Mario Pardo

Subduction and collision processes in the Central Andes constrained by converted seismic phases

The Central Andes are the Earths highest mountain belt formed by ocean–continent collision. Most of this uplift is thought to have occurred in the past 20 Myr, owing mainly to thickening of the continental crust, dominated by tectonic shortening. Here we use P-to-S (compressional-to-shear) converted teleseismic waves observed on several temporary networks in the Central Andes to image the deep structure associated with these tectonic processes. We find that the Moho (the Mohorovičić discontinuity—generally thought to separate crust from mantle) ranges from a depth of 75 km under the Altiplano plateau to 50 km beneath the 4-km-high Puna plateau. This relatively thin crust below such a high-elevation region indicates that thinning of the lithospheric mantle may have contributed to the uplift of the Puna plateau. We have also imaged the subducted crust of the Nazca oceanic plate down to 120 km depth, where it becomes invisible to converted teleseismic waves, probably owing to completion of the gabbro–eclogite transformation; this is direct evidence for the presence of kinetically delayed metamorphic reactions in subducting plates. Most of the intermediate-depth seismicity in the subducting plate stops at 120 km depth as well, suggesting a relation with this transformation. We see an intracrustal low-velocity zone, 10–20 km thick, below the entire Altiplano and Puna plateaux, which we interpret as a zone of continuing metamorphism and partial melting that decouples upper-crustal imbrication from lower-crustal thickening.

Reappraisal of great historical earthquakes in the northern Chile and southern Peru seismic gaps

A critical reappraisal of great historical interplate earthquakes in the occidental margin of South America, including southern Peru and northern Chile, is carried out.A spacetime distribution of the earthquakes associated to the seismotectonics regions defined by the rupture zones of the greatest events (1868, Mw = 8.8 and 1877, Mw = 8.8) is obtained. Both regions are seismic gaps that are in the maturity state of their respective earthquake cycles. The region associated to the 1868 earthquake presents a notable seismic quiescence in the present century.

The 1985 central chile earthquake: a repeat of previous great earthquakes in the region?

A great earthquake (surface-wave magnitude, 7.8) occurred along the coast of central Chile on 3 March 1985, causing heavy damage to coastal towns. Intense foreshock activity near the epicenter of the main shock occurred for 11 days before the earthquake. The aftershocks of the 1985 earthquake define a rupture area of 170 by 110 square kilometers. The earthquake was forecast on the basis of the nearly constant repeat time (83 � 9 years) of great earthquakes in this region. An analysis of previous earthquakes suggests that the rupture lengths of great shocks in the region vary by a factor of about 3. The nearly constant repeat time and variable rupture lengths cannot be reconciled with time- or slip-predictable models of earthquake recurrence. The great earthquakes in the region seem to involve a variable rupture mode and yet, for unknown reasons, remain periodic. Historical data suggest that the region south of the 1985 rupture zone should now be considered a gap of high seismic potential that may rupture in a great earthquake in the next few tens of years.

Steep subduction geometry of the Rivera Plate beneath the Jalisco Block in western Mexico

The morphology of the Rivera plate subducted beneath the Jalisco block in western Mexico is determined from accurately located hypocenters of locally recorded microearthquakes, and from earthquakes with magnitude mb≥4.5 recorded at teleseismic distances. The hypocenters of these latter earthquakes are relocated, and for five of them the focal depth is constrained by the inversion of long-period body waves. The Wadati-Benioff zone inferred from these data indicates a steep subduction of the Rivera plate that resembles the geometry of subduction of the Cocos plate beneath Central America. It is, however, very different from the shallower and almost subhorizontal subduction of the Cocos plate observed in southern Mexico, southeast of this region. The Rivera plate is comparable to the Juan de Fuca plate in terms of the small areal extent, young seafloor age, low relative velocity, and low teleseismic activity in the subduction zone. This study shows that the dip of both the Juan de Fuca and Rivera plates are similar once they are decoupled from the overriding continental crust. The downgoing Rivera plate initially starts with a dip of ∼10° down to a depth of 20 km and then increases gradually to a constant dip of ∼50° below a depth of 40 km. Intermediate-depth seismicity is low in this zone associated with the subduction of the slow (2 cm/yr) and young (9 m.y.) Rivera plate. The maximum depth extent of earthquakes observed in the Rivera subduction zone is about 130 km. The andesitic, calc-alkaline Colima volcano appears to be directly related to the subduction of the Rivera plate. To the NW of this volcano, the observed Quaternary volcanism in the Jalisco block, which is parallel to the trench, may also be explained by the subduction of the Rivera plate.

