Limits of stresses in continental crusts and their relation to the depth‐frequency distribution of shallow earthquakes

Publisher Website
Google Scholar
Abstract
Based on published results of experiments on low‐temperature, low‐pressure frictional sliding and creep at higher temperature and pressure, theoretical curves of yield strength versus depth corresponding to maximum crustal stresses (STRESSMAX) are calculated. These curves are compared to the frequency‐depth distribution of earthquakes (DEFREQ) in several tectonic areas. Both sets of curves have a very similar form and show a prominent peak. From the similarity it is concluded that it is basically the temperature and the water content of the entire crust, not the properties of a particular layer, which determine the shape of maxima of the DEFREQ curves. The peaks of DEFREQ curves are generally at 5‐ to l0‐km depth and agree with STRESSMAX peaks of wet upper crust only. At this depth range the high stresses provide an increased ‘cracking potential’, resulting in an increased number and an increased stress drop of earthquakes. More and stronger barriers/asperities seem to exist at these depths, causing large earthquakes to nucleate in this high‐strength region. The stresses required to overcome the strength maximum are built up from below by ductile creep. The lower crust is considered to be a stress and viscosity minimum; in orogenic zones, strong interaction with plumes from the mantle may take place, and lateral movements are probable.