Jean-Bernard Minster

Data Citation and Peer Review

A scientific publication is fundamentally an argument consisting of a set of ideas and expectations supported by observations and calculations that serve as evidence of its veracity. An argument without evidence is only a set of assertions. Consider the difference between the statement “The hairy woodpecker population is declining in the northwest region of the United States” and the statement “Hairy woodpecker populations in the northwest region of the United States have declined by 11% between 1992 and 2003, according to data from the Institute for Bird Populations (http://www.birdpop.org/).” Both or neither of these statements could be true, but only the second one can be verified. Scientific papers do, of course, present specific data points as evidence for their arguments, but how well do papers guide readers to the body of those data, where the the datas integrity can be further examined? In practice, a chasm may lie across the path of a reviewer seeking the source data of a scientific argument.

TeraShake2: Spontaneous Rupture Simulations of Mw 7.7 Earthquakes on the Southern San Andreas Fault

Abstract Previous numerical simulations (TeraShake1) of large ( M w 7.7) southern San Andreas fault earthquakes predicted localized areas of strong amplification in the Los Angeles area associated with directivity and wave-guide effects from northwestward-propagating rupture scenarios. The TeraShake1 source was derived from inversions of the 2002 M w 7.9 Denali, Alaska, earthquake. That source was relatively smooth in its slip distribution and rupture characteristics, owing both to resolution limits of the inversions and simplifications imposed by the kinematic parameterization. New simulations (TeraShake2), with a more complex source derived from spontaneous rupture modeling with small-scale stress-drop heterogeneity, predict a similar spatial pattern of peak ground velocity (PGV), but with the PGV extremes decreased by factors of 2–3 relative to TeraShake1. The TeraShake2 source excites a less coherent wave field, with reduced along-strike directivity accompanied by streaks of elevated ground motion extending away from the fault trace. The source complexity entails abrupt changes in the direction and speed of rupture correlated to changes in slip-velocity amplitude and waveform, features that might prove challenging to capture in a purely kinematic parameterization. Despite the reduced PGV extremes, northwest-rupturing TeraShake2 simulations still predict entrainment by basin structure of a strong directivity pulse, with PGVs in Los Angeles and San Gabriel basins that are much higher than predicted by empirical methods. Significant areas of those basins have predicted PGV above the 2% probability of exceedance (POE) level relative to current attenuation relationships (even when the latter includes a site term to account for local sediment depth), and wave-guide focusing produces localized areas with PGV at roughly 0.1%–0.2% POE (about a factor of 4.5 above the median). In contrast, at rock sites in the 0–100-km distance range, the median TeraShake2 PGVs are in very close agreement with the median empirical prediction, and extremes nowhere reach the 2% POE level. The rock-site agreement lends credibility to some of our source-modeling assumptions, including overall stress-drop level and the manner in which we assigned dynamic parameters to represent the mechanical weakness of near-surface material. Future efforts should focus on validating and refining these findings, assessing their probabilities of occurrence relative to alternative rupture scenarios for the southern San Andreas fault, and incorporating them into seismic hazard estimation for southern California.

Multi-year monitoring of rift propagation on the Amery Ice Shelf, East Antarctica

We use satellite imagery from four sensors (Multi-angle Imaging SpectroRadiometer (MISR), Enhanced Thematic Mapper (ETM), and RADARSAT and ERS Synthetic Aperture Radar (SAR) to monitor the lengths of two rifts on the Amery Ice Shelf, from 1996 to 2004. We find that the rifts have each been propagating at a steady annual rate for the past 5 years. Superimposed on this steady rate is a seasonal signal, where propagation rates are significantly higher in the summer period (i.e., September–April) than in the winter period (i.e., April–September). Possible causes of this summer-winter effect are changing properties of the ice melange, which fills the rifts, and seasonal changes in ocean circulation beneath the ice shelf

