John Nábělek

Underplating in the Himalaya-Tibet Collision Zone Revealed by the Hi-CLIMB Experiment

Himalayan-Tibetan Underplate The Himalayas formed from the collision of India with Eurasia beginning about 50 million years ago, but the fate and position of the subducted Indian crust was not well defined until the Hi-CLIMB seismic experiment was initiated. The centerpiece of the project is an 800-kilometer-long, closely spaced, linear array of broadband seismographs, extending from the Ganges lowland, across the Himalayas, and onto the central Tibetan plateau. Nábělek et al. (p. 1371) present images of the crust and upper mantle of the Southern Tibetan plateau underthrust northward by the Indian plate, in which they trace the base of the Indian plate to 31°N. The character of the crust-mantle interface in this region suggests that the Indian crust is at least partly decoupled from the mantle beneath. A seismic study delineates the position and local thickening of the Indian plate underlying the Himalayas and southern Tibet. We studied the formation of the Himalayan mountain range and the Tibetan Plateau by investigating their lithospheric structure. Using an 800-kilometer-long, densely spaced seismic array, we have constructed an image of the crust and upper mantle beneath the Himalayas and the southern Tibetan Plateau. The image reveals in a continuous fashion the Main Himalayan thrust fault as it extends from a shallow depth under Nepal to the mid-crust under southern Tibet. Indian crust can be traced to 31°N. The crust/mantle interface beneath Tibet is anisotropic, indicating shearing during its formation. The dipping mantle fabric suggests that the Indian mantle is subducting in a diffuse fashion along several evolving subparallel structures.

Spectral analysis of seismic noise induced by rivers: A new tool to monitor spatiotemporal changes in stream hydrodynamics

to 20 dB (relative to (m/s) 2 /Hz) for all the stations located along a steep 30-km-long narrow and deeply incised channel of the Trisuli River, a major trans-Himalayan river. The early summer increase in high-frequency energy is modulated by a 24-h periodicity where the minimum of seismic noise level is reached around noon and the maximum is reached late in the evening. A detailed study of seismic noise amplitude reveals a clear correlation with both regional meteorological and hydrological data along the Trisuli River. Seasonal increase in ambient noise coincides with the strong monsoon rainfall and a period of rapid melting of snow and ice in the high elevations. The observed 24-h cyclicity is consistent with the daily fluctuation of the precipitation and river discharge in the region. River-induced seismic noise is partly generated by stream turbulence, but this mechanism fails to explain the observed clockwise hysteresis of seismic noise amplitude versus water level. This pattern is better explained if a significant part of the observed seismic noise is caused by ground vibrations generated by bed load transport. This points out the potential of using background seismic noise to quantify in continuous river bed load and monitor its spatial variations, which remain difficult with classical approaches.

Moment-tensor analysis using regional data: application to the 25 March, 1993, Scotts Mills, Oregon, earthquake

In this paper we outline a procedure we use for routine moment-tensor analysis of regional data from broadband seismic stations in northwestern North America and apply it to the mo- ment magnitude 5.5, March, 1993, Scotts Mills, Oregon, earth- quake. The results compare favorably with those obtained from teleseismic data. We found that the earthquake occurred at a depth of 13-15 km and had a mechanism with approximately equal amounts of reverse and right-lateral strike-slip components. The estimated stress drop of 40 bar is average on a world-wide basis, supporting the view that the rather large damage was caused primarily by poor construction and not by exceptional properties of the source. The Scotts Mills earthquake is most likely related to the Mr. Angel Fault. This fault is a part of the Gales Creek-Mr. Angel structural lineament (GCMAL) extending about 150 km across the Willamette Valley. At present data are not sufficient to estimate the likelihood of an earthquake involv- ing the entire GCMAL, but given its length an earthquake of magnitude 7 is conceivable. The results of this study, together with investigations of other earthquakes, suggest that sparse broadband networks can be used efficiently for determining source parameters of earthquakes of magnitude greater than 4.0 in regions with infrequent seismicity.

