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Base computarizada de datos sobre Tectónica y Geología estructural de Centroamerica y el Caribe (Tectobase)
(Universidad de Costa Rica, 1985) Aguilar D., Álvaro; Alvarado V., Fernando; Obando A., Luis; Protti, Marino
Presentación de proyecto de banco computarizado sobre la bibliografía publicada sobre sobre Tectónica y Geología Estructural de Centroamérica y el Caribe (TECTOBASE)
Interpretación preliminar de la secuencia sísmica de potrero grande (marzo 1988) y su relación con la secuencia sísmica de sierpe (marzo de 1987)
(Universidad Nacional, Costa Rica, 1992) Protti, Marino; Guendel, Federico; Montero, Carlos; Brenes, Jorge
El sismo de magnitud 5.3 grados en la escala de Ritcher se convierte en el más importante después de los terremotos de abril y junio de 1983, dado que es el primero que se registra en la recién instalada Red Sismográfica Nacional, operada por el OVSICORI-UNA. Este documento realiza una interpretación preliminar sobre las características epicentrales e ipocentrales del sismo. Así como el estudio de las intensidades del evento principal. Se presenta el mapa de intensidades según la escala de Mercalli, combinando la información del mapa con una gira efectuada a la zona de mayores intensidades. En la gira a la zona sur del país se utilizó un cuestionario de intensidades suministrado por los reporteros sísmicos del OVSICORI-UNA, como parte del proyecto de investigación "Sistematización de reportes de intensidad sísmica (UNA-CONICIT)". Durante el trabajo de campo se utilizó una encuesta para determinar la intensidad máxima alcanzada por el sismo en los lugares visitados.
The geometry of the Wadati-Benioff zone under southern Central America and its tectonic significance: results from a high-resolution local seismographic network
(Elsevier, 1994) Protti, Marino; Gündel, Federico; McNally, Karen
We present here a detailed geometry of the Wadati—Benioff zone under Costa Rica, obtained from seismicity recorded by a dense local seismographic network jointly operated by the Costa Rica Volcanological and Seismological Observatory, National University, and the Charles F. Richter Seismological Laboratory, University of California, Santa Cruz. Underneath the Nicaragua—Costa Rica border the Wadati—Benioff zone smoothly contorts (from steep to shallow dip angles, NW to SE), but does not show evidence of a brittle tear, as postulated by others. However, further to the SE, NE of Puerto Caldera, Costa Rica, the Wadati—Benioff zone does show a segmentation (the Quesada Sharp Contortion) at intermediate depths (h > 70 km). NWof this sharp contortion the deepest portion of the seismically active slab dips at about 800 and reaches maximum depths ranging from 200 km, near the Nicaragua—Costa Rica border, to 135 km under Ciudad Quesada. To the SE the deeper portion of the Wadati—Benioff zone dips at about 60°and the seismicity does not extend below depths ranging from 125 km, behind the volcanic arc, to 50 km, east of Quepos. In southern Costa Rica, east of 83°55’W, we find no evidence of the Wadati—Benioff zone deeper than 50 km. The obtained geometry and other known tectonic features related to the subduction of the Cocos plate under the Caribbean plate along the southern terminus of the Middle America Trench (Nicaragua and Costa Rica) correlate well with along-trench variations in age of the subducted Cocos plate. Some of these tectonic features are: (1) the shallowing of Middle America Trench bathymetry from NW to SE; (2) variations in the energy release within the subducted slab; (3) differences in coupling between Cocos and Caribbean plates; (4) the termination of the Central America Volcanic Chain in central Costa Rica; (5) distinct stress field variations on the overriding Caribbean plate. The subduction of the Cocos Ridge under southern Costa Rica is partially responsible for some of these features (shallowing of the Middle America Trench in southern Costa Rica) and for the high uplift rates of the outer arc. However, as the presence of the Panama Fracture Zone limits the subducted extension of the Cocos Ridge to less than 100 km from the trench, we propose that the overall geometry of the Wadati—Benioff zone is controlled by abrupt along-trench changes in the age of the subducted Cocos plate.
