Petatlan Earthquake Research Articles

Spectral analysis and earthquake scaling of the Petatlan earthquake (M s =7.6) aftershocks, recorded at stations along the pacific coast and between the coast and Mexico City

Spectral parameters have been estimated for 214 Petatlan aftershocks recorded at stations between Petatlan and Mexico City and between Petatlan and Acapulco. The spectral parameters were used to obtain empirical relations for the estimation of seismic moment from coda length and fromM L . Stress drops, using Brune's model, were calculated for these aftershocks. Six events with large stress drop are located within a previously suggested asperity, and seven more suggest a boundary zone at the intersection of the Petatlan and Zihuatanejo aftershock rupture volumes. Stress drops increase with increasing seismic moment up to 1020 dyne-cm but appear to be constant at greater moment values. The peak horizontal velocity times distance of aftershocks recorded near the coast and between the coast and Mexico City (30 to 270 km away), scales linearly with seismic moment, and predicts well the peak horizontal values of large (M s ≥7.0) coastal thrust events recorded on rock sites at Mexico City. Peak horizontal velocity is a straightforward measurement, thus this relation allows us to evaluate expected ground motion between the Pacific coast and Mexico City from the seismic moment of subduction related earthquakes along the coast.

Correlation between the fault rupture pattern and associated microseismic activity of the Petatlan earthquake

SUMMARY The fault rupture pattern of the 1979 March 14, Petatlan, Mexico, earthquake (M,=7.6) was analysed in previous studies using local and teleseismic data. Here, we demonstrate that the local data are consistent with the complex source of this event inferred from recent P-wave inversion studies. The result is particulary important since this correlation is obtained from the analysis of the pre- and post-seismic activity as recorded by a local array. Although local seismic records are unable to establish the rupture complexity of a mainshock, the foreshock and aftershock activity of the Petatlan earthquake provided some of the source characteristics of this event.

Finite-fault analysis of the 1979 March 14 Petatlan, Mexico, earthquake using teleseismic P waveforms

SUMMARY Vertical, teleseismic P waves recorded for the 1979 March 14 Petatlan, Mexico, earthquake were used to derive the distribution of coseismic slip using a linear finite-fault inversion scheme that solves for the amount of slip in each of a series of consecutive time windows. Data recorded by six stations of the Global Digital Seismograph Network were inverted in addition to digitized analogue long-period recordings available from nine Worldwide Standardized Seismograph Network stations. The digital data include four broad-band and short-period velocity waveforms reconstructed from the short- and long-period components. The time-window approach allows for a variable rise time on the fault and accounts for the source multiplicity evident in the recorded P waveforms. Synthetic tests conducted using the inversion method on the limited data set, however, reveal that the data are insufficient to identify the exact dislocation duration on the fault. The method is thus implemented by prescribing the fault rise time using five consecutive 1 s time windows. The coseismic slip inferred from the P waves shows a small 70 cm peak near the earthquake hypocentre and a large zone of dislocation (1.2 m maximum) further south-east. The slip pattern covers depths from 3 to 25 km and is located south-east of other recent large interplate ruptures on the Michoacan segment of the Mexican subduction zone. This result indicates that the 1979 Petatlan earthquake broke an independent, adjacent portion of the Cocos-North America plate boundary. The seismic moment of 1.5 × 1027 dyn cm inferred from the P waves is approximately one-half the long-period moment estimated by other investigators from the observed surface waves. Although the discrepancy is within the uncertainty of the seismic-moment estimates, it may suggest the presence of a component of slow interplate motion that did not radiate significant P-wave energy.

