Fine structure of a postfailure Wadati-Benioff zone

Journal of Geophysical Research, 1991

We study source parameters of large, shallow Mexican subduction zone earthquakes (95%V to 106øW) which occurred between 1928 and 1986 by modeling the P waves recorded on Galitzin-Wilip seismograph in DeBilt (DBN), Holland. For post-1962 events the source parameters retrieved from DBN seismograms alone agree well with those obtained from long-period World-Wide Standardized Seismograph Network records, giving us confidence in our results for pre-1962 events. All earthquakes are shallow 0t---10 to 20 km). With few exceptions the sources in Oaxaca (95%V to 99øW) are very simple. To the northwest of • they are simple as well as complex. The ratio of surface wave to body wave seismic moment 0VlosfMoP), which is a measure of long-to short-period radiation, is smaller in Oaxaca (--1.5 + 0.5) than in the regions northwest of 99%V (--3.1 __+ 1.3). These results suggest a change in the plate interface characteristics near 99%V. The sharp change in the rupture mode and the intersection of the O'Gorman Fracture Zone (OFZ) with the trench occur near 99øW. Two strike-slip events offshore, close to OFZ, suggest a segmentation of the subducting plate near 99mAr. The age of the plate near the trench in Oaxaca is not well known; it is possible that it does not increase continuously from northwest to southeast in the region but jumps across 99mAr. If so, then the older age of the subducted plate southeast of 99'W may be the cause of the distinct rupture mode of the Oaxaca earthquakes. The length of the Benioff zone, which is greatest below Oaxaca (• 400 km) and decreases toward the northwest, can be explained by the correlation between the length of the subducted slab and the product of the lithosphere age and convergence rate. The relative complexity of sources, the weaker background seismicity, and the lesser number of afh•rshocks northwest of Oaxaca may be explained by a stronger interface coupling resulting from subduction of younger oceanic slabs --5 to 13 m.y. old) in this region. This, however, explains neither larger Mos/MoP values northwest of Oaxaca nor the low stress drop estimates obtained from the analysis of near-field strong-motion data for the Michoacan earthquake of 1985, both of which indicate weaker coupling of the interface. Thus the issue of whether subduction of very young plates (•. 10 m.y. old) results in strong or weak coupling remains unsolved from the presently available Mexican data. 1982; Astiz and Kanamori, 1984; Beroza et al., 1984; Tajima, 1984; LeFevre and McNally, 1985; Eissler et al., 1986; Priestley and Masters, 1986; Ekstr6m and Dziewonski, 1986; Houston and Kanamori, 1986; Riedesel et al. 1986; Mendoza and Hartzell, 1988, Astiz et al. 1987; Yamamoto and Mitchell, 1988]. The quality and quantity of the data used in these analyses (seismograms from short-and long-period World-Wide Standardized Seismograph Network (WWSSN) stations, IDA stations, and broadband intermediate-and short-period digital stations) are impressive. For example, several research groups have studied the September 19, 1985 (Ms = 8.1), Michoacan earthquake. Although the data set and the methodologies used differ, it is reassuring to note that a fairly consistent picture of the source emerges from these studies. The source characteristics of some recent large Mexican earthquakes have also been studied from the analysis of strong-motion data obtained in and near the epicentral zone [e.g., Anderson et al., 1986; Yomogida, 1988; Singh et al., 1988, 1990a, b]. Thus we may surmise that the source characteristics of recent, large Mexican subduction zone earthquakes are reasonably well understood at long periods and, for some events, even at short periods. For events between 1904 (when Wiechert seismographs were installed) and 1962 (when WWSSN became operational) the quality and the number of available seismograms are relatively poor. Yet, in view of the small time span covered by WWSSN and other modem networks, it is very important to estimate source characteristics of earthquakes that occurred in this period. With this in view, Singh et al. [1984a] carried out a qualitative analysis of P waves of Wiechert seismograms recorded at a few European stations (mostly Uppsala (UPP) and G6ttingen (GOT)). They also modeled P waves of nine events which occurred between 1932 and 1981; seven of these were recorded on the Galitzin-Wilip seismograph at Stuttgart (STU), and two were recorded on the Press-Ewing seismograph at UPP. The inferred simplicity or complexity of the sources from visual inspection of small-amplitude P waves recorded at European Wiechert seismographs is, obviously, subject to uncertainties.