Mechanics of back arc deformation in Costa Rica: Evidence from an aftershock study of the April 22, 1991, Valle de la Estrella, Costa Rica, earthquake (Mw=7.7)
(American Geophysical Union, 1994-10) Protti, Marino; Schwartz, Susan
The April 22, 1991, Mw=7.7 Valle de la Estrella, Costa Rica, earthquake represents back arc thrusting of the Caribbean plate beneath the Panama block along the North Panama Thrust Belt. Large back arc thrusting events are quite rare, occurring in only two other locations along the Sunda Arc and JapanS ea. To betteru nderstandth e mechanicso f back arc thrusting, we constrain the faulting geometry associated with the 1991 Costa Rica earthquake using aftershock locations and focal mechanisms obtained from a threecomponent portable digital network deployed in and around the aftershock area following the mainshock. The spatial distribution of aftershocks reveals a complicated faulting geometry in the rupture area. Focal mechanisms determined from inversion of P wave and tangentially and radially polarized S wave (SH and SV, respectively) amplitudes recorded by this temporary network confirm fault complexity and indicate active thrust, normal and strike-slip faults in the back arc of Costa Rica. Most of the thrust events are confined to the southern portion of the aftershock zone in the vicinity of the mainshock. Their distribution suggests the existence of a near-horizontal basal fault plane at a depth of about 15 km, with many imbricate faults having steeper dips extending from the basal plane toward the surface. Events with strike-slip mechanisms locate northwest of the thrust events and define a SW-NE trending, left-lateral strike-slip fault zone that represents the NW termination of the mainshock rupture and possibly the maximum NW extension of the Panama block. The superposition of the aftershock locations on a geologic map of the region shows that aftershocks are restricted to occur in the older, more competent rock units (volcanic and volcaniclastic rocks interbedded with carbonates) of the back arc sedimentary basin. Shallow events (depth<5 km) occur only where these oldest units are exposed at the surface. This suggests that (1) exposure of the lower units results from repeated earthquake slip on the shallow crustal faults imaged by the aftershocks and (2) as much as 7 km of basin fill material, overlying the lower units, northeast of the mainshock, does not deform seismically but, instead, folds and possibly faults aseismically.
Correlation between the age of the subducting Cocos Plate and the geometry of the Wadati-Benioff zone under Nicaragua and Costa Rica.
(Universidad Nacional, Costa Rica, 1995) Protti, Marino; Güendel, Federico; McNally, Karen
High- resolution seismicity data from Costa Rica and southern Nicaragua have been used to image the geometry of the Wadati-Benioff zone under southern Central America. The data include 9,514 events with computed horizontal and vertical errors smaller than 4 and 5 km respectively. We found that under the Nicaragua- Costa Rica border the Wadati- Benioff zone contorts (from steep to shallow dip angles, northwest to southeast) but does not show evidence of a brittle tear, as postulated by others. Further to the southeast-northeast from Puerto Caldera- the Wadati-Benioff zone does, however, show a tear (the Quesada Sharp Contortion) at intermediate depths (h >70 km). Overall the dip angle of the Wadati-Benioff zone decreases from 84° under Nicaragua to 60° under central Costa Rica. The maximum depth of interaplate Wadati-Benioff zone earthquakes also decreases from northwest (-200 km under Nicaragua) to southeast (-125 km under central Costa Rica). In southern Costa Rica, east of 83°55 W, we find no evidence of the Wadati-Benioff zone below 50 km. This geometry of the Wadati-Benioff zone and other tectonic features related to the subduction of the Cocos plate beneath the Caribbean plate have been integrated into a model that correlates them with along- trench variations in age of the subducted Cocos plate. These tectonic featuresare (1) the shallowing of the Middle America Trech bathymetry from northwest to southeast, (2) differences in coupling between Cocos and Caribbean plates, (3) the termination of the Central America Volcanic Chain in central Costa Rica, and (4) distinct stress field variations on the overriding Caribbean plate. The subduction of the Cocos Ridge under southern Costa Rica is partially responsible for somo of these features (shallowing of the Middle America Trench in southern Costa Rica) and for the high uplift rates of the outer arc, However, since the presence of the Panama Fracture Zone limits the subducted extension of the Cocos Ridge to less that 100 km from the trench, we propose that the overall geometry of the Wadati- Benioff zone is controlled by abrupt along trench changes in the age of the subducted Cocos plate.
