Guerrero Seismic Gap Research Articles

Tsunami deposits highlight high-magnitude earthquake potential in the Guerrero seismic gap Mexico

Globally, the largest tsunamigenic earthquakes have occurred along subduction zones. Devastating events exceeding magnitude 9, such as those in Chile, Sumatra, and Japan, struck in regions lacking instrumental records of similar events. Despite the absence of such events along the 1000-kilometer-long Mexican subduction zone, historical and geologic evidence suggests the occurrence of a magnitude 8.6 tsunamigenic earthquake. However, the Guerrero seismic gap has not experienced a high-magnitude earthquake in over 100 years. Here we present results on analyses of sediment grain size, geochemistry, microfossils, magnetic properties, and radiometric and optical stimulated luminescence dating conducted along the Guerrero coast. We provide evidence of a 2000-year history of large tsunamis triggered by potentially large earthquakes. Numerical modeling supports our findings, indicating a magnitude >8 event around the year 1300 in the Guerrero seismic gap. This evidence underscores the importance of assessing earthquake and tsunami potential using long-term evidence and instrumental observations along subduction zones globally.

A Source Study of the Mw 7.0 Acapulco, Mexico, Earthquake of 8 September 2021

AbstractThe Acapulco earthquake of 2021 broke a segment of the southeast Guerrero seismic gap along the Mexican subduction thrust. The rupture initiated offshore Acapulco (16.770° N, 99.942° W) and propagated down-dip toward northeast. This source directivity is confirmed from both (1) an analysis of local and regional recordings as a function of azimuth and (2) kinematic inversion of near-source, band-pass filtered (0.025–0.5 Hz) displacement seismograms and Global Positioning System static coseismic displacement vectors. The inversion reveals little slip near the hypocenter (<0.5 m) and significant slip distributed over an area of ∼184 km2, with the large slip patches in the northeast part of the fault. The estimated average slip and static stress drop are 260 cm and 18.6 MPa, respectively. Moment rate function reported by National Earthquake Information Center–U.S. Geological Survey from finite-fault modeling is simple, and it resembles other Mexican subduction earthquakes in the 7.0 ≤ M ≤ 7.5 range. Moment rate spectrum is well fit by the Brune ω−2 source model. Radiated seismic energy from teleseismic P waves is 7.5×1014 J, and ER/M0 is 2.1×10−5. Radiated energy enhancement factor—a measure of source complexity—is small, 5.8, similar to other Mexican subduction thrust earthquakes. Seismograms at DeBilt of the 2021 and the 11 May 1962 Acapulco earthquakes show an extraordinary similarity, seldom seen at M 7.0 level. The 2021 earthquake seems a repeat of the 1962 earthquake. The slip deficit since 1962 corresponding to a plate convergence rate of 6.2 cm/yr and perfect coupling is 366 cm. Thus, the seismic slip of 260 cm during that 2021 earthquake suggests a coupling ratio of 0.7, greater than 0.3 and 0.5 reported from geodetic measurements. Large moment release in the southeast seismic gap appears to have a periodicity of ~60 yr. Because 60 yr have elapsed since the last sequence earthquakes (1957 MS 7.5; 1962 MS 7.0 and 6.8), a renewal of large earthquakes in the region may be expected.

The First Written Accounts of Pre-Hispanic Earthquakes in the Americas

Abstract The first written evidence of earthquakes in the Americas is represented as pictograms in codices and annals drafted in the first few years after the Spanish conquest. The main source of earthquakes in pre-Hispanic times comes from the codex Telleriano-Remensis. There are 12 earthquakes reported in the Telleriano-Remensis, from 1460 to 1542. Earthquakes reported in this codex show the date of occurrence of the earthquake and other natural phenomena and social events. However, there is little information on the damage or on the precise location where the earthquakes were felt. Thus, although they represent the first written chronology of earthquakes in the Americas, little seismological information can be extracted from them. In contrast, there are other accounts written after the Spanish conquest in 1521 that report political and social events that occurred prior to the downfall of the Aztec empire. In some of these annals, there is information on the locations affected by the earthquakes and on the damage caused. In one case, these historical accounts reveal damage and ground effects that suggest the presence of a crustal earthquake near Mexico City. A second interesting earthquake is dated in 1507. The reports indicate damage and landslides near the Guerrero seismic gap, in the region occupied by the Yope culture. This earthquake appears to be a great earthquake in this part of the subduction zone where no large subduction earthquakes have occurred at least in the past 120 yr.

