Karen C. McNally

The geometry of the Wadati-Benioff zone under southern Central America and its tectonic significance: results from a high-resolution local seismographic network

Abstract 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 > 70km ). NW of this sharp contortion the deepest portion of the seismically active slab dips at about 80° 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.

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

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), Perez Zeledon earthquake.

Quantitative Estimates of Interplate Coupling Inferred from Outer Rise Earthquakes

AbstractInterplate coupling plays an important role in the seismogenesis of great interplate earthquakes at subduction zones. The spatial and temporal variations of such coupling control the patterns of subduction zone seismicity. We calculate stresses in the outer rise based on a model of oceanic plate bending and coupling at the interplate contact, to quantitatively estimate the degree of interplate coupling for the Tonga, New Hebrides, Kurile, Kamchatka, and Marianas subduction zones. Depths and focal mechanisms of outer rise earthquakes are used to constrain the stress models. We perform waveform modeling of body waves from the GDSN network to obtain reliable focal depth estimates for 24 outer rise earthquakes. A propagator matrix technique is used to calculate outer rise stresses in a bending 2-D elastic plate floating on a weak mantle. The modeling of normal and tangential loads simulates the total vertical and shear forces acting on the subducting plate. We estimate the interplate coupling by searching for an optimal tangential load at the plate interface that causes the corresponding stress regime within the plate to best fit the earthquake mechanisms in depth and location.We find the estimated mean tangential load

A Crustal Velocity Model for Locating Earthquakes in Monterey Bay, California

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Analysis of the December 1998 Santa Cruz Mountains, California, Earthquake Sequence

over 125–200 km width ranging between 166 and 671 bars for Tonga, the New Hebrides, the Kuriles, and Kamchatka. This magnitude of the coupling stress is generally compatible with the predicted shear stress at the plate contact from thermal-mechanical plate models byMolnar andEngland (1990), andVan den Buekel andWortel (1988). The estimated tectonic coupling,Ftc, is on the order of 1012–1013 N/m for all the subduction zones.Ftc for Tonga and New Hebrides is about twice as high as in the Kurile and Kamchatka arcs. The corresponding earthquake coupling forceFec appears to be 1–10% of the tectonic coupling from our estimates. There seems to be no definitive correlation of the degree of seismic coupling with the estimated tectonic coupling. We find that outer rise earthquakes in the Marianas can be modeled using zero tangential load.

VARIABLE RUPTURE MODE OF THE SUBDUCTION ZONE ALONG THE ECUADOR-COLOMBIA COAST

At 5:04 P.M. on Tuesday, October 17, 1989 local time (10/18/89 00:04:15.23 UT) a large earthquake ruptured a 40-km segment of the San Andreas fault in the Santa Cruz Mountains in northern California. The magnitude Ms was calculated at 7.1 by the National Earthquake Information Service using data from 18 stations. This report is based on information made available to geophysicists at the C. F. Richter Seismological Laboratory at the University of California at Santa Cruz (UCSC), as of 10 days following the main shock. The Santa Cruz Mountains (Loma Prieta) earthquake was the most severe in the continental U.S. since 1952, when a very large earthquake (Ms = 7.7) broke along the White Wolf fault near Bakersfield, Calif. It was the largest event on the San Andreas since the 1906 (M = 8.3) San Francisco event.

Correlation between the age of the subducting Cocos plate and the geometry of the Wadati-Benioff zone under Nicaragua and Costa Rica

The seismicity of the Monterey Bay displays a sparse distribution of events with a majority in northern Monterey Bay, on the San Gregorio fault. The paucity of near-shore and offshore seismic recording instruments and the use of velocity models from inland regions for earthquake hypocentral locations and focal mechanisms have led to uncertainties and inaccuracies for seismic events on major faults that crosscut the Bay. New three-component seismic data were acquired during 1997–1999 by the Monterey Bay Aquarium Research Institute (MBARI) Margin Seismology project using ocean-bottom digital corehole and standard seismometers, and coastal digital RefTek instruments obtained from IRIS-PASSCAL by the University of California, Santa Cruz. We have analyzed phases from earthquakes in the Monterey Bay vicinity located by these instruments and used them to supplement the adjacent coastal stations of the permanent Northern California Seismic Network. A new one-dimensional velocity model for the region requires slow velocities from 2 to 6 km that we attribute to sheared granites observed in the Salinian Block in the center of Monterey Bay. Velocities in the 10–16 km layer are consistent with continental crustal velocities. A sharp increase in velocity at ∼16 km suggests a boundary that results from underplating of oceanic crust. This underplated zone appears to extend to a depth of ∼27–30 km where we observe normal upper mantle velocities near 8.0 km/sec. New details from the ocean-bottom corehole and coastal RefTek instruments for events along the SGF and Monterey Bay fault zones hint at local fine-scale structures and have implications for tectonic history and plate reconstruction interpretation.

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

Zhou, Y., McNally, K.C. and Lay, T., 1993. Analysis of the 1986 Mr. Lewis, California, earthquake: preshock sequencemainshock-aftershock sequence. Phys. Earth Planet. Inter.. 75: 267-288. The 1986 Mt. Lewis earthquake (M L = 5.7) occurred on a right-lateral fault northeast of and oblique to the Calaveras fault in a region that had not experienced significant seismicity since 1943. Data from the nearby Lawrence Livermore Seismic Network and selected U.S. Geological Survey stations are used to relocate events within 15 km of the mainshock epicenter during the period 1980-1987, using the master-event method. Beginning 17 months before the mainshock, 22 events ruptured in the depth range 5-9 km within 1.4 km and northwest of the mainshock epicenter, in an area subsequently almost devoid of aftershocks. This cluster of preshock activity is clearly separated both spatially and temporally from the background activity in the surrounding area. Composite focal mechanisms for the preshocks and for nearby aftershocks suggest that there are two slightly different focal mechanisms amongst the preshocks, one being similar to the mainshock and aftershocks and one being rotated in strike. Cross-correlations of digitally recorded short-period waveforms of 10 of the clustered preshocks (Mu. = 1.5-2.5) reveal that the average inter-event peak cross-correlation between seismograms is 0.62. Six nearby early aftershocks show an average inter-event peak cross-correlation between seismograms of 0.54. Only a few aftershocks have cross-correlations of 0.6 or higher, which implies that the events were slightly further apart or multiple mechanisms were active during the early aftershock period. No significant differences are observed between the spectra of the preshocks and aftershocks. The aftershock area expanded along the strike with time, ultimately defining a north-south fault plane, 11 km in length, extending from 3 to 10 km in depth. The mainshock appears to have originated at the base of the seismogenic zone and ruptured bilaterally along strike and updip. A forward modeling technique is used to model teleseismic body waveform data of the mainshock. The long-period data (P and SH) are consistent with a point source strike-slip earthquake with a teleseismic moment of 3.9 × 1017 N m. We infer that the mainshock involved the rupture of an asperity in the central portion of the aftershock zone.