Douglas A. Wiens

Tomography of the Source Area of the 1995 Kobe Earthquake: Evidence for Fluids at the Hypocenter?

Seismic tomography revealed a low seismic velocity (−5%) and high Poissons ratio (+6%) anomaly covering about 300 square kilometers at the hypocenter of the 17 January 1995, magnitude 7.2, Kobe earthquake in Japan. This anomaly may be due to an overpressurized, fluid-filled, fractured rock matrix that contributed to the initiation of the Kobe earthquake.

Tsunami earthquakes: Slow thrust‐faulting events in the accretionary wedge

The November 20, 1960, Peru, October 20, 1963, Kurile and June 10, 1975, Kurile earthquakes are classified as tsunami earthquakes based on anomalously large tsunami excitation relative to earthquake magnitude. Long-period surface wave analysis indicates double-couple (faulting) mechanisms for all three events rather than single-force mechanisms indicative of submarine landslides. The earthquakes have shallow depths (< 15 km) and are located near the trench axis and seaward of most other thrust zone events beneath the accretionary prism. Body waveform inversion indicates very shallowly dipping thrust faulting mechanisms for the three events, with dip angles of 6°–8°. Surface wave spectral amplitudes and deconvolution of SH waveforms suggests anomalously long source durations and large seismic moments relative to MS. Specifically, the 1963 Kurile event (MS 7.2) shows a duration of 85 s and a moment of 6.0 × 1027 dyn cm (MW 7.8), the 1975 Kurile event (MS 7.0) shows a duration of 60 s and a moment of 2.0 × 1027 dyn cm (MW 7.5), and the 1960 Peru event (MS 6.75) shows a time function consisting of four subevents with a total duration of 110–130 s and a seismic moment of 3.4 × 1027 dyn cm (MW 7.6). Estimated rupture velocities are about 1 km/s or less, but there is no evidence of unusually low stress drops. The August 1, 1968, Philippines event, previously classified as a tsunami earthquake, shows none of the anomalous source properties, and teleseismic tsunami height measurements are sparse; we do not consider this event a tsunami earthquake. Most of the “anomalous” tsunami excitation results from underestimation of earthquake size by MS due to the long source duration; the tsunami heights are not significantly anomalous relative to seismic moment. The slow nature of these events may result from rupture through the sedimentary rock along the basal decollement of the accretionary prism. Standard scaling laws when adjusted for the slow seismic velocity in the source region show an MW - MS relationship similar to that observed for the tsunami earthquakes and predict MS saturation at about 7.3 rather than 8.0 for typical events.

Long-term eruptive activity at a submarine arc volcano

Three-quarters of the Earths volcanic activity is submarine, located mostly along the mid-ocean ridges, with the remainder along intraoceanic arcs and hotspots at depths varying from greater than 4,000 m to near the sea surface. Most observations and sampling of submarine eruptions have been indirect, made from surface vessels or made after the fact. We describe here direct observations and sampling of an eruption at a submarine arc volcano named NW Rota-1, located 60 km northwest of the island of Rota (Commonwealth of the Northern Mariana Islands). We observed a pulsating plume permeated with droplets of molten sulphur disgorging volcanic ash and lapilli from a 15-m diameter pit in March 2004 and again in October 2005 near the summit of the volcano at a water depth of 555 m (depth in 2004). A turbid layer found on the flanks of the volcano (in 2004) at depths from 700 m to more than 1,400 m was probably formed by mass-wasting events related to the eruption. Long-term eruptive activity has produced an unusual chemical environment and a very unstable benthic habitat exploited by only a few mobile decapod species. Such conditions are perhaps distinctive of active arc and hotspot volcanoes.

The depth distribution of mantle anisotropy beneath the Tonga subduction zone

Abstract Shear-wave splitting recorded by a temporary deployment of broadband seismometers located above the Tonga subduction zone yields strong constraints on the depth distribution of anisotropy in the mantle beneath the Tonga back-arc region. Splitting parameters obtained for local S and teleseismic SKS phases were modeled using a ray-based method that predicts shear-wave splitting on individual phase paths and determines the best-fitting values of anisotropic strength, orientation and depth extent. Splitting in teleseismic SKS phases is identical to that in S phases from local earthquakes that occur at the base of the transition zone, demonstrating that there is no significant splitting due to anisotropy in the lower mantle. Splitting times for local S phases do not vary significantly with depth, and for models in which the strength of anisotropy is laterally uniform, they rule out anisotropy in the transition zone. Finally, the observed splitting indicates that anisotropy of more than 0.8% exists in the upper mantle, with a fast symmetry axis at 60°W, roughly parallel to the absolute plate motion vector of the subducting Pacific lithosphere.

