American Geophysical Union (AGU)

Tide gauge data from Key West to Norfolk were used to identify a monthly signal in sea level that is uncorrelated with local shelf‐trapped processes. Time series (1955–1975) of local winds, sea level slope, and river runoff were used in a regression model of sea level to separate a local response and a residual signal. The monthly means of the residual contribution were investigated for their relationship to the seasonal fluctuations of the Gulf Stream. In the Florida Channel, lower sea level is found to correspond to increased flow of the Florida Current. During July and August a marked fall in residual sea level, unrelated to the coastal winds, is found from Key West to Charleston, suggesting that the transport of the Gulf Stream increases both in and north of the Florida Channel during this time. Measured long‐term monthly surface currents at Diamond Shoals, Cape Hatteras, which markedly increase in summer to high velocities, tend to substantiate this claim. An additional wintertime low in residual sea level occurs north of the Florida Channel. The wintertime low does not result from steric heating within the upper 100–150 m of water, nor does it, in contrast to the summertime low, appear to coincide with increased northward surface flow. Monthly mean Sverdrup transport was computed across the North Atlantic between 15°N and 35°N. When compared at the same latitude, residual sea level and Sverdrup transport, both interpreted as indices of Gulf Stream transport, generally disagree in phase during summer. However, north of the Florida Channel they are consistent during winter, assuming that lower sea level at this time reflects increased flow in the stream.

The microwave limb sounder (MLS) on the Upper Atmosphere Research Satellite (UARS) is the first satellite experiment using limb sounding techniques at microwave frequencies. Primary measurement objectives are stratospheric ClO, O₃, H₂O, temperature, and pressure. Measurements are of thermal emission: all are performed simultaneously and continuously and are not degraded by ice clouds or volcanic aerosols. The instrument has a 1.6‐m mechanically scanning antenna system and contains heterodyne radiometers in spectral bands centered near 63, 183, and 205 GHz. The radiometers operate at ambient temperature and use Schottky‐diode mixers with local oscillators derived from phase‐locked Gunn oscillators. Frequency tripling by varactor multipliers generates the 183‐ and 205‐GHz local oscillators, and quasi‐optical techniques inject these into the mixers. Six 15‐channel filter banks spectrally resolve stratospheric thermal emission lines and produce an output spectrum every 2 s. Thermal stability is sufficient for “total power” measurements which do not require fast chopping. Radiometric calibration, consisting of measurements of cold space and an internal target, is performed every 65‐s limb scan. Instrument in‐orbit performance has been excellent, and all objectives are being met.

We present a technique that greatly improves the precision in measuring temporal variations of crustal velocities using an earthquake doublet, or pair of microearthquakes that have nearly identical waveforms and the same hypocenter and magnitude but occur on different dates. We compute differences in arrival times between seismograms recorded at the same station in the freqency domain by cross correlation of short windows of signal. A moving‐window analysis of the entire seismograms, including the coda, gives δ(t), the difference in arrival times versus running time along the seismogram. The time resolution of the method is an order of magnitude better than the digitization interval. The δ(t) technique is illustrated with a pair of microearthquakes, M = 1.7 and 2.0, that occurred before and after the Coyote Lake, California, earthquake (M = 5.9) of August 6, 1979, and on the same segment of the Calaveras fault that ruptured during the earthquake. The coda wave arrivals for some stations are progressively delayed for the second earthquake in the doublet, so that its seismogram appears as a stretched version of the earlier event. We interpret this systematic variation in δ(t) along the coda as a change in the average S velocity in the upper crust in the time interval between the two doublets. S wave velocities appear to have decreased by 0.2% in an oblong region 5–10 km in radius at the south end of the aftershock zone.

