A. Iglesias

Nonvolcanic tremor observed in the Mexican subduction zone

Nonvolcanic tremor (NVT) activity is revealed as episodes of higher spectral amplitude at 1–8 Hz in daily spectrograms from the continuous seismological records in Guerrero, Mexico. The analyzed data cover a period of 2001–2007 when in 2001–2002 a large slow slip event (SSE) had occurred in the Guerrero-Oaxaca region, and then a new large SSE occurred in 2006. The tremor burst is dominated by S-waves. More than 100 strong NVT bursts were recorded in the narrow band of ~40 × 150 km^2 to the south of Iguala City and parallel to the coastline. Depths of NVT hypocenters are mostly scattered in the continental crust between 5 and 40 km depth. Tremor activity is higher during the 2001–2002 and 2006 SSE compared with that for the “quiet” period of 2003–2005. While resistivity pattern in Guerrero does not correlate directly with the NVT distribution, gravity and magnetic anomaly modeling favors a hypothesis that the NVT is apparently related to the dehydration and serpentinization processes.

S wave velocity structure below central Mexico using high‐resolution surface wave tomography

Shear wave velocity of the crust below central Mexico is estimated using surface wave dispersion measurements from regional earthquakes recorded on a dense, 500 km long linear seismic network. Vertical components of regional records from 90 well-located earthquakes were used to compute Rayleigh-wave group-velocity dispersion curves. A tomographic inversion, with high resolution in a zone close to the array, obtained for periods between 5 and 50 s reveals significant differences relative to a reference model, especially at larger periods (>30 s). A 2-D S wave velocity model is obtained from the inversion of local dispersion curves that were reconstructed from the tomographic solutions. The results show large differences, especially in the lower crust, among back-arc, volcanic arc, and fore-arc regions; they also show a well-resolved low-velocity zone just below the active part of the Trans Mexican Volcanic Belt (TMVB) suggesting the presence of a mantle wedge. Low densities in the back arc, inferred from the low shear wave velocities, can provide isostatic support for the TMVB.

Rupture history of September 30, 1999 intraplate earthquake of Oaxaca, Mexico (MW=7.5) from inversion of strong‐motion data

Near-source strong motions are inverted to estimate the rupture history of intraslab, normal-faulting September 30, 1999, Oaxaca, Mexico earthquake. Two focal mechanisms (Harvard and NEIC CMT solutions) are tested for the source geometry. The inversion with the NE dipping fault plane of the Harvard solution best matches the data (strike=295°, dip=50°, rake=−82°). We estimated the slip distribution on the fault and the associated rupture front propagation, as well as the rise time. The inversion results show that the rupture mainly propagated from ESE to WNW and slightly downdip, with an average rupture velocity of about 3 km/s. The rise time ranges between 1 and 2 s. The slip distribution on the fault is mainly concentrated in two interconnected patches with a maximum slip of 2.5 m located about 20 km and 40 km WNW of the hypocenter. Most of the slip is released at an average depth of 45 km. A smaller area with a maximum slip of 1.5 m is also observed close to the hypocenter. The total co-seismic moment released is equal to 1.8 × 1020 Nm.

Size of Popocatepetl Volcano explosions (1997–2001) from waveform inversion

Several volcanic explosions have been recorded since April 1997 at broadband seismic stations located around the Popocatepetl volcano, Mexico. We have inverted waveforms of ten of these explosions to estimate the following source parameters: depth, duration, magnitude and direction of the single force, F. The crustal structure used in generating Greens function at nearest stations is derived from the inversion of teleseismic receiver functions at the broadband permanent station PPIG, located 5 km north of the volcano. This inversion reveals a low velocity zone at ∼8 km beneath the summit with high Poisson ratio, possibly related to the magma chamber. We find that F scales with τ, the duration of the source-time function, as F ∝ tau². Based on this relationship we determine an impulse magnitude scale, Mk. This magnitude is tied to the Mount Saint Helens initial explosive phase of May 18, 1980, whose magnitude is estimated as 4.6. Mk of the ten Popocatepetl explosions ranges between 1.8 and 3.2. Finally, we also propose an equivalent formula for rapid estimation of magnitude of future Popocatepetl explosions, which requires filtered amplitudes at PPIG.

