Paul M. Davis

Cenozoic climate change as a possible cause for the rise of the Andes

Causal links between the rise of a large mountain range and climate have often been considered to work in one direction, with significant uplift provoking climate change. Here we propose a mechanism by which Cenozoic climate change could have caused the rise of the Andes. Based on considerations of the force balance in the South American lithosphere, we suggest that the height of, and tectonics in, the Andes are strongly controlled both by shear stresses along the plate interface in the subduction zone and by buoyancy stress contrasts between the trench and highlands, and shear stresses in the subduction zone depend on the amount of subducted sediments. We propose that the dynamics of subduction and mountain-building in this region are controlled by the processes of erosion and sediment deposition, and ultimately climate. In central South America, climate-controlled sediment starvation would then cause high shear stress, focusing the plate boundary stresses that support the high Andes.

SKS splitting beneath continental rift zones

We present measurements of SKS splitting at 28 digital seismic stations and 35 analog stations in the Baikal rift zone, Siberia, and adjacent areas, and at 17 stations in the East African Rift in Kenya and compare them with previous measurements from the Rio Grande Rift of North America. Fast directions in the inner region of the Baikal rift zone are distributed in two orthogonal directions, NE and NW, approximately parallel and perpendicular to the NE strike of the rift. In the adjacent Siberian platform and northern Mongolian fold belt, only the rift-orthogonal fast direction is observed. In southcentral Mongolia, the dominant fast direction changes to rift-parallel again, although a small number of measurements are still rift-orthogonal. For the axial zones of the East African and Rio Grande Rifts, fast directions are oriented on average NNE, that is, rotated clockwise from the N-S trending rift. All three rifts are underlain by low-velocity upper mantle as determined from teleseismic tomography. Rift-related mantle flow provides a plausible interpretation for the rift-orthogonal fast directions. The rift-parallel fast directions near the rift axes can be interpreted by oriented magmatic cracks in the mantle or small-scale mantle convection with rift-parallel flow. The agreement between stress estimates and corresponding crack orientations lends some weight to the suggestion that the rift-parallel fast directions are caused by oriented magmatic cracks.

Earthquake and Ambient Vibration Monitoring of the Steel-Frame UCLA Factor Building

Dynamic property measurements of the moment-resisting steel-frame University of California, Los Angeles, Factor building are being made to assess how forces are distributed over the building. Fourier amplitude spectra have been calculated from several intervals of ambient vibrations, a 24-hour period of strong winds, and from the 28 March 2003 Encino, California ( ML =2.9), the 3 September 2002 Yorba Linda, California ( ML =4.7), and the 3 November 2002 Central Alaska ( Mw =7.9) earthquakes. Measurements made from the ambient vibration records show that the first-mode frequency of horizontal vibration is between 0.55 and 0.6 Hz. The second horizontal mode has a frequency between 1.6 and 1.9 Hz. In contrast, the first-mode frequencies measured from earthquake data are about 0.05 to 0.1 Hz lower than those corresponding to ambient vibration recordings indicating softening of the soil-structure system as amplitudes become larger. The frequencies revert to pre-earthquake levels within five minutes of the Yorba Linda earthquake. Shaking due to strong winds that occurred during the Encino earthquake dominates the frequency decrease, which correlates in time with the duration of the strong winds. The first shear wave recorded from the Encino and Yorba Linda earthquakes takes about 0.4 sec to travel up the 17-story building.

Asymmetric upwarp of the asthenosphere beneath the Baikal rift zone, Siberia

In the summer of 1991 we installed 27 seismic stations about lake Baikal, Siberia, aimed at obtaining accurately timed digital seismic data to investigate the deep structure and geodynamics of the Baikal rift zone and adjacent regions. Sixty-six teleseismic events with high signal-to-noise ratio were recorded. Travel time and Q analysis of teleseisms characterize an upwarp of the lithosphere-asthenosphere boundary under Baikal. Theoretical arrival times were calculated by using the International Association of Seismology and Physics of the Earths interior 1991 Earth model, and travel time residuals were found by subtracting computed arrival times from observed ones. A three-dimensional downward projection inversion method is used to invert the P wave velocity structure with constraints from deep seismic sounding data. Our results suggest that (1) the lithosphere-asthenosphere transition upwarps beneath the rift zone, (2) the upwarp has an asymmetric shape, (3) the velocity contrast is −4.9% in the asthenosphere, (4) the density contrast is −0.6%, and (5) the P wave attenuation contrast t* is 0.1 s.

