M. J. S. JOHNSTON, U. S. Geological Survey

High-resolution strain data have been obtained near active faults, in the near-field of many moderate to large earthquakes and in active volcanic regions. To avoid near-surface noise, these recordings were made with borehole dilatometers (resolution 10E-10) and tensor borehole strainmeters (resolution 10E-9). Many aspects of transient crustal strain are now becoming clear. Strain transients resulting from fault creep events observed at the Earth's surface indicate these events are a near-surface soil failure phenomena driven by deeper fault slip. Slow and silent earthquakes are observed but are not common in the few networks in the world capable of detecting them. Also uncommon are indications of large-scale fault slip waves and indications of pore fluid pressure changes. For the majority of ordinary earthquakes, observed co-seismic offsets are generally in good agreement with expectations from elastic dislocation theory. Post-seismic deformation occurs for the larger events and continues, in some cases, with a moment comparable to that of the main shock. Preseismic strain from hours to seconds (or less) before the main shock is not apparent at the present resolution. Precursory slip for the largest events is minor, if it occurs, having a moment less than 0.1% of that of the main event. This constraint on the size and amount of fault slip triggering major rupture makes prediction of the onset time and final magnitude of earthquakes a difficult task. These various data support an inhomogeneous fault failure model for which the rupturing process is inhomogeneous and controlled by points of higher strength. Rupture initiation apparently occurs at smaller regions of higher strength which, when broken, allow runaway catastrophic failure. Intrusive events of both magmatic and hydrothermal origin can be tracked in volcanic regions. Triggered seismicity and VLP earthquakes appear to have triggered diffusion and small intrusive events as their origin.