We study the origin of the background seismic noise averaged over long time by cross correlating of the vertical component of motion, which were first normalized by 1‐bit coding. We use 1 year of recording at several stations of networks located in North America, western Europe, and Tanzania. We measure normalized amplitudes of Rayleigh waves reconstructed from correlation for all available station to station paths within the networks for positive and negative correlation times to determine the seasonally averaged azimuthal distribution of normalized background energy flow (NBEF) through the networks. We perform the analysis for the two spectral bands corresponding to the primary (10–20 s) and secondary (5–10 s) microseism and also for the 20–40 s band. The direction of the NBEF for the strongest spectral peak between 5 and 10 s is found to be very stable in time with signal mostly coming from the coastline, confirming that the secondary microseism is generated by the nonlinear interaction of the ocean swell with the coast. At the same time, the NBEF in the band of the primary microseism (10–20 s) has a very clear seasonal variability very similar to the behavior of the long‐period (20–40 s) noise. This suggests that contrary to the secondary microseism, the primary microseism is not produced by a direct effect of the swell incident on coastlines but rather by the same process that generates the longer‐period noise. By simultaneously analyzing networks in California, eastern United States, Europe, and Tanzania we are able to identify main source regions of the 10–20 s noise. They are located in the northern Atlantic and in the northern Pacific during the winter and in the Indian Ocean and in southern Pacific during the summer. These distributions of sources share a great similarity with the map of average ocean wave height map obtained by TOPEX‐Poseidon. This suggests that the seismic noise for periods larger than 10 s is clearly related to ocean wave activity in deep water. The mechanism of its generation is likely to be similar to the one proposed for larger periods, namely, infragravity ocean waves.

We estimate shear wave velocities in the shallow subsurface throughout Japan by applying seismic interferometry to the data recorded with KiK‐net, a strong motion network in Japan. Each KiK‐net station has two receivers; one receiver on the surface and the other in a borehole. By using seismic interferometry, we extract the shear wave that propagates between these two receivers. Applying this method to earthquake‐recorded data at all KiK‐net stations from 2000 to 2010 and measuring the arrival time of these shear waves, we analyze monthly and annual averages of the near‐surface shear wave velocity all over Japan. Shear wave velocities estimated by seismic interferometry agree well with the velocities obtained from logging data. The estimated shear wave velocities of each year are stable. For the Niigata region, we observe a velocity reduction caused by major earthquakes. For stations on soft rock, the measured shear wave velocity varies with the seasons, and we show negative correlation between the shear wave velocities and precipitation. We also analyze shear wave splitting by rotating the horizontal components of the surface sensors and borehole sensors and measuring the dependence on the shear wave polarization. This allows us to estimate the polarization with the fast shear wave velocity throughout Japan. For the data recorded at the stations built on hard rock sites, the fast shear wave polarization directions correlate with the direction of the plate motion.

Four intersecting ∼300‐km‐long reversed refraction lines within northwestern Alberta and northeastern British Columbia have been interpreted for crustal and upper mantle seismic velocity structure in the Peace River Arch (PRA) region of the Western Canada Sedimentary Basin. The data have been modeled using a two‐dimensional ray trace forward modeling approach based on asymptotic ray theory to match travel times and amplitudes of first and coherent later arrivals. In addition, an inversion of first arrival travel times along a fan shot profile has been performed to constrain crustal thickness northwest of the arch in a region not sampled by the reversed profiles. The major features of the structural models are (1) weak to moderate lateral variations in crustal structure with no evidence of significant layering or thick low‐velocity zones within the crust, (2) an average sub‐basement RMS crustal velocity of 6.6 km/s, average upper mantle velocity of 8.25 km/s and average crustal thickness of 40 km, (3) regional variations in structure which appear related to the dominant N‐S trending cratonic structure, including crustal thickness, upper crustal and upper mantle velocities and P_mP character, and (4) subtle variations in structure that may be associated with the E‐W trending Devonian axis of the PRA, including a shallowing of high lower crustal velocities, thickening of the crust, and an anisotropic P_mP character beneath the arch and a weak trend in the dip of intracrustal reflectors away from the center of the PRA region. Evidence from the refraction and other geophysical data for the presence of a local crustal expression of the PRA is weak, but suggests a thermal as opposed to flexural origin for its anomalous vertical movements.