Journal of South American Earth Sciences, 2015

Aftershock sequences along the Mexican subduction margin (between coordinates 110ºW and 91ºW) were analyzed by means of the p value from the OmorieUtsu relation and the b value from the GutenbergeRichter relation. We focused on recent medium to large (M w > 5.6) events considered susceptible of generating aftershock sequences suitable for analysis. The main goal was to try to find a possible correlation between aftershock parameters and plate characteristics, such as displacement rate, age and segmentation. The subduction regime of Mexico is one of the most active regions of the world with a high frequency of occurrence of medium to large events and plate characteristics change along the subduction margin. Previous studies have observed differences in seismic source characteristics at the subduction regime, which may indicate a difference in rheology and possible segmentation. The results of the analysis of the aftershock sequences indicate a slight tendency for p values to decrease from west to east with increasing of plate age although a statistical significance is undermined by the small number of aftershocks in the sequences, a particular feature distinctive of the region as compared to other world subduction regimes. The b values show an opposite, increasing trend towards the east even though the statistical significance is not enough to warrant the validation of such a trend. A linear regression between both parameters provides additional support for the inverse relation. Moreover, we calculated the seismic coupling coefficient, showing a direct relation with the p and b values. While we cannot undoubtedly confirm the hypothesis that aftershock generation depends on certain tectonic characteristics (age, thickness, temperature), our results do not reject it thus encouraging further study into this question.

Geophysical Journal International, 2007

We use coseismic displacements and aftershock information from Global Positioning System (GPS) measurements at 27 sites in western Mexico and a 12-station local seismic network to determine the characteristics of the 2003 January 22 Mw = 7.2 subduction thrust earthquake near Tecomán, Colima, Mexico. Estimates of the earthquake moment, slip direction and best-fitting slip distribution are derived by optimizing the fit to the GPS displacements for a 3-D finite element mesh that simulates the study area. The calculated moment release is 9.1 × 1019 N m (Mw = 7.2), with maximum slip of 2 m at a depth of 24 km and a maximum rupture depth of 35-40 km. The inversion indicates that coseismic rupture extended downdip from depths of 9 to 40 km along a 80 km along-strike region that is bounded by the edges of the Manzanillo Trough. The optimal solution is robust with respect to plausible changes in the subduction interface geometry and differing subsets of the data. A comparison of the cumulative post-seismic slip that can be inferred separately from earthquake aftershocks and GPS measurements within a year of the earthquake indicates that 95 per cent or more of the post-seismic deformation was aseismic. Near-term post-seismic measurements indicate that slip propagated downdip to areas of the subduction interface beneath the coastline within days following the earthquake, as also occurred after the nearby Mw = 8.0 Colima-Jalisco subduction earthquake in 1995. The similar behaviours and locations of the 1995/2003 earthquake sequence to two earthquakes in June of 1932 suggests that thrust earthquakes along the subduction interface northwest of the Manzanillo Trough may trigger earthquakes in the vicinity of the Manzanillo Trough; however, our modelling of Coulomb stress changes caused by the 1995 earthquake indicate that it induced only modest unclamping of the subduction interface in the vicinity of the Tecomán rupture. In addition, GPS measurements indicate that elastic shortening characterized areas onshore from the Tecomán rupture from mid-1997 up until the time of the rupture, consistent with progressively stronger clamping of the subduction interface during this period. This precludes any obvious triggering relationship with the 1995 earthquake. The apparent coincidence of the edge of both the 1932 and 1995/2003 rupture sequences with the edge of the Manzanillo Trough may indicate that the trough is a mechanical barrier to along-strike rupture propagation. This implies a limit to the area of potential slip and hence rupture magnitude during future large earthquakes in this region.

