Seismicity on and near the boundaries of the Rivera plate is examined in order to determine the subduction regime of the Rivera plate and its influence on the earthquake behavior of the northernmost section of the mid‐America trench. In particular, we address the question of a Rivera‐Cocos boundary.There have been several large historic earthquakes in the coastal areas of the Mexican states Colima and Jalisco, but the last large event was in June 1932 (the 1932 Jalisco earthquake, Ms = 8.1). The present quiescence has lasted longer than the average recurrence interval for the Mexican subduction zone, and the Jalisco area is now categorized as a seismic gap. However, if subduction in this area is dominated by the motion of the Rivera plate, which is subducting at half the rate of the Cocos plate, the deficiency of earthquake activity may not be unusual. In an attempt to better delineate the Rivera‐Cocos boundary,intermediate‐sized earthquakes along and near the Rivera fracture zone and the mid‐America trench were relocated with the joint epicenter determination method. We found that in general, the catalog locations are fairly accurate, with the average change in epicenter upon relocation being 12 km. Seismicity along the Rivera fracture zone follows the physiographic trend of the fracture zone, but east of its intersection with the East Pacific Rise, seismicity is more diffuse. Two intermediate‐sized earthquakes in this diffuse area have focal mechanisms suggesting that they represent Rivera‐Cocos boundary motion. Reexamination of arrival time data for the 1932 Jalisco earthquake yields an epicenter at 19.57°N, 104.42°W, close to the boundary zone inferred from these two earthquakes. We conclude that the possibility that the 1932 Jalisco earthquake broke the northernmost section of the Cocos‐North American plate interface, as opposed to the Rivera‐North American plate interface, cannot be ruled out.
Abstract. A single force mechanism is investigated as the source of long-period seismic radiation from the 1975 Kalapana, Hawaii, earthquake (Ms = 7
We 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.
Although hypocenters of earthquakes on the island of Hawaii are now routinely assigned to within 5 km, depth was a poorly determined parameter until the early 1960's. However, the 1950–1960 period was very active both in volcanic eruptions and large earthquakes. Source depths for the 12 largest Hawaiian earthquakes (magnitude 6 or greater) since 1940 are estimated from the ratios of body and surface wave amplitudes recorded at Pasadena, California. Excitation functions for Rayleigh waves are calculated as a function of source depth for the two dominant periods in the Pasadena records, 8s and 20s. Theoretical body wave amplitudes are determined from synthetic seismograms. Calculated ratios are very sensitive to source depth; for example, amplitudes of 8‐s Rayleigh waves diminish by a factor of 300 between depths of 10 km and 50 km. This is a much larger effect than the fault geometry, which we estimate to be a factor of 4 between representative focal mechanisms. Estimated depths for post‐1960 earthquakes agree fairly well with the instrumental depths. In general, large earthquakes near the volcanic flanks and fault systems are shallow (≤20 km). Two earthquakes of magnitude 6 occurred under the volcanoes Mauna Loa (in 1950) and Kilauea (in 1951); they preceded major eruptions by 3 days and 14 months, respectively, and had the largest depth estimates at 40–55 km and 35–50 km. MS values assigned from global amplitudes are compared with those assigned from Pasadena amplitudes alone, for 70 events in 1973–1974 with 5.1≤ MS ≤ 6.0. The global values are only slightly larger (0.05 magnitude units) than the Pasadena values, indicating that Pasadena amplitudes are on the average representative of the event magnitude.
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