An airborne scanning laser altimetry survey of Long Valley, California

Between 28 September and 7 October 1995, we conducted an airborne laser altimetry experiment over the Long Valley caldera, California, in which each of two scanning laser altimeters (dubbed SLICER and RASCAL) were flown in a NASA T-39 jet aircraft. Operating concurrently were a Global Positioning System (GPS) guidance system and dual frequency receivers for precise navigation and post-flight calculation of the airplane trajectory relative to a ground station, and an inertial navigation system (INS) for attitude determination. Reduction of raw laser ranges requires merging the differential kinematic GPS aircraft trajectory and the INS data with the laser data, and determination of the atmospheric delay. Data geolocation consists of obtaining the centre location and the mean elevation within each footprint in a geodetic coordinate system. The elevation of Crowley Lake is recovered to an accuracy of ∼3 cm or better from 3 km above ground level and crossover analysis indicates that the elevation estimates are consistent from pass to pass. We test our geolocation procedures by comparing laser-derived elevations with those determined in situ for recognizable ground features. A comparison of laser and GPS-derived positions shows that the horizontal accuracy is better than the diameter of the footprint and vertical accuracy is within the error inherent in the range measurement. A comparison of SLICER elevation data with digital elevation models (DEMs) of the region shows that the DEM data provides surface topography to within stated accuracy limits. Although research continues to utilize the full potential of laser altimetry data, our results constitute a successful demonstration that the technique may be used to perform geodetic monitoring of surface topographic changes.

Near real-time radar interferometry of the Mw 7.1 Hector Mine Earthquake

The Hector Mine Earthquake (Mw 7.1, 16 Oc- tober 1999) ruptured 45 km of previously mapped and un- mapped faults in the Mojave Desert. The ERS-2 satellite imagedtheMojaveDeserton15Septemberandagain on20 October, just 4 days after the earthquake. Using a newly- developed ground station we acquired both passes and were able to form an interferogram within 20 hours of the second overflight. Estimates of slip along the main rupture are 1-2 meters greater than slip derived from geological mapping. The gradient of the interferometric phase reveals an inter- esting pattern of triggered slip on adjacent faults as well as a 30 mm deep sink hole along Interstate 40.

An investigation into the forces that drive ice-shelf rift propagation on the Amery Ice Shelf, East Antarctica

For three field seasons (2002/03, 2004/05, 2005/06) we have deployed a network of GPS receivers and seismometers around the tip of a propagating rift on the Amery Ice Shelf, East Antarctica. During these campaigns we detected seven bursts of episodic rift propagation. To determine whether these rift propagation events were triggered by short-term environmental forcings, we analyzed simultaneous ancillary data such as wind speeds, tidal amplitudes and sea-ice fraction (a proxy variable for ocean swell). We find that none of these environmental forcings, separately or together, correlated with rift propagation. This apparent insensitivity of ice-shelf rift propagation to short-term environ- mental forcings leads us to suggest that the rifting process is primarily driven by the internal glaciological stress. Our hypothesis is supported by order-of-magnitude calculations that the glaciological stress is the dominant term in the force balance. However, our calculations also indicate that as the ice shelf thins or the rift system matures and iceberg detachment becomes imminent, short- term stresses due to winds and ocean swell may become more important.

Seismicity and deformation associated with ice-shelf rift propagation

Previous observations have shown that rift propagation on the Amery Ice Shelf (AIS), East Antarctica, is episodic, occurring in bursts of several hours with typical recurrence times of several weeks. Propagation events were deduced from seismic swarms (detected with seismometers) concurrent with rapid rift widening (detected with GPS receivers). In this study, we extend these results by deploying seismometers and GPS receivers in a dense network around the tip of a propagating rift on the AIS over three field seasons (2002/03, 2004/05 and 2005/06). The pattern of seismic event locations shows that icequakes cluster along the rift axis, extending several kilometers back from where the rift tip was visible in the field. Patterns of icequake event locations also appear aligned with the ice-shelf flow direction, along transverse-to-rift crevasses. However, we found some key differences in the seismicity between field seasons. Both the number of swarms and the number of events within each swarm decreased during the final field season. The timing of the slowdown closely corresponds to the rift tip entering a suture zone, formed where two ice streams merge upstream. Beneath the suture zone lies a thick band of marine ice. We propose two hypotheses for the observed slowdown: (1) defects within the ice in the suture zone cause a reduction in stress concentration ahead of the rift tip; (2) increased marine ice thickness in the rift path slows propagation. We show that the size-frequency distribution of icequakes approximately follows a power law, similar to the well-known Gutenberg-Richter law for earthquakes. However, large icequakes are not preceded by foreshocks nor are they followed by aftershocks. Thus rift-related seismicity differs from the classic foreshock and aftershock distribution that is characteristic of large earth quakes.