The March 25, 1990 (Mw=7.0, ML=6.8), earthquake at the entrance of the Nicoya Gulf, Costa Rica: Its prior activity, foreshocks, aftershocks, and triggered seismicity
(American Geophysical Union, 1995-10-10) Protti, Marino; McNally, Karen; González, Victor; Montero, Carlos; Velasco, Aaron; Segura, Juan; Brenes, Jorge; Barboza, Vilma; Malavassi, Eduardo; Gündel, Federico; Simila, Gerald; Rojas, Daniel; Mata, Antonio; Schillinger, Walter; Pacheco, Javier Francisco
On March 25, 1990 a large earthquake (Mw = 7.0, ML = 6.8) occurred at the entrance of the Nicoya Gulf, Costa Rica, at 1322:55.6 UTC, producing considerable damage in central Costa Rica and generating much interest about whether or not the Nicoya seismic gap (Nishenko, 1989) had broken. The local country-wide seismographic network recorded 6 years of activity prior to this large earthquake, 16 hours of foreshocks, the mainshock, and its aftershocks. This network is operated jointly by the Costa Rica Volcanological and Seismological Observatory at the National University (OVSICORI-UNA), and the Charles F. Richter Seismological Laboratory at the University of California, Santa Cruz (CFRSL-UCSC). We obtained high resolution locations from this network and located the mainshock at 9°38.5′N, 84°55.6′W (depth is 20.0 km) and the largest foreshock (Mw = 6.0, March 25, 1990, at 1316:05.8 UTC) at 9°36.4′N, 84°57.1′W (depth is 22.4 km). We find that the aftershock zone abuts the southeast boundary of the Nicoya seismic gap, suggesting that the seismic gap did not rupture. Since the installation of the local network in April 1984 to March 24, 1990, nearly 1900 earthquakes with magnitudes from 1.7 to 4.8 (318 with magnitude 3.0 or larger) have been located at the entrance of the Nicoya Gulf, one of the most active regions in Costa Rica. The March 25 earthquake occurred at the northwest edge of this region, where a sequence of foreshocks began 16 hours prior to the mainshock. The spatial-temporal distribution of aftershocks and directivity analysis of the mainshock rupture process using teleseismic records both indicate a southeast propagating rupture. The mainshock ruptured an asperity of approximately 600 km2 of area, with this area expanding to 4000 km2 after 7 days. We present evidence that suggests that the ruptured asperity is produced by the subduction of a seamount. Inversion of teleseismic broadband and long-period P and SH waves yields a thrust faulting mechanism with the shallow plane striking 292°, dipping 26°, and with a rake of 88°, in agreement with the subduction of the Cocos plate under the Caribbean plate. Local first motions for the largest foreshock and the mainshock agree with this solution. We also present evidence suggesting that the March 25, 1990, earthquake triggered and reactivated several seismic swarms in central Costa Rica and temporally decreased the activity in the epicentral area of the July 3, 1983 (Ms = 6.2), Pérez Zeledón earth
The March 25, 1990 (Mw=7.0 Ml=6.8) earthquake at the entrance of the Nicoya Gulf, Costa Rica: its prior activity, foreshocks, aftershocks and triggered seismicity
(1995-10-10) Protti, Marino; McNally, Karen; Pacheco, Javier; González, Victor; Montero, Carlos; Segura, Juan; Brenes, Jorge; Barboza, Vilma; Malavassi, Eduardo; Güendel, Federico; Simila, Gerald; Rojas, Daniel; Velasco, Aaron; Mata, Antonio; Schillinger, Walter
