Abstract: Southern Mexico was struck by four earthquakes with Mw > 6 and numerous smaller earthquakes in September 2017, starting with the 8 September Mw 8.2 Tehuantepec Earthquake beneath the Gulf of Tehuantepec offshore Chiapas and Oaxaca. We study whether this M8.2 earthquake triggered the three subsequent large M>6 quakes in southern Mexico to improve understanding of earthquake interactions and time-dependent risk. All four large earthquakes were extensional despite the subduction of the Cocos plate at the convergent plate boundary. The traditional definition of aftershocks: likely an aftershock if it occurs within two rupture lengths of the main shock soon afterwards. Two Mw 6.1 earthquakes, half an hour after the M8.2 beneath the Tehuantepec gulf and on 23 September near Ixtepec in Oaxaca, both fit as traditional aftershocks, within 200 km of the main rupture. The 19 September Mw 7.1 Puebla earthquake was ~600 km away from the M8.2 shock, outside the standard aftershock zone. Geodetic measurements from interferometric analysis of synthetic aperture radar (InSAR) and time-series analysis of GPS station data constrain finite fault static slip models for the M8.2, M7.1, and M6.1 Ixtepec earthquakes. We include open-ocean tsunami waveforms for the M8.2 inversions. We analyzed InSAR data from Copernicus Sentinel-1A and -1B satellites and JAXA ALOS-2 satellite. Our Bayesian (AlTar) static slip model for the M8.2 quake shows significant slip extended > 150 km and possible 220 km NW from the hypocenter and there is a high probability that the slip extended to depths of at least 70 km indicating slab pull stress state. Our slip model for the M7.1 earthquake is similar to the v2 NEIC FFM. Inversions for the M6.1 Ixtepec quake confirm shallow depth in the upper-plate crust and show centroid is about 30 km SW of the preliminary NEIC epicenter but consistent with cluster relocations. The NEIC updated epicenter and Mexican SSN location are closer to the InSAR-constrained location.
Slidecast:
https://vimeo.com/276974759
Abstract: Typical subduction zones are characterised by curved thrust fault geometries that merge with the bathymetry under very shallow angles of narrow subduction wedges. Additionally, complicated networks of fault branches at high angles to the megathrust in the overriding and oceanic plates potentially generate strong gaining effects of vertical sea-floor displacements, making tsunami generation more likely. We present high-resolution physics-based numerical simulations of the 2004 Sumatra-Andaman earthquake, including non-linear frictional failure on a megathrust-splay fault system, off-fault plasticity, seismic wave propagation up to 2.2 Hz in 3D media and bathymetry. We specifically analyse splay fault slip transfering into vertical sea-floor displacement which may be required to to generate large events despite the fact that observations suggest high-fluid pressure and low stress drops limiting the available energy. The earthquake scenario matches coseismic slip and horizontal and vertical surface displacements inferred from observations. We find a high sensitivity of splay fault activation and plasticity effects to the orientation of the background stress field in conjuncture with fault geometry. Considering the geometric complexity of subduction zones and their potentially characteristically long rupture duration, invariably leads to huge differences in element sizes and many thousands of time steps. Our largest model consists of up to 221 million elements and 111 billion degrees of freedom. Such high fidelity simulations were performed using recent advances applied to SeisSol (www.seissol.org), specifically through a novel clustered local-time-stepping scheme extended to the dynamic rupture implementation (Uphoff et al., SC17).
Slidecast:
https://vimeo.com/276934728
Abstract: Better characterization of induced microseismic events can reveal important geomechanical parameters and support the assessment of seal integrity for fluid injection operations. The spatio-temporal evolution of seismicity in conjunction with source parameter analysis can provide more detailed insight into reservoir behavior. We perform a detailed characterization of microseismic events at the Illinois Basin – Decatur Project (IBDP), in Decatur, Illinois. About 4800 microseismic events were located with deep borehole sensors during the injection of 1 Mio tons of CO2 during 3 years. Using a waveform cross-correlation method, we can distinguish between events occurring in the sandstone reservoir and events only some tens of meters deeper in the adjacent uppermost crystalline basement. Full-waveform modeling can be used to identify observations of different phase arrivals. Further analysis of source parameters such as Brune stress drop and b-value, as well as the general evolution of microseismic clusters reveal a fluid-driven behavior of seismicity within a cluster, and a punctual hydraulic connection between reservoir and basement. Focal mechanisms are estimated for selected events by combining recordings from surface and downhole sensors. In addition, temporal changes in attenuation as measured from microseismic waveforms may also carry information about the progression of the CO2 plume. We apply a multiple empirical Green’s function approach on repeating multiplets to compare spectral ratios. The method yields systematic spatial variations of Q. However, no temporal variations of Q can be resolved from the microseismic data alone. However, as part of seismic monitoring, we aim to establish a new data analysis workflow integrating active and passive seismic data for a more holistic 4D seismic monitoring system. The larger aperture and higher resolution achieved by combination of all available seismic data may allow for a more precise mapping of the injected fluid.
