Geologic structure of the Taltal Area, Northern Chile, in relation to the earthquake of December 28, 1966

The Taltal area, which lies within the coastal cordillera of northern Chile, is dominated by a group of major active faults that cut a eugeosynclinal section of predominantly Jurassic andesites overlying Paleozoic metamorphic and plutonic rocks and intruded by Late Mesozoic plutons. The Atacama fault, a suggested regional strike-slip fracture parallel to the coastline, has been obliquely cut and left-laterally offset 10 km by the Taltal fault, which passes through the town of Taltal. Three distinctive features were found to be consistently offset 10 km by the Taltal fault: the easternmost strand of the Atacama fault, an intrusive contact, and a unique volcanic unit. Former continuity of the Atacama fault through the Taltal region is proposed, and subsequent disruption by the Taltal fault appears to have caused major structural readjustments in the still-active Atacama fault zone. The tentative offshore epicenter and aftershock distribution of the December 28 earthquake are not directly correlative with faults that have been mapped in the nearby on-shore areas; this lack of correlation is not surprising in view of the suggested depths of hypocenters in the lower crust or upper mantle

Locations of small earthquakes near the trifurcation of the San Jacinto fault southeast of Anza, California

About 100 small earthquakes (M ≈ 1/2 to 2) which occurred near the trifurcation of the San Jacinto fault southeast of Anza, California, have been accurately located using five- and six-station arrays with dimensions of about 10 km. The pattern of epicenters is complex and extends several km outside of the area outlined by the traces of faulting. Patterns of seismicity observed on opposite sides of the San Jacinto fault are significantly different. On the southwest side, a concentration of foci lies at a depth of about 4-7 km along the projected extension of the Coyote Creek fault a few km northwest of the last surface evidence of faulting. On the northeast side, earthquakes are concentrated at depths between 10 and 15 km. A group of the latter events recorded about 1 week after the magnitude 4.7 earthquake of May 21, 1967 forms a linear pattern parallel to the San Jacinto fault with depths from 3 to 15 km. This pattern may represent the zone of energy release or slip for that earthquake and possibly the plane of the San Jacinto fault at depth, although the epicenters are located about 2 to 3 km to the northeast of the trace of the San Jacinto fault. Most of the earthquakes located in this study are not aftershocks in the usual sense, i.e., easily correlated with a preceding large earthquake. They represent a complex pattern of seismicity which has continued at least for the last 3 years on the micro-earthquake level and for the last 30 years on the macroseismic level

National Seismic System Science Plan

Recent developments in digital communication and seismometry are allowing seismologists to propose revolutionary new ways to reduce vulnerability from earthquakes, volcanoes, and tsunamis, and to better understand these phenomena as well as the basic structure and dynamics of the Earth. This document provides a brief description of some of the critical new problems that can be addressed using modem digital seismic networks. It also provides an overview of existing seismic networks and suggests ways to integrate these together into a National Seismic System. A National Seismic System will consist of a number of interconnected regional networks (such as southern California, central and northern California, northeastern United States, northwestern United States, and so on) that are jointly operated by Federal, State, and private seismological research institutions. Regional networks will provide vital information concerning the hazards of specific regions. Parts of these networks will be linked to provide uniform rapid response on a national level (the National Seismic Network). A National Seismic System promises to significantly reduce societal risk to earthquake losses and to open new areas of fundamental basic research. The following is a list of some of the uses of a National Seismic System