Aaron Ferris

High resolution image of the subducted Pacific (?) plate beneath central Alaska, 50–150 km depth

Abstract A receiver function transect across the Alaska Range images the subducting Pacific plate at 50–150 km depth. Across a 200 km long array of 30 receivers, the largest observed P-to-S conversions come from the top of the subducting slab. This signal is coherent across the array and is strongly asymmetric, requiring a complicated interface at the top of the slab. Waveform inversion shows that the conversion is generated by a 11–22 km thick low velocity zone at the top of the slab, as much as 20% slower than the surrounding mantle. The velocity of this zone increases with increasing depth of the slab, approaching velocities of the mantle near 150 km depth. All intermediate depth earthquakes occur within the zone, along a plane dipping 5° steeper. The layer is too thick to represent metamorphosed oceanic crust, as proposed for other subduction zones. It may represent a thick serpentinized zone or, more likely, a thick exotic terrane subducting along with the Pacific plate.

Mantle compensation of active metamorphic core complexes at Woodlark rift in Papua New Guinea.

In many highly extended rifts on the Earth, tectonic removal of the upper crust exhumes mid-crustal rocks, producing metamorphic core complexes. These structures allow the upper continental crust to accommodate tens of kilometres of extension, but it is not clear how the lower crust and underlying mantle respond. Also, despite removal of the upper crust, such core complexes remain both topographically high and in isostatic equilibrium. Because many core complexes in the western United States are underlain by a flat Moho discontinuity, it has been widely assumed that their elevation is supported by flow in the lower crust or by magmatic underplating. These processes should decouple upper-crust extension from that in the mantle. In contrast, here we present seismic observations of metamorphic core complexes of the western Woodlark rift that show the overall crust to be thinned beneath regions of greatest surface extension. These core complexes are actively being exhumed at a rate of 5–10 km Myr-1, and the thinning of the underlying crust appears to be compensated by mantle rocks of anomalously low density, as indicated by low seismic velocities. We conclude that, at least in this case, the development of metamorphic core complexes and the accommodation of high extension is not purely a crustal phenomenon, but must involve mantle extension.

Crustal thickness variation in south-central Alaska

Crustal thicknesses have been determined by receiver function analysis of broadband teleseismic waveforms recorded during the Broadband Experiment Across the Alaska Range (BEAAR). Typical crust beneath the northern lowlands is 26 km thick, while beneath the mountains it is 35–45 km thick. The transition from thick to thin crust coincides with the location of the Hines Creek fault, a major tectonostratigraphic boundary. Crustal thicknesses determined by receiver functions agree with those predicted from topography assuming Airy type isostasy, suggesting that the Alaska Range is compensated by its crustal root. North of the range, however, the crust is systematically thinner than predicted by simple Airy isostasy. A crustal density contrast of 4.6% across the Hines Creek fault, 2700 kg m−3 to the north and 2830 kg m−3 to the south, explains the observed difference between the crustal thicknesses predicted by simple Airy isostasy, and the crustal thicknesses determined by receiver function analysis.

Analysis and localization of blue whale vocalizations in the Solomon Sea using waveform amplitude data.

During the Woodlark Basin seismic experiment in eastern Papua New Guinea (1999-2000), an ocean-bottom seismic array recorded marine mammal vocalizations along with target earthquake signals. The array consisted of 14 instruments, 7 of which were three-component seismometers with a fourth component hydrophone. They were deployed at 2.0-3.2 km water depth and operated from September 1999 through February 2000. While whale vocalizations were recorded throughout the deployment, this study focuses on 3 h from December 21, 1999 during which the signals are particularly clear. The recordings show a blue whale song composed of a three-unit phrase. That song does not match vocalization characteristics of other known Pacific subpopulations and may represent a previously undocumented blue whale song. Animal tracking and source level estimates are obtained with a Bayesian inversion method that generates probabilistic source locations. The Bayesian method is augmented to include travel time estimates from seismometers and hydrophones and acoustic signal amplitude. Tracking results show the whale traveled northeasterly over the course of 3 h, covering approximately 27 km. The path followed the edge of the Woodlark Basin along a shelf that separates the shallow waters of the Trobriand platform from the deep waters of the basin.