Seismic attenuation tomography of the Tonga‐Fiji region using phase pair methods

The anelastic structure of the region surrounding the Tonga slab and Lau back arc spreading center in the southwest Pacific is studied using data from 12 broadband island stations and 30 ocean bottom seismographs. Two differential attenuation methods determine δt* over the frequency band 0.1 to 3.5 Hz for earthquakes in the Tonga slab. The S-P method measures the difference in spectral decay between P and S waves arriving at the same station. The P-P method measures the difference in spectral decay for P waves with different paths through the upper mantle. Eight hundred sixty phase pairs are used to invert for two-dimensional 1/Qα structure using a nonnegative least squares algorithm. A grid search method determines the Qα/Qβ ratio most compatible with both the S-P and P-P differential measurements. The highest attenuation (Qα = 90) is found within the upper 100 km beneath the active portions of the Lau Basin extending westward to the Lau Ridge. These regions probably delineate the source region for the back arc spreading center magmas, expected to be within the upper 100 km based on petrological considerations. The high attenuation regions also correlate well with zones of low P wave velocity determined by regional velocity tomography. Somewhat lower attenuation is found beneath the Fiji Plateau than beneath the Lau Basin. The entire back arc is characterized by a gradual decrease in attenuation to a depth of 300 to 400 km. The slab is imaged as a region of low attenuation (Qα > 900) material. A Qα/Qβ ratio of 1.75 provides the best fit between the S-P and P-P data sets upon inversion. Spectral stacking shows no frequency dependence within the frequency band analyzed.

Simultaneous teleseismic and geodetic observations of the stick–slip motion of an Antarctic ice stream

Long-period seismic sources associated with glacier motion have been recently discovered, and an increase in ice flow over the past decade has been suggested on the basis of secular changes in such measurements. Their significance, however, remains uncertain, as a relationship to ice flow has not been confirmed by direct observation. Here we combine long-period surface-wave observations with simultaneous Global Positioning System measurements of ice displacement to study the tidally modulated stick–slip motion of the Whillans Ice Stream in West Antarctica. The seismic origin time corresponds to slip nucleation at a region of the bed of the Whillans Ice Stream that is likely stronger than in surrounding regions and, thus, acts like an ‘asperity’ in traditional fault models. In addition to the initial pulse, two seismic arrivals occurring 10–23 minutes later represent stopping phases as the slip terminates at the ice stream edge and the grounding line. Seismic amplitude and average rupture velocity are correlated with tidal amplitude for the different slip events during the spring-to-neap tidal cycle. Although the total seismic moment calculated from ice rigidity, slip displacement, and rupture area is equivalent to an earthquake of moment magnitude seven (Mw 7), seismic amplitudes are modest (Ms 3.6–4.2), owing to the source duration of 20–30 minutes. Seismic radiation from ice movement is proportional to the derivative of the moment rate function at periods of 25–100 seconds and very long-period radiation is not detected, owing to the source geometry. Long-period seismic waves are thus useful for detecting and studying sudden ice movements but are insensitive to the total amount of slip.

Implications of oceanic intraplate seismicity for plate stresses, driving forces and rheology

Abstract Recent studies of earthquakes occurring within the oceanic lithosphere provide valuable information about stresses in the lithosphere, plate tectonic driving forces, and the rheology of the lithosphere, asthenosphere and mantle. Focal mechanisms indicate that oceanic lithosphere older than 35 million years is almost entirely in deviatoric compression. Both compressional and extensional events occur in younger lithosphere. In young lithosphere mechanisms generally indicate compressive stress in the spreading direction or extension oblique to the spreading direction; extension in the spreading direction is not observed. Using the constraint of compression in the spreading direction in old lithosphere, models of the stresses produced by a combination of ridge push and basal drag forces require the magnitude of the drag be less than a few bars for rapidly moving plates and a few tens of bars for slow moving plates. Assuming that basal drag results from mantle return flow, the upper limits on drag can be converted to constraints on the viscosity structure of the asthenosphere and mantle using simple two-dimensional models. If return flow occurs in a mantle of viscosity 10 22 poise, comparable to the results from glacial rebound studies, the predicted basal drag is too high unless a thin asthenosphere with viscosity less than 10 19 to 10 20 poise (depending on flow depth) is present. The intraplate stress data thus are consistent with the idea of oceanic plates largely decoupled from the underlying mantle. The strength of the lithosphere is constrained by the maximum depth of oceanic intraplate seismicity, which increases with lithospheric age and appears to be bounded by a 700°-800°C isotherm. This limiting depth is approximately equal to the flexural elastic thickness of the lithosphere and is consistent with experimental olivine rheologies which predict rapid weakening at high temperatures. Similar phenomena are important for estimating the fraction of seismic and aseismic slip on transform faults and in determining the extent of rupture for large trench normal faulting events.

