Playa Azul Research Articles

U-Pb Ages of Zircon Grains in the Playa Azul Beach Sediments, Guerrero State, Mexican Pacific

ABSTRACT The mineralogy of bulk sediments, U-Pb ages and chemistry of 195 detrital zircon grains recovered in the Playa Azul beach, Mexican Pacific coast were performed to infer their provenance. The bulk sediments were composed of minerals like quartz, feldspar, titanite, plagioclase, zircon, and magnetite. The average Th/U ratio in zircon grains was ~ > 0.2, indicated an igneous origin. The chondrite normalized rare earth element (REE) patterns of zircons were depleted in low REE (LREE) and enriched in heavy REE (HREE), with positive cerium and negative europium anomalies, indicating a granitoid source. U-Pb ages of zircon grains revealed the predominance of Cenozoic and Mesozoic ages in samples PAC2 (~ 33.8 - 61.8 Ma, n = 90 and ~ 67 - 132 Ma, n = 10, respectively) and PAC19 (~ 0.1 -39.6 Ma, n = 55 and ~ 67 - 251 Ma, n = 20, respectively). Minor peaks were represented by Palaeozoic (n = 9) and Precambrian (n = 11) ages in PAC19. Zircon ages and their morphology indicated that they were mostly derived from the nearby terranes. The source terranes, which supplied Cenozoic zircons to the beach area were the coastal Cenozoic plutons and Cuicateco terrane. The Mesozoic zircons in the Playa Azul coastal sediments were derived from the Mixteca (Acatlan Complex), Guerrero, and Xolapa terranes, located along the Mexican Pacific coastal zone. The Proterozoic zircons were represented by the coastal Oaxacan Complex. In addition, the Arteaga Complex in the Guerrero State, adjacent to the playa Azul beach was the potential source for the Eocene zircons.

Estructura forestal de una zona de manglar en la laguna de Coyuca de Benítez, Guerrero

Los manglares son un recurso forestal de gran importancia en los humedales, tanto por su diversidad y estructura maderable como por los beneficios ambientales y socioeconómicos que representan. Su composición depende de factores ambientales, condiciones edáficas, acción antrópica y de la capacidad de interrelacionarse entre sí para compartir un espacio. En el estado de Guerrero existen solo registros que indican áreas de distribución de bosques de manglar y descripción de especies. El objetivo fue conocer la importancia ecológica del manglar adjunto a las comunidades de El Carrizal y Playa Azul, municipio Coyuca de Benítez, Guerreo, a través de la obtención de su densidad y abundancia relativa, diversidad y volumen maderable. De julio a diciembre del 2016, se obtuvo la composición estructural de la vegetación asociada al manglar, con base en 10 unidades de 10 ×10 m (100 m2) y una intensidad de muestreo de 4 %. Se identificaron nueve especies forestales asociadas al ecosistema de manglar. Se determinó un volumen total de 10.30 m3 rollo total árbol, de los cuales Laguncularia racemosa (mangle blanco) aporta el mayor volumen con 8.05 m3 rollo total árbol; mientras que para el área total de la zona de humedal el valor fue de 218.92 m3 rollo total árbol. Con esta investigación se logró conocer la composición estructural de las especies de flora en el área de estudio, y la importancia ecológica del manglar constituido por L. racemosa, representada como alternativa de conservación para la zona.

Primer registro del colimbo mayor (Gavia immer Brünnich, 1764) en Oaxaca, México

Registramos por primera vez al colimbo mayor (Gavia immer) en las costas del estado de Oaxaca, México. Observamos un individuo veraneante en Playa Azul (13/07/2014), municipio de Salina Cruz, Oaxaca. El individuo presentó plumaje no reproductivo y se observó en buen estado. Debido a la escasez o falta de registros se puede considerar que la región representa el límite de distribución natural para la especie, a lo que se suma un irregular esfuerzo de observación.