Fault systems of the 1971 San Fernando and 1994 Northridge earthquakes, southern California: Relocated aftershocks and seismic images from LARSE II

We have constructed a composite image of the fault systems of the M 6.7 San Fernando (1971) and Northridge (1994), California, earthquakes, using industry reflection and oil test well data in the upper few kilometers of the crust, relocated aftershocks in the seismogenic crust, and LARSE II (Los Angeles Region Seismic Experiment, Phase II) reflection data in the middle and lower crust. In this image, the San Fernando fault system appears to consist of a decollement that extends 50 km northward at a dip of ∼25° from near the surface at the Northridge Hills fault, in the northern San Fernando Valley, to the San Andreas fault in the middle to lower crust. It follows a prominent aseismic reflective zone below and northward of the main-shock hypocenter. Interpreted upward splays off this decollement include the Mission Hills and San Gabriel faults and the two main rupture planes of the San Fernando earthquake, which appear to divide the hanging wall into shingle- or wedge-like blocks. In contrast, the fault system for the Northridge earthquake appears simple, at least east of the LARSE II transect, consisting of a fault that extends 20 km southward at a dip of ∼33° from ∼7 km depth beneath the Santa Susana Mountains, where it abuts the interpreted San Fernando decollement, to ∼20 km depth beneath the Santa Monica Mountains. It follows a weak aseismic reflective zone below and southward of the main-shock hypocenter. The middle crustal reflective zone along the interpreted San Fernando decollement appears similar to a reflective zone imaged beneath the San Gabriel Mountains along the LARSE I transect, to the east, in that it appears to connect major reverse or thrust faults in the Los Angeles region to the San Andreas fault. However, it differs in having a moderate versus a gentle dip and in containing no mid-crustal bright reflections.

Models of ground deformation from vertical volcanic conduits with application to eruptions of Mount St. Helens and Mount Etna

We present simple analytical models of ground deformation from inflation of vertical volcanic conduits or pipes. We compare three cases corresponding to (1) a pressurized pipe with closed end at the top that resists the internal pressure, (2) a pressurized pipe with top open for which the internal deformation is mainly dislocation of the cylindrical walls, and (3) a pipe-shaped region that dilates. For the closed pipe we use Eshelbys inclusion theory to model deformation from a thin, pressurized, elongated, prolate ellipsoid in a full-space, which we express in terms of double forces. To satisfy surface boundary conditions, we superimpose image solutions by using double forces derived from Mindlins half-space solution for the point force. For the open pipe we apply the Volterra integral to dislocation across a cylindrical surface and generalize it to the half-space using Mindlins point force solution. These two solutions show marked differences from the line of dilatation solution. Ratios of maximum horizontal deformation to maximum vertical deformation for the pipe models are significantly greater than values for the center or line of dilatation models. This, and the more gradual fall off of the deformation with distance may be used as diagnostics for discriminating between pipe-like sources and dilatational sources on volcanoes, where both components of the deformation field are available. As examples, we compare the closed pipe model with deformation associated with dome building on Mount St. Helens volcano, and tilt predicted by the open pipe model with tilt measured during a period of explosive eruptive activity on Mount Etna, Sicily, in 1995. Comparison between model values and the measurements suggests that the effective elastic moduli of the volcanic cones are very low.

