Steve Roecker

Receiver functions for the Tien Shan Analog Broadband Network: Contrasts in the evolution of structures across the Talasso-Fergana Fault

We determined the structure of the crust beneath the central and western Tien Shan by analyzing broadband (1–20 s) analog seismograms of converted P-SV phases generated by earthquakes at teleseismic distances and recorded by 14 seismograph stations. The one-dimensional structures that best explain the waveforms reveal pronounced differences in crustal velocities east and west of the Talasso-Fergana fault. Specifically, the transition zone between the crust and the mantle east of the fault is about 2 times broader than that west of the fault. Also, velocities at depths between 10 and 35 km are about 10% lower in the east, although the depth range of these lower velocities is not well resolved. The Talasso-Fergana fault is an important boundary for other observables; in addition to the structural discontinuities observed at the surface, the areas east of the Talasso-Fergana fault are associated with abnormally low mantle velocities, outcrops of basalt, low Q, and short-wavelength variations in anisotropy. Integrating these observations, we interpret the broad mantle gradient as being due to vertical intrusions of mantle material into the lower crust. Likewise, the low velocities in the midcrust could be thermally induced or due to the introduction of magmatically derived fluids. The gross contrast in structure east and west of the Talasso-Fergana fault could reflect a contrast in the dynamics of mountain building in these two regions. We postulate that while crustal shortening is the dominant mechanism controlling topography west of the Talasso-Fergana fault, vertical uplift caused by a mass deficiency in the upper mantle may contribute significantly to the generation and maintenance of the high elevations east of the fault.

Seismic azimuthal anisotropy beneath the Pakistan Himalayas

Teleseismic S, SKS and SKKS data, collected from a temporary broadband array across the Himalayan front in Pakistan, are analyzed for shear-wave splitting parameters. The SKS and SKKS phases have ray paths originating from both the South Pacific and Colombia which have azimuths approximately 40° apart with respect to the Pakistan array. If significant seismic azimuthal anisotropy is present we should observe splitting associated with one of these ray paths. No evidence was seen for any shear-wave splitting beneath any of the stations in the array. Teleseismic S waves were also used in order to provide better azimuthal coverage for the shear-wave splitting measurements. We were able to correct for any source-side anisotropy when needed. No receiver-side splitting was observed in any of the S wave data. The lack of shear-wave splitting beneath the Pakistan array indicates that there is no appreciable large-scale azimuthal anisotropy beneath this part of the Himalayas. Therefore, if there is any significant strain in the upper mantle beneath this area, it must either be vertically oriented, or, if horizontal, vertically vary in such a way that the integrated effect on S wave splitting is null.

Three-Dimensional Seismic Attenuation Structure around the SAFOD Site, Parkfield, California

Abstract We present models of the three-dimensional (3D) seismic attenuation structure, both Q p and Q s , for a 16 km 2 area centered on the San Andreas Fault Observatory at Depth (SAFOD). The P - and S -wave t * -values used in the inversion were determined from local earthquake data recorded by seismic network and portable array stations within the Parkfield region by inverting arrival spectra for source parameters, t * , and site response. Two techniques for determining the site response, the joint and alternating methods, were compared and it was found that the alternating method significantly underestimated site response variations. The t * -values were inverted to obtain 3D frequency-independent Q p and Q s models using 3D V p and V s models and associated event locations. A shallow low- Q area ( Q p and Q s about 50–75) on the southwest edge of both models is attributed to the low-velocity Cenozoic sedimentary rocks that overlie the Salinian basement rock. A high- Q feature ( Q p and Q s about 250 to 300) abuts this area and is interpreted as the Salinian basement. Adjacent to the San Andreas fault (SAF) trace, on its southwest side, there is a low- Q feature ( Q p and Q s about 50–80) attributed to a wedge of sedimentary rocks; uniformly low Q p - and Q s -values suggest that the wedge is fluid rich. A low- Q basin feature ( Q p and Q s about 50–75) on the northeast side of the SAF is interpreted as a fluid rich zone. Beneath this area there is a high- Q feature ( Q p and Q s about 220–300), which may be caused by crack closure due to increased pressure with depth in the rocks of the Franciscan formation. Given these high Q -values, it seems unlikely that this area acts as a fluid pathway for fluids entering the fault zone from the east into the seismogenic zone of the SAF.