On March 25, 1990 a large earthquake( Mw=7.0, ML=6.8) occurreda t the entranceo f the Nicoya Gulf, CostaR ica, at 1322:55.6U TC, producingc onsiderabled amagei n centralC osta Rica andg eneratingm uchi nteresta boutw hethero r not the Nicoya seismicg ap (Nishenko,1 989) had broken.T he local country-wides eismographince tworkr ecorded6 yearso f activityp rior to thisl argee arthquake1, 6 hourso f foreshockst,h e mainshocka, ndi ts aftershocksT.h is networki s operatedjo intly by the CostaR ica Volcanologicala nd SeismologicaOl bservatorya t the National University( OVSICORI-UNA), andt he CharlesF . RichterS eismologicaLl aboratorya t the Universityo f California,S antaC ruz (CFRSL-UCSC).W e obtainedh igh resolutionlo cations from thisn etworka ndl ocatedt hem ainshocka t 9ø38.5'N,8 4ø55.6'W( depthi s 20.0 km) andt he largestf oreshock(M w=6.0,M arch2 5, 1990,a t 1316:05.8U TC) at 9ø36.4'N,8 4ø57.1'W( depth is 22.4 km). We find that the aftershockz one abutst he southeasbt oundaryo f the Nicoya seismic gap, suggestingth at the seismicg ap did not rupture.S incet he installationo f the local networki n April 1984 to March 24, 1990, nearly 1900 earthquakews ith magnitudesfr om 1.7 to 4.8 (318 with magnitude3 .0 or larger)h aveb eenl ocateda t the entranceo f the Nicoya Gulf, one of the mosta ctiver egionsi n CostaR ica. The March 25 earthquakeo ccurreda t the northweste dgeo f this region,w herea sequencoe f foreshockbs egan1 6 hoursp rior to the mainshockT. he spatialtemporadl istributiono f aftershockasn dd irectivitya nalysiso f the mainshockru pturep rocess usingt eleseismicre cordsb othi ndicatea southeasptr opagatingru pture.T he mainshockru ptured ana speriotyf approximat6e0ly0k m2 ofa reaw, itht hisa reae xpanditnog4 000k m2 afte7r days. We presenet videnceth ats uggesttsh att her uptureda sperityis producedb y the subductioonf a seamountI.n versiono f teleseismicb roadbanda ndl ong-periodP and SH wavesy ieldsa thrust faultingm echanismw ith the shallowp lanes triking2 92ø , dipping2 6ø , andw ith a rakeo f 88ø , in agreemenwt ith the subductiono f the Cocosp lateu ndert he Caribbeanp late.L ocal first motions for the largestf oreshocka ndt he mainshocka greew ith thiss olutionW. e alsop resente vidence suggestintgh at the March 25, 1990,e arthquaketr iggereda ndr eactivateds everals eismics warmsin centralC ostaR ica andt emporallyd ecreasedth e activityi n the epicentraal reao f the July 3, 1983 (Ms=6.2), P6rez Zeled6n earthquake.
Seafloor structural observations, Costa Rica accretionary prism
(1996-04-15) McAdoo, Brian G.; Orange, Daniel L.; Silver, Eli; McIntosh, Kirk; Abbott, Lon; Galewsky, Joseph; Kahn, Leslie; Protti, Marino
By studying seafloor morphology we can make associations between near surface defornation, fluid flow and the overakk structural framework of accretionary prisms. In February, 1994 a DS/RV ALVIN program to the Costa Rica accretionary prism investigated the relationship of fluid seepage and sediment deformation by using the distribution of chemosynthetic communities and heat flow anormalies as indicators of fluid flow. The active normal faults that cut the hemipelagic section on the Cocos plate may provide conduits for fluids that cause the regional heat flow to be extremely low. These normal faults intersect the toe of the prism at an oblique angle, creating localized regions of increased deformation. Positive heat flow anormalies observed at the deformation. Positive heat flow anormalies observed at the deformation front indicate of flow, however, we discovered ni seep communities indicative of focused flow. The seaward-most seep communities discovered are in a region of active out-of-sequence thrusts that cut a sediment apron which covers the complex to within 3 km of the prism toe. Vents occur consistently at the base of the fault scarps. Dives in a mud diapir show extensive seep communities, pock marks, and authigenic carbonates. Evidence of a fluid release is on the crest which implies a low viscosity fluid migrating upward in the center of the structure. Normal faults on the upper slope can be seen in cross- section in the walls of a submarine canyon. The faults cut the slope apron and displace the seafloor, actively maintaining the critical taper of the prism.