Geophysical Journal International, 1998

The most prominent feature of the regional seismic wave¢eld from about 150 to over 1000 km is usually the Lg phase. This arrival represents trapped S-wave propagation within the crust as a superposition of multiple re£ections, and its amplitude is quite sensitive to the lateral variation in the crust along a propagation path. In an environment where the events occur in a subduction zone, such as the western coast of Mexico, quite complex in£uences on the character of the regional wave¢eld arise from the presence of the subduction zone. The great 1985 Michoacan earthquake (M W~8 .1), which occurred in the Mexican subduction zone, was one of the most destructive earthquakes in modern history and its notable character was that at Mexico City, located over 350 km from the epicentre, there was strong ground shaking almost comparable to that in the epicentral region that lasted for several minutes. Considerable e¡ort has been expended to explain the origin of the unusual observed waves that caused the severe damage in the capital city during the destructive earthquake. The nature of the propagation process in this region can be understood in part by using the detailed strong-motion records from the 1995 Copala, Guerrero (M W~7 .4) earthquake near the coast to the south of Mexico City, which also had an enhanced amplitude in the Valley of Mexico. Numerical modelling of both P and S seismic waves in 2-D and 3-D heterogeneous crustal models for western Mexico using the pseudospectral method provides direct insight into the nature of the propagation processes through the use of sequences of snapshots of the wave¢eld and synthetic seismograms at the surface. A comparison of di¡erent models allows the in£uences of di¡erent aspects of the structure to be isolated. 2-D and 3-D modelling of the 1985 Michoacan and 1995 Copala earthquakes clearly demonstrates that the origin of the long duration of strong ground shaking comes from the Sn and Lg wave trains. These S-wave arrivals are produced e¤ciently from shallow subduction earthquakes and are strongly enhanced during their propagation within the laterally heterogeneous waveguide produced by the subduction of the Cocos Plate beneath the Mexican mainland. The amplitude and duration of the Lg coda is also strongly reinforced by transmission through the Mexican Volcanic Belt from the ampli¢cation of S waves in the low-velocity sur¢cial layer associated with S-toP conversions in the volcanic zone. The further ampli¢cation of the large and long Lg wave train impinging on the shallow structure in the basin of Mexico City, with very soft soil underlain by nearly rigid bedrock with a strong impedance contrast, gives rise to the destructive strong ground shaking from the Mexican subduction earthquakes.

Geophysical Journal International, 2013

To improve earthquake location, we create a 3-D a priori P-wave velocity model (3-DVM) that approximates the large velocity variations of the Ecuadorian subduction system. The 3-DVM is constructed from the integration of geophysical and geological data that depend on the structural geometry and velocity properties of the crust and the upper mantle. In addition, specific station selection is carried out to compensate for the high station density on the Andean Chain. 3-D synthetic experiments are then designed to evaluate the network capacity to recover the event position using only P arrivals and the MAXI technique. Three synthetic earthquake location experiments are proposed: (1) noise-free and (2) noisy arrivals used in the 3-DVM, and (3) noise-free arrivals used in a 1-DVM. Synthetic results indicate that, under the best conditions (exact arrival data set and 3-DVM), the spatiotemporal configuration of the Ecuadorian network can accurately locate 70 per cent of events in the frontal part of the subduction zone (average azimuthal gap is 289 • ± 44 •). Noisy P arrivals (up to ± 0.3 s) can accurately located 50 per cent of earthquakes. Processing earthquake location within a 1-DVM almost never allows accurate hypocentre position for offshore earthquakes (15 per cent), which highlights the role of using a 3-DVM in subduction zone. For the application to real data, the seismicity distribution from the 3-D-MAXI catalogue is also compared to the determinations obtained in a 1-D-layered VM. In addition to good-quality location uncertainties, the clustering and the depth distribution confirm the 3-D-MAXI catalogue reliability. The pattern of the seismicity distribution (a 13 yr record during the inter-seismic period of the seismic cycle) is compared to the pattern of rupture zone and asperity of the M w = 7.9 1942 and the M w = 7.7 1958 events (the M w = 8.8 1906 asperity patch is not defined). We observe that the nucleation of 1942, 1958 and 1906 events coincides with areas of positive Simple Bouguer anomalies and areas where marine terraces are still preserved on the coastal morphology. From north to south: (1) the 1958 rupture zone is almost aseismic and is attributed to a zone of high coupling; (2) south of the Galera alignment (perpendicular to the trench), the 1942 rupture zone presents moderate seismicity, deeper on the seismogenic interplate zone, and abutting on the Jama cluster (to the south). This cluster is facing the Cabo Pasado cap and positive Bouguer anomalies on the overriding margin. We suspect that this cluster reflects a zone of local asperity (partial coupling). South of the Jama cluster, the spherical aseismic zone in the Bahia area is interpreted as having a low seismic coupling (steady creep motion or slow slip events). We suspect that the site that generated the three M > 7 events (1896, 1956 and 1998) correspond to a small patch of strong coupling. To the south, in the Manta-Puerto Lopez zone, the seismicity is mainly organized in earthquake swarms (1998, 2002, 2005). Although slow slip events have been observed in the area (Vallée et al. submitted), we infer from the coastline shape, the marine terraces and the high positive Bouguer anomalies that the seismicity here might reveal a significant amount of seismic coupling.