The coda of seismic waves consists of that part of the signal after the directly arriving phases. In a finite medium, or in one that is strongly heterogeneous, the coda is dominated by waves which have repeatedly sampled the medium. Small changes in a medium which may have no detectable influence on the first arrivals are amplified by this repeated sampling and may thus be detectable in the coda. We refer to this use of multiple‐sampling coda waveforms as coda wave interferometry. We have exploited ultrasonic coda waves to monitor time‐varying rock properties in a laboratory environment. We have studied the dependence of velocity on uniaxial stress in Berea sandstone, the temperature dependence of velocity in granite and in aluminum, and the change in velocity due to an increase of water saturation in sandstone. There are many other possible applications of coda wave interferometry in geophysics, including dam and volcano monitoring, time‐lapse reservoir characterization, earthquake relocation, and stress monitoring in mining and rock physics.

Temporal variation in shear wave anisotropy was detected in a monitoring experiment using an accurately controlled, routinely operated signal system (Accurately Controlled Routinely Operated Signal System, ACROSS). We conducted an experiment that lasted for 15 months between January 2000 and April 2001 at a site near the Nojima fault, which ruptured during the 1995 Kobe earthquake (M_w 7.2) [Yamaoka et al., 2001; Ikuta et al., 2002]. Two vibration sources that generated 2 × 10⁵ N with centrifugal force, which were firmly fixed on the ground, were used to emit elastic waves. Seismometers deployed at the bottom of 800‐m‐ and 1700‐m‐deep boreholes near the ACROSS sources were used to receive the signal. We extracted small temporal changes in the travel time of the P and S wave by calculating cross‐spectral density among the records every hour. During the experiment, sudden delays in travel times for the S wave were observed when the 2000 western Tottori earthquake (M_w 6.6) and the 2001 Geiyo earthquake (M_w 6.4) occurred. Their epicenters were 165 and 215 km away from the site, respectively. The travel times of the S waves between the surface and the bottom of 800‐m‐deep borehole abruptly slowed down and gradually speeded up with each earthquake. The delay in S was about 0.4% and 0.1% of the absolute travel time for the western Tottori earthquake and the Geiyo earthquake, respectively. The delays were polarized in a direction perpendicular to the Nojima fault in both cases. This indicates that the density of the cracks parallel to the fault increased in association with the earthquakes. These cracks can be regarded as opened by the increase in pore pressure. The small changes in P wave velocities support this interpretation. An additional experiment to determine the static anisotropy revealed that there was a preferred orientation of the cracks that was enhanced with the strong shaking of earthquakes in the middle distance.

The 2008 M7.2 Iwate‐Miyagi Nairiku earthquake occurred in NE Japan where dense seismic networks exist. Using the data from these networks, we determined the coseismic seismic velocity change associated with this earthquake by two different methods: ambient noise interferometry and vertical array interferometry of coda wave. The purpose of this article is to reveal the spatial distribution and the cause of the velocity change by integrating these two approaches. Ambient noise interferometry revealed a coseismic velocity decrease of Rayleigh wave by 0.1–0.5% at 0.25–0.5 Hz. We also estimated the spatial distribution of the velocity change by a tomographic inversion. The velocity decrease was distributed in and around the focal area. In the second method, we applied cross‐correlation analysis to coda waves observed by the KiK‐net vertical array. We detected a shear velocity decrease of approximately 5% in shallow layers up to a few hundred meters depth. Quantitative comparison of the two results reveals that the 5% shear velocity decrease in shallow layers can explain the 0.1–0.5% decrease of Rayleigh wave velocity. The distribution of the velocity decrease is similar to that of the strong ground motion and the static stress change. However, a significant velocity increase is not observed in the compression area, where a velocity increase is expected. Based on the observations, we consider that the primary factor affecting the velocity change is damage in shallow layers due to strong motion. The effect of the static stress change might be masked by the larger effect of the strong motion.