3D Modeling of Long‐Term Slow Slip Events Along the Flat‐Slab Segment in the Guerrero Seismic Gap, Mexico

AbstractDuring the last two decades, quasi‐periodic long‐term slow slip events (SSEs) of magnitude up to Mw7.5 have been observed about every 4 years in the Guerrero Seismic Gap, Mexico. We present numerical simulations of the long‐term SSE cycles along the 3D slab geometry of central Mexico. Our model accounts for the hydrated oceanic crust in the framework of rate‐and‐state friction and captures the major source characteristics of the long‐term SSEs occurring between 2001 and 2014, as inferred from geodetic observations. Synthetic surface deformation calculated from simulated fault slip is in good agreement with the cumulative GPS displacements. Our results suggest that the flat‐slab segment of the Cocos plate aids the large magnitudes and long recurrence interval of the long‐term SSEs. We conclude that 3D slab geometry is an important factor in improving our understanding of the physics of slow slip events.

Shallow slow earthquakes to decipher future catastrophic earthquakes in the Guerrero seismic gap

The Guerrero seismic gap is presumed to be a major source of seismic and tsunami hazard along the Mexican subduction zone. Until recently, there were limited observations at the shallow portion of the plate interface offshore Guerrero, so we deployed instruments there to better characterize the extent of the seismogenic zone. Here we report the discovery of episodic shallow tremors and potential slow slip events in Guerrero offshore. Their distribution, together with that of repeating earthquakes, seismicity, residual gravity and bathymetry, suggest that a portion of the shallow plate interface in the gap undergoes stable slip. This mechanical condition may not only explain the long return period of large earthquakes inside the gap, but also reveals why the rupture from past M < 8 earthquakes on adjacent megathrust segments did not propagate into the gap to result in much larger events. However, dynamic rupture effects could drive one of these nearby earthquakes to break through the entire Guerrero seismic gap.

Adjoint slip inversion under a constrained optimization framework: revisiting the 2006 Guerrero slow slip event

SUMMARYTo shed light on the prevalently slow, aseismic slip interaction between tectonic plates, we developed a new static slip inversion strategy, the ELADIN (ELastostatic ADjoint INversion) method, that uses the adjoint elastostatic equations to compute the gradient of the cost function. ELADIN is a 2-step inversion algorithm to efficiently handle plausible slip constraints. First it finds the slip that best explains the data without any constraint, and then refines the solution by imposing the constraints through a Gradient Projection Method. To obtain a self-similar, physically consistent slip distribution that accounts for sparsity and uncertainty in the data, ELADIN reduces the model space by using a von Karman regularization function that controls the wavenumber content of the solution, and weights the observations according to their covariance using the data precision matrix. Since crustal deformation is the result of different concomitant interactions at the plate interface, ELADIN simultaneously determines the regions of the interface subject to both stressing (i.e. coupling) and relaxing slip regimes. For estimating the resolution, we introduce a mobile checkerboard analysis that allows to determine lower-bound fault resolution zones for an expected slip-patch size and a given stations array. We systematically test ELADIN with synthetic inversions along the whole Mexican subduction zone and use it to invert the 2006 Guerrero Slow Slip Event (SSE), which is one of the most studied SSEs in Mexico. Since only 12 GPS stations recorded the event, careful regularization is thus required to achieve reliable solutions. We compared our preferred slip solution with two previously published models and found that our solution retains their most reliable features. In addition, although all three SSE models predict an upward slip penetration invading the seismogenic zone of the Guerrero seismic gap, our resolution analysis indicates that this penetration might not be a reliable feature of the 2006 SSE.