Pn-wave observations in Costa Rica
(Universidad Nacional Autónoma de México, 1998-02-18) Quintero, Ronnie; Kulhánek, Ota
Tiempos de arribos, medidos en la Red Sismográfica de Costa Rica, fueron empleados para extraer información sobre la corteza y el manto por debajo de Costa Rica. Datos de tiempos de arribo de la onda P y S de 100 sismos locales y regionales ocurridos durante el período 1984-1990 fueron usados para obtener la velocidad Pn, la razón de Poisson, la anisotropía de la velocidad Pn y la correción de las estaciones. La razón de Poisson para la capa superior del manto por debajo de Costa Rica es 0.265 y la velocidad de Pn encontrada es de 7.81 km/sec. La presente investigación no muestra variaciones anisotrópicas significantes para la onda Pn. La profundidad del Moho se calcula en 34 km, aproximadamente.
ANTOLOGÍA SOBRE POLITIONATOS: COMPUESTOS DE AZUFRE EXISTENTES EN LOS LAGOS CRATÉRICOS DE VOLCANES ACTIVOS
(Revista de Vulcanología, Sismología y Tectónica., 1999) Martínez-Cruz, María
En la siguiente recopilación bibliográfica sobre los oxianiones de azufre llamados politionatos, con fórmula general SxO6 2-, se contemplan algunas de sus propiedades físico-químicas, y se hace énfasis principalmente en el hecho de que se ha encontrado que los cambios en la concentración total de los politionatos son un buen indicador de cambios en la actividad fumarólica subacuática de varios lagos cratéricos. La variación en la concentración de los ácidos politiónicos presentes en los lagos cratéricos, se ha observado y relacionado con actividad sísmica y la producción de erupciones freáticas en los volcanes Poás en Costa Rica; Kusatsu Shirane en Japón, y el Ruapehu en Nueva Zelanda. Los oxianiones conteniendo al azufre en varios estados de oxidación han sido encontrados en ambientes tales como fumarolas volcánicas, fuentes termales, en el agua sumamente ácida de algunos lagos cratéricos, sedimentos de lagos, aguas residuales de industrias mineras y solfataras. El monitoreo de la variación en la concentración de ciertas especies químicas en los sistemas volcánicos constituye una herramienta de gran importancia en la predicción de actividad volcánica.
Seismic cycle and plate margin deformation in Costa Rica: GPS observations from 1994 to 1997
(American Geophysical Union, 1999-12-10) Lundgren, P.; Protti, Marino; Donnellan, Andrea; Heflin, M.; Hernández, E.; Jefferson, D.
Global Positioning System (GPS) observations in Costa Rica from 1994 to 1997 reveal a complex pattern of motion consistent with the superposition of seismic cycle and secular plate margin deformation. In the south, velocity vectors are consistent with motion of the Panama Block plus postseismic deformation following the 1991 Limon earthquake and interseismic strain due to partial locking of the Middle America Trench (MAT) thrust. In the northwest, sites west of the volcanic arc are moving to the NW as a forearc sliver. Superimposed on this sliver motion are vertical and horizontal interseismic deformations from the adjacent Nicoya segment of the MAT. We apply two different inverse methods to understand the source of the seismic strain in NW Costa Rica. We compare fault-locking models derived using a singular value decomposition inversion with that of a simulated annealing global optimization approach. Both methods yield similar models for partial locking of the thrust interface beneath the Nicoya Peninsula. Our results define an area of nearly fully locked fault beneath the outer coast of the southern portion of the peninsula, with somewhat lower coupling beneath the northern half and with low coupling elsewhere. These initial results show the promise for detailed imaging of the locked portion of a thrust interface responsible for future large subduction zone earthquak
Analysis of seismic and acoustic observations at Arenal Volcano, Costa Rica, 1995–1997
(Elsevier Science, 2000-08) Hagerty, M. T.; Schwartz, S.Y.; Garcés, M. A.; Protti, Marino
