Space geodetic data recorded rates and directions of motion across the convergent boundary zone between the oceanic Nazca and continental South American plates in Peru and Bolivia. Roughly half of the overall convergence, about 30 to 40 millimeters per year, accumulated on the locked plate interface and can be released in future earthquakes. About 10 to 15 millimeters per year of crustal shortening occurred inland at the sub-Andean foreland fold and thrust belt, indicating that the Andes are continuing to build. Little (5 to 10 millimeters per year) along-trench motion of coastal forearc slivers was observed, despite the oblique convergence.

Abstract. Two years of continuous GPS data from several sites in South America indicate that Arequipa in the southern Peruvian Andes has a velocity of 13+3 mm/yr (two standard errors) to the northeast with respect to stable South America. We interpret these data as reflecting a combination of elastic strain accumulation associated with a locked Nazca-South America subduction zone and a small amount of crustal shortening across the fold and thrust belt on the eastern margin of the Andes. Models of elastic strain accumulation for fully locked and partly locked subduction zones constrain shortening in the eastern Andes to 0-3 mm/yr (fully locked) and 0-12 mm/yr (partly locked), slower than some geologic estimates averaged over millions of years.

The source of the geological disturbance responsible for the midcontinent gravity high is here reexamined on the basis of gravimetric, seismological, and geological data in consort, and both the size of the structure and the lithologies responsible are at variance to the results of earlier studies that did not employ long‐range, seismological data. Its extension under Lake Superior, implied recently on gravity and seismic grounds and a new seismic inference that flanking areas that extend laterally are typified by gabbroic rather than granitic velocities, infers a disturbance laterally more extensive than that of the high and hence the name used, the central North American rift system. A refined method of analysis of existing long‐range crust‐upper mantle profile data has made possible the inclusion of this seismic data for the first time. The analysis method rests on the recognition of the existence of a common finite and discrete regional suite of velocities determined from shorter reversed profiles traversing key areas, the assumption of local planarity of interfaces, which then are approximated piecewise, the existence of a functional relationship between compressional velocity and bulk density for crustal materials of the region, and, finally, on the admission that long ‘reversed’ profiles are in the main unreversed and must be treated so that the resulting models simultaneously satisfy the seismic, gravimetric, and geologic data. A high velocity, 6.9 km/sec, forms the ‘core’ of the gravity high imbedded in material of 6.4‐km/sec compressional velocity. The inferred density contrast, 0.14 gm/cm3, is substantially smaller than any used previously and results in a 40% increase of volume and an increase in the thickness of the anomalous body. The latter is in accord with minimum depths (25–30 km) estimated from Pn time term. The 6.9‐km/sec material is associable with rocks of Mellen gabbro type, and the 6.4‐km/sec material with rocks of Duluth gabbro type on the basis of geologic field relations and extrapolated seismic ‘contact outcrops’ supplemented with high pressure and temperature velocity data. The nature of the structural model suggests the 6.9‐km/sec material as intruded into the 6.4‐km/sec material, which in turn (at least in Wisconsin) is replaced in the upper tens of kilometers with 6.1‐km/sec material at a lateral distance of several hundred kilometers from the high. Models across the axial zone of the gravity high in Lake Superior, Wisconsin‐Minnesota, and Iowa all show the anomalous high‐velocity mass increases in width upward, producing a velocity reversal in the vicinity of the gravity high. A preliminary search for analogous structure among the more modern rift or ridge systems shows that, on the basis of average width, length, compressional velocity, velocity contrast, gravimetric and magnetic expression, structure, etc., the most analogous tectonic feature (at least on the basis of present information) is the Red Sea rift rather than the continental rift valleys of Africa.

SUMMARY A microearthquake survey was conducted in the central Andes of Peru, east of the city of Lima, to study the seismicity and style of tectonic deformation of the Peruvian Andes. Although most of the stations forming the temporary seismographic network were located on the high Andes, the vast majority of the microearthquakes recorded occurred to the east of the mountain belt: on the Huaytapallana fault in the Eastern Cordillera and beneath the western margin of the sub‐Andes. Thus the sub‐Andes appear to be the physiographic province subject to the most intense seismic deformation. Focal depths of the crustal events in this region range generally from 15 to 35 km and some events beneath the sub‐Andes appear to be as deep as 40‐50 km. The fault‐plane solutions of events in the sub‐Andean margin show thrust faulting on steep planes oriented roughly north‐south, similar to that observed in teleseismic earthquakes studied using body wave modelling. The Huaytapallana fault in the Cordillera Oriental also shows relatively high seismicity along a NE‐SW trend that agrees with the fault scarp and the east‐dipping nodal plane of two large earthquakes that occurred on this fault on 1969 July 24 and October 1. Microearthquakes of intermediate depth recorded during the experiment show a flat seismic zone about 25 km thick at a depth of about 100 km. This agrees with recent observations showing that beneath Peru the slab first dips at an angle of about 30° to a depth of 100 km and then flattens following a quasi‐horizontal trajectory. Fault‐plane solutions of intermediate‐depth microearthquakes have horizontal T axes oriented east‐west suggesting slab pull is the dominant force in the downgoing slab.

Using recordings from 15 portable instruments and six permanent stations operated during two field investigations of microearthquakes in southern Peru, we determined locations of 888 shallow and intermediate depth earthquakes and 56 fault plane solutions.

A microearthquake survey of crustal seismicity conducted in 1985 on the Eastern Cordillera and the Amazonian foothills in Central Peru gives a description of the present tectonic activity related to the uplift of the Andes. Hypocenters on the Huaytapallana fault are shallower than 10 km, and their focal mechanism is in agreement with the fault trace corresponding to the earthquakes of 1969 and suggests a reverse strike slip movement on a plane striking N130°. An active tectonic zone is evidenced along a NW‐SE direction passing through the Amauta Subandean region. The vertical distribution of hypocenters and the comparison of shallow and deep focal mechanisms suggest a reverse fault dipping to the west. Depths vary between 0 and 20 km. and never exceed 32 km. The Huaytapallana Cordillera is uplifted by thrust faults on both sides.

In 1968, the Carnegie Institution of Washington together with North and South American collaborators carried out a reconnaissance explosion seismic experiment to investigate the apparently highly anomalous crustal structure under the Peru-Bolivia altiplano. The data of this experiment have been reinterpreted by ray-tracing in a spherical Earth so as to fit as closely as possible arrival times, relative amplitudes, cusps, etc., of seismograms displayed in record section. The resultant model confirms the previous average model consisting of three major refractors: the sedimentarymetamorphic layer 4 9 km thick and 4.54.9 km s-' velocity; the ' granitic ' layer with 6 .O-6.1 km s-' velocity down to 26-30 km depth; and the ' gabbroic ' layer reaching depths of 68-70 km below sea level with 6.8-6.9 km s-' velocity. However, in order to account for relatively large amplitudes in the secondary arrivals with apparent velocities close to the first arrivals, two low-velocity zones are postulated within the crust under the Peru-Bolivia altiplano. In Peru, the shallow and thinner lowvelocityzone with boundaries at 9 km and 12 km depth is between materials of 6.0 km s-' and 6.

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