Seafloor morphology along the active margin in Guerrero, Mexico: Probable earthquake implications

Submarine geomorphic features on the active margin along the Mexican Pacific coast are the result of subduction process. We analyze bathymetric data and present an overview of the continental slope geomorphology in the Guerrero seismic gap. For the first time we identify and describe submarine landforms in this area, including small-scale forearc basins, trench basin segmentation, narrow accretionary wedge and seamounts. We propose that the presence and formation of forearc basins are related to seamount subduction as well as the origin of higher relief above subducted seamounts. Here, forearc basins and trench sediments are sourced from submarine canyons. Notably, the thickness of sediments along the trench changes due to the presence of seamounts. Some parts of the trench seem to contain little sedimentary fill, which may increase friction along these segments of the subduction zone.

Spatial Distribution of Radiated Seismic Energy of Three Aftershocks Sequences at Guerrero, Mexico, Subduction Zone

AbstractGeneral characteristics of seismic energy release of thrust earthquakes in Mexico have been reviewed in the past; however, a detailed analysis can contribute to a better understanding of the mechanisms that control its distribution along the Guerrero, Mexico, subduction zone. To address it, we obtain the source spectra of the 2012 Mw 7.5 Ometepec‐Pinotepa Nacional, the 2014 Mw 7.2 Papanoa, and the 2018 Mw 7.2 Pinotepa Nacional earthquakes, as well as of their M≥4.0 aftershocks to estimate their seismic moment M0 and radiated seismic energy ES. The first and the last sequences occurred at the southern border of the Guerrero seismic gap, a region where no significant earthquake (M&gt;7.0) has occurred at least in the last century; whereas the second sequence was located at the northern edge of the same seismic gap. The mean value of the log of radiated seismic energy scaled with the seismic moment, log(e˜)=log(ES/M0), for this set of earthquakes is −5.05±0.25. We classify the analyzed events into four regions, two in the southern edge of the gap and two in the northern one. At both ends, there is one region that shows regular values of log(e˜) (−4.64±0.25 and −4.62±0.25), whereas the other one shows low values of log(e˜) (−5.40±0.25 and −5.55±0.25) that could be related to a possible slow‐rupture behavior. These last regions are identified near the trench at southern Guerrero coast and immediately outside the northern end of the seismic gap. The distribution of log(e˜) is spatially heterogeneous along the trench, suggesting variations on the shear strength and coupling at the interface.

Evidence of directivity during the earthquakes of 8 and 10 May 2014 (Mw 6.5, 6.1) in the Guerrero, Mexico seismic gap and some implications

We report on rupture directivity during 8 and 10 May 2014 earthquakes (Mw 6.5, 6.1), which occurred on the Mexican subduction thrust in the Guerrero seismic gap. The two events were located near each other and SE of the area that had ruptured in an Mw 7.2 earthquake on 18 April 2014. Near-source recordings show that the rupture during the 8 May propagated roughly along the strike towards ESE, while the directivity during the 10 May event was in the opposite direction. Analyses of the ground motions produced by the two earthquakes at regional distances (spectral ratios, ratios of peak ground accelerations and velocities, observed ground motion parameters, and radiated seismic energy) exhibit strong azimuthal variation, consistent with the directivity. As expected, the motions were enhanced in the forward direction. Since the attenuation of seismic waves from coastal earthquakes is slower along the inland path as compared to the coastal path, the ground motions during the 8 May earthquake at inland sites in the forward direction were exceptionally large. It follows that the combination of the two effects during future large/great earthquakes in the gap may lead to destructive ground motions at inland sites even at regional distances. The combined effect may have been one of the causes of the unprecedented number of deaths and injuries, and damage in Mexico City during the 1985 Michoacan earthquake (Mw 8.0) which ruptured a nearby subduction interface with a ESE directivity.

Relating the long-term and short-term vertical deformation across a transect of the forearc in the central Mexican subduction zone