In November 1995, we installed five, three-component broadband seismometers and electronic tiltmeters around the circumference of Arenal Volcano, a young stratovolcano in Costa Rica that exhibits strombolian activity. With the addition of two continuous-recording GPS receivers deployed in May 1995, these instruments provide continuous monitoring of seismicity and ground deformation at an active volcano over a very wide bandwidth. In addition, during April–May 1997, we deployed a small, linear array of co-located three-component seismometers and broadband microphones. This paper presents a comprehensive analysis of all the seismic and acoustic data collected thus far. Seismic signals are primarily of two types: (1) longperiod (1–3 Hz) transients associated with summit explosions; and (2) harmonic tremor that contains regularly spaced spectral peaks (0.9, 1.8, 2.7, 3.6, 4.5, 5.4, 6.3, and 7.1 Hz) and lasts up to several hours. The explosion signals appear to originate in a small volume that is located at shallow depth beneath the vent and does not migrate with time. No unambiguous long-period seismic signals T . 5 s associated with volcanic processes at Arenal have been observed during the three-year deployment period. The spectra of summit explosions show distinct signatures at each site, suggesting significant path and/or site modification of the waveforms. In contrast, the harmonic tremor signals show no variation in the frequency content at the five sites, nor on the three seismic components at each site (Hagerty et al., 1997). This, and the fact that harmonic tremor is recorded in the acoustic channels as well, demonstrates that harmonic tremor is not a seismic propagation effect and that pressure disturbances propagate within the magma–gas mixture inside of volcanic conduits. These pressure waves are sensitive to the flow velocity and to small changes in the gas content of the magma–gas mixture. Observations and synthetic tests are presented that challenge the notion that harmonic tremor is a superposition of repeated gas explosions at shallow depth. We propose that equilibrium degassing of the melt creates a stable, stratified magma column where the void fraction increases with decreasing depth and that disruption of this equilibrium stratification is responsible for observed variations in the seismic efficiency of explosions. q2000 Elsevier Science B.V. All rights reserved.
An improved P-wave velocity reference model for Costa Rica
(Universidad Nacional Autónoma de México, 2001-01-01) Quintero, Ronnie; Kissling, Edi
Derivamos un modelo de velocidad unidimensional para la onda P en Costa Rica, el cual puede servir en la rutina de localización de eventos sísmicos y como modelo de referencia para tomografía en 3 dimensiones. La inversión para la velocidad es realizada usando 822 sismos con buena ubicación y 14 774 observaciones de la onda P, las cuales fueron obtenidas combinando datos de tiempos de arribo de 10 335 eventos en el periodo 1984-1997 colectados por el Observatorio Vulcanológico y Sismológico de la Universidad Nacional de Costa Rica (OVSICORI-UNA) y 3510 eventos en el periodo 1992-1998 colectados por la Red Sismológica Nacional (RSN). Durante el proceso de fusión, se tomó un cuidado estricto para reducir el número de errores en los datos, y en particular, para adaptar, corregir y completar los parámetros de las estaciones sísmicas. Un modelo de velocidad unidimensional es prerequisito para tal proceso de fusión cuando la consistencia y calidad tienen prioridad sobre la totalidad del resultante juego de datos. Los datos finales para el periodo 1984-1998 en Costa Rica consiste de 11 848 eventos locales con 13 2331 observaciones de ondas P y 86 018 de ondas S.
Along-strike variability in the seismogenic zone below Nicoya Peninsula, Costa Rica
(American Geophysical Union, 2002) Newman, Andrew V.; Schwartz, Susan; González Salas, Victor; Deshon, Heather R.; Protti, Marino; Dorman, LeRoy
At the subduction zone in northwestern Costa Rica, the seismogenic zone lies directly beneath the Nicoya Peninsula, allowing for near source seismic studies of earthquake activity. We located 650 earthquakes along the seismogenic plate interface using a dense seismic network in the vicinity of the Nicoya Peninsula. Using these data we constrained the updip limit of the seismogenic zone there and found a transition in depth, 10 km in the south to 20 km in the north, that occurs where the subducting oceanic crust changes from warmer Cocos-Nazca Spreading center (CNS) origin to colder East Pacific Rise (EPR) origin. We argue that the temperature of the incoming oceanic crust controls the seismogenic updip limit beneath Nicoya, Costa Rica; subducting colder oceanic crust deepens the seismogenic updip limit.
