Vladimir Kostoglodov

Transient fault slip in Guerrero, southern Mexico

The Guerrero region of southern Mexico has ac- cumulated more than 5 m of relative plate motion since the last major earthquake. In early 1998, a continuous GPS site in Guerrero recorded a transient displacement. Modeling indicates that anomalous fault slip propagated from east to west along-strike of the subduction megathrust. Campaign GPS and leveling data corroborate the model. The moment release was equivalent to an Mw≥6.5 earthquake. No M> 5 earthquakes accompanied the event, indicating the frictional regime is velocity-strengthening at the location of slip.

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.

Seismicity and structure of the Kamchatka subduction zone

The configuration of the Pacific plate subducted beneath the Kamchatka peninsula and the stress distribution in the Kamchatka subduction zone (KSZ) were studied using the catalog of the Kamchatka regional seismic network, focal mechanism solutions estimated from P wave first motions, the formal inversion of long-period waveforms, and centroid moment tensor solutions. To the south of ∼55°N, the slab shows an approximately constant dip angle of ∼55°. To the north of ∼55°N, the dip of the slab becomes shallower reaching ∼35°. The maximum depth of seismicity, Dm, varies from ∼500 km depth near 50°N to ∼300 km depth at ∼55°N. The volcanic front is almost linear along the main part of the KSZ whereas it is sharply shifted landward to the north of ∼55°N. The variation of Dm is apparently consistent with the standard empirical relation Dm=ƒ(ϕ), where ϕ is the thermal parameter of the subducted slab. To the north of ∼55°N, the slab is offset toward the northwest, and it is sharply deformed in a narrow contorted zone which is ∼30 km wide (∼56°N, ∼161°E). To the north of this contortion, Dm decreases to ∼100 km. The landward shift of the northern part of the slab is reflected by a sharp deviation of the volcanic front to the northwest which follows the ∼90–160 km isodepth range of the subducted slab. The observed value of Dm in the northern segment significantly diverges from the global relation Dm=ƒ(ϕ). We interpret this as an effective decrease of the thermal thickness of the subducted lithosphere.

The 2006 slow slip event and nonvolcanic tremor in the Mexican subduction zone

The last decade featured an explosive sequence of discoveries of slow slip events (SSE) and nonvolcanic tremor (NVT) in different subduction zones and continental faults. Many observations show that SSE is usually associated with an increased NVT activity but it is not clear yet if those events are the result of the same process or are independent expressions of a common underlying seismotectonic source. A large SSE in Central Mexico occurred in 2006 during the Meso-American Subduction Experiment (MASE) which provided continuous observations of the NVT for the years 2005-2007. GPS and abundant seismic data show that although the NVT energy increased notably during the 2006 SSE, the two phenomena were separated spatially and not completely synchronized in time. Significant NVT episodes occur during the period between SSEs, suggesting again that large slow slip events and NVT observed in the Mexican subduction zone are of different origins. The results presented here contribute to uncovering the nature of these two separate phenomena that have been indistinguishable in some other regions.

Seismotectonic constraints on the convergence rate between the Rivera and North American plates

There are two significantly different types of models for the convergence rate between the Rivera and North American plates. The first type, the high-rate model (Bandy, 1992), predicts convergence rates of approximately 5.0 cm/yr near the southern end of the Rivera-North America subduction zone and between 2.0 and 3.0 cm/yr at its northern end. In contrast, the second type, the low-rate model (e.g., DeMets and Stein, 1990), predicts convergence rates of between 2.0 and 3.3 cm/yr near the southern end of the Rivera-North America subduction zone and between 0.6 and 1.7 cm/yr at its northern end. Seismotectonic relationships, which relate seismic characteristics of subduction zones (maximum magnitudes, maximum seismic depths, etc.) to plate tectonic parameters (convergence rates, age of the oceanic lithosphere, etc.) provide a means of distinguishing between the two different models. Three such relationships suggest that the Rivera-North American and Cocos-North American convergence rates should be roughly equal across the Rivera-Cocos plate boundary, favoring the high-rate model. Employing the high-rate model, one can evaluate the magnitude and distribution of the strike-slip component of forearc motion, Vss, produced by oblique convergence between the Rivera and North American plates. The analysis indicates both a progressive increase and clockwise reorientation of Vss northwestward along the plate contact zone of the Rivera-North America subduction zone. Such a distribution in Vss should produce a northwestward movement of and NW-SE oriented extension within the interior of the Jalisco Block, consistent with previous proposals of Jalisco Block motions. Also, such a distribution in Vss should produce a slight clockwise rotation of the Jalisco Block in the vicinity of Bahia de Banderas, consistent with paleomagnetic data.