GEOMETRY OF THE BENIOFF ZONE AND STATE OF STRESS IN THE OVERRIDING PLATE IN CENTRAL MEXICO

Analysis of depths and focal mechanisms of 16 small earthquakes in Central Mexico, along with previous data, shows that (a) the subducted Cocos plate becomes subhorizontal between 110 and 275 km from the trench reaching a depth of about 50 km, and (b) the bottom part of the overriding continental plate is in tensional stress regime. Neotectonic structures in Central Mexico and stress orientations (estimated from borehole elongations, cinder cone alignments, and fault-slip analysis) in the Trans Mexican Volcanic Belt, which lies in the northeastern portion of the area under study, also indicate the same stress regime. Absolute motion of the North American plate in the region has a component normal to the trench of 20 mm/yr which, if true, would result in compressional stress in the upper plate if the trench position is fixed in an absolute frame of reference. If we allow for seaward retreat of the trench then the tensional stress in the overriding plate and the observed geometry of the Benioff zone can be explained. This, however, implies that seaward retreat of a trench is possible even for a young (∼15 m.y. old) subducting slab. Alternatively, tectonic erosion of the leading edge of the continent, which has been proposed to explain truncated igneous and metamorphic continental terrane, could give rise to the tensional stress but would not explain the geometry of the Benioff zone.

The Mw 7.7 Tocopilla Earthquake of 14 November 2007 at the Southern Edge of the Northern Chile Seismic Gap: Rupture in the Deep Part of the Coupled Plate Interface

The slip distribution of the Mw 7.7 Tocopilla earthquake was obtained from the joint inversion of teleseismic and strong-motion data. Rupture occurred as underthrusting at the base of the seismically coupled plate interface, mainly between 35 and 50 km depth. From the hypocenter, located below the coast 25 km south of the town of Tocopilla, the rupture propagated 50 km northward and 100 km southward. Overall, the slip distribution was dominated by two slip patches, one near the hypo- center and the other 70 km to the south where slip reached its maximum value (3 m). An additional branch of moderate slip propagated at shallower depth toward the west near the northern tip of the Mejillones peninsula. Rupture velocity remained close to 2:8 km=sec, with a total rupture duration of 45 sec. The first 2 weeks of aftershocks located with a local seismic network display a strong correlation with the slip distri- bution. The 2007 rupture ended below the Mejillones peninsula, where the 1995 An- tofagasta rupture also ended (Ruegg et al., 1996; Delouis et al., 1997; Pritchard et al., 2006). This corroborates the role of barrier played by this structure. The downdip end of the seismically coupled zone at 50 km depth, evidenced by previous studies for the 1995 event, is also confirmed. The 2007 Tocopilla earthquake contributed only mod- erately to the rupturing of the great northern Chile seismic gap, which still has the capacity for generating a much larger underthrusting event.

A DOUBLE-LAYERED SEISMIC ZONE IN ARICA, NORTHERN CHILE

A double layered seismic zone is determined in Arica, northern Chile using locally recorded events. At depths >100 km two planes of seismicity can be observed: one dipping at ∼30°E with ∼10 km of thickness and a second parallel plane 20–25 km deeper, with the same average thickness. Fault plane solutions for both layers show a wide variability, even between nearby events. The genesis of the Arica double seismic zone seems to be independent of the age, the relative convergence rate and direction of the Nazca plate, because all of these parameters are almost the same along the whole northern Chile and a clear separation of the seismicity into two layers is only observed around the Arica elbow. Moreover, it cannot be responsible of the double layered seismic zone because no similar significant changes are observed in the other well studied double seismic zones.