Analysis of Seismic Activity in the Crust from Earthquake Relocation in the Central Tien Shan

Using P - and S -wave arrivals from local earthquakes recorded by a temporary broadband seismic network (ghengis) and a Kyrgyzstan broadband seismic network (knet), we determined the source parameters of 1938 earthquakes occurring from 1997 to 1998 in the central Tien Shan and adjacent areas, based on one- dimensional and three-dimensional velocity models. The results indicate that seismic activity is not restricted to only central Tien Shan but also penetrates into the edges of the Kazakh platform and the Tarim basin to the north and south, respectively. It implies that most range-bounding faults in the central Tien Shan are active and they play a significant role in the tectonic activity of the mountain belt. However, seismic activity of the Talas-Fergana fault in the study area is different; its central segment seems more active than its eastern segment. We infer that this part of the fault may be activated by neighboring faults, whereas its eastern segment could be temporally locked. The earthquakes at the depths of 30–40 km beneath the east-central Tien Shan reveal an interaction between the upper and mid-lower crust. This suggests that brittle failure is possible to greater depth within the crust. Another possibility is that the seismic activity in the mid-lower crust is affected by a positive buoyancy force created by mantle upwelling of this area. These observations indicate that dynamic processes such as small-scale mantle convection or mantle upwelling, which resulted in lithospheric thinning or removal, still occur beneath the central Tien Shan.

Three‐dimensional velocity structure and hypocenters of earthquakes beneath the Hazara Arc, Pakistan: Geometry of the underthrusting Indian Plate

The three-dimensional P and S wave velocity structures and hypocenters of 420 events beneath the western Hazara Arc are obtained simultaneously by inverting travel time data observed at fifteen Tarbela seismic stations. In general, the P and S wave velocity distribution of the top layer (0–6 km depth) correlates well with surface geology. Within this layer we find a low-velocity region beneath the Hazara Thrust Zone (HTZ) corresponding to the underthrusted Murree Formation, and there are high-velocity regions south of the Main Mantle Thrust (MMT) which are associated with the exposed Cambrian, late Paleozoic, and Tertiary granites. A low-velocity zone immediately to the west of the Hazara-Kashmir Syntaxis (HKS) indicates the existence of a Miocene foreland basin which is covered by late stage southeasterly directed thrusts along the Hazara Arc and is consistent with the idea that the HKS is detached from the lower crust. From the Salt Range to the HTZ, the Indian plate dips at a shallow angle, about 2°–3° to the northeast. North of the HTZ the underthrusting Indian plate dips gently to the northeast with an increased slope of 5° to 8° until it reaches the Indus-Kohistan Seismic Zone (IKSZ). Along the NW trending IKSZ the Indian plate bends more steeply to the northeast beneath a seismically active midcrustal wedge directed to the southwest. The larger events in the IKSZ are interpreted as occurring on a major thrust zone that can be followed to a depth of 24 km. The IKSZ appears to consist of an upper seismic zone (from the surface to about 8 km) and a lower seismic zone (12 km to 24 km) separated by an aseismic region about 4 km thick. The lower IKSZ may represent the leading edge of a southwestward directed slab which has not yet ruptured the surface. Hypocenters of relocated earthquakes indicate that the HTZ is about 30 km wide with most of the larger microearthquakes occurring at 12–14 km. Seismicity along the HTZ suggests that the Panjal, and Murree thrusts are active.

Mapping of low P wave velocity structures in the subducting plate of the central New Hebrides, southwest Pacific

Arrival times of compressional (P) and shear (S) waves generated by earthquakes located in the New Hebrides subduction zone and recorded by local and regional arrays of seismographs are used to determine large-scale one- and three-dimensional elastic wave velocity structures of the subduction zone between 15° and 20°S and from the surface to about 250 km depth. The results obtained from inverting the locally and regionally recorded arrival times individually corroborate each other, and they are inverted jointly in order to improve the resolution of shallow to intermediate depth structures. The results for one-dimensional structure indicate a gradual increase of velocity with depth until a 9% reversal appears between 60 and 100 km depth. The three-dimensional structure determined from the joint inversion shows that these low velocities lie within a sizable seismic gap in the descending Benioff zone. Taken with other observations such as the attenuation of high-frequency shear waves travelling across this gap and the locations of active volcanoes at the surface, we infer that the low-velocity region represents a thermal anomaly of about 750°C which alters the physical properties of the descending plate in this region. At shallower depths there is evidence of low-velocity structures included in the descending plate: one beneath north Malekula island and another between Malekula and Efate islands corresponding to the large embayment of the leading edge of the upper plate. The locations of these structures combined with previous investigations of the area lead us to infer that these low velocities are due to the subduction of small-scale features such as seamounts and accretionary wedges.