2014

We present a combined method, using sP depth-phase data and double-difference arrival times, to determine the precise hypocenter locations of earthquakes that occur under the Pacific Ocean outside of the area covered by the land-based seismic network. We assess the effectiveness of the combined method using a data set of P-and S-wave arrival times and sP depth phase from suboceanic earthquakes recorded by both land-based seismic stations and offshore seismic stations (OFS). The hypocenters of the offshore earthquakes relocated using the combined method are consistent with those determined using the standard location method and OFS data. The differences in the hypocenters relocated by the two methods are less than 4 km. We applied the method to the subduction region that underlies the Kanto district, central Japan, and located a large number of earthquakes that occurred beneath the Pacific Ocean. We then determined the detailed 3D seismic velocity structure by inverting a large number of arrival times of P-and S-waves and sP depth phase from the relocated earthquakes in the study region. High-velocity anomalies related to the cold subducting Pacific slab and low-velocity anomalies related to the hot mantle wedge are clearly imaged. Beneath active volcanoes, low-velocity zones are visible from the surface to a depth of 100 km, reflecting fluids released by dehydration of the subducting Pacific slab. Strong lateral heterogeneities are revealed on the upper boundary of the Pacific slab beneath the forearc region. The low-velocity areas under the offshore region are associated with low seismicity and weak interplate coupling. A lowvelocity layer is imaged along the upper boundary of the Philippine Sea slab in the northern part of Kanto district, which may reflect dehydration of the slab. Our tomographic images indicate that the overlaying Philippine Sea plate has effects on the spatial distribution of active volcanoes related to the subducting Pacific slab in the study region.

International Journal of Geophysics, 2020

In this study, we present a velocity model for the area of the 2007 Pisco-Peru earthquake ( Mw = 8.0 ) obtained using a double-difference tomography algorithm that considers aftershocks acquired for 6 months. The studied area is particularly interesting because it lies on the northern edge of the Nazca Ridge, in which the subduction of a large bathymetric structure is the origin of geomorphological features of the central coast of Peru. Relocated seismicity is used to infer the geometry of the subduction slab on the northern flank of the Nazca Ridge. The results prove that the geometry is continuous but convex because of the subduction of the ridge, thereby explaining the high uplift rates observed in this area. Our inferred distribution of seismicity agrees with both the coseismic and postseismic slip distributions.

Geophysical Journal International, 1999

The coupled plate interface of subduction zones-commonly called the seismogenic zone-has been recognized as the origin of fatal earthquakes. A subset of the aftershock series of the great Antofagasta thrust-type event (1995 July 30; M w =8.0) has been used to study the extent of the seismogenic zone in northern Chile. To achieve reliable and precise hypocentre locations we applied the concept of the minimum 1-D model, which incorporates iterative simultaneous inversion of velocity and hypocentre parameters. The minimum 1-D model is complemented by station corrections which are influenced by near-surface velocity heterogeneity and by the individual station elevations. By relocating mine blasts, which were not included in the inversion, we obtain absolute location errors of 1 km in epicentre and 2 km in focal depth. A study of the resolution parameters ALE and DSPR documents the importance of offshore stations on location accuracy for offshore events. Based on precisely determined hypocentres we calculate a depth of 46 km for the lower limit of the seismogenic zone, which is in good agreement with previous studies for this area. For the upper limit we found a depth of 20 km. Our results of an aseismic zone between the upper limit of the seismogenic zone and the surface correlates with a detachment zone proposed by other studies; the results are also in agreement with thermal studies for the Antofagasta forearc region.