The Seismic Alert System for Mexico City: An Evaluation of Its Performance and a Strategy for Its Improvement

The seismic alert system (sas) for Mexico City has now been in operation for about 15 years. The sas takes advantage of the fact that the city is located more than 300 km from the foci of many of the potentially damaging earthquakes. The system consists of 15 accelerometers located along the coast of the State of Guerrero, above a segment of subduction plate boundary that is a mature seismic gap. An algorithm estimates the magnitudes of earthquakes from the near-source accelerograms and issues public and restricted alerts for earthquakes with M ≥6 and 5 ≤ M < 6, respectively. An evaluation of the sas’s performance during 1991–2004 reveals a surprisingly high rate of failure and false alerts. This poor performance results from an inadequate detection algorithm and a limited areal coverage by the sas. This renders the alert system of limited use. In this article we propose an alternative strategy for detecting earthquakes potentially damaging to Mexico City that differs substantially from the one presently implemented by the sas. Although our analysis is based on close-to-source accelerograms of 45 Mexican earthquakes and the corresponding peak accelerations recorded at a reference site in Mexico City (cu), there is no restriction on the distance to the field station, except that its location should provide sufficient alert time to Mexico City. Based on these data, an attenuation relation is derived to compute expected peak acceleration at cu ( A red) from root-mean-square acceleration ( A rms) at a field station. The relation permits specification of an A red threshold, given the peak acceleration at cu ( A cu) for which an alert is desired along with prescribed probabilities of failure and false alert. We find that the use of bandpass-filtered (0.2–1.0 Hz) accelerograms leads to an improved performance of the sas. The choice of the filter is guided by the frequency band of amplification of seismic waves in the lake-bed zone of Mexico City. We think that a single level of general public alert may be the best option. A good choice appears to be an alert for A cu ≥ 2 gal (for 0.2–1.0 Hz bandpass-filtered accelerograms) with 1% probability of failure. To accomplish this we must set A red ≥ 0.8 gal. The data since 1985 suggest that such an alert would occur about once or twice a year and the event will be felt by most persons in the lake-bed zone. The proposed algorithm, along with an array of sensors located 30 to 40 km apart and distributed in a roughly semicircular arc of 310-km radius centered at Mexico City, should considerably improve the areal coverage and performance of the sas and potentially save thousands of lives.

Near-Trench Mexican Earthquakes Have Anomalously Low Peak Accelerations

It has previously been reported that regional seismograms of earthquakes that occur near the Middle America trench are relatively deficient at high frequencies. Based on this observation, an algorithm has been proposed for detecting potentially tsunamigenic earthquakes and issuing tsunami alerts. It is reasonable to expect relatively low peak accelerations during these earthquakes. In this note, we present evidence that this is indeed the case. This explains why the seismic alert system for Mexico City, with sensors located along the coast, does not trigger during some earthquakes. Low peak accelerations from near-trench earthquakes also have important implications in seismic hazard estimation.

A Source and Wave Propagation Study of the Copalillo, Mexico, Earthquake of 21 July 2000 (Mw 5.9): Implications for Seismic Hazard in Mexico City from Inslab Earthquakes

The Copalillo earthquake of 21 July 2000 ( M w 5.9) is the closest, well-located inslab event to Mexico City ever to be recorded. In this study, we analyze local and regional broadband and accelerometric recordings to determine the source parameters of the earthquake and the attenuation of ground motion with distance and to obtain a preliminary estimate of the seismic hazard posed to the city by such events. Our results show that the earthquake occurred at a depth of about 50 km, most probably in the subducted oceanic crust. The waveform inversion discriminates between the two nodal planes; the fault plane defined by the following: strike, 305°; dip, 32°; and rake, -80°. The rupture propagated nearly unilaterally along the strike toward northwest with a small downdip component. The observed source spectrum can be well explained by an ω 2 -source model with M 0 = 6.0 × 10 25 dyne cm and a stress drop of 360 bar. We find that high-frequency ground motion ( f > 3 Hz), which is related to A max during inslab earthquakes, is not amplified at Ciudad Universitaria (CU), a hill-zone site in the Valley of Mexico that is known to suffer amplification at low frequencies (0.1 f M w 7.0 event could give rise to an A max value of 30-40 gal. The CU recordings indicate that the A max value of 30 gal could have a return period of about 40 yr, about the same as from shallow-dipping thrust earthquakes along the Mexican subduction zone, which have been regarded as posing the highest hazard for the city. An inslab earthquake with an A max value of about 40 gal could cause heavy damage to small buildings at certain locations of the city. We conclude that seismic hazard from inslab earthquakes to Mexico City has so far been underestimated.