The magmatic plumbing system of the Askja central volcano, Iceland, as imaged by seismic tomography

The magmatic plumbing system beneath Askja, a volcano in the central Icelandic highlands, is imaged using local earthquake tomography. We use a catalog of more than 1300 earthquakes widely distributed in location and depth to invert for the P wave velocity (Vp) and the Vp/Vs ratio. Extensive synthetic tests show that the minimum size of any velocity anomaly recovered by the model is ~4 km in the upper crust (depth < 8 km below sea level (bsl)), increasing to ~10 km in the lower crust at a depth of 20 km bsl. The plumbing system of Askja is revealed as a series of high-Vp/Vs ratio bodies situated at discrete depths throughout the crust to depths of over 20 km. We interpret these to be regions of the crust which currently store melt with melt fractions of ~10%. The lower crustal bodies are all seismically active, suggesting that melt is being actively transported in these regions. The main melt storage regions lie beneath Askja volcano, concentrated at depths of 5 km bsl with a smaller region at 9 km bsl. Their total volume is ~100 km3. Using the recorded waveforms, we show that there is also likely to be a small, highly attenuating magmatic body at a shallower depth of about 2 km bsl.

A shallow double seismic zone beneath the central New Hebrides (Vanuatu): evidence for fragmentation and accretion of the descending plate?

A shallow double seismic zone (SDSZ) has been found in the descending Australian plate beneath the central part of the New Hebrides island arc, directly above a large gap in intermediate depth seismicity and between two seismic boundaries. Ambient seismicity occurs mostly in the upper part of the SDSZ, while earthquakes in the lower part occur in clusters (swarms or aftershocks of large earthquakes). The distance between the upper and lower levels of the SDSZ is 50–70 km, and they are joined at 80 km depth by a near-horizontal band of seismicity. Thrust-faulting mechanisms predominate for earthquakes in the upper level of the SDSZ. Those in the lower level, however, appear to be normal faulting, despite their being aftershocks of large thrust events. We suggest that with the absence of a pull from the detached lithosphere the upper part of the Australian plate in the region of the SDSZ is resistant to subduction, and thus the downward displacements caused by large earthquakes in the adjoining regions result in a localized rebound. The location of the aftershocks within the plate suggests that a new plate boundary is forming, which will eventually replace that outlined by the residual seismicity in the upper level. Thus the leading edge is decoupling, and the boundary will eventually shift back to the lower level of the SDSZ.

Fault-magma interactions during early continental rifting: seismicity of the Magadi-Natron-Manyara basins, Africa

Although magmatism may occur during the earliest stages of continental rifting, its role in strain accommodation remains weakly constrained by largely 2D studies. We analyze seismicity data from a 13-month, 39-station broadband seismic array to determine the role of magma intrusion on state-of-stress and strain localization, and their along-strike variations. Precise earthquake locations using cluster analyses and a new 3D velocity model reveal lower crustal earthquakes beneath the central basins and along projections of steep border faults that degas CO2. Seismicity forms several disks interpreted as sills at 6-10 km below a monogenetic cone field. The sills overlie a lower crustal magma chamber that may feed eruptions at Oldoinyo Lengai volcano. After determining a new ML scaling relation, we determine a b-value of 0.87 ± 0.03. Focal mechanisms for 65 earthquakes, and 13 from a catalogue prior to our array reveal an along-axis stress rotation of ∼60° in the magmatically active zone. New and prior mechanisms show predominantly normal slip along steep nodal planes, with extension directions ∼ N90°E north and south of an active volcanic chain consistent with geodetic data, and ∼ N150°E in the volcanic chain. The stress rotation facilitates strain transfer from border fault systems, the locus of early stage deformation, to the zone of magma intrusion in the central rift. Our seismic, structural, and geochemistry results indicate that frequent lower crustal earthquakes are promoted by elevated pore pressures from volatile degassing along border faults, and hydraulic fracture around the margins of magma bodies. Results indicate that earthquakes are largely driven by stress state around inflating magma bodies.

Seismic Imaging of Microblocks and Weak Zones in the Crust Beneath the Southeastern Margin of the Tibetan Plateau

Haijiang Zhang1, Steve Roecker2, Clifford H. Thurber3 and Weijun Wang4 1Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 2Department of Earth and Environmental Sciences, Rensselaer Polytechnic Institute, Troy, New York 3Department of Geoscience, University of Wisconsin-Madison, Madison, WI 4Institute of Earthquake Science, China Earthquake Administration, Beijing, 1,2,3USA 4China