 1100 m/sec exists in the east-central part of the Tai- chung basin. We estimate the thicknesses of sedimentary layers above the pre- Tertiary bedrock at 48 sites to fit calculated peak and trough frequencies of horizontal-to-vertical spectral ratios of Rayleigh waves to observed peak and trough frequencies, assuming the same S-wave velocities estimated using array records. The pre-Tertiary bedrock depth was estimated to be about 5 to 6 km in this region. The estimated thickness of the layer with VS 1100 m/sec is largest in the east-central part of the basin and rapidly decreases to less than 400 m in the northeastern and western parts inside the basin. The estimated S-wave velocity structures reasonably explain arrival time of initial P and S waves of aftershock records observed by Higashi et al. (2001).

Three-Dimensional Finite-Difference Waveform Modeling of Strong Motions Observed in the Sendai Basin, Japan

We perform three-dimensional (3D) finite-difference (FD) waveform modeling of strong motions in the frequency range 0.2 to 1.67 Hz observed in the Sendai basin, Japan, during the Japan Meteorological Agency magnitude ( M J) 5.0 1998 Miyagiken-Nanbu earthquake. In a previous, we estimated S -wave velocity structures above the pre-Tertiary bedrock at six sites in the Sendai basin based on array records of microtremors. To interpolate these velocity structures in space we conduct single-station microtremor measurements at a total of 61 locations and estimate the S -wave velocity structure at each site by modeling the horizontal-to-vertical spectral ratio of fundamental mode Rayleigh waves. An initial 3D model of the basin is constructed using the velocity structures estimated from both array and single-station microtremor measurements, along with other information such as surface geology. This model encompasses a region 33 km long, 30 km wide, and 19 km deep. The final model is obtained through a trial-and-error process by fitting 3D FD synthetic waveforms to the bandpass-filtered (0.2 Hz to 1.67 Hz) displacement records 12 stations for the 1998 Miyagiken-Nanbu earthquake. We compute the synthetics using a fourth-order staggered-grid 3D FD method with variable grid spacing. As the 3D model is modified, the source parameters (strike, dip, rake, and seismic moment) are estimated by a grid search method using 1D site-specific models derived from the modified 3D model. The observed waveforms are reproduced well at most stations by the final 3D basin model. This agreement suggests the validity of the final 3D basin model for theoretical strong-motion prediction of large earthquakes in the frequency range from 0.2 to 1.67 Hz. Manuscript received 7 June 2000.

Estimation of S-Wave Velocity Structures in and around the Sendai Basin, Japan, Using Array Records of Microtremors

We conducted array measurements of microtremors at six sites in and around the Sendai basin to estimate deep S -wave velocity structures above the seismological bedrock (pre-Tertiary bedrock). After estimating phase velocities of microtremors in the frequency range from about 0.5 to 3 Hz, we succeeded to estimate the deep structures above the pre-Tertiary bedrock at four soil sites (KATA, OKIN, ARAH, and MYG15) and only the upper part just beneath the Pliocene layer at the other soil site (TRMA) using a Rayleigh-wave inversion technique. At TRMA and a rock site TAMA, we roughly estimate the deep S -wave velocity structures using empirical site amplification factors derived from strong-motion records. The deepest bedrock depth is 1 km, and the shallowest depth is 200 m among these six sites. The difference of the bedrock depth between the eastern side and the western side of an active thrust fault, the Nagamachi-Rifu line, is only 120 m. The difference of the bedrock depth between two sites in the east of the basin reaches about 500 m, although any geological boundaries or buried faults have not been mapped. Since the deep S -wave velocity structure of the Sendai basin had been basically unknown, we delineate it for the first time in this article using array measurement of microtremors.

Fossil shore platforms and drowned gravel beaches; evidence for high-frequency sea-level fluctuations in the distal Alpine foreland basin