Geochemistry of beach sands along the western Gulf of Mexico, Mexico: Implication for provenance

This paper contributes to understanding the intractable problems in provenance study due to hydraulic sorting and geochemical heterogeneity in medium, fine, and very fine sands. For this purpose, detrital modes, major, trace, and rare earth element (REE) compositions of recent sands from the Playa Azul, Tecolutla, and Nautla beach areas of the western Gulf of Mexico have been investigated. Marked geochemical and petrographic differences occur among the three beach sands, even though they are separated just by 45km. The average quartz-feldspar-lithic fragment (QtFL) ratios for the Playa Azul, Tecolutla, and Nautla sands are respectively Qt69–F10–L21, Qt57–F11–L32, and Qt37–F5–L58. The volcanic lithic fragment (Lv)−sedimentary lithic fragment (Ls)−[plutonic lithic (Lp)+metamorphic lithic fragments (Lm)] ternary diagram indicates that the Nautla sands are dominated by volcanic detritus, while the Tecolutla sands are dominated by sedimentary and volcanic detritus. The Playa Azul sands are dominated largely by sedimentary detritus. Geochemically, the three beach sands are quite distinctive from each other. The Playa Azul sands are higher in SiO2 content (∼64–84wt.%) than in the Tecolutla sands (SiO2=∼47–69wt.%). The Nautla sands are very low in SiO2 content (<46wt.%). The contrasting geochemical compositions among the three beach areas are also confirmed by significant variations in Al2O3/TiO2, Na2O/K2O, K2O/Al2O3, Rb/Al2O3, and Cr/Ni ratios. The CIA values (∼39–69; chemical index of alteration) for the three beach areas suggest low to moderate weathering nature.In the three beach sands studied, the decrease in grain size is accompanied by a gradual decrease in SiO2 content and an increase in TiO2, Al2O3, Fe2O3, MgO, Zr, Hf, Cr, and V contents. Similarly, the ∑REE content increases with decreasing grain size. However, very fine sands in the Playa Azul and Nautla beaches are different in ∑REE content. This observation suggests that the provenance is more important in controlling the geochemical composition of beach sands than the grain size. The zirconium concentration in beach sands, however, is not related to the grain size.The comparison of REE patterns of beach sands with those of source rocks located relatively close to the study areas suggests that the Playa Azul sands were derived from felsic rocks, whereas a mixed provenance with contributions from felsic and intermediate rocks is more likely for the Tecolutla sands. In contrast, the REE distribution patterns of Nautla sands resemble those derived from basalt and basaltic andesite. However, selective concentration of magnetite grains in beach sands increases the LREE content and fractionates Eu resulting in a europium anomaly that is more negative than that displayed by Nautla sands. All of the above observations suggest that rivers delivering sands to the beaches are the important factors in controlling the composition of beach sands and that longshore currents play a less significant role.

Alkaloids, coumarins and sesquiterpenes from Esenbeckia conspecta Kunt (Rutaceae)

1. Subject and sourceThe wood of Esenbeckia conspecta Kunth (Rutaceae) was collected in May 1996near Playa Azul, Michoaca´n, Me´xico. The plant was identified by M.C. Clara HildaRamos and a voucher specimen has been deposited at the National Herbarium(MEXU, voucher CH-124) at the Instituto de Biologi´a, UNAM, Me´xico.2. Previous workSpecies of Esenbeckia Kunth (Rutaceae) have been shown to contain quinolinealkaloids [furoquinolines (Rios and Delgado, 1992a), acridones (Bevalot et al., 1984)and quinolones (Guilhon et al., 1994)], indolic alkaloids and amides (Nakatsu et al.,1990), coumarins and furocoumarins (Trani et al., 1997), limonoids (Dreyer, 1980),sesquiterpenoids, polyprenols, phloroglucinols and triterpenes with friedelane, lupane(Rios and Delgado, 1992b) and dammarane (Mata et al., 1998) skeleta, sterols, fla-vonoids (Kubo, 1991) and cinnamic acid derivatives (Guilhon et al., 1994) as the

Coda wave attenuation parallel and perpendicular to the Mexican Pacific coast

We calculated the quality factor, Q c, at frequencies from 6 to 24 Hz using coda waves of 97 aftershocks of the Petatlan, Mexico, earthquake (March 14, 1979; M S=7.6). The data were recorded parallel (between Acapulco and Playa Azul) and perpendicular (between Petatlan and Mexico City) to the coast. The results are the following: at 12 and 24 Hz there is no significant difference in the attenuation ( Q c −1) along the two paths; at 6 Hz, Q c −1 has a large scatter in both directions. This observation indicates strong site effects at this frequency; average Q c −1 is slightly higher between Petatlan–Acapulco (toward SE) than between Petatlan–Playa Azul (toward NW); and at high frequencies, Q c −1 remains essentially constant perpendicular to the coast. These results show that the large seismic wave amplifications in Mexico City are caused by shallow site effects.