Author(s): Ramirez-Herrera, MT; Gaidzik, K; Forman, S; Kostoglodov, V; Burgmann, R; Johnson, CW | Abstract: Earthquake-cycle deformation, which includes earthquake ruptures, interseismic strain, and transient slow slip events, spans spatial scales ranging from fractions of a meter to thousands of kilometers. Similarly, temporal scales range from seconds during an earthquake rupture to thousands of years of strain accumulation between earthquakes. We discuss results regarding the vertical crustal deformation associated with both slow and rapid crustal deformation across a transect of the central Mexican subduction forearc in the Guerrero seismic gap, where the Cocos plate underthrusts the North America plate. This sector of the subduction zone is characterized by a flat-slab geometry with zones of sharp bending-unbending of the slab, irregularly distributed seismicity, and exceptionally large slow slip events. We used the river network, topography, geomorphic features, and morphometry on a transect across the forearc to assess Quaternary crustal deformation. The Papagayo drainage network shows that the forearc has been uplifted since the late Cenozoic (~25 Ma), and that rates of uplift increased since the beginning of the Holocene. Uplift is not homogeneous but shows a trend of increase away from the coast. This vertical deformation is strongly influenced by subduction processes. Thus, the Papagayo River network is strongly controlled by Holocene earthquake cycle processes. This is particularly true for the southern section of the drainage basin, where E-W-striking left-lateral strike-slip faults with a vertical component offset the course of the main river. These faults are accommodating part of the oblique plate convergence at the Mexican subduction zone. We measured the height of a series of terraces and dated quartz extracts by optically stimulated luminescence, and we calculated long-term rates of uplift ranging from 0.5 to 4.9 mm/yr. We discuss associations of forearc topography, faults, and long-term crustal deformation with the Cocos slab geometry, distribution of slow slip events, and earthquake-cycle deformation.

A geologic recipe for transient slip within the seismogenic zone: Insight from the Guerrero seismic gap, Mexico

A geologic recipe for transient slip within the seismogenic zone: Insight from the Guerrero seismic gap, Mexico

GPS deformation related to the Mw 7.3, 2014, Papanoa earthquake (Mexico) reveals the aseismic behavior of the Guerrero seismic gap

AbstractWe present the study of 21 continuous GPS (cGPS) stations in Mexico during the time interval that goes from 1 October 2013 to 31 December 2014. The area under investigation produced a slow slip event (SSE) that started in February 2014 and lasted at least until December 2014. Superimposed on this transient signal, the Mw7.3 Papanoa earthquake struck the region on 18 April 2014 and affected multiple stations. Thanks to an independent component analysis (ICA) technique we are able to separate the postseismic deformation associated with this earthquake from the deformation related to the ongoing SSE. We infer the slip distributions associated with the three tectonically related processes: the coseismic and postseismic slip and the SSE. The inferred postseismic slip distribution reduces the amount of slip related to the SSE in the updip portion of the slab. The moment released by the postseismic processes (afterslip and aftershocks) is estimated to be [8.06 ± 0.24] × 1019 Nm, equivalent to [71 ± 4]% of the moment associated with the main shock. More than 88% of the postseismic moment is released aseismically and the afterslip spatially correlates with the Guerrero seismic gap, suggesting that the region is controlled by stable sliding behavior and rate‐strengthening frictional properties.

Re-Evaluating the Geological Evidence for Late Holocene Marine Incursion Events along the Guerrero Seismic Gap on the Pacific Coast of Mexico

Despite the large number of tsunamis that impact Mexico’s Pacific coast, stratigraphic studies focusing on geological impacts are scanty, making it difficult to assess the long-term risks for this vulnerable region. Surface samples and six cores were taken from Laguna Mitla near Acapulco to examine sedimentological and geochemical evidence for marine incursion events. Sediment cores collected from behind the beach barrier are dominated by intercalated layers of peat and inorganic sediments, mostly silt and clay, with little or no sand. Sand- and shell-rich clastic layers with high levels of sulfur, calcium, and strontium only occur adjacent to the relict beach ridge remnants near the center of the lagoon. With the exception of one thin fine sand layer, the absence of sand in the near-shore cores and the predominance of the terrigenous element titanium in the inorganic layers, evidently eroded from the surrounding hillslopes, suggests that these large-grained intervals do not represent episodic marine incursions, but rather were likely formed by the erosion and redeposition of older marine deposits derived from the beach ridge remnants when water levels were high. These results do not support the occurrence of a large tsunami event at Laguna Mitla during the Late Holocene.