ABSTRACT The Eocene Nummulitic Limestone Formation records progressive backstepping of a shallow marine carbonate ramp at the distal margin of the Early Tertiary Alpine foreland basin during flexurally induced transgression of the European foreland plate. Analysis of paleoshoreline features preserved along the basal unconformity of the basin in southeastern France enables identification of rapid fluctuations in the rate of relative sea-level rise. In the Champsaur district of Les Hautes Alpes, the Priabonian Nummulitic Limestone Formation unconformably overlies pre-Tertiary bedrock across a stepped transgressive ravinement surface. This surface consists of a succession of subplanar bedrock terraces that are separated by high-angle risers. The terraces display an overall staircase geometry, stepping upwards in the direction of shoreline transgression. Progradational gravelly beachface deposits overlie the terraces, and are themselves abruptly overlain either by transgressive shoreface facies or by younger beachface units. The prograded beaches are arranged in an overall landward-stepping stacking pattern. The terraces are interpreted as a flight of fossil wavecut shore platforms that record episodic landward migration of the paleoshoreline onto a paleotopographic high. Mapping of the relict paleoshorelines indicates successive episodes of terrace cutting, beach progradation, drowning, and shoreline backstepping on a late Eocene rocky coastline. The formation of the platforms and preservation of drowned gravel beaches is interpreted as a response to abrupt variations in the rate of relative sea-level rise during transgression of the region by the distal shoreline of the Alpine foreland basin. Periods of slow rise or stillstand enabled platform cutting and subsequent beach progradation, whereas pulses of rapid sea-level rise caused abrupt drowning and landward shift of the shoreline. We speculate that these high-frequency episodic fluctuations in relative sea level are a consequence of glacioeustatic sea-level oscillations of < 0.5 m.y. duration superimposed upon a steady relative sea-level rise resulting from the background flexural subsidence.

Statistical spectral model of earthquakes in the eastern Tohoku district, Japan, based on the surface and borehole records observed in Sendai

Abstract Strong motions of 18 earthquakes (3.4 ≦ MJ ≦ 7.1, MJ: JMA magnitude) in the eastern Tohoku district, Japan, have been observed at 12 borehole sites within a 20- × 20-km region in Sendai. In our previous study, we defined a Pliocene layer, whose S-wave velocity VS is greater than 500 m/sec and whose N value of the standard penetration test is greater than 50, as engineering bedrock in Sendai and calculated 304 engineering bedrock waves (hypocentral distance X = 15 to 300 km) by removing the site response between the engineering bedrock and the surface. As the second stage of our study toward a quantitative prediction of strong ground motions of horizontal components, we propose here a statistical spectral model of the engineering bedrock waves by introducing the site response between the pre-Tertiary bedrock (VS ≈ 3000 m/sec) and the engineering bedrock (VS ≈ 500 to 700 m/sec) and a frequency-dependent Q into Boore's spectral model. We separate the site response between the pre-Tertiary bedrock and the engineering bedrock, an attenuation function, and source spectra from the engineering bedrock spectra by assuming an ω−2 model with the seismic moment M0 from the Harvard CMT solution estimated for earthquakes of MJ ≧ 5.3. Initial values of corner frequency f0 are determined based on a previous empirical M0 − f0 relationship in this district. Using six moderate-sized earthquakes (5.3 ≦ MJ ≦ 6.0), we first estimate frequency-dependent Q to be Q = 110f0.69 (f: frequency) by minimizing the standard deviation of the site response, which is defined as the ratio of observed engineering bedrock spectra with respect to the estimated attenuation and the assumed source spectra. The averaged site response is simultaneously estimated to be 1 at 0.1 Hz, 5 at 1 Hz, and 3 at 20 Hz. We then invert f0 and cutoff frequencies fmax for all 18 earthquakes and M0 for seven small-sized earthquakes of MJ &amp;lt; 5.3 by minimizing the difference between the model and observed spectra. The average Brune stress drop obtained from an M0 − f0 relationship estimated from 17 earthquakes except for the smallest earthquake with M0 less than 1021 dyne·cm is 200 bars. The estimated M0 − f0 relationship is identical to the one used to calculate initial values of f0 so that we do not need to perform the inversion iteratively. The obtained stress drop for subduction zone earthquakes in the eastern Tohoku district is consistent with other previous studies. The dependence of the inverted fmax on M0 is not significant, and the logarithmic average of fmax is found to be 13.5 Hz. By using these controlling parameters and the M0 − f0 relationship obtained from a regression analysis, acceleration spectra on the engineering bedrock in Sendai can be predicted statistically from X and M0 or MJ by considering standard deviations of the site response, f0, and fmax.