S wave coda Q from 3 to 20 Hz and P wave Q for foreshocks and aftershocks of the Petatlan Earthquake

In order to understand better the variations of mechanical properties close to the source and the failure process in the upper portion of the crust in a geologically very young subduction zone, the seismic S wave coda Qc from 3 to 20 Hz and P wave half pulse width Qp from 6 to 12 Hz were determined for selected foreshocks and aftershocks of the 1979 Petatlan, Mexico, Ms = 7.6 earthquake. To limit the region being sampled, the quality factor of S coda, Qc, was obtained from the first 10 s following the S onset. Qc exhibited a strong spatial, temporal, and frequency dependence, and in the small foreshock portion of the aftershock area, Qc and (to a lesser extent) Qp were significantly lower before the mainshock than afterward. The frequency dependence n, in Qc (ƒ) = Qo ƒn, was practically constant for aftershocks in four different regions (n = 1.32±0.03) and for foreshocks (n = 1.43±0.07). Only in one peripheral region‐station pair did we find a significantly different n (n = 0.97±0.09). Our observations indicate that Qc, and perhaps the frequency dependence n, are sensitive to the physical property changes in the source region and might be useful as predictors of large earthquakes in a shallow subduction zone environment.

Moment magnitude scale for local earthquakes in the Petatlan region, Mexico, based on recorded peak horizontal velocity

AbstractAssigning moment magnitude to local earthquakes based on peak recorded amplitude and hypocentral distance is the principal concern of this paper. For 38 aftershocks of the Petatlan earthquake (14 March 1979) with moments between 2.8 × 1018 and 1.4 × 1022 dyne-cm, we developed an empirical relationship for the estimation of moment based on peak horizontal velocity (V) and hypocentral distance (R). This empirical relationship in conjunction with the definition of moment magnitude (Hanks and Kanamori, 1979) allows direct estimation of moment magnitude, knowing peak horizontal velocity and hypocentral distance. We call this estimation of moment magnitude “Mw′” to distinguish it from its original definition. Equation for Mw′ for our data set isM w ' = 2 . 2 7 + 0 . 7 7 log ( V ⋅ R ) + 6 . 5 9 × 1 0 - 3 R ,where V is in millimeters/second, and R is in kilometers.In order to be able to estimate the moment magnitude from coda duration (Tc), we sought and found correlation between log(Tc) and log of moment (M0) as followslog M 0 = ( 2 . 2 5 ± 0 . 1 7 ) log ⁡ T c + ( 15.88 ± 0.29 ) ,where M0 is in dyne-cm, and Tc is in seconds.

Fine Structure of a Postfailure Wadati‐Benioff Zone

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 subduction zones.

An ocean bottom seismometer study of shallow seismicity near the Mid‐America Trench offshore Guatemala

Five ocean bottom seismometers recorded seismicity near the Mid‐America Trench offshore Guatemala for 27 days in 1979. The array was emplaced in the lower slope region, just above the topographic trench, in the area investigated during Deep Sea Drilling Project legs 67 and 84. Approximately 170 events were recorded by three or more seismometers, and almost half were located with statistical hypocentral errors of less than 10 km. Most epicenters were located immediately landward of the trench axis, and many were further confined to a zone northwest of the array. In terms of depth, most events were located within the subducting Cocos plate rather than in the overlying plate or at the plate‐plate boundary. Their apparent concentration in the lower crust and upper mantle may suggest that the upper crust does not have the strength to support earthquake‐generating stresses. The data permit construction of a magnitude‐duration scale, calibrated with mb magnitudes for events located by the World‐Wide Standard Seismograph Network (WWSSN) and recorded by our array and by the network recording foreshocks and aftershocks of the 1979 Petatlan earthquake. Most magnitudes ranged between 3.0 and 4.0 mb, and the threshold magnitude of locatable events was about 2.8 mb. Two distinct composite focal mechanisms were determined. One appears to indicate high‐angle reverse faulting in the subducting plate, in a plane parallel to trench axis strike. The other, constructed for some earthquakes in the zone northwest of the array, seems to show normal faulting along possible fault planes oriented quasi‐perpendicular to the trench axis. The normal faulting is consistent with the segmentation of the Cocos plate that has been proposed from land evidence. Such segmentation might be evidenced offshore by normal faulting along planes subperpendicular to trench strike. Alternatively, the seismicity zone and associated normal faulting mechanism may be the subsurface expression of the tectonics responsible for the San Jose Canyon, a prominent submarine canyon located farther upslope. Finally, projection of our seismicity sample and of well‐located WWSSN events from 1954 to 1980 onto a plane perpendicular to the trench axis shows a distinct gap between the shallow seismicity located by our array, and the deeper Wadati‐Benioff zone seismicity located by the WWSSN. We tentatively ascribe this gap to inadequate sampling, but we suggest that it requires further investigation.