Slow Slip History for the MEXICO Subduction Zone: 2005 Through 2011

To further our understanding of the seismically hazardous Mexico subduction zone, we estimate the first time-dependent slip distributions and Coulomb failure stress changes for the six major slow slip events (SSEs) that occurred below Mexico between late 2005 and mid-2011. Slip dist ributions are the first to be estimated from all continuous GPS data in central and southern Mexico, which better resolves slow slip in space and time than was previously possible in this region. Below Oaxaca, slip during previously un-modeled SSEs in 2008/9 and 2010/11 extended farther to the west than previous SSEs. This constitutes the first evidence that slow slip accounts for deep slip within a previously noted gap between the Oaxaca and Guerrero SSE source regions. The slip that we estimate for the two SSEs that originated below Guerrero between 2005 and 2011 agrees with slip estimated in previous, mostly static-offset SSE modeling studies; however, we show that both SSEs migrated eastward toward the Oaxaca SSE source region. In accord with previous work, we find that slow slip below Guerrero intrudes up-dip into the potentially seismogenic region, presumably accounting for some of the missing slip within the well-described Guerrero seismic gap. In contrast, slow slip below Oaxaca between 2005 and 2011 occurred mostly down-dip from the seismogenic regions defined by the rupture zones of large thrust earthquakes in 1968 and 1978 and released all of the slip deficit that accumulated in the down-dip region during this period.

Papanoa, Mexico earthquake of 18 April 2014 (Mw7.3)

Papanoa earthquake broke the plate interface NW of the Guerrero seismic gap. In this region, previous large earthquakes occurred in 1943 (MS 7.4), 1979 (Mw7.4) and 1985 (Mw7.5). The earthquake was recorded in the near-source region by several accelerographs. Severe damage was reported in Papanoa (where PGA of ~ 0.9 g was recorded on one of the horizontal components at a soft site) and other nearby coastal towns. It was also felt strongly in Mexico City where the ground motions were comparable to those recorded during the 1979 and 1985 events. A careful analysis of the near-source data, including P-wave polarization, yields an epicenter at 17.375 ºN, 101.055 ºW, close to the coast, near the town of Papanoa. Effective duration of ground motion at near-source coastal stations to the NW of the epicenter is 10-15 s, while it is 20-35 s to the SE, demonstrating rupture directivity towards Zihuatanejo. Three (in some cases only two) bursts of high-frequency radiation are visible in the accelerograms. Near-field records show that the slip was small during the initial 2-3 s of rupture which, subsequently, cascaded in two or three larger subevents. Slip inversion using teleseismic waves, along with GPS data from a couple of near-source sites, reveals that the rupture mainly consisted of two subevents. The first one was centered close to the hypocenter and had a radius of ~ 15 km. The second subevent, roughly of the same dimension as the first, was centered ~ 25 km SSE of Zihuatanejo. Previous analysis of three slow slip events (SSEs) in the region (2001-2002; 2006; 2009-2010) had revealed that this region had a high inter-SSE coupling ratio (&gt; 0.5) with a slip deficit about four times greater than in the adjacent NW Guerrero seismic gap (Radiguet et al., 2012). It seems that the large slip patch corresponding to the first subevent of the 2014 earthquake experienced a cumulative slip of ~ 20 cm during the SSEs, suggesting that seismic and SSE slip may share the same area of the interface. Alternatively, the SSE slip may have occurred over an area surrounding the large slip patch, a physically more plausible model. Epicenters of aftershocks (M ≥ 3.5), which occurred in the next 36 hours, define a rectangular area of ~ 40 km • 70 km, oriented ~ N75ºE; about half of this area lies onshore. This area encloses the inverted slip region. More than half of the aftershock area overlaps with that of the 1979 earthquake and a small fraction with that of the 21 September 1985 earthquake. As we only know the slip distribution of the 2014 earthquake, it is not known if the two large-slip patches had also slipped similarly during the previous earthquakes. The earthquake was followed by two moderately large events that occurred on 8 May (Mw6.5) and 10 May (Mw6.1). The epicenters of these events fall near Tecpan, within the NW Guerrero seismic gap (which extends from 100 ºW to 101 ºW), outside the aftershock area of the Papanoa earthquake. No large earthquake has occurred in this part of the gap, between Papanoa and Acapulco, since the events of 1899 (MS7.5) and 1911 (MS7.6). However, seismicity in the region (at Mw ≥ 5 level) appears normal. A few moderate earthquakes of unusually large duration and deficient high-frequency radiation have been identified near the trench. In contrast to the Papanoa - Zihuatanejo region, in this segment the inter-SSE coupling ratio from ~ 10 km inland towards sea is very low (&lt; 0.2) and the slip deficit is about one-fourth that of the Papanoa- Zihuatanejo region (Radiguet et al., 2012). As a consequence, the expected recurrence period of large/great earthquakes may be relatively long, in agreement with the seismicity of the Papanoa-Acapulco region.