Tomographic evidence for a subducted seamount beneath the Gulf of Nicoya, Costa Rica: The cause of the 1990 Mw = 7.0 Gulf of Nicoya earthquake
(American Geophysical Union, 2002-04-26) Husen, S.; Kissling, Edi; Quintero, Ronnie
Tomographic images constrain the existence of a subducted seamount beneath the Gulf of Nicoya, Costa Rica. The subducted seamount is found at a depth of 30 km within the rupture area of the March 25, 1990, Mw = 7.0 Gulf of Nicoya earthquake. The Gulf of Nicoya earthquake was a typical thrust-type subduction earthquake and occurred on a shallow dipping thrust fault parallel or along the boundary between the subducting Cocos plate and the overriding plate. Precise relocation of the mainshock and its aftershocks in a 3-D P-wave velocity model shows that the area of the mainshock rupture is coincident with the imaged subducted seamount. Most of the aftershocks are relocated within or close to the inferred subducted seamount above the subducting oceanic plate. We interpret the subducted seamount as an asperity whose rupture caused the 1990 Gulf of Nicoya earthquake. INDEX TERMS: 1734 History of Geophysics: Seismology; 6982 Radio Science: Tomography and imaging; 7209 Seismology: Earthquake dynamics and mechanics; 7230 Seismology: Seismicity and seismotectonics
Duration-amplitude distribution of volcanic tremor
(American Geophysical Union, 2003) Benoit, John P.; McNutt, Stephen; Barboza, Vilma
The duration-amplitude distribution of volcanic tremor was examined in eight volcanoes and one geothermal area. An exponential model, implying a scale-bound source process, is found to be a better fit to the data than a power law (scale invariant) model. The exponential model well describes tremor associated with magmatic and phreatic eruptions, shallow and deep source regions, and geothermal sources. We tested the exponential model described by: dðDRÞ ¼ dtelDR , where d is the duration of tremor greater than or equal to a particular amplitude DR, dt is the total duration of tremor, and the inverse of l is the characteristic amplitude of the distribution. l1 takes on values between 0.003 and 7.7 cm2. Our results show that the characteristic amplitude for eruptive tremor is greater than for noneruptive tremor, that for deep tremor is greater than for shallow tremor, and that for tremor associated with magmatic eruptions is greater than for tremor associated with phreatic eruptions. The exponential scaling of tremor suggests that tremor is not simply composed of a series of low-frequency events closely spaced in time. Further, the exponential scaling requires the source to be scale bound; the amplitude variations of tremor are distributed about a constant characteristic amplitude. We propose that exponential scaling of tremor amplitude is caused by fixed source geometry driven by variable excess pressures. The exponential scaling of tremor demonstrates that tremor source processes are fundamentally different from those for earthquakes.
Seismogenic zone structure of the southern Middle America Trench, Costa Rica
(Journal of Geophysical Research, 2003-10) DeShon, H.R.; Schwartz, S.Y.; Bilek, S.L.; Dorman, LeRoy; González, Victor; Protti, Marino; Flueh, E.R.; Dixon, T.H.
The shallow seismogenic portion of subduction zones generates damaging large and great earthquakes. This study provides structural constraints on the seismogenic zone of the Middle America Trench offshore central Costa Rica and insights into the physical and mechanical characteristics controlling seismogenesis. We have located 300 events that occurred following the MW 6.9, 20 August 1999, Quepos, Costa Rica, underthrusting earthquake using a three-dimensional velocity model and arrival time data recorded by a temporary local network of land and ocean bottom seismometers. We use aftershock locations to define the geometry and characteristics of the seismogenic zone in this region. These events define a plane dipping at 19 that marks the interface between the Cocos Plate and the Panama Block. The majority of aftershocks occur below 10 km and above 30 km depth below sea level, corresponding to 30–35 km and 95 km from the trench axis, respectively. Relative event relocation produces a seismicity pattern similar to that obtained using absolute locations, increasing confidence in the geometry of the seismogenic zone. The aftershock locations spatially correlate with the downdip extension of the oceanic Quepos Plateau and reflect the structure of the main shock rupture asperity. This strengthens an earlier argument that the 1999 Quepos earthquake ruptured specific bathymetric highs on the downgoing plate. We believe that subduction of this highly disrupted seafloor has established a set of conditions which presently limit the seismogenic zone to be between 10 and 35 km below sea level.