Dynamic rupture of asperities and stress change during a sequence of large interplate earthquakes in the Mexican subduction zone

Abstract We investigate the spatial and temporal variations of shear stress due to the successive failures over an extensive segment of the Mexican subduction zone during a sequence of large interplate earthquakes that occurred over a period of 13 yr. For this purpose, we develop 3D dynamic rupture models incorporating a shallowly dipping fault located above the subducting plate. The spatial distribution of dynamic stress drop over the fault has been estimated for each of the events, through an inversion procedure using some of the previously derived kinematic fault parameters as observational constraints. The results revealed quite heterogeneous stress changes during these earthquakes coming from medium to high dynamic stress drop due to the rupture of a few patch-like asperities and from stress increase in between and around them. Two weak asperities located southeast of the Michoacan segment were ruptured first by the 1979 Petatlan event. The 1981 Playa Azul event ruptured two asperities in the central zone with a stress drop higher than 80 bars. The largest 1985 Michoacan earthquake resulted from the rupture of two large-size, strong asperities located at both sides of the 1981 fault zone with high stress drop of 80 to 100 bars and from another two asperities at depth. Two days after this largest event, two asperities were broken during the Zihuatanejo aftershock in the southeastern adjacent zone. Many aftershocks of these large events tend to be distributed in the zones of stress increase outside the asperities, while only small numbers of aftershocks have been observed within these asperity zones. It appears that several major asperities that existed in this extensive segment have been ruptured successively so as to fill unbroken gaps on the plate interface. Thus, the stress change left over from the previous earthquake has dominant effects on the next event in this subduction zone.

Volcaniclastic Sequences With Continental Affinities Within the Late Jurassic‐Early Cretaceous Guerrero Intra‐Oceanic Arc Terrane (Western Mexico)

The Guerrero terrane in western Mexico consists of upper Jurassic‐lower Cretaceous volcano‐plutonic and volcano‐sedimentary arc sequences developed in an intra‐oceanic setting and accreted to North America late in the Early Cretaceous. The Playa Azul sequence belongs to the southern Pacific Province of the Guerrero terrane (PPGT), which extends along the Pacific coast from Puerto Vallarta to Acapulco. This sequence is composed of pyroclastic rocks and rare lava flows interlayered with red detrital rocks and coastal calcarenites. The red beds yielded Early Cretaceous dinosaur footprints. The whole sequence is capped by Albian to Cenomanian reefal limestones. The rare basalts are olivine‐phyric and include altered amphibole‐bearing cumulates. The most common facies are amphibole‐biotitephyric andesites and dacites. The welded, or not, tuffs have K‐feldspar, quartz, and plagioclase phenocrysts, while their groundmass includes biotite and zircon. Amphibole ranges in composition from pargasitic hornblende to edenite similar to volcanic rocks of islands arcs and continental margins. The mafic rocks are Ti‐, Ta‐, and Nb‐depleted and belong to mild tholeiitic and calc‐alkaline suites. They show transitional geochemical characteristics between lavas from oceanic island arc and active continental margins. The felsic rocks belong to calc‐alkaline suites. The Playa Azul rocks exhibit a wide range of ϵNd ratios, but the basalts have ϵNd ratios that range from +9 to +2. The predominance of hornblende‐biotite‐phyric felsic rocks in the Playa Azul sequence suggest that it was built on a thick basement. The low ϵNd ratios of some mafic rocks imply that the sediments played a key role in the genesis of some basalts. A basement formed of continental crust is highly unlikely because the felsic rocks do not show continental contamination. Thus, the Playa Azul sequence was probably built on an oceanic crust thickened by sediments such as the Arteaga complex. Compared to the neighboring arc sequences along the Pacific coast, the Playa Azul sequence differs by its subaerial environment, the predominance of hornblende‐biotite felsic rocks, and a wide diversity of the rock facies. However, like all the other igneous rocks of the Guerrero terrane, the Playa Azul lavas derived from a mantle source that was little affected by crustal components. A geodynamic model of the PPGT is proposed based on these differences and similarities.