Seismicity and average velocities beneath the Argentine Puna Plateau

A network of 60 seismographs was deployed across the Andes at ∼23.5°S. The array was centered in the backarc, atop the Puna high plateau in NW Argentina. P and S arrival times of 426 intermediate depth earthquakes were inverted for 1-D velocity structure and hypocentral coordinates. Average velocities and v p /v s in the crust are low Average mantle velocities are high but difficult to interpret because of the presence of a fast velocity slab at depth. Although the hypocenters sharply define a 35° dipping Benioff zone, seismicity in the slab is not continuous. The spatial clustering of earthquakes is thought to reflect inherited heterogeneties of the subducted oceanic lithosphere. Additionally, 57 crustal earthquakes were located. Seismicity concentrates in the fold and thrust belt of the foreland and Eastern Cordillera, and along and south of the El Toro-Olacapato-Calama Lineament (TOCL). Focal mechanisms of two earthquakes at this structure exhibit left lateral strike-slip mechanisms similar to the suggested kinematics of the TOCL. We believe that the Puna north of the TOCL behaves like a rigid block with little internal deformation, whereas the area south of the TOCL is weaker and currently deforming.

Statistical examination of the existence and relative motion of the Jalisco and Southern Mexico Blocks

The continental lithosphere of Mexico south of the Trans-Mexican volcanic belt may consist of two lithospheric blocks; the Jalisco and Southern Mexico Blocks. The existence of these two blocks can be examined employing the statistical F test (formulated to test for the presence of significant misclosure around a plate circuit) to plate motion data derived from marine magnetic anomaly lineations, earthquake slip vectors, and transform fault azimuths. The result of the F test applied to the Pacific-Cocos-North American plate circuit indicates that there is no significant (at the 99% confidence level) misclosure around this plate circuit. Therefore if the Southern Mexico Block exists, its motion relative to the surrounding lithospheric plates is too small to be resolved from these data. In contrast, the result of the F test applied to the Rivera-Pacific-North American plate circuit indicates the presence of a significant misclosure about the Rivera-Pacific-North American plate circuit, the cause of which is uncertain. One interpretation of the misclosure around the Rivera-Pacific-North American plate circuit is that it is due to the presence of an independent Jalisco Block. However, it is more likely that this misclosure is, instead, primarily due to the effects of recent changes in the relative motion between the Pacific and Rivera plates. Assuming the second explanation is correct, a new Rivera-North American Euler pole location (21.8°N, 110.4°W) is determinable solely from earthquake slip vectors located along the Rivera-North American boundary.

The October 15, 1997 Punitaqui earthquake (Mw=7.1): a destructive event within the subducting Nazca plate in central Chile

Abstract The 1943 Illapel seismic gap, central Chile (30–32°S), was partially reactivated in 1997–1998 by two distinct seismic clusters. On July 1997, a swarm of offshore earthquakes occurred on the northern part of the gap, along the coupled zone between Nazca and South American plates. Most of the focal mechanisms computed for these earthquakes show thrust faulting solutions. The July 1997 swarm was followed on October 15, 1997 by the Punitaqui main event (Mw=7.1), which destroyed the majority of adobe constructions in Punitaqui village and its environs. The main event focal mechanism indicates normal faulting with the more vertical plane considered as the active fault. This event is located inland at 68-km depth and it is assumed to be within the oceanic subducted plate, as are most of the more destructive Chilean seismic events. Aftershocks occurred mainly to the north of the Punitaqui mainshock location, in the central-eastern part of the Illapel seismic gap, but at shallower depths, with the two largest showing thrust focal mechanisms. The seismicity since 1964 has been relocated with a master event technique and a Joint Hypocenter Determination (JHD) algorithm, using teleseismic and regional data, along with aftershock data recorded by a temporary local seismic network and strong motion stations. These data show that the 1997 seismic clusters occurred at zones within the Illapel gap where low seismicity was observed during the considered time period. The analysis of P and T axis directions along the subduction zone, using the Harvard Centroid Moment Tensor solutions since 1977, shows that the oceanic slab is in a downdip extensional regime. In contrast, the Punitaqui mainshock is related to compression resulting from the flexure of the oceanic plate, which becomes subhorizontal at depths of about 100 km. Analog strong motion data of the Punitaqui main event show that the greatest accelerations are on the horizontal components. The highest amplitude spectra of the acceleration is in the frequency band 2.5–10 Hz, in agreement with the energy band responsible for the collapsed adobe constructions. The isoseismal map derived from the distribution of observed damage show that a high percentage of destruction is due to the proximity of the mainshock, the poor quality of adobe houses and probably local site amplification effects.