Fluid geochemistry and seismic activity in the period 1998-2002 at Turrialba Volcano: Costa Rica
(Universidad de Florencia, Italia, 2004) Tassi, Franco; Vaselli, Orlando; Barboza, Vilma; Fernández, Erick; Duarte, Eliecer
Turrialba Volcano, located in Central-Southern Costa Rica, has been characterized, since the last period of eruptive activity in 1884-1886, by a weak and discontinuous fumarolic activity in the western area of its summit. During the 1998-2002 period, fumaroles discharging from central and West craters were collected for chemical analyses of major and trace gas compounds, 13C/12C in CO2 and 18O/16O and D/H (in one fumarolic condensate), isotopic ratios. Geophysical measurements (seismic activity and ground deformation), monitored in the same period, were compared to geochemical data to define the status of the volcanic system. Chemical and isotopic characteristics of fumaroles of Turrialba Volcano seem to be related to interaction processes between a magmatic source and a shallower hydrothermal aquifer. Since February 1997, seismicity at Turrialba Volcano gradually increased, while since August 2001 new fumaroles start to discharge from a new fracture system located in the area between central and West craters. Since September 2001, strong compositional changes of gas discharges have been recorded at central crater. These occurrences are possibly due to variations in the permeability of the conduit system feeding the fumaroles. Heat pulse episodes from a magmatic source have possibly caused the increase of vapour pressure at depth and, consequently, favoured the uprising of the magmatic fluids toward the surface. The observed evolution of chemical and physical parameters suggests that to forecast a possible renewal of the volcanic activity in the near future a full program of both geochemical and geophysical surveillance must be provided at Turrialba Volcano.
A long-term record of polythionates in the acid crater-lake of Poás volcano: Changes in the subaqueous input of fumarolic gases
(IAVCEI. General Assembly. Pucón-Chile, 2004-01) Martínez, M.; Van Bergen, M. J.; Fernández, E.; Takano, B.; Malavassi, E.; Barboza, V.; Miura, Y.; Van der Laat, R.; Duarte, E.; Valdés, J.; Sáenz, W.
Seismogenic zone structure beneath the Nicoya Peninsula, Costa Rica, from three-dimensional local earthquake P- and S-wave tomography
(Geophysical Journal International, 2006) Deshon, Heather R.; Schwartz, Susan; Newman, Andrew; González, Victor; Protti, Marino; Dorman, LeRoy; Dixon, Timothy; SAMPSON, DANIEL; Flueh, Ernst R.
The subduction plate interface along the Nicoya Peninsula, Costa Rica, generates damaging large (Mw > 7.5) earthquakes. We present hypocenters and 3-D seismic velocity models (VP and VP/VS) calculated using simultaneous inversion of P- and S-wave arrival time data recorded from small magnitude, local earthquakes to elucidate seismogenic zone structure. In this region, interseismic cycle microseismicity does not uniquely define the potential rupture extent of large earthquakes. Plate interface microseismicity extends from 12 to 26 and from 17 to 28 km below sea level beneath the southern and northern Nicoya Peninsula, respectively. Microseismicity offset across the plate suture of East Pacific Rise-derived and Cocos-Nazca Spreading Center derived oceanic lithosphere is ∼5 km, revising earlier estimates suggesting ∼10 km of offset. Interplate seismicity begins downdip of increased locking along the plate interface imaged using GPS and a region of low VP along the plate interface. The downdip edge of plate interface microseismicity occurs updip of the oceanic slab and continental Moho intersection, possibly due to the onset of ductile behaviour. Slow forearc mantle wedge Pwave velocities suggest 20–30 per cent serpentinization across the Nicoya Peninsula region while calculated VP/VS values suggest 0–10 per cent serpentinization. Interpretation of VP/VS resolution at depth is complicated however due to ray path distribution.We posit that the forearc mantle wedge is regionally serpentinized but may still be able to sustain rupture during the largest seismogenic zone earthquakes.

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