Seismic reflection profiling across Tertiary extensional structures in the eastern Amargosa Desert, southern Nevada, Basin and Range province

Outcrops, shallow well control, and coincident geophysical surveys are used to interpret a seismic reflection profile in the Amargosa Desert, within the Basin and Range province, of southern Nevada. The east-west-trending, 27-km-long seismic line crosses all or parts of three Tertiary subbasins, revealing that basin growth occurred by progressive shifts of basin-bounding faults. The reflection line images Tertiary strata that is rotated by steeply dipping listric faults and that noses into normal faults. A shallow (less than 100 to 200 m deep), laterally continuous, flat-lying, low-frequency reflector, interpreted as a Tertiary basalt flow, suggests that little vertical deformation has occurred within the easternmost of the small Tertiary basins since the eruption of the flow about 10 million years ago. Moderately dipping reflections within the pre-Tertiary bedrock may image Mesozoic thrust faults. The reflection data indicate that, whereas the top of the reflective lower crust shallows to the west, possibly in the direction of increasing crustal extension, the Moho is relatively flat between 30 and 33 km deep. Apparent bright-spot reflections from the lower crust are interpreted as evidence for ductile shearing of the lower crust, not for active magma chambers. Doming of the lower crust resembles that observed elsewhere in the Basin and Range province and is consistent with ductile flow in the lower crust.

Propagation Characteristics of Intermediate-Period (1-10 Seconds) Surface Waves in the Kanto Plain, Japan

Propagation characteristics of the intermediate-period (1-10 s) surface waves predominated in strong motion records are analyzed on the basis of an array observation deployed in metropolitan Tokyo area, Japan. Seismic energy source is the main shock of the 1984 western Nagano earthquake (M 6.8) with a focal depth of 2 km; epicentral distances (Δ) for the stations are about 157-205 km. Phase analyses using the sub-array at KOG (Δ_??_7175 km) indicate that dominant phases in the period range 1-10 s are Love and Rayleigh waves with clear dispersion. Apparent of their phases are lying in the range of 800-2, 100 m/s. They are matched with the fundamental mode of Love waves for transverse motion but are stretching over the two modes of Rayleigh waves (M11 and M21) for radial component; the crustal model, on which the normal mode solution is based, includes thick (-52, 000 m) sedimentary layers lying on the Pre-Tertiary bedrock. The above results in conjunction with direction of wave propagation and particle orbit of the ground motion suggest that the intermediate-period surface waves are perturbed by other phases such as diffracted surface waves which are owed to lateral heterogeneities in the uppermost crust. Correspondence of major phases in the strong motion records of the long-span array gives rise to inter-station group velocities of the surface waves of the intermediate-period. Thus derived group enable us to interpret that the well-dispersed later phases are on a branch near the Airy phase of which propagation velocity is 300-800 m/s.

Source of Detritus in Gueydan (Catahoula) Formation, Southern Texas Gulf Coast: ABSTRACT

The Gueydan Formation in outcrop is pastel-colored tuffaceous clay, sandstone, conglomerate, bentonite, and minor ash. The clay, bentonite, and ash are composed predominantly of silicic pyroclastic debris or the alteration products of such debris. Sandstone and conglomerate are composed largely of debris from volcanic and hypabyssal rocks plus minor detritus from sedimentary rocks and reworked Gueydan deposits. Bailey (1926) reviewed the problem of the source of the igneous material and, although the evidence was inconclusive, favored a source near Duval and McMullen Counties. The writers' study suggests that sand and gravel debris of igneous rocks was derived from the Big Bend region of Texas and adjacent parts of northern Mexico; the fine pyroclastic material may have come from the same area or farther west. Cross-bed data show that streams flowed east-southeast to the Gulf during the time of Gueydan deposition; hence, detritus was derived from terrane in an updip direction. Pebbles of soda-rich trachyte and trachyandesite (latite), rhyolite porphyry, and welded tuff in the Gueydan are similar in texture and composition to rocks exposed in the Big Bend region and adjacent Mexico. Although the Gueydan lacks detritus from some rocks exposed there, the rocks which are not represented as pebbles are mafic types that weather rapidly. Boulder-size clasts in the Gueydan were transported as stream bed-load and possibly for short distances by mudflows; they are not proof of a local source. Cretaceous foraminifers and rock fragments in the Gueydan show that pre-Tertiary bedrock was exposed locally cross the drainage area. Upper level winds today and presumably throughout the Tertiary moved eastward across Texas; hence, the source volcanoes were toward the west. Although ash may have travelled to the site of deposition by wind, much was reworked by streams after initial deposition and subsequently modified by soil-forming processes. End_of_Article - Last_Page 2166------------