Tectonic Model of the Pacific-North American Plate Boundary in the Gulf of Alaska from Broadband Analysis of the 1979 St. Elias, Alaska, Earthquake and its Aftershocks

The St. Elias, Alaska earthquake of 28 February, 1979 (Ms 7.2) is reanalyzed using broadband teleseismic body waves and long-period surface waves because of unresolved questions about its depth, focal mechanism, seismic moment, and location in a seismic gap. Teleseismic waveforms are simultaneously inverted to determine the source mechanism, seismic moment, rupture history and centroid depth. These data are well modeled with a point source propagating in the ESE direction with an average kinematic rupture velocity of 2.5 km/s. The best-fitting source mechanism indicates underthrusting on a NE-dipping plane. The mainshock depth of 24 km and the depth of aftershocks determined from inversions are consistent with locations on the gently dipping main thrust of the Pacific-North American plate boundary. These depths are substantially different from those of earlier body wave studies and regional seismic network aftershock depth determinations but are in accord with the Harvard Centroid-Moment Tensor and International Seismological Centre determinations. The seismic moment determined from body waves is 9.4×1019 N-m (Mw 7.3). The spatial and temporal distribution of moment release indicates that the St. Elias earthquake was a complex rupture consisting of two distinct subevents within 38 s of the initial onset, followed by low moment release during the next 34 s. Earlier studies indicated an unusual amount of surface wave energy at very long periods (> 200 s) that led some workers to suggest that St. Elias was a “slow” earthquake. Our broadband modeling does not require more than 34 s of additional moment release after the first two subevents. Moreover, we are able to match the phase and amplitude of 200-s Love and Rayleigh waves with a thrust fault point source of moment 1.3×1020 N-m (Mw 7.4) located at the body wave centroid. The moment difference is not discernible with body waves for moment evenly distributed over 72 s. Thus, the St. Elias earthquake is not slow with respect to 200-s surface waves but is complex with regard to the broadband body waves. Upper plate structure apparently controlled the gross characteristics of rupture. The rupture direction parallels mapped upper plate faults. Rupture propagated unilaterally to the ESE, with little initial moment release, as a shallow, north-dipping thrust that later changed to more steeply NE dipping with a large right-lateral strike-slip component. The locations and source mechanisms of these subevents and locations of aftershocks define a shallow dipping surface at the eastern edge of the Pacific plate. Moreover, the component of strike-slip motion increases with time in the mainshock implying that the transition to strike-slip faulting occurs along the plate interface. The estimated nucleation point of the second subevent coincides with a large concentration of aftershocks interpreted as representing a barrier to continuous rupture associated with the northern-most boundary of the Yakutat terrane. Joint relocation of aftershocks suggests that the main plate boundary may be offset vertically by 5–10 km as a result of this structure. The southern part of the aftershock zone, while containing many aftershocks, appears not to have ruptured coseismically, but may have failed later by aseismic creep as seen in geodetic measurements. Faults associated with the Malaspina fault system (the onshore extension of the Aleutian trench) appear to be the surface expression of the underthrusting plate boundary; however, upper plate deformation is widespread because of the collision of the Yakutat terrane. The convergence direction may explain the lack of a highly active Wadati-Benioff zone downdip of the St. Elias zone. The neotectonic deformation of the Chugach-St. Elias mountains is probably related to collision and subduction of the Yakutat terrane: A terrane in the process of accreting and subducting will cause considerable upper plate deformation over a wide zone. Once subduction of a terrane has begun, deformation may then become localized.

Geometry of continental normal faults: Seismological constraints

Teleseismic body waves from large earthquakes are used to study the downdip geometry of continental normal faults in the Aegean. Waveform modeling techniques together with rigorous statistical tests are applied to put firm bounds on the amount of downdip curvature of these faults and the role of coseismic slip on a basal detachment. Synthetic modeling shows that good azimuthal station coverage and inclusion of SH waves are necessary to resolve fault curvature. The data indicate ruptures of the Aegean events occurred on planar faults extending across the entire brittle portion of the crust. No seismogenic low-angle detachment faulting at the base of the upper crust was detected for these events. Decoupling of the brittle upper crust from the plastic lower crust probably occurs aseismically in a ductile fashion.

The 1993 Klamath Falls, Oregon, earthquake sequence: Source mechanisms from regional data

We use regional broadband seismograms to obtain seismic moment-tensor solutions of the two September 20, 1993, Mw=6, Klamath Falls, Oregon earthquakes, their foreshock and largest aftershocks (My>3.5). Several sub-groups with internally consistent solutions indicate activity on several fault segments and faults. From the estimated moment-tensors and depths of the main shocks and from the aftershock distribution we deduce that both main shocks occurred on an east-dipping normal fault, pos- sibly related to the Lake of the Woods fault system. Rotation of T-axes between the two main shocks is consistent with the two dominant trends of the aftershocks and mapped faults. We pro- pose that a change in fault strike acted as temporary barrier sepa- rating the rupture of the main shocks. Empirical Greens function analysis shows that the first main event had a longer rupture du- ration (half-duration 1.7 s) than the second (1.2 s). In December, vigorous shallow activity commenced near Klamath Lakes west- ern shore, 5-10 km east of the primary aftershock zone. It ap- pears a Mw=5.5 aftershock occurring the day before, though within the primary aftershock zone, triggered the activity.