Coseismic slip of two large Mexican earthquakes from teleseismic body waveforms: Implications for asperity interaction in the Michoacan Plate Boundary Segment

Teleseismic body waves recorded for the October 25, 1981, Playa Azul and September 21, 1985, Zihuatanejo earthquakes in western Mexico were inverted to derive the distributions and depths of coseismic slip. Broadband P wave displacements and intermediate‐period records were included to incorporate a wide frequency band in the analysis. For the Zihuatanejo earthquake, digital SH records located away from the nodal directions were used in the inversion process. Nonnodal SH waveforms were not available for the Playa Azul earthquake, but additional azimuthal coverage was obtained by including selected worldwide analog P wave records. The results for the Playa Azul earthquake indicate that rupture occurred in two separate zones both updip and downdip of the point of initial nucleation with the majority of the slip concentrated in a circular region (15‐km radius) downdip from the hypocenter. The maximum slip in this downdip region exceeds 3.6 m and is at a depth of about 16 km. The computed seismic moment is 7.14 × 1026 dyn cm. Coseismic slip is observed to occur entirely within the area of reduced slip separating the two main shallow sources of the Michoacan earthquake that occurred almost 4 years later on September 19, 1985. For the Zihuatanejo earthquake the P and SH data suggest rupture over a larger area (30‐km radius) with a lower peak slip (2 m) and encompassing depths between 12 and 26 km. Slip is concentrated in an area adjacent to one of the major sources of the Michoacan earthquake and represents the southeastern continuation of rupture along the Cocos‐North America plate boundary. The corresponding seismic moment is 1.35 × 1027 dyn cm. The zones of peak slip observed for the Playa Azul, Zihuatanejo, and Michoacan earthquakes are interpreted as asperity regions that control the cessation and generation of large earthquakes within the Michoacan segment of the plate boundary. The stress drops within each of these asperities are observed to be very similar and may reflect a characteristic asperity strength for this portion of the subduction zone.

Slip distribution of the 19 September 1985 Michoacan, Mexico, earthquake: Near-source and teleseismic constraints

Abstract We simultaneously invert the strong-motion velocity records and the long- and intermediate-period teleseismic P waveforms of the 19 September 1985 Michoacan, Mexico, earthquake to recover the distribution of slip on the fault using a point-by-point constrained and stabilized, least-squares inversion method. A fault plane with strike fixed at 300° and dip fixed at 14° is placed in the region of the earthquake hypocenter and divided into 120 subfaults. Rupture is assumed to propagate at a velocity of 2.6 km/sec away from the hypocenter. Synthetic near-source ground motions and teleseismic P waveforms for pure strike-slip and dipslip dislocations are calculated for each subfault. The observed data are then inverted to obtain the amount of strike-slip and dip-slip displacement required of each subfault. We also invert the data sets using a time-window procedure where the subfaults are allowed to slip up to three times. This approach relaxes the constraint of fixed subfault rupture time imposed by a constant rupture velocity. Inversion of the strong-motion data alone yields a slip model similar to the solution previously obtained using only teleseismic waveforms. This result supports the use of teleseismic waveform data for the derivation of fault dislocation models in the absence of strong-motion recordings. Our simultaneous inversion of both data sets suggests that rupture during the Michoacan earthquake was controlled largely by the failure of three major asperities located along the length and down the dip of a 150-km segment of the Cocos-North America plate boundary. The solution contains three major source regions including an 80 km by 55 km source near the hypocenter with a peak slip of 6.5 meters. Two additional sources are present on the southeast portion of the fault about 70 km away from the hypocenter. One of these sources, with a peak slip of 5 meters, covers a 45 km by 60 km area and extends downdip from a depth of about 10 km to 24 km. The third source region is somewhat smaller (30 km by 60 km area, 3.1-meters peak slip) and extends further downdip at depths between 27 km and 39 km. Aftershock activity following the earthquake was associated mainly with the two shallow sources. These two sources are separated by the aftershock zone of the 1981 Playa Azul earthquake.