Crust and upper mantle structure of the Transantarctic Mountains and surrounding regions from receiver functions, surface waves, and gravity: Implications for uplift models

[1] This study uses seismic receiver functions, surface wave phase velocities, and airborne gravity measurements to investigate the structure of the Transantarctic Mountains (TAM) and adjacent regions of the Ross Sea (RS) and East Antarctica (EA). Forty-one broadband seismometers deployed during the Transantarctic Mountain Seismic Experiment provide new insight into the differences between the TAM, RS, and EA crust and mantle. Combined receiver function and phase velocity inversion with niching genetic algorithms produces accurate crustal and upper mantle seismic velocity models. The crustal thickness increases from 20 ± 2 km in the RS to a maximum of 40 ± 2 km beneath the crest of the TAM at 110 ± 10 km inland. Farther inland, the crust of EA is uniformly 35 ± 3 km thick over a lateral distance greater than 1300 km. Upper mantle shear wave velocities vary from 4.5 km s � 1 beneath EA to 4.2 km s � 1 beneath RS, with a transition between the two at 100 ± 50 km inland near the crest of the TAM. The � 5k m thick crustal root beneath the TAM has an insufficient buoyant load to explain the entire TAM uplift, suggesting some portion of the uplift may result from flexure associated with a buoyant thermal load in the mantle beneath the edge of the TAM lithosphere.

Combined Receiver-Function and Surface Wave Phase-Velocity Inversion Using a Niching Genetic Algorithm: Application to Patagonia

A combined inversion of body wave receiver functions and Rayleigh wave phase velocities using a niching genetic algorithm (NGA) increases the unique- ness of the solution over separate inversions and also facilitates explicit parameteri- zation of layer thickness in the model space. This parameterization requires fewer layers and a priori constraints for modeling more complex structures than traditional linearized inversion. NGAs solve for a suite of locally optimal solutions in addition to isolating the globally optimal solution by using an evolutionary paradigm. This nonlinear examination of the error space provides an opportunity to examine trade- off within the model space. The method, when applied to synthetic data, locates interfaces within 2 km of the test structure and identifies appropriate structure and velocity. Applying this method to data from a deployment of five broadband stations in Chilean Patagonia yields a regional model of crustal structure that is consistent with the geological history of the region. Sediment thickness is inversely proportional to crustal thickness, with sediment thicknesses reaching more than 4 km, where the crustal thickness thins to 28 km. This is consistent with previous geological studies, which suggests that the Rocas Verdes basin in western Patagonia formed by crustal thinning, isostatic compensation, and subsequent sedimentation.

P and S velocity structure of the upper mantle beneath the Transantarctic Mountains, East Antarctic craton, and Ross Sea from travel time tomography

P and S wave travel times from teleseismic earthquakes recorded by the Transantarctic Mountains Seismic Experiment (TAMSEIS) have been used to tomographically image upper mantle structure beneath portions of the Transantarctic Mountains (TAM), the East Antarctic (EA) craton, and the West Antarctic rift system (WARS) in the vicinity of Ross Island, Antarctica. The TAM form a major tectonic boundary that divides the stable EA craton and the tectonically active WARS. Relative arrival times were determined using a multichannel cross-correlation technique on teleseismic P and S phases from earthquakes with mb ≥ 5.5. 3934 P waves were used from 322 events, and 2244 S waves were used from 168 events. Relative travel time residuals were inverted for upper mantle structure using VanDecars method. The P wave tomography model reveals a low-velocity anomaly in the upper mantle of approximately δVp = −1 to −1.5% in the vicinity of Ross Island extending laterally 50 to 100 km beneath the TAM from the coast, placing the contact between regions of fast and slow velocities well inland from the coast beneath the TAM. The magnitude of the low-velocity anomaly in the P wave model appears to diminish beneath the TAM to the north and south of Ross Island. The depth extent of the low-velocity anomaly is not well constrained, but it probably is confined to depths above ∼200 km. The S wave model, within resolution limits, is consistent with the P wave model. The low-velocity anomaly within the upper mantle can be attributed to a 200–300 K thermal anomaly, consistent with estimates obtained from seismic attenuation measurements. The presence of a thermal anomaly of this magnitude supports models invoking a thermal buoyancy contribution to flexurally driven TAM uplift, at least in the Ross Island region of the TAM. Because the magnitude of the anomaly to the north and south of Ross Island may diminish, the thermal contribution to the uplift of the TAM could be variable along strike, with the largest contribution in the Ross Island region. The tomography results reveal faster than average velocities beneath East Antarctica, as expected for cratonic upper mantle.