Seismic evidence of nonlinear crustal deformation during a large slow slip event in Mexico

Repeated cross-correlations of ambient seismic noise indicate a long-term seismic velocity change associated with the 2006 M7.5 slow-slip event (SSE) in the Guerrero region, Mexico. Because the SSE does not radiate seismic waves, the measured velocity change cannot be associated with the response of superficial soil layers to strong shaking as observed for regular earthquakes. The perturbation observed maximized at periods between 7 s and 17 s, which correspond to surface waves with sensitivity to the upper and middle crust. The amplitude of the relative velocity change (∼10−3) was much larger than the volumetric deformation (∼10−6) at the depths probed (∼5-20 km). Moreover, the time dependence of the velocity perturbation indicated that it was related to the strain rate rather than the strain itself. This suggests that during strong slow-slip events, the deformation of the overlying crust shows significant nonlinear elastic behavior.

Slow slip events and strain accumulation in the Guerrero gap, Mexico

Note: Best student presentation award Reference EPFL-TALK-183540 Record created on 2013-02-01, modified on 2016-08-09

The October 9, 1995 Colima‐Jalisco, Mexico Earthquake (Mw 8): An aftershock study and a comparison of this earthquake with those of 1932

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.

Propagation of the 2001–2002 silent earthquake and interplate coupling in the Oaxaca subduction zone, Mexico

The aseismic slow slip event of 2001–2002 in Guerrero, Mexico, with an equivalent magnitude MW ~ 7.5, is the largest silent earthquake (SQ) among many recently recorded by GPS in different subduction zones (i.e. Japan, Alaska, Cascadia, New Zealand). The sub-horizontal and shallow plate interface in Central Mexico is responsible for specific conditions for the ~100 km long extended transient zone where the SQs develop from ~80 to ~190 km inland from the trench. This wide transient zone and relatively large slow slips of 10 to 20 cm displacements on the subduction fault result in noticeable surface displacements of 5–6 cm during the SQs. Continuous GPS stations allow one to trace the propagation of SQs, and to estimate their arrival time, duration and geometric attenuation. These propagation parameters must be accounted in order to locate source of slow slips events and to understand the triggering effect that they have on large subduction earthquakes. We use long-baseline tiltmeter data to define new time limits (onset and duration) for the SQs and continuous records from 8 GPS stations to determine the propagation of the 2001–2002 SQ in Central Mexico. Data from the CAYA and IGUA GPS stations, separated by ~170 km and located along the profile perpendicular to the trench, are used to determine that the surface deformation from the 2001–2002 SQ started almost instantaneously. It propagated parallel to the coast at ~2 km/day with an exponential attenuation of the horizontal surface displacement and a linear decrease of its duration with distance. Campaign data obtained yearly from 2001 to 2005 at the Oaxaca GPS network have been modeled according to a propagation of the 2001–2002 SQ step-like displacement anomaly. This modeling shows that the SQ ceased gradually in the central part of the Oaxaca segment of the subduction zone (west of Puerto Angel, PUAN) and then it apparently triggered another SQ in SE Oaxaca (between PUAN and Salina Cruz, SACR). The estimated horizontal velocities for inter-event epochs at each GPS site are used to assess an average interplate coupling in the Central Oaxaca subduction zone.

Using systematically characterized low-frequency earthquakes as a fault probe in Guerrero, Mexico

Studies of low-frequency earthquakes (LFEs) have focused on detecting events within previously identified tectonic tremor. However, the principal LFE detection tools of matched-filter searches are intrinsically incapable of detecting events that have not already been characterized previously as a template event. In this study, we therefore focus on generating the largest number possible of LFE templates by uniformly applying a recently developed LFE template detection method to a 2.5 yearlong data set in Guerrero, Mexico. Using each of the detected templates in a matched-filter search, we then form event families that each represents a single source. We finally develop simple, empirical statistics to select the event families that represent LFEs. Our resulting catalog contains 1120 unique LFE sources and a total of 1,849,486 detected LFEs over the 2.5 yearlong data set. The locations of the LFE sources are then divided into subcatalogs based on their distance from the subduction trench. Considering each LFE as a small unit of slip along the subduction interface, we observe discrete episodes of LFE activity in the region associated with large slow-slip events; this is in direct contrast to the near-continuous activity observed 35 km farther downdip within the previously identified LFE/tremor sweet spot.