Source characteristics of earthquakes in the Michoacan seismic gap in Mexico

AbstractWe investigated the source characteristics of large earthquakes which occurred in the Michoacan, Mexico, seismic gap during the period from 1981 to 1986 in relation to historical seismicity in the region. The rupture pattern of the Michoacan gap during this period can be characterized by a sequential failure of five distinct asperities. Before 1981, the Michoacan gap had not experienced a large earthquake since 1911 when an MS = 7.8 earthquake occurred. The recent sequence started in October 1981 with the Playa Azul earthquake which broke the central part of the gap. Body-wave modeling indicates that the Playa Azul earthquake is 27 km deep with a seismic moment of 7.2 × 1027 dyne-cm. It is slightly deeper than the recent Michoacan earthquakes, and its stress drop is higher, suggesting a higher stress level at depths in the Michoacan gap. The seismic moment of the 19 September 1985 (Mw = 8.1) earthquake was released in two distinct events, with the rupture starting in the northern portion of the seismic gap and propagating to the southeast with low moment release through the area already broken by the 1981 Playa Azul earthquake. The rupture propagated further southeast with an Mw = 7.5 event on 21 September 1985. Another aftershock occurred on 30 April 1986 to the northwest of the 19 September main shock. Body-wave modeling indicates that this event has a simple source 10 sec long at 21 km depth, and fault parameters consistent with subduction of the Cocos plate (ϑ = 280°, δ = 12°, and λ = 70°) and M0 = 2.0 to 3.1 × 1026 dyne-cm (Mw = 6.8 to 6.9). Although this distribution of asperities is considered characteristic of the Michoacan gap, whether the temporal sequence exhibited by the 1981 to 1986 sequence is also characteristic of this gap or not is unclear. It is probable that, depending on the state of stress in each asperity, the entire gap may fail in either a single large event with a complex time history or a sequence of moderate to large events spread over a few years. The seismic moment and the time since the last earthquake in Michoacan (in 1911) fit an empirical relation between moment and recurrence time found for the Guerrero-Oaxaca region of the Mexico subduction zone.

Source mechanism of the September 19, 1985 Michoacan Earthquake and its implications

Moment tensor inversion of low frequency teleseismic data for the September 19, 1985 Michoacan, Mexico earthquake gave an average strike 298°, slip 268°, and seismic moment 10.3×1027 dyne‐cm. Low frequency body waves clearly show the complex nature of this event. Cross‐correlation of the September 19 P‐wave with that of the smaller, and colocated October 25, 1981 Playa Azul earthquake, show that the Michoacan earthquake consisted of two subevents separated in time by 25 s, and located approximately 30 km northwest and 40 km southeast respectively, of the Playa Azul epicenter. Body wave inversion of the first subevent gave strike 298°, slip 266°, and seismic moment 3.0×1027 dyne‐cm. The second subevent had a comparable seismic moment. The cumulative moment for large earthquakes occurring along the Michoacan‐northwest Guerrero coast indicate that seismic displacement along this segment of the Mexican subduction zone is similar to that occurring on other portions, indicating that the "former" Michoacan gap was not anomalously aseismic.

Source characteristics of the 1985 Michoacan, Mexico Earthquake at periods of 1 to 30 seconds

Source characteristics of the Sept. 19, 1985 Michoacan, Mexico earthquake and its aftershock on Sept. 21 were inferred from broadband and short‐period teleseismic GDSN records. We Fourier‐transformed the P waves, corrected for instrument response, attenuation, geometrical spreading, and radiation pattern (including the depth phases), and then averaged to obtain the teleseismic source spectrum from 1 to 30 s. The Michoacan source spectrum is enriched at 30 s and depleted at 1 to 10 s relative to an average source spectrum of large interplate subduction events. Source spectra for the Sept. 21 aftershock, 1981 Playa Azul, 1979 Petatlan, and 1978 Oaxaca events follow a trend similar to that of the 1985 Michoacan event. This spectral trend may characterize the Mexican subduction zone.A station‐by‐station least‐squares inversion of the Michoacan earthquake records for the source time function yielded three source pulses, which we interpreted as events on the fault plane. The first two are similar in moment, and the third contains only 20% of the moment of the first. Directivity is evident in the timing. At each station, we measured the time differences between the pulses, and performed a least‐squares nonlinear estimation of the strike, distance, and time separation between the events to locate them relative to one another. The second event occurred 26 s after the first, and 82 km southeast of it, indicating southeastward rupture along the trench. The third event occurred 21 s after the second, and about 40 km seaward of it. The two large events are also seen in the near‐field strong motions.The mainshock records, spectrum, and time functions contain less high frequency radiation than those of the 1985 Valparaiso, Chile earthquake. Apparently, the Michoacan earthquake ruptured two relatively smooth, strong patches which generated large 30 s waves, but small 1 to 10 s waves. Such behavior contrasts with the Valparaiso event which had a more complex rupture process and generated more 1 to 5 s energy. This difference is consistent with the higher near‐field accelerations recorded for the Valparaiso event.