Slidecast:
https://vimeo.com/276939321
Abstract: We measured shear wave anisotropy under the Geometry of Cocos (GECO) experiment, deployed at the eastern end of the Trans-Mexican Volcanic Belt. A tear in the south Cocos slab has been proposed [Dougherty and Clayton, 2014]. Two splitting parameters are used to quantify anisotropy. These are the delay time (delta t) and the fast polarization direction (phi). The shear wave splitting method of Silver and Chan [1991] was used. A time window containing the SKS or PKS phase is selected. Then, the north-south and east-west components are rotated by 1 degree, ranging from -90º to 90º. For each rotation, the solution is searched in 0.05s increments, and the crosscorrelation and autocorrelation between the two traces is obtained. Next the eigenvalues are calculated for every possible combination of (delta t) and (phi). Finally, the minimum eigenvalue is chosen as the best solution. We check our results in two ways: by correcting the original records using the measured (delta t) and (phi) in order to obtain a weaker SKS signal in the transverse component, and by comparing the waveforms and the arrival time difference between the fast and the slow components. The orientation of (phi), measured for western stations, is explained by subslab, entrained mantle flow under flat Cocos slab. While going from west to east. the slab geometry changes from flat to steeper subduction, and may be accompanied by a possible tear in the slab. For the eastern stations, (phi) is rotated 25º clockwise relative to stations in the west. This phenomenon may be accounted for by mantle flow between the possible tear and by rollback of the slab. The orientation of the fast axes where the slab subducts steeply is consistent with both subslab entrained flow, and with corner flow in the mantle wedge between the Cocos and North American plates.
Slidecast:
https://vimeo.com/276938525
Abstract: The distinct sensitivities of seismic velocity and attenuation (1/Q) to water content, melt, and major element composition yield important constraints on mid-ocean ridge processes and the associated mantle flow pattern, melt distribution, and the interaction of spreading centers with hotspots. Beneath the Juan de Fuca ridge, both body-wave and surface-wave studies observed strong attenuation and slow velocity anomalies. However, it is still under debate which mechanism is favored: Either the direct effects of in situ melt extending to depths of 150 km or more, or the pre-melting effects of a hydrous mantle upwelling with about 200 ppm of water in which melting commences at about 100 km. Neither the depth-integrated S-wave attenuation nor the averaged 1-D shear attenuation for surface waves can provide further constraints; to resolve the debate a 3-D attenuation structure is required. Surface-wave tomography of 3-D attenuation structure is known as a challenging task due to difficulties in separating elastic focusing effects on seismic amplitudes from intrinsic attenuation and the relatively small effects of attenuation on amplitude over short distances. Taking this into account, we introduced the finite-frequency kernels for attenuation and imaged 3D shear attenuation and velocity structures in the vicinity of the Juan de Fuca plate. Owing to the extensive arrays of ocean bottom seismometers (OBS) with high quality, long deployment periods, and novel noise removal techniques, we obtained 3-D attenuation and shear velocity model with the best resolution to date of any spreading center based on fundamental-mode Rayleigh waves. With 3-D models of shear attenuation and shear velocity, we evaluate the two competing paradigms and shall present a summary of the inversion results and favored interpretation.
Slidecast:
https://vimeo.com/276930248
Abstract: The Beichuan area suffered the most serious seismic damage in the 2008 Wenchuan earthquake although the Beichuan is over 100 km away from the instrumental epicenter of the mainshock. The mechanism for this peculiar phenomenon remains unclear even though nearly 10 years has passed since the Wenchuan shock. For this purpose, we construct a spontaneous rupture model in which Gaochuan right bend (GRB) in the middle of Xingxiu-Beichuan fault, a major seismogenic fault for the Wenchuan event, is included. The simulated results show that the complex geometry of the GRB played a first-order role in controlling the rupture propagation. While rupture is initiated at the epicenter of the Wenchuan mainshock, it propagates spontaneously northeastward at the speed slower than the shear wave speed of local medium. When the rupture front spreads near the end of Yingxiu-Gaochuan fault, a new rupture is renucleated at the curve section of the Gaochuan bend, and propagates in the NE direction with the speed greater than the S wave velocity. In particular, this rupture transition from subshear to supershear speed does not need time delay, much different from the case of fault step over which were studied well by previous workers. Due to the curved geometry of GRB, the stress regime on the fault section favors the supershear rupture formation. Once supershear rupture occurs along the Beichuan fault, seismic waves were focused and largely amplified with the form of Mach waves. The numerical result also illustrated that high values of spatial distribution of the strong ground motion acceleration are mainly located in the Beichuan area, directly leading to grave seismic catastrophe. Therefore, this work may give some insight into why the most serious hazard occurred in the Beichuan area, suggesting that comprehensive investigation of fault geometry is important in understanding earthquake dynamics and seismic hazard assessment.
Slidecast:
https://vimeo.com/276934346
