Symptomatic Features of Intraplate Earthquakes

Available geological, geophysical, and seismological data for 39 cases were compared and analyzed to understand the cause of intraplate arthquakes. The results revealed common fea-tures which can be used to test models for their seismogene-sis. Intraplate seismicity occurs in the vicinity of "stress concentrators " within pre-existing zones of weakness, most commonly failed rifts. These stress concentrators are struc-tures where plate tectonic stresses can cause a localized build-up of stresses and, ultimately, earthquakes. These include intersecting faults, buried plutons, and rift pillows. One of the intersecting faults was often found to be tens of kilome-ters in length and oriented optimally with respect to the max-imum horizontal stress direction. Intraplate arthquakes have longer return periods compared to their plate-boundary counterparts. They occur in failed rifts of all ages, whereas those not associated with rifts occur only in the Precambrian crust. The former have shorter eturn periods (_<500 years) compared to the latter (thousands of years).