Investigation of Microearthquakes, Macroseismic Data, and Liquefaction Associated with the 1867 Wamego Earthquake in Eastern Kansas

Historical felt earthquakes and instrumentally recorded microseismicity in eastern Kansas and western Missouri have been attributed to the movement of the Nemaha Ridge and Humboldt fault zone (nrhf). Our investigations of the nrhf have concentrated on relocation of microearthquakes in the Kansas catalog, re-evaluation of the 1867 earthquake felt reports, and field studies to determine the presence or absence of sedimentary evidence of earthquake-induced liquefaction. Microearthquakes in the Kansas catalog have been relocated in a joint inversion with 3D seismic velocity variations. This improved set of hypocenters affirms the loose association of seismicity with the basement nrhf structure, even though seismicity does not seem to follow a single, well-defined fault feature. Overall, the association of microearthquakes with the nrhf suggests that it remains a potential source of large earthquakes. Our field investigations confirm that sedimentary deposits with moderate liquefaction susceptibility are present in the vicinity of Wamego and Wabaunsee, Kansas. Soft-sediment deformation features, including flame and dish structures, are present in the late Holocene floodplain deposits of the Kansas River. These features may have formed by liquefaction or by processes unrelated to seismic activity, such as sediment dewatering. We found several clastic dikes that may be attributed to seismically induced liquefaction. Our initial results suggest that liquefaction features are present but may not be pervasive in this region. These data imply that the 5.2 magnitude of the 1867 Wamego earthquake may characterize the seismic source in this location. However, additional field investigations are still necessary to complete the assessment for liquefaction features.

Analysis of cetacea vocalizations using ocean bottom seismic array observations, western Woodlark Basin, Papua New Guinea.

Over a 6‐month period in 1999 an array of seven ocean bottom seismometers and seven bottom mounted hydrophones covered a 30‐km2 area at 2–3‐km water depth in the western Woodlark Basin. The array’s main task was the detection of microearthquakes associated with nearby active tectonics. However, it also made fortuitous broadband recordings of Cetacea vocalizations that we associate with finback whales. Each of these signals has a 10–15‐s duration and they exhibit a repetitive pattern with 10–15‐min period. The signals are well recorded across both the seismometers and hydrophones, which were sampled at 64 and 125 Hz, respectively. Time‐varying spectral analysis demonstrates that the signals are frequency modulated in the 20–30‐Hz band with most energy occurring at ∼22 Hz. We employ array‐based methods (e.g., optimal beam forming techniques) and acoustic transmission loss simulations to determine range and bearing of the acoustic sources. Preliminary analysis indicates that the acoustic sources originate near the array, and secondary signals may represent back‐scattered energy from the short‐wavelength, high‐topographic features associated the active tectonics with in the basin.

Augmenting probabilistic inversion for whale localization with propagation models.

Tracking of marine mammals is important for the evaluation of species geographic range and for investigating the relationship between vocalization and behavior patterns. The use of ocean bottom seismic and hydrophone arrays is gaining interest in marine mammal studies due to their relatively long deployments (6–12 months) and low environmental impact. A Bayesian inversion method generates a probability density function for the source location by comparing travel time differences observed at ocean bottom arrays with those from a theoretical model. The method also accounts for uncertainty arising from difficulties in evaluating arrival times, instrument location, or choice of theoretical model. This method can be augmented with a similar comparison of observed arrival amplitudes and transmission loss results from parabolic equation propagation modeling. Due to acoustic convergence zones, the amplitude data may specifically contain information regarding the source’s range from the array. Uncertainty in the p...

Use of cross correlation to estimate Green’s functions in the Pakistan‐Himalayas

Recent theoretical and observational work indicates that the time derivative of the Green’s function between two points can be estimated from ambient seismic noise recordings. Continuous seismograms from a temporary deployment of nine broadband seismographs in NE Pakistan (1992) are cross correlated and averaged over 73 days to obtain Green’s function estimates of the longitudinal, radial, and transverse components for all station combinations. In general, the time‐averaged correlation functions show dispersion characteristics, which are analyzed using Hilbert transform filtering and wavelet transforms. Preliminary results show group velocities between 1.5 and 3.5 km/s for the period band between 2 and 20 s. In some cases, the noise correlation functions exhibit asymmetry between causal and acausal portions. This is interpreted as preferred directionality of the diffusive noise field. By comparing the peak energy at causal and acausal delay times, it appears the noise field propagates predominantly from t...