Four detailed (average station interval = 5 km) gravity transects were recently conducted in the Pacific coastal region of Mexico. A differential GPS technique was used to determine the elevation and coordinates of the gravity stations. The profiles are oriented northeast‐southwest and extend from the coast up to ∼60 km inland. The Bouguer gravity anomaly is decreasing consistently along every profile from 60–80 mGal at the coast with an approximately constant regional gradient of −2.2 mGal/km normal to the trench. A plot of the gravity anomaly against the distance from the trench axis demonstrates that the regional slope in the gravity anomaly is shifting gradually (20–25 mGal) inland along the coast of Guerrero from the southeast (Atoyac) to the northwest (Petatlán ‐ Zihuatanejo). A model cross section of the Mexican subduction zone (MSZ) based on the tomography inversion for the Guerrero region shows that the gravity anomaly values and the regional anomaly trend can be explained mostly by the effect of the density contrast between the slab and the continental crust. The upper surface of the subducted slab (USS) and the seismogenic contact zone between the upper plate and the slab is traced clearly in several seismicity cross sections based on the data of the regional seismic network in Guerrero. The depth and shape of the USS revealed from the seismicity and gravity anomaly data for the same profiles are in good agreement. This correlation may be fairly useful when applied to gravity profiles in order to estimate the depth of the USS and the seismogenic contact in other parts of the MSZ which lack reliable seismicity data.

Data from portable seismographs and a permanent local network (called RESCO) are used to locate the aftershocks of the October 9, 1995 Colima‐Jalisco earthquake (Mw 8.0). The maximum dimension of the aftershock area, which is rectangular in shape, is 170 km × 70 km. Our study shows that the mainshock nucleated ∼24 km south of Manzanillo, near the foreshock of October 6, 1995 (Mw 5.8), and propagated ∼130 km to the NW and ∼40 km to SE. The aftershock area lies offshore and is oriented parallel to the coast. The observed subsidence of the coast is a consequence of this offshore rupture area. The aftershocks reach unusually close to the trench (within 20 km). This may be due to lack of sediments with high pore pressure at shallow depth. There are some similarities between this earthquake and the two great earthquakes of 1932 (3 June, Ms 8.1; 18 June, Ms 7.8) which occurred in this region. In both cases the aftershocks were located offshore and the coastline subsided. The sum of seismic moments and the rupture lengths of the 1932 events (1.8×1021 N‐m and 280 km, respectively), however, were greater than the 1995 earthquake. Also a comparison of seismograms of 1932 and 1995 earthquakes show great differences. It seems that the 1995 event is not a repeat of either June 3 or June 18, 1932 earthquakes.

On April 22, 1991, a large earthquake (M w = 7.7) occurred along the Caribbean coast of Costa Rica and western Panama. The rupture area of the fault mapped from the aftershocks is 45x85 km 2. The distribution of aftershocks and the local geological record suggest that faulting occurred on a blind thrust sheet that shallows toward the northeast. Uplift of the Caribbean coast ranging from 1.5 m near Puerto Lim6n and decreasing gradually toward the southeast was observed along the Caribbean. Northwest of Puerto Lim6n no significant coastal uplift was observed. This observation agrees with the aftershock data suggesting the rupture did not extend to the northwest of this location. The Lim6n earthquake also triggered aftershocks on secondary faults in the crust. These events are apparently associated with a family of imbricate thrust and strike-slip faults that lie in the eastern piedmont of the Talamanca Cordillera. The historical seismicity indicates that the Caribbean coast has been the site of several historical earthquakes with magnitudes greater than 7.0. On April 26, 1916, another earthquake (M s = 6.9) took place in the same region. Summing the scalar seismic moment release along the Caribbean coast, the average rate of slip is approximately 0.8 cm/yr, compared with a value of 0.4 to 0.8 cm/yr along the Pacific subduction zone, depending on the estimated width of the seismogenic zone. Therefore a large fraction of the relative plate motion between the Cocos and Caribbean plates (9.8 cm/yr) appears to be taken up by crustal deformation in the back arc. The tectonic regime in the area appears to be dominated by the collision of the buoyant Cocos ridge with the subduction zone. The absence of a Wadati-Benioff zone where the Cocos ridge collides with the trench suggests the slab does not subduct beneath the Osa Peninsula; this is supported by the Pliocene gap of volcanism present in Costa Rica. Thus the predicted relative motion between the Cocos and Caribbean plates appears to be absorbed by a low rate of seismic moment release in the forearc and by a broad zone of Paper number 94TC02546. 0278-7407/95/94TC-02546510.00 active crustal shortening and underthrusting in the back arc. This type of tectonic deformation resembles more a collisional regime than a typical subduction zone environment. 20-40-ß ß ß ß 24, Apr 22, Apr ß ß ß ß ß ß e ß ß A B gm , 0 20 40 60 80 Km 60, 1993. Bowland, C.L., Seismic stratigraphy and structure of the western Colombian basin, Caribbean Sea, M.S. thesis, 248 pp., Univ. of Tex., 1984. Brune, J., Seismic moment, seismicity, and rate of slip along major fault zones, J. Geophys. Res., 68, 777-784, 1968. Camacho, E., E1 tsunami del 22 de abril de 1991 en Bocas del Toro, Panamfi, Rev. Geol. Am. Cent., Spec. Vol., 61-64, 1994. Camacho, E. and V. Viquez, Historical and instrumental seismicity of the Caribbean region of Panama, U.S. Geol. Surv. Prof Pap., in press, 1994. Chinn, D.S., and B.L. Isacks, Accurate source depths and focal mechanisms of shallow earthquakes in western South America, Tec...

A total of 5270 shallow and intermediate‐depth earthquakes recorded by the 32 stations of the regional seismic network of the Geophysical Service of Russia are used to assess the P‐wave velocity structure beneath the Kamchatka peninsula in the Western Pacific. The tomographic inversion is carried out in three steps. First, a 1‐D tomographic problem is solved in order to obtain an initial velocity model. Based on the 1‐D velocity model, 3‐D tomographic inversions with homogeneous and heterogeneous starting models are obtained. The Conrad (15 km depth) and Moho (35 km depth) discontinuities determined from the 1‐D tomographic inversion, and the upper boundary of the subducting slab are taken into account in the heterogeneous starting model for the traveltimes and ray‐path determinations. Both velocity structure and hypocentral locations are determined simultaneously in the inversion. The spacing of the grid nodes is a half‐degree in the horizontal direction and 20–50 km in the vertical direction. A detailed P‐wave tomographic image is determined down to a depth of 200 km. The resulting tomographic image has a prominent low‐velocity anomaly that shows a maximum decrease in P‐wave velocity of approximately 6 per cent at 30 km depth beneath a chain of active volcanoes. At depth, low‐velocity anomalies are also observed in the mantle wedge extending down to a depth of approximately 150 km. These anomalies are apparently associated with the volcanic activity. The sedimentary basin of the Central Kamchatsky graben, to the west of the volcanic front, and the accretionary prism at the trench correlate with shallow low‐velocity anomalies. High‐velocity anomalies observed at a depth of 10 km may be associated with the location of metamorphic basements in the Ganalsky–Valaginskoe uplift and upper crust of Shipunsky cape. The results also suggest that the subducted Pacific plate has P‐wave velocities approximately 2–7 per cent higher than those of the surrounding mantle and a thickness of approximately 70 km.

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