Time‐space variation of seismic S‐wave coda attenuation (Qc−1) and magnitude distribution (b‐values) for the Petatlan Earthquake

Average S‐wave coda‐Q (Qc) at 6 Hz and to about 10 sec after the S‐wave arrival shows: (1) temporal variation positively correlated with the magnitude distribution parameter b for foreshocks and aftershocks of the 1979 Petatlan, Mexico earthquake; (2) spatial variation through the aftershock area, positively correlated with b, high Qc ‐b values at the two activity centers, intermediate values in between and landward, and low values toward the Middle America Trench, which remain low for two weeks after the main shock; and (3) five‐foreshock average Qc = 135±1.6% and eleven‐aftershock average Qc= 175±1.3% for the eastern asperity. The b values—and possibly Qc values—seem to correlate with the level of aftershock activity.

Correlation of foreshocks and aftershocks and asperities

A close correlation in spatial distribution of local seismic activity and energy release patterns before and after the 1979 Petatlan, Mexico earthquake suggests heterogeneity within the fault plane of this major low-angle thrust event associated with subduction along the Middle America Trench. A simple two-asperity model is proposed to account for the complexity. Foreshocks and aftershocks of the neighboring 1981 Playa Azul earthquake showed a similar pattern. As both events occurred at the junction of the Orozco Fracture Zone and the Middle America Trench, we speculate that the observed complex fault plane is caused by subduction of the rugged ocean floor of the Orozco Fracture Zone. Short-term precursory seismicity prior to the Petatlan earthquake can be explained by using the asperity model and migration of a slip front from the south-east to the north-west across the main shock source region.

Aftershock activity of the Petatlan earthquake: The first 54 hours

Abstract Aftershocks of the 14 March 1979, Petatlan, Mexico, earthquake ( M S = 7.6) were recorded by a temporary seismic network deployed by the Hawaii Institute of Geophysics. This paper describes the aftershock activity in the first 54 hr following the main shock. The aftershock area expanded to about 4800 km 2 at the end of the first day and did not significantly increase during the second day. Only an additional 26 per cent enlargement occurred by the 36th day. The aftershock region contains two zones of relatively higher seismicity. The ratio of slip between these two zones alternates (1.75, 0.72, 1.73) for time periods determined by the minima and maxima of the slip difference between them. The total slip during the first 54 hr is about 3 mm. The aftershock b value for the rupture region is 1.49, higher than the foreshock b value of 1.11 obtained by the same local network. This result is consistent with the lowered state of stress of the area after the major shock.

Local seismicity preceding the March 14, 1979, Petatlan, Mexico Earthquake (Ms = 7.6)

Local seismicity surrounding the epicenter of the March 14, 1979, Petatlan, Mexico earthquake was monitored by a network of portable seismographs of the Hawaii Institute of Geophysics from 6 weeks before to 4 weeks after the main shock. Prior to the main shock, the recorded local seismic activity was shallow and restricted within the continental plate above the Benioff zone. The relocated main shock hypocenter also lay above the Benioff zone, suggesting an initial failure within the continental lithosphere. Four zones can be recognized that showed relatively higher seismic activity than the background. Activity within these zones has followed a number of moderate earthquakes that occurred before or after the initial deployment of the network. Three of these moderate earthquakes were near the Mexican coastline and occurred sequentially from southeast to northwest during the three months before the Petatlan earthquake. The Petatlan event occurred along the northwestern extension of this trend. We infer a possible connection between this observed earthquake migration pattern and the subduction of a fracture zone because the 200‐km segment that includes the aftershock zones of the Petatlan earthquake and the three preceding moderate earthquakes matches the intersection of the southeastern limb of the Orozco Fracture Zone and the Middle America Trench. The Petatlan earthquake source region includes the region of the last of the three near‐coast seismic activities (zone A). Earthquakes of zone A migrated toward the Petatlan main shock epicenter and were separated from it by an aseismic zone about 10 km wide. We designate this group of earthquakes as the foreshocks of the Petatlan earthquake. These foreshocks occurred within the continental lithosphere and their observed characteristics are interpreted as due to the high‐stress environment before the main shock. Pre‐main shock seismicity of the Petatlan earthquake source region shows a good correlation with the aftershocks in their spatial distribution. This suggests that an asperity existing along the Benioff zone may have affected both the pre‐main shock activity in the continental lithosphere and the aftershocks along the Benioff zone. Although major thrust earthquakes at trenches occur along Benioff zones, in the present study we find little activity on this interplate boundary before the Petatlan earthquake. The overlying continental block, on the contrary, is very active seismically. Our data suggest that the activity is probably governed by the stress transmitted from below due to coupling between two plates and the heterogeneity within the continental lithosphere. The continental material is probably the more likely place for precursors.