Modeling the topography of the salar de Uyuni, Bolivia, as an equipotential surface of Earth's gravity field

The salar de Uyuni is a massive dry salt lake that lies at the lowest point of an internal drainage basin in the Bolivian Altiplano. A kinematic GPS survey of the salar in September 2002 found a topographic range of only 80 cm over a 54 × 45 km area and subtle surface features that appeared to correlate with mapped gravity. In order to confirm the correlation between topography and gravity/geopotential, we use local gravity measurements and the EGM96 global geopotential model to construct a centimeter-level equipotential surface corresponding to the elevation of the salar. Our comparison of GPS survey elevations with the equipotential surface estimate shows that 63% of the variance of the GPS elevations can be explained by equipotential surface undulations (and long-wavelength error) in the EGM96 model alone, with an additional 30% explained by the shorter-wavelength equipotential surface derived from local gravity. In order to establish a physical connection between topography and the geopotential, we also develop and test a simple surface process model that redistributes salt via the dissolution, transport, and redeposition of salt by precipitated water. Forcing within the model pushes the system to evolve toward constant water depth, with the salt surface approximating the shape of the local equipotential surface. Since the model removes almost all topographic relief with respect to the equipotential surface within a matter of decades, it appears that observed (~5 cm amplitude, ~5 km wavelength) residual topography is actively maintained by a process independent of gravity-driven fluid flow.

Model for nonlinear wave propagation derived from rock hysteresis measurements

We develop a method for modeling nonlinear wave propagation in rock at intermediate strain levels, that is, strain levels great enough that nonlinearity cannot be neglected but low enough that the rock does not incur macroscopic damage. The constitutive model is formulated using a singular-kernel endochronic formalism which satisfies a number of observational constraints, including producing a power law dependence of attenuation (Q−1) on strain amplitude. One free parameter represents cubic anharmonicity, and we set it to agree with laboratory observations of harmonic distortion. Another free parameter controls the amount of hysteresis; it is set to approximate laboratory stress-strain curves. The resulting phenomenological model provides a convenient means to parameterize laboratory observations in a form suitable for efficient wave propagation calculations. We solve one-dimensional wave propagation problems for this constitutive model using both finite difference and pseudospectral methods. Quasi-harmonic wave propagation in the Berea sandstone model shows several departures from results obtained with nonlinear elasticity: (1) more rapid decay with distance of the fundamental frequency component due to nonlinear, amplitude-dependent attenuation; (2) enhanced excitation of the third-order harmonic, in agreement with laboratory observations, and saturation, with propagation distance, of the harmonics. This behavior reflects competing effects of harmonic amplitude growth via nonlinear energy transfer from the source frequency and amplitude-dependent energy dissipation due to hysteresis. We also find that a two-frequency source function generates harmonics with frequencies which can be expressed as linear combinations of integer multiples of the source frequencies, in agreement with published laboratory results.

Rapid Determination of Near‐Fault Earthquake Deformation Using Differential LiDAR

Improved near‐field measurements of earthquake slip and deformation patterns have the potential for expanding our understanding of fault behavior and the relationship of active faulting to topography. Current techniques for obtaining these measurements—including field observation, Global Navigation Satellite Systems displacement estimation, and optical or radar remote sensing—have limitations that can be mitigated by the inclusion of results from differential airborne Light Detection and Ranging (LiDAR) analysis of the rupture zone. The 2005 airborne LiDAR survey of the southern San Andreas, San Jacinto, and Banning faults (the B4 survey) mapped 1100 km of the most seismically active fault systems in southern California for the purpose of providing a baseline for determining slip from a future earthquake. We used the B4 survey to develop a processing algorithm that yields rapid estimates of near‐fault ground deformation using simultaneous cross correlation of both topography and backscatter intensity from pre‐earthquake and simulated postearthquake LiDAR datasets. We show robust recovery of the direction and magnitude of an applied synthetic slip of 5 m in the horizontal and 0.5 m in the vertical within our area of study, with clear discrimination between areas with and without applied slip. We also successfully recovered more complex deformation from a modeled fault stepover in the same study area. Our results indicate that we should be able to recover slip to accuracies of better than 20 cm in the horizontal and 1 cm in the vertical, at a spatial resolution of ≤15 m for LiDAR datasets with sample densities as low as 0.5 points/m2.