ISSN:0094-8276    

DOI:10.1029/GL013i006p00565

年代:1986

数据来源: WILEY

ISSN:0094-8276    

DOI:10.1029/GL013i006p00567

年代:1986

数据来源: WILEY

摘要:

Analysis of body waves and long‐period surface waves from the September 1985 earthquake in coastal Michoacan, Mexico shows that the event was an interplate subduction event with a low dip angle fault plane (δ=9°) striking parallel to the Mid‐America trench (ϕ=288°) and a small component of left lateral motion (λ=72°) with a point source depth of 17 km, and a seismic moment in excess of 1 × 1028dyn cm. The earthquake was a multiple event, with a second source of identical moment, fault geometry, and depth occurring approximately 26 s after the first. Directivity in the body wave time function indicates that the second event occurred roughly 100 km to the southeast of the first. This suggests that the earthquake first broke the northern portion of the Michoacan gap, propagated with low moment release through the rupture zone of the 1981 Playa Azul earthquake, and then broke the remaining asperity in the southern section of the gap. The seismic moment determined from Rayleigh and Love waves is between 1.0 ‐ 1.7 × 1028dyn cm (MW= 7.9 ‐ 8.1), the largest moment determined to date for a Mexico subduction earthquake. Comparison of seismograms at Pasadena with records of other large Mexico events shows that the Michoacan earthquake is basically the same size as the 1932 Jalisco, Mexico earthquake, and clearly larger than other significant events in Mexico since 1932. The seismic moment and the time since the last large earth‐quake in Michoacan (in 1911) fit an empirical relation between moment and recurrence time found for the Guerrero‐Oaxaca region of the subduction zone. The large aftershock on September 21 (Ms=7.5) has the same geometry as the mainshock, a somewhat larger source depth (22 km), a simple time function, and a seismic moment between 2.9 ‐ 4.7 × 1027

ISSN:0094-8276    

DOI:10.1029/GL013i006p00569

年代:1986

数据来源: WILEY

摘要:

The disastrous earthquake of Sept 19, 1985 (Ms=8.1) broke a segment of the plate boundary along the Mexican subduction zone known as the Michoacan gap for which the seismic potential was uncertain. This earthquake was followed by another major earthquake on Sept 21 (Ms=7.5). Teleseismic, long‐period seismograms show that the first earthquake consisted of two subevents separated in time by 27 s; the local strong‐motion data indicate that the second subevent occurred about 95 km SE of the first one. Teleseismic records of the Sept 21 shock can be modeled by a single source. Aftershock areas, slips, and stress drops of the two earthquakes are 170×50 and 66×33 km², 220 and 330 cm, and 19 and 43 bars, respectively. In hindsight, it seems very likely that the earthquake of June 7, 1911 (Ms=7.9) occurred in the same region; if so, its rupture area was probably only 60% of that of the Sept 19 earthquake. Furthermore, the earthquake of Sept 21 appears to have broken the updip part of the interface which ruptured in the 1979 Petatlan earthquake (Ms=7.6). These observations suggest a variable mode of rupture in the

ISSN:0094-8276    

DOI:10.1029/GL013i006p00573

年代:1986

数据来源: WILEY

摘要:

The hypocentral distribution of locally recorded aftershocks of the great (Ms=8.1) Michoacan, Mexico, earthquake of September 19, 1985, defines a narrow Wadati‐Benioff zone structure, roughly 10 km thick, dipping 14° at N23°E. This is in good agreement with the source geometry obtained by waveform modeling of the 1985 Michoacan mainshock and the large 1979 Petatlán earthquake in the adjoining region. We inverted for the crustal velocity structure in the epicentral region by applying the Levenberg‐Marquardt non‐linear least squares algorithm to our local aftershock data. The velocity model consists of a layer with linearly increasing velocity in depth overlying a dipping, constant velocity halfspace. Our hypocentral location program uses a velocity model of the same form together with ray tracing. The earthquake hypocentral resolution obtained with this program is significantly better than that from conventional approaches (HYPO) and looks very promising for use in velocity structures with an important dipping interface like subducti

ISSN:0094-8276    

DOI:10.1029/GL013i006p00577

年代:1986

数据来源: WILEY

摘要:

We describe acceleration signals recorded for nine aftershocks of the September 19, 1985 Michoacan earthquake. To obtain this data set, three A‐700 Teledyne‐Geotech digital strong‐motion instruments were operated temporarily at two sites on the José María Morelos (La Villita) Dam, and at a site located at about 12 km to the west of the town of Zihuatanejo. Peak horizontal accelerations of 0.005 g to 0.031 g were recorded at epicentral distances between 10 and 75 km, for earthquakes with magnitude (mb) between 4.5 and 5.3. It was observed that the peak accelerations recorded at a site on the embankment of the dam (near the crest ) are approximately three times those recorded on the abutment bedrock portion of the dam. Although these sites were spatially separated by no more than 300 m, differences among their records are also significant. Waveforms recorded at the embankment site look more complex than those from the abutment site. This fact, as well as the higher peak accelerations on the embankment, provides evidence of a strong influence of the structure of the dam on the ground motion at the embankme

ISSN:0094-8276    

DOI:10.1029/GL013i006p00581

年代:1986

数据来源: WILEY

摘要:

The size and source complexity of the great (Ms= 8.1) Michoacan, Mexico earthquake can be attributed to both a restriction of the downdip width of the seismic interface between the overriding and downgoing plates, and a uniform distribution of asperities. The seismogenic evidence of strain accumulation during the 20 years prior to the 19 September, 1985 mainshock closely resembles that observed prior to the smallerMW= 7.6 earthquakes in the adjacent Colima (1973) and Petatlán (1979) regions: the subsequent rupture zones are seismically quiescent (mb≥4.0) for ∼ 2.5 ‐ 4 years prior to the mainshocks; the locked thrust interface is loaded by aseismic slip and normal faulting within the downgoing plate below 25‐30 km. There is evidence that the 1981 Playa Azul earthquake (MW= 7.3) is an integral stage of the evolutionary process of strain accumulation and release in the Michoacan area and could thus be considered a foreshock to the great 1985 ea

ISSN:0094-8276    

DOI:10.1029/GL013i006p00585

年代:1986

数据来源: WILEY

摘要:

This paper describes the structural damage in Mexico City caused by the September 19, 1985 earthquake. Photographs which illustrate various features of structural behavior are included. One explanation is presented as to why buildings with fundamental periods of elastic vibration considerably below the predominant two‐second period of the ground motion were most vulnerable to damag

ISSN:0094-8276    

DOI:10.1029/GL013i006p00589

年代:1986

数据来源: WILEY

摘要:

The extensive damage to high‐rise buildings in Mexico City during the September 19, 1985 earthquake is primarily due to the intensity of the ground shaking exceeding what was previously considered credible for the city by Mexican engineers. There were two major factors contributing to the catastrophe, resonance in the sediments of an ancient lake that once existed in the Valley of Mexico, and the long duration of shaking compared with other coastal earthquakes in the last 50 years. Both of these factors would be operative again if the Guerrero seismic gap ruptured in a single earthquak

ISSN:0094-8276    

DOI:10.1029/GL013i006p00593

年代:1986

数据来源: WILEY

摘要:

Source characteristics of the Sept. 19, 1985 Michoacan, Mexico earthquake and its aftershock on Sept. 21 were inferred from broadband and short‐period teleseismic GDSN records. We Fourier‐transformed the P waves, corrected for instrument response, attenuation, geometrical spreading, and radiation pattern (including the depth phases), and then averaged to obtain the teleseismic source spectrum from 1 to 30 s. The Michoacan source spectrum is enriched at 30 s and depleted at 1 to 10 s relative to an average source spectrum of large interplate subduction events. Source spectra for the Sept. 21 aftershock, 1981 Playa Azul, 1979 Petatlan, and 1978 Oaxaca events follow a trend similar to that of the 1985 Michoacan event. This spectral trend may characterize the Mexican subduction zone.A station‐by‐station least‐squares inversion of the Michoacan earthquake records for the source time function yielded three source pulses, which we interpreted as events on the fault plane. The first two are similar in moment, and the third contains only 20% of the moment of the first. Directivity is evident in the timing. At each station, we measured the time differences between the pulses, and performed a least‐squares nonlinear estimation of the strike, distance, and time separation between the events to locate them relative to one another. The second event occurred 26 s after the first, and 82 km southeast of it, indicating southeastward rupture along the trench. The third event occurred 21 s after the second, and about 40 km seaward of it. The two large events are also seen in the near‐field strong motions.The mainshock records, spectrum, and time functions contain less high frequency radiation than those of the 1985 Valparaiso, Chile earthquake. Apparently, the Michoacan earthquake ruptured two relatively smooth, strong patches which generated large 30 s waves, but small 1 to 10 s waves. Such behavior contrasts with the Valparaiso event which had a more complex rupture process and generated more 1 to 5 s energy. This difference is consistent with the higher near‐field accelerations recorded for the V

ISSN:0094-8276    

DOI:10.1029/GL013i006p00597

年代:1986

数据来源: WILEY