Q of Lg Waves in the Central Mexican Volcanic Belt

From seismograms of nine shallow, coastal earthquakes recorded at a pair of broadband stations, we estimate Q of Lg waves in the part of central Mexican Volcanic Belt (mvb) that includes the Valley of Mexico. The two stations straddle the central mvb and are located on Cretaceous limestone. A weighted least-square fit to the Q −1 ( f ) data in the frequency range 0.25 to 8 Hz yields Q ( f ) = 98 f 0.72 . This estimate of Q is lower than the corresponding Q in the forearc region that is given by Q ( f ) = 273 f 0.66 . Note that our estimate of Q ( f ) corresponds to a 200-km-wide zone of the mvb. The result of this study sheds light on the characteristics of seismic waves as they traverse through the mvb where they undergo dramatic amplification in the Valley of Mexico. It also provides one of the critical elements needed in the estimation of expected ground motions at sites to the north of the mvb from future coastal earthquakes. The lower Q of Lg waves in the mvb as compared with the forearc region seems correlated with lower resistivity reported in the mvb relative to the forearc region.

An Estimate of Shear-Wave Q of the Mantle Wedge in Mexico

We utilize earthquake recordings at two broadband seismographs located on the coast of the Gulf of Mexico to estimate Q of the mantle wedge. From the data at a station situated in the Laguna Verde nuclear power plant (lvig), about 100 km northwest of Veracruz, and at the eastern edge of the Mexican Volcanic Belt, we estimate an upper bound of shear-wave Q of the mantle wedge, Q ( f ) ∼ 120 f 0.75 (0.1 ≤ f ≤ 10 Hz), as compared with Q ( f ) = 251 f 0.58 for the average path through subducted slab and continental lithosphere. Curiously, the estimated Q of the mantle wedge at station scig, which is located on the Yucatan Block, is about the same as the average Q in southern Mexico. There are several possible explanations for the difference: (1) very few recordings at scig, (2) a site effect at scig masking the effect of low Q of the mantle wedge, and (3) relatively high-mantle Q beneath the Yucatan Block. The third possibility is supported by surface-wave tomography that reveals a thick, cold, mantle lithosphere below the Yucatan Block and near absence of mantle lithosphere in the backarc region of central Mexico. A higher density of seismographs along the gulf coast is needed to resolve these issues. Our study predicts diminished ground motions at the Laguna Verde nuclear power plant if the seismic waves pass through the mantle wedge or traverse below the Popocatepetl or Orizaba volcanoes.

Intraslab versus Interplate Earthquakes as Recorded in Mexico City: Implications for Seismic Hazard

We study the relative importance of interplate and intraslab earthquakes in the seismic hazard of Mexico City by analyzing accelerograms recorded at the hill-zone site of CU (1964–2012) and the lake-bed site of SCT (1985–2012). Amax exceeded 6 gal during 20 earthquakes at CU during this period. Of these, eight were intraslab events so that the exceedance rate of Amax ≥ 6 gal from both types of earthquakes is roughly about the same. The estimated return period of Amax of 30 gal from the two types of earthquakes is ∼100 yrs. If we consider high-frequency (2.5–8.5 Hz) acceleration (AmaxHF) at CU, then the top 7 out of the 20 events are all intraslab earthquakes. Even at the lake-bed site of SCT, the AmaxHF values are, generally, associated with intraslab earthquakes. It follows that the risk from both types of earthquakes to low-rise construction in the city needs careful assessment.