Tectonic Setting and Source Parameters of the September 19, 1985 Michoacan, Mexico Earthquake

Analysis of body waves and long‐period surface waves from the September 1985 earthquake in coastal Michoacan, Mexico shows that the event was an interplate subduction event with a low dip angle fault plane (δ=9°) striking parallel to the Mid‐America trench (ϕ=288°) and a small component of left lateral motion (λ=72°) with a point source depth of 17 km, and a seismic moment in excess of 1 × 1028 dyn cm. The earthquake was a multiple event, with a second source of identical moment, fault geometry, and depth occurring approximately 26 s after the first. Directivity in the body wave time function indicates that the second event occurred roughly 100 km to the southeast of the first. This suggests that the earthquake first broke the northern portion of the Michoacan gap, propagated with low moment release through the rupture zone of the 1981 Playa Azul earthquake, and then broke the remaining asperity in the southern section of the gap. The seismic moment determined from Rayleigh and Love waves is between 1.0 ‐ 1.7 × 1028 dyn cm (MW = 7.9 ‐ 8.1), the largest moment determined to date for a Mexico subduction earthquake. Comparison of seismograms at Pasadena with records of other large Mexico events shows that the Michoacan earthquake is basically the same size as the 1932 Jalisco, Mexico earthquake, and clearly larger than other significant events in Mexico since 1932. The seismic moment and the time since the last large earth‐quake in Michoacan (in 1911) fit an empirical relation between moment and recurrence time found for the Guerrero‐Oaxaca region of the subduction zone. The large aftershock on September 21 (Ms=7.5) has the same geometry as the mainshock, a somewhat larger source depth (22 km), a simple time function, and a seismic moment between 2.9 ‐ 4.7 × 1027 dyn cm (Mw = 7.6 ‐ 7.7).

Seismogenesis of the 1985 Great (Ms=8.1) Michoacan, Mexico Earthquake

The size and source complexity of the great (Ms = 8.1) Michoacan, Mexico earthquake can be attributed to both a restriction of the downdip width of the seismic interface between the overriding and downgoing plates, and a uniform distribution of asperities. The seismogenic evidence of strain accumulation during the 20 years prior to the 19 September, 1985 mainshock closely resembles that observed prior to the smaller MW = 7.6 earthquakes in the adjacent Colima (1973) and Petatlán (1979) regions: the subsequent rupture zones are seismically quiescent (mb ≥4.0) for ∼ 2.5 ‐ 4 years prior to the mainshocks; the locked thrust interface is loaded by aseismic slip and normal faulting within the downgoing plate below 25‐30 km. There is evidence that the 1981 Playa Azul earthquake (MW = 7.3) is an integral stage of the evolutionary process of strain accumulation and release in the Michoacan area and could thus be considered a foreshock to the great 1985 earthquake.

Stress distribution and subduction of aseismic ridges in the Middle America Subduction Zone