Geophysical Journal International, 2013

We study the distribution of the aftershocks of Tocopilla M w 7.7 earthquake of 2007 November 14 in northern Chile in detail. This earthquake broke the lower part of the seismogenic zone at the southern end of the Northern Chile gap, a region that had its last megathrust earthquake in 1877. The aftershocks of Tocopilla occurred in several steps: the first day they were located along the coast inside the co-seismic rupture zone. After the second day they extended ocean-wards near the Mejillones peninsula. Finally in December they concentrated in the South near the future rupture zone of the Michilla intermediate depth earthquake of 2007 December 16. The aftershock sequence was recorded by the permanent IPOC (Integrated Plate Boundary Observatory in Chile) network and the temporary task force network installed 2 weeks after the main event. A total of 1238 events were identified and the seismic arrival times were directly read from seismograms. Initially we located these events using a single event procedure and then we relocated them using the double-difference method and a crosscorrelation technique to measure time differences for clusters of aftershocks. We tested a 1-D velocity model and a 2-D one that takes into account the presence of the subducted Nazca Plate. Relocation significantly reduced the width of the aftershock distribution: in the inland area, the plate interface imaged by the aftershocks is thinner than 2 km. The two velocity models give similar results for earthquakes under the coast and a larger difference for events closer to the trench. The surface imaged by the aftershocks had a length of 160 km. It extends from 30 to 50 km depth in the northern part of the rupture zone; and between 5 and 55 km depth near the Mejillones peninsula. We observed a change in the dip angle of the subduction interface from 18 • to 24 • at a depth of 30 km. We propose that this change in dip is closely associated with the upper limit of the rupture zone of the main event. We also studied the focal mechanisms of the aftershocks, most of them were thrust events like the mainshock. As the aftershock activity was significantly reduced, on 2007 December 13, an M L 6.1 event occurred offshore of the Mejillones peninsula reactivating the seismicity. Three days later the Michilla intraslab earthquake of M w 6.8 ruptured an almost vertical fault with slab-push mechanism. The aftershocks locations of this event define a planar zone about 11 km in depth, situated right bellow the subduction interface.

Geophysical Journal International, 1995

The stress distribution along the subducting Nazca plate in northern Chile is analysed using focal mechanism solutions obtained from the inversion of long-period P, SV, and SH waveforms of 15 earthquakes (mb 2 5.5), and from 212 events with reported focal mechanisms, which occurred between 1962 and 1993. A joint hypocentral determination was carried out to control the depth of 261 events (mh 2 5.0) recorded at teleseismic distances. A change from tensional to compressional stress field along the upper part of the subducting slab is associated with the maximum depth extent of the coupled zone. This change occurs in northern Chile at -200-250 km from the trench, at depths of -60 f 10 km. This depth is larger than the maximum depth observed for the thrusting interplate events (40 + 10 km), probably meaning that, at depths of between 40 and 60 km, large low-dip angle thrust events do not nucleate. Seismic slip, however, probably extends down to 40 km in depth. The shallow dip angle (up to 60 km in depth) of the Wadati-Benioff zone does not show variations along the strike of the trench. However, a gradual southward flattening of the slab is observed at distances greater than 200-250 km from the trench. This change, observed from about 21"S, could be associated with a younger and probably more buoyant lithosphere than that observed to the north of this latitude. There are two gaps located between the three main clusters of seismicity; these gaps are clearly not related to detachments in the descending litosphere. The first cluster is located in and beneath the seismogenic interplate contact, and is characterized by reverse and thrust faulting events over a scarce tensional activity. In the second cluster, the compressional seismicity is scarce for teleseismic events and is located beneath the normal faulting events. The third cluster corresponds to tensional events. Therefore, these gaps in seismicity could be associated with alternating changes from compressional to tensional stress field in the subducting slab.