Source study of three moderate size recent earthquakes in the Guerrero seismic gap

We study three recent earthquakes of different types in the Guerrero seismic gap zone with M ≥ 6.5: the 15 July 1996 Guerrero near-coast interplate earthquake (Mw = 6.6), the 18 April 2002 Guerrero near-trench interplate earthquake (Mw = 6.7), and the 11 December 2011 Guerrero normal-faulting inslab earthquake (Mw = 6.5). We compute the slip distributions, estimate source parameters, and model strong ground motions with the finite-fault stochastic method. We use different methods to estimate source parameters in order to observe differences in stress drop and radiated seismic energy among these events. The similarity in seismic magnitude gives us the opportunity to compare our results for the three different types of earthquake and interpret them in terms of the tectonic environments and seismic hazard. We analyze the peak ground accelerations and their relation with the stress drop. We simulated ground motions with the stochastic method. The model parameters are validated against recordings and a stress drop of 3, 15, and 70 MPa is estimated for the near-trench interplate, near-coast interplate, and normal-faulting inslab events, respectively. The near-trench interplate event has the lowest radiated seismic energy and the lowest rupture velocity. This is reflected in the energy to moment ratio of 2.17 × 10−6, 4.52 × 10−6, and 3.96 × 10−6 for the near-trench interplate, near-coast interplate, and normal-faulting inslab events, respectively. We define asperities using two different criteria: (1) based on average displacement and (2) maximum displacement. The asperity area for the near-coast and near-trench interplate events represents about 23 and 24–25 % of the total rupture area, which in the case of the normal-faulting inslab event, it is only 19 and 23 % based on average and maximum displacement, respectively.

Patterns of significant seismic quiescence on the Mexican Pacific coast

Many authors have proposed that the study of seismicity rates is an appropriate technique for evaluating how close a seismic gap may be to rupture. We designed an algorithm for identification of patterns of significant seismic quiescence by using the definition of seismic quiescence proposed by Schreider (1990). This algorithm shows the area of quiescence where an earthquake of great magnitude may probably occur. We have applied our algorithm to the earthquake catalog on the Mexican Pacific coast located between 14 and 21degrees of North latitude and 94 and 106degrees West longitude; with depths less than or equal to 60km and magnitude greater than or equal to 4.3, which occurred from January, 1965 until December, 2014. We have found significant patterns of seismic quietude before the earthquakes of Oaxaca (November 1978, Mw=7.8), Petatlán (March 1979, Mw=7.6), Michoacán (September 1985, Mw=8.0, and Mw=7.6) and Colima (October 1995, Mw=8.0). Fortunately, in this century earthquakes of great magnitude have not occurred in Mexico. However, we have identified well-defined seismic quiescences in the Guerrero seismic-gap, which are apparently correlated with the occurrence of silent earthquakes in 2002, 2006 and 2010 recently discovered by GPS technology.

Rapid Estimation of Fault Parameters for Tsunami Warning along the Mexican Subduction Zone: A Scenario Earthquake in the Guerrero Seismic Gap

