Significance Recent destructive megathrust earthquakes and tsunamis in Japan and Sumatra indicate the difficulty of forecasting these events. Geodetic monitoring of the offshore regions of the subduction zones where these events occur has been suggested as a useful tool, but its potential has never been conclusively demonstrated. Here we show that slow slip events, nondestructive events that release energy slowly over weeks or months, are important mechanisms for releasing seismic strain in subduction zones. Better monitoring of these events, especially those offshore, could allow estimates of the size of future earthquakes and their potential for damaging tsunamis. However, the predictive value of slow slip events remains unclear.

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 earthquake.

Abstract. 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 earthquakes.

By studying seafloor morphology we can make associations between near surface deformation, fluid flow and the overall 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 anomalies 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 anomalies observed at the deformation front indicate diffuse fluid flow, however, we discovered no 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 on a mud diapir show extensive seep communities, pock marks, and authigenic carbonates. Evidence of 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.

Recent measurements of ground deforma tion at an active volcano show great promise for elucidating the processes that lead to vol canic eruptions. A long-term effort to continu ously monitor ground deformation over a very wide bandwidth using state-of-the-art geodetic, seismic, and acoustic instruments at Arenal Volcano in Costa Rica is producing high-quality recordings of harmonic tremor and explosive events and offers an exciting opportunity to explore possible lunar peri odicities in volcanic eruptivity.While the physical processes of earth quake rupture and subsequent seismic wave generation are fairly well characterized, those related to volcanic eruptions are not yet understood and pose an exciting chal lenge to Earth scientists. In contrast to earth quakes, which can largely be described from a seismological perspective, volcanoes are complex, multicomponent systems that truly require a multidisciplinary approach to study. The integrated seismic, geodetic, and acoustic investigation of Arenal Volcano is a step toward this goal and will yield rich infor mation about the inner workings of this dan gerous class of explosive volcanoes.Arenal was selected for monitoring be cause of its high level of activity, which en sures rich geophysical observations, its young age (-3000 years, Borgia et al [1988]), which offers the possibility of observing a transition in the eruptive behavior, and for the importance of studying explosive stratovolcanoes that threaten population centers throughout the world. We have collected two years of continuous seismic and geodetic data at 5 sites distributed around the circum ference of the volcano and have made periFor more information, contact M. Hagerty,