Geodetic analysis of dike intrusion and motion of the magma reservoir beneath the summit of Kilauea Volcano, Hawaii: 1970–1985

We use leveling and trilateration data collected on Kilauea volcano to constrain the location of deformation sources caused by magma accumulation, intrusion, and eruption. For the 13 inflationary epochs examined, combinations of an expanding point source and one or two opening rectangular dislocations mimic inflation of the summit reservoir and formation of dike(s), respectively. The combined model adequately accounts for the deformation data and is consistent with the seismicity observed during each epoch. For 10 deflationary epochs, however, the data require only a contracting point source. Confidence in these results is gained by noting that locations of the sources of both inflation and deflation are coincident, within the observed uncertainties of the data, the function of network geometry, and the inversion procedure. It appears, therefore, that magma accumulation at Kilauea volcano may be characterized by the growth of dikes during inflation of the summit reservoir. Drainage of the reservoir, on the other hand, is not accompanied by significant closure of dikes. In contrast to previous studies (e.g., Fiske and Kinoshita, 1969; Dvorak et al., 1983) that do not include the dislocation (or dike growth) component of summit magma accumulation and concluded that the source of inflation migrates over a 5 km2 area, we find that a single magmatic reservoir source accounts for data collected during all inflationary and deflationary epochs, results, which compare favorably with those obtained from the point ellipsoid model, can be used to estimate the distribution of stresses within the volcano in the near field of the source.

SKS splitting beneath southern California

Measurements of SKS phase splitting were obtained from nineteen seismic stations in southern California. The fast polarization directions are 53° at the southern end of the Great Valley, 82±8° in the western Transverse Ranges and northern Peninsular Ranges, 95±4° in Mojave Desert, and 70° on San Clemente Island. The splitting time ranges from 0.8 to 1.8 seconds, which is consistent with an anisotropic layer of 100 to 200 km thick for 4% anisotropy.

ATTENUATION AND VELOCITY OF P-WAVES IN THE MANTLE BENEATH THE EAST-AFRICAN RIFT, KENYA

Abstract We present the results of mantle P velocity and attenuation tomography from the combined KRISP 1985 and KRISP 1990 teleseismic data sets. We incorporate the results from the KRISP 1990 refraction survey to remove crustal effects from the teleseismic data and thereby better image the lithospheric and asthenospheric structure in the mantle beneath the rift. We find a broad 6% low-velocity anomaly that extends beyond the limits of the rift, as defined by its bounding faults, and superimposed on this a narrower 6% low-velocity zone that is confined to the rift zone. The latter anomaly has a total contrast of 12% compared to nearby unrifted cratonic lithosphere The broad structure persists at all depths to 165 km, and is aligned with the rift, while the narrower structure varies with depth and does not extend along the entire length of the rift covered by our array. The attenuation signal is noisier than the travel time signal, but shows the greatest attenuation beneath the rift and at the southern and northern ends of our array.

Comparison of shear wave splitting and finite strain from the India‐Asia collision zone

We investigate whether observations of shear wave splitting in Asia may be related to distributed strain of the lithosphere or to strain caused by motion of the Asian lithosphere over the deep mantle. The HS2-NUVEL angular velocity for Eurasia, and that of Minster and Jordan [1978], predict “absolute” plate motions that are a factor of at least 4 slower than the NNE relative motion of India with respect to stable Eurasia. Shear of the upper mantle beneath the south Asian lithosphere is therefore likely to be dominated by the velocity field caused by the internal deformation of Asia, rather than by plate motion. If that shear causes SKS splitting, fast polarization directions in tectonic Asia should be aligned approximately in the direction of the motion of Asian lithosphere relative to the deep mantle, which for southern Asia is NNE. In general, however, fast polarization directions are not aligned with this direction, and in some cases are nearly orthogonal to it. We calculate finite deformation of the Asian lithosphere, in response to its indentation by India and, from the deformation, predict the orientations of fast polarization directions aligned with the principal axes of elongation. These predictions agree with the gross features of the anisotropy in Tibet and the Tien Shan. Orientations of anisotropy in the Baikal region of Siberia are consistent with the orientations of the local principal axes of active strain. Orientations of anisotropy in Mongolia, however, are inconsistent both with the azimuth of the velocity of this region with respect to the deeper mantle and with the orientation of the calculated maximum elongation of the region. We conclude that SKS splitting observations from Asia agree better with those predicted by calculations of internal lithospheric deformation due to indentation by India than with strain predicted from motion of the lithosphere over the deeper mantle. These results place constraints on the depth range where shear wave splitting occurs, which we propose lies predominantly within the lithospheric mantle.