Online Material: Color figures; synthetic‐data links. The devastating tsunami produced by the Sumatra–Andaman earthquake ( M w 9.2) of 24 December 2004 heightened the interest of the scientific community in tsunami early warning systems. Global efforts have been successful in achieving a robust ocean basin‐wide tsunami warning in which there is enough time to obtain accurate source parameters, determine the tsunami potential, model the tsunami propagation through the oceans, and update it with real‐time sea‐surface height observations. However, local systems are still not reliable. In such cases, rapid estimation of accurate source parameters that are expeditious in discriminating a tsunamigenic earthquake and its potential tsunami are essential. The 2011 Tohoku–Oki earthquake ( M w 9.0) revealed that tsunami‐warning techniques based on traditional seismic instrumentation could lead to an underestimation of the event magnitude (Ozaki, 2011), which can translate into tragic loss of life and infrastructure (e.g., Cyranosky, 2011). The Pacific coast of Mexico has been struck by at least 26 historic tsunamis originating from local subduction earthquakes (Sanchez and Farreras, 1993; Farreras, 1997). Tide‐gauge recording in Mexico began in 1952. Since then, the maximum height recorded is 3 m for the 1985 Michoacan earthquake ( M w 8.0). However, historic documents suggest that tsunamis as high as 8–11 m may have hit the Mexican Pacific coast during the Guerrero–Oaxaca earthquake of 28 March 1787, M w 8.4–8.6 (Sanchez and Farreras, 1993; Farreras, 1997; Nunez‐Cornu et al. , 2008; Suarez and Albini, 2009) and the Colima–Jalisco earthquake of 22 June 1932, M w 7.0 (Sanchez and Farreras, 1993; Farreras, 1997). An effective tsunami early warning system for the Mexican Pacific coast should be able to emit a reliable alert in less than 10 minutes, given that the peak in the tsunami height may reach the coast about 10–15 minutes after the …

Slow slip event in the Mexican subduction zone: Evidence of shallower slip in the Guerrero seismic gap for the 2006 event revealed by the joint inversion of InSAR and GPS data

Slow slip events (SSEs) in subduction zones have been observed in the last decade with continuous GPS stations. Some of them could be related to the lateral segmentation of subduction interface that seems to be a critical parameter for the propagation of large subduction earthquakes. In 2006, one of the largest SSEs recorded so far was captured by a dozen continuous GPS stations, in the Guerrero area (Mexico) along the Mexican subduction zone. Previous studies based on these data suggested a lateral variation of the updip depth of the SEE at the Guerrero seismic gap, but suffered from a lack of resolution east of the gap. Here, we show the ability of InSAR technique to capture a part of the 2006 SSE cumulative displacement east of the Guerrero gap by a stacking approach. We processed long strip Envisat interferograms corrected for orbital errors and interseismic signal using GPS data. We first use a forward modelling approach to test InSAR sensitivity to the amount of slip, depth and width of the slipping area on the subduction interface. Due to its high spatial resolution, InSAR allows one to comprehensively sample the North–South spatial wavelength of the SSE deformation, complementing the sparse GPS network. InSAR locates the maximum of uplift and subsidence caused by the SSE more precisely than the GPS data, giving better constraints on the updip slip limit of the SSE. We then inverted the InSAR and GPS data separately to understand how each inversion resolves the slip at depth. Finally, we performed a joint inversion of InSAR and GPS data, which constrained the SSE slip and its location on the plate interface over the entire Guerrero area. The joint inversion shows significant lateral variation of the SSE slip distribution along the trench with a shallower updip edge in the Guerrero seismic gap, west of Acapulco, and a deeper slip edge further east.

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

Global positioning system (GPS) time series in Guerrero (Mexico) reveal the existence of large slow slip events (SSEs) at the boundary between the Cocos and North American plates. In this study, we examined the last three SSEs that occurred in 2001/2002, 2006 and 2009/2010, and their impact on the strain accumulation along the Guerrero subduction margin. GPS displacements were inverted to retrieve the slip distribution during each SSE and the inter‐SSE coupling of the subduction interface. The three analyzed SSEs have equivalent moment magnitudes of between 7.50 and 7.65, their lateral extents are variable, and they all show significant slip in the Guerrero seismic gap. During the inter‐SSE epochs, the interplate coupling is high in the area where slow slip subsequently occurs. In the Guerrero gap, the shallow portion of the plate interface from the trench to the coast is weakly coupled. The average slip deficit accumulated in the Guerrero gap over a period of 12 years, which corresponds to three cycles of SSE, is only 1/4 of the slip deficit accumulated on both sides of the gap. Moreover, the regions of large slip deficit coincide with the rupture areas of recent large earthquakes. We conclude that the SSEs in the Guerrero gap release a significant part of the strain accumulated during the inter‐SSE period. If large subduction thrust earthquakes occur in the Guerrero gap, their recurrence time is probably increased compared to adjacent regions.