Seismicity and fault interaction, Southern San Jacinto Fault Zone and adjacent faults, southern California: Implications for seismic hazard

The southern San Jacinto fault zone is characterized by high seismicity and a complex fault pattern that offers an excellent setting for investigating interactions between distinct faults. This fault zone is roughly outlined by two subparallel master fault strands, the Coyote Creek and Clark-San Felipe Hills faults, that are located 2 to 10 km apart and are intersected by a series of secondary cross faults. Seismicity is intense on both master faults and secondary cross faults in the southern San Jacinto fault zone. The seismicity on the two master strands occurs primarily below 10 km; the upper 10 km of the master faults are now mostly quiescent and appear to rupture mainly or solely in large earthquakes. Our results also indicate that a considerable portion of recent background activity near the April 9, 1968, Borrego Mountain rupture zone (M_L=6.4) is located on secondary faults outside the fault zone. We name and describe the Palm Wash fault, a very active secondary structure located about 25 km northeast of Borrego Mountain that is oriented subparallel to the San Jacinto fault system, dips approximately 70° to the northeast, and accommodates right-lateral shear motion. The Vallecito Mountain cluster is another secondary feature delineated by the recent seismicity and is characterized by swarming activity prior to nearby large events on the master strand. The 1968 Borrego Mountain and the April 28, 1969, Coyote Mountain (M_L=5.8) events are examples of earthquakes with aftershocks and subevents on these secondary and master faults. Mechanisms from those earthquakes and recent seismic data for the period 1981 to 1986 are not simply restricted to strike-slip motion; dipslip motion is also indicated. Teleseismic body waves (long-period P and SH) of the 1968 and 1969 earthquakes were inverted simultaneously for source mechanism, seismic moment, rupture history, and centroid depth. The complicated waveforms of the 1968 event (M_o=1.2 × 10^(19) Nm) are interpreted in terms of two subevents; the first caused by right-lateral strike-slip motion in the mainshock along the Coyote Creek fault and the second by a rupture located about 25 km away from the master fault. Our waveform inversion of the 1969 event indicates that strike-slip motion predominated, releasing a seismic moment of 2.5 × 10^(17) Nm. Nevertheless, the right-lateral nodal plane of the focal mechanism is significantly misoriented (20°) with respect to the master fault, and hence the event is not likely to be associated with a rupture on that fault. From this and other examples in southern California, we conclude that cross faults may contribute significantly to seismic hazard and that interaction between faults has important implications for earthquake prediction.

Mantle dynamics beneath the discrete and diffuse plate boundaries of the Juan de Fuca plate: Results from Cascadia Initiative body wave tomography

We use the delay times of teleseismic S phases recorded by ocean bottom seismometers during the plate-scale Cascadia Initiative community experiment to constrain the heterogeneity of seismic velocity structure beneath young oceanic lithosphere. Our study area covers the entire Juan de Fuca (JdF) and Gorda plates, from their creation at the JdF and Gorda Ridges to their subduction beneath the North American continent, and the entire length of the Blanco transform fault. The range of the observed Vs anomalies requires variations in the melt fraction of the asthenosphere. The data require that low Vs anomalies extend to depths of at least 200 km, which is within the carbonatite melting regime. In the upper 200 km of the mantle, Vs increases rapidly to the east of the JdF Ridge, while there is no clear relationship with the age of the lithosphere in the Gorda region. The distribution of melt is asymmetric about both the JdF and Gorda Ridges. Dynamic upwelling – due to the buoyancy of the mantle – and accompanying downwelling can explain the rapid decrease in melt fraction to the east of the JdF Ridge, the asymmetry about the JdF Ridge, and the sinuous pattern of upwelling near the Blanco transform fault. Finally, mantle flow beneath the diffuse Gorda and Explorer plate boundaries is distinct from that beneath the discrete plate boundary of the JdF Ridge. In particular, shear between the Pacific and JdF plates appears to dominate mantle deformation over seafloor spreading beneath the Gorda Ridge.