Seismicity and rates of relative motion on the plate boundaries of Western North America

Abstract

The consistency of earthquake data and plate tectonic models and other estimates of fault slip, may be tested by estimating rates of motion from the earthquakes using the concept of seismic moment. The contribution of individual events may simply be summed, but a generally better estimate of long-term average slip rate is obtained by integration over magnitudefrequency of occurrence relations. Estimates of fault motion rates associated with earthquakes are possible within about a factor of 2 using this approach if the major sources of uncertainty are given careful consideration; e.g. incompleteness and inaccuracies in the earthquake data; empirical moment-magnitude relations and the effect of their stochasticity; fault 'widths' or depth extents; the recurrence relations; maximum magnitudes and the form of truncation at the maximum magnitude. A formulation for the recommended magnitude density truncation is developed. Application of the latter method to the earthquake data of the offshore transform faults of the Juan de Fuca ridge system, the Queen Charlotte fault zone and the northern Vancouver Island area in each case gives good agreement with rates from plate tectonic models. For the southern San Andreas fault and Gulf of California area there is also good agreement with previous estimates obtained by summing the contributions of individual events. However, the displacement rate in the margin convergence zone of southern British Columbia, Washington and Oregon computed from the seismicity is at least a factor of 10 lower than from plate models and from other convergence estimates, and primarily aseismic slip is suggested. The fault motions as a function of time computed from the seismicity records have also been plotted and compared to the longterm average rates. The plots permit estimates of the minimum present accumulated elastic strain, and show if there are any temporal relations among earthquake displacements on different fault zones. htroduc tionA basic understanding of the origin and nature of earthquakes in a region and detailed evaluation of the associated seismic risk requires development of valid tectonic models. Tectonic

Seismic hazard in western Canada from GPS strain rates versus earthquake catalog

“…For example, Anderson [1979] found an overall agreement between observed catalog statistics and those derived from fault slip rates in southern California. Similarly, Hyndman and Weichert [1983] found that fault slip rates derived from earthquake catalogs and from plate tectonic models agreed well for offshore western Canada. However, both studies noted cases of significant disagreement between catalog statistics and geological estimates, which they attributed to aseismic deformation, elastic strain, or under-sampling of the earthquake catalog.…”

“…A factor 6-20 increase of the catalog moment rate (equation (2)) requires: a magnitude-moment parameter increase (d by about 0.8-1.3 or c by about 0.1-0.2) much larger than those permitted by earthquake data [Ristau et al, 2005]; a multiplication of the asymmetry factor 8 by 6-20, well beyond the range of magnitude uncertainties [e.g., Hyndman and Weichert, 1983]; or a maximum magnitude increase of about 1.1-1.8, leading to unrealistic M X = 8.1-9.3.…”

“…The statistical seismic moment rate _ M 0 S is derived by integrating the cumulative truncated GR distribution up to the maximum magnitude M X [Hyndman and Weichert, 1983]:…”

“…where 8 is a correction for the asymmetry of the stochastic magnitude -moment relation (8 = 1.27 assuming a standard error of 0.2 on magnitudes [Hyndman and Weichert, 1983]), and c and d are the parameters of the magnitude (m) -moment (M 0 ) relation:…”

Characteristic scales of earthquake rupture from numerical models

“…A simple example is an oceanic transform fault, bounded by midocean ridge segments. In that case, a scaling change is associated with the maximum magnitude, which is characteristic of the fault area (Hyndman and Weichert, 1983). Smaller scale fault discontinuities, as well as jogs and bends have been shown put strong constraints on rupture propagation and arrest (Nielsen and Knopoff, 1998).…”