Vincent S. Cronin

Types and kinematic stability of triple junctions

A triple junction is kinematically stable if the orientation of each plate boundary remains constant relative to other boundaries in the triple junction during a finite time-interval. Instantaneous relative-velocity vectors have been used elsewhere to indicate the relative motion of plates during a finite time-interval — a technique that is not generally valid where finite relative plate motion is not circular. In the typical case in which all three plates have non-zero velocities around different plate-specific poles of rotation, the direction and magnitude of relative plate velocity varies systematically with time. Five plate-boundary types are considered in evaluating triple-junction stability: ridges (R), right-lateral and left-lateral transform faults (FR and FL) and trenches in which the overriding plate is clockwise (TC) or anticlockwise (TA) from the trench, judged by a rotation around the triple junction. One hundred twenty five triplet combinations of boundary types are possible. Some combinations display symmetry with other combinations. Grouping all similar configurations, 25 types of triple junctions can be distinguished, of which 19 types may be stable when finite relative motion is circular. Vectorial descriptors of the geometry of ridges, transform faults and trenches permit the listing of general conditions for the kinematic stability of triple junctions, under conditions of both circular and non-circular relative motion. The TJ1 model for the evolution of RRR triple junctions provides estimates of the variation in the geometry of triple junctions in which spreading is orthogonal and near-symmetric. RRR triple junctions at which spreading is orthogonal and symmetric are always stable when the finite relative motion of all corresponding plates is circular; however, TJ1 indicates that RRR triple junctions are not generally stable when relative motion is non-circular. In general, triple junctions are not kinematically stable, but evolve with changes in the finite relative motion of plates.

The cycloid relative-motion model and the kinematics of transform faulting

Abstract Adequate models of instantaneous plate tectonics currently exist; however, a model to describe the continuous relative motion of lithospheric plates during a finite time interval has been necessary. The small-circle relative-motion model suggests that a particle on a given plate follows a circular path around a pole of relative motion, as viewed from another plate during finite relative displacement. It can be shown that the small-circle model is not generally valid in describing the finite relative motion of plates, so this motion must trace a curve that is more complex than a circle. A new approach has been developed that permits improved modeling of finite relative plate motion. The first-order cycloid relative-motion model (CYC1) is based upon the idea that each plate is in motion around a plate-specific pole (elsewhere termed an “absolute” or “no-net-torque” pole). The observed relative motion between two plates is the result of the two plate-specific motions. Combining the two rotations, a point on one plate moves along a cycloid figure of rotation around two axes as viewed from another plate. A cycloid (a two-axis curve) is the simplest spherical curve that is more complex than a circle (a one-axis curve). The mathematical expression of cycloid relative motion involves a series of coordinate transformations. The small-circle model is a subset of the cycloid model. CYC1 indicates that significant variations in the direction and velocity of relative motion can occur even when the governing axial positions and angular velocities are constant. Hence, a variety of boundary interactions are possible during finite relative displacements that would not have been predicted by a small-circle model of finite relative plate motion. The cycloid model indicates that transform faults are not simply lines of pure slip along which crust is conserved. Systematic changes in the amount of convergence or divergence along the length of a given transform fault are predicted. This gradient is probably reflected in local boundary deformation, and is predicted to be a factor controlling the length of a given fault segment. Transform faults are generally in motion relative to the corresponding relative pole and plate-specific poles. The relative velocity along a given transform fault and the spreading rate across the adjacent ridge segments vary as the radius from the fault to its relative pole changes. The curvature of a given transform fault is also predicted to change with finite displacement, as the fault moves with respect to its relative pole. Motion of a given transform fault with respect to its relative pole and to the plates that it bounds suggests that the traces of the resulting fracture zones should be complex curves that are not symmetrical across the ridge axis. Improved modeling of finite relative plate motion should enhance understanding of the structural, stratigraphic, magmatic, and metamorphic evolution of plate margins.

Relocation analysis of earthquakes near Nanga Parbat‐Haramosh Massif, northwest Himalaya, Pakistan

Locations of earthquakes that have occurred between 1970 and 1988 in the northwest Himalaya of Pakistan, as computed by the ISC, were analyzed to improve knowledge of the spatial distribution of seismicity near the Nanga Parbat-Haramosh Massif (NPHM). A set of 15 earthquakes were relocated using the multiple-event relocation technique of Jordan and Sverdrup [1981]. Earthquakes located using records from at least 25 to 30 stations tend to be well-located in the ISC database: smaller error ellipses and shorter distance of relocation. These analyses indicate that earthquakes with magnitudes mb > 4.0 have occurred since 1970 along the NPHM.

Current and future difficulties in the practice of engineering geology

Abstract Slosson, J.E., Williams, J.W. and Cronin, V.S., 1991. Current and future difficulties in the practice of engineering geology. In: M. Arnould and H.W. Smedes (Editors), Applied Geosciences for Low-level Radioactive and Chemical Wastes. Eng. Geol., 30: 3–12. The next five to ten years will be critical to the survival of engineering geology as a profession. A crisis currently exists in the quality of work being done in many sectors of engineering geology. If the professional geologic community does not accept the challenge to improve its overall performance record, the contributions that the engineering geologist can make will not be sought or valued by society. The opportunity to appropriately include geology in construction and land-use decisions will become more limited to non-existent if the work now assigned to the enginering geologist is relinquished to the geotechnical and civil engineering community.

Instantaneous velocity of mid-ocean ridges

Abstract Intuition suggests that all points on the same mid-ocean ridge should rotate around the relative pole of the two-plate system at the same instantaneous angular velocity. Contrary to intuition, the instantaneous angular velocity of a ridge varies from one point to another along the ridge, given the general case in which two plates move around different plate-specific poles of rotation. The variation in the instantaneous angular velocity of a ridge is a function of the motion characteristics of the plates and the position of the ridge relative to the poles of plate motion. The length or orientation of individual ridge segments is predicted to vary over time, leading to local changes in the shape of the ridge. The gradient in instantaneous angular velocity for the fast-spreading East Pacific Ridge, between the Cocos and Pacific plates, is an order of magnitude greater than the gradient along the Mid-Atlantic Ridge, between the North American and African plates. This great contrast in ridge instantaneous velocity gradients may be reflected in the contrasting ridge geometries of the East Pacific and Mid-Atlantic Ridges.

Liabilities Related to the Natural Hazards of Coastal California

Those consultants involved in design, construction, and maintenance as well as in storm damage repair in coastal zones should be fully apprised of the professional responsibility and legal liability that they have or will probably incur. The coastal zone is the most environmentally dynamic area in California and is subject to a variety of natural hazards which are exacerbated during periods of climatic extremes. Most professionals have not had sufficient training in marine geology, coastal engineering, and engineering geology to fully understand and evaluate the natural hazards which exist in the coastal zone. Thus, many costly errors have been and are still being made. Architects, engineers, and geologists should be fully cognizant of their exposure to liability when providing professional services. They should be aware that codes frequently do not meet current prudent professional standards and that just meeting the codes will not protect one from being sued for malpractice. Fraud and gross negligence have now been joined by the principle of simple negligence as grounds for legal action against professionals whose work proves to be erroneous, lacking in data, or inadequate. The “act of God” is no longer a valid excuse for professional error, omission, or negligence. The professionals working in the coastal area have an implied professional responsibility and attendant legal liability related to landslides and other slope failures, erosion, flooding, foundation failures, and seismic hazards as well as for added destruction caused by wave action, tidal action, and longshore transport.