Analysis of the Petatlan aftershocks: Numbers, energy release, and asperities

The Petatlan earthquake of March 14, 1979 (Ms = 7.6), occurred between the Middle America trench and the Mexican coast, 15 km southwest of Petatlan, Guerrero, Mexico. From seismograms recorded on smoked paper, FM, and digital tapes, we have identified 255 aftershocks with coda lengths greater than 60 s that occurred 11 hours to 36 days after the main shock. Based on these events, the aftershock epicentral area defined during the period between 11 and 60 hours was about 2000 km2; between 11 hours and 6 days it was about 2400 km2. Although the area grew to 6060 km2 in 36 days, most of the activity was still confined within the area defined after 6 days. This suggests that the smaller aftershock area might represent an asperity. The distribution of events and energy release per unit area confirm the existence of heterogeneity in the aftershock area. Thus our data support the concept of an inhomogeneous rupture area that includes an asperity, as suggested by Chael and Stewart (1982) to account for the differences they computed for the body and surface wave moments from WWSSN data. However, the combination of the moments Reichle et al. (1982) report for body and surface waves from IDA data and the rupture areas reported in this paper results in a solution that is most physically realizable in terms of stress drop and slip. We calculate stress drops of 5 and 15 bars, the former for the average over the entire area, the latter for the asperity, and an average slip of 60 cm for the entire area and 120 cm for the asperity. These values for slip are 30% and 60%, respectively, of the convergence of the Cocos plate relative to the North America plate during the 36‐year period between the last two major earthquakes in the Petatlan area. Hypocenters of the aftershocks define a zone about 25 km thick, dipping 15° with an azimuth of N20°E, which is perpendicular to the Middle America trench. Most aftershocks are below the main shock. The b value estimated for aftershocks in the range 3.0 ≤ MCL ≤ 4.5 is 60% higher than one determined similarly by Hsu (1981) for the foreshocks.

Patterns of local seismicity preceding the petatlan earthquake of 14 March 1979

abstract Local seismic activity (M > 2.3) during the 412-day period preceding the Petatlan earthquake of 14 March 1979 (Ms = 7.6) was monitored by a seismic network deployed by the Hawaii Institute of Geophysics. These data allow us to revise the location of that event, and to study its pattern of foreshocks. The foreshock hypocenters are concentrated above the ocean/continent plate boundary and landward of the hypocenter of the main shock. The spatial distribution of foreshocks suggests that the foreshocks occurred along existing zones of weakness within the continental plate. During the 112-day period preceding the Petatlan event, foreshocks were concentrated within 100 km of the epicenter of that earthquake. The pattern of precursory seismic activity we observed during that period is similar to that observed during the same time period prior to the Oaxaca earthquake of 29 November 1978. However, the 17-hr period of seismic quiescence (for events with M > 2.8) preceding the Oaxaca earthquake is not found in our data where a magnitude 4 foreshock occurs within 28 min of and 2 km from the Petatlan event. This suggests that the spatially larger set of foreshocks may have greater diagnostic value than does the sequence of precursory events within 30 km of the main shock.