The regional distribution of stresses associated with the subduction of the Cocos plate is inferred from a synthesis of 190 earthquake focal mechanisms, body and surface wave analyses of large earthquakes, and seismicity distributions. Broad patterns of consistent behavior are found across the region, from the Rivera Plate boundary in the northwest to the Guatemala/El Salvador border in the southeast, and are used as a framework to evaluate evidence for variations in local stresses due to the subduction of two aseismic ridges, the Tehuantepec Ridge and the Orozco Fracture Zone. Information which bears on the seismic potential at locations of aseismic ridge subduction is particularly important in that no large (Ms ≥ 7.5) earthquakes have occurred historically. We identify three major zones with consistent patterns in focal mechanisms and hypocentral distributions of seismicity. The first, closest to the trench and reflecting the mechanical interaction of the converging plates, is a zone of shallow thrust earthquakes extending 100–150 km inland from the trench. The second is a zone of normal faulting, beginning at about 200 km inland from the trench, h ≥ 60 km, which extends continuously along the entire length of the descending plate throughout the region. The third distinct zone exhibits a relatively low level of activity and separates the zones of thrust and normal faulting at about 150–200 km inland from the trench. This zone extends from the Rivera plate boundary in the northwest to the Guatamala region in the southeast. At this point, the quiet region pinches out, and the thrust and normal faulting zones abut and overlap. Superimposed on this overall pattern, we find locally only minor changes in areas of aseismic ridge subduction, aside from the prominent seismic slip gaps. Furthermore, on October 25, 1981, the Playa Azul earthquake (Ms = 7.3) occurred in the midregion of the Orozco Fracture Zone. Body and surface wave analyses of this event show a simple source rupture and shallow thrust fault mechanism, as are found elsewhere in the region. The seismic moment is MO = 1.3 × 1020 N m; the calculated stress drop is 4.5 MPa, not extraordinarily high, as might be expected in these pervasive seismic gaps. An event in the Tehuantepec Ridge region, on January 24, 1983, Ms = 6.7, was a large normal faulting event, but further interpretation is ambiguous due to the proximity of other normal faulting. We conclude that while aseismic slip may be occurring in the areas of ridge subduction, the possibility of large thrust earthquakes cannot be ruled out, due to the overall similarities with adjacent regions.

Correlation of foreshocks and aftershocks and asperities

A close correlation in spatial distribution of local seismic activity and energy release patterns before and after the 1979 Petatlan, Mexico earthquake suggests heterogeneity within the fault plane of this major low-angle thrust event associated with subduction along the Middle America Trench. A simple two-asperity model is proposed to account for the complexity. Foreshocks and aftershocks of the neighboring 1981 Playa Azul earthquake showed a similar pattern. As both events occurred at the junction of the Orozco Fracture Zone and the Middle America Trench, we speculate that the observed complex fault plane is caused by subduction of the rugged ocean floor of the Orozco Fracture Zone. Short-term precursory seismicity prior to the Petatlan earthquake can be explained by using the asperity model and migration of a slip front from the south-east to the north-west across the main shock source region.

Playa Azul, Michoacan, Mexico, earthquake of 25 October 1981 (Ms = 7.3)

abstract The Playa Azul earthquake broke the central part of the Michoacan gap along the Mexican subduction zone. The aftershock area estimated from field data for a 6-day period beginning 19 hr after the main shock is 40 km × 20 km. Preliminary focal mechanism of the main shock is consistent with a shallow-dipping thrust fault. Although there is some evidence of seismic quiescence for 5 1 2 yr followed by a foreshock about 3 months before the main shock and an above average microseismic activity in the western part of the epicentral region during the last 4 1 2 months or more, the data is not sufficient quality to assert this conclusion. The Michoacan gap was previously assigned category 3 (Singh et al. , 1981) based on the classification of McCann et al. (1979). The seismic potential of the unbroken segments of this gap (∼45 km to NW and ∼35 km to the SE of the Playa Azul aftershock area) remains uncertain. Corresponding to a rupture length of 40 km, each segment could generate an earthquake of similar magnitude to the Playa Azul earthquake. Unfortunately, the recurrence period of large earthquakes in the region is not known and great/major earthquakes cannot be ruled out.

Eastern Pacific Hurricane Season of 1971

Abstract The 1971 season was characterized by recurrent and persistent patterns of activity. A record 12 hurricanes and six tropical storms were counted. Satellite pictures and ship reports provided most of the clues to developing storms. U.S. Air Force Weather Reconnaissance provided most of the location, intensity, and structure information after development was underway. Several of the hurricanes and storms rate special attention: Agatha, a small violent May hurricane, hit the village of Playa Azul, Mexico; Bridget did $40 million damage at Acapulco, Mexico; Denise, Francene, Olivia, and Priscilla had reported surface winds of 100 kt or more; Monica was described by reconnaissance (which could not penetrate) as the strongest ever seen in the area; Katrina was an elusive ministorm that caused floods at Los Mochis, Mexico; Lily, a violent, recurving hurricane, took 12 lives in Mexico and caught several ships in her hurricane-force winds; and Olivia had an earlier existence as Caribbean hurricane Irene.