Zack Spica

Lateral heterogeneity imaged by small‐aperture ScS retrieval from the ambient seismic field

Interpreting core-related body wave travel-times is challenging for seismologists because Earths heterogeneities are averaged over thousands of kilometers and the sparsity of earthquake measurements makes these heterogeneities difficult to localize. We show how the ambient seismic wave field can be used to overcome these limitations. We present a regional-scale analysis of core-mantle boundary reflections (ScS) under Mexico. We show that body wave arrivals (i.e., P and ScS) are retrieved from higher-order cross-correlations (C3), a technique that provides a more uniform and controlled source distribution using the scattered waves of the coda of classical ambient field cross-correlations (C1). Then, we extract ScS travel times along a dense linear array in Mexico and find that lithospheric lateral heterogeneity, such as the subducting Cocos slab beneath Mexico, may have a strong impact on ScS travel times. In parallel, we show that lateral heterogeneity such as a possible ultra low velocity zone (ULVZ) near the core mantle boundary might also affect, although to a lesser extent, the travel time anomalies. Our results and interpretation are supported through numerical simulations that accounts for slab and ULVZ properties.

Shallow VS Imaging of the Groningen Area from Joint Inversion of Multimode Surface Waves and H/V Spectral Ratios

The Groningen gas field in the northern Netherlands is subject to production-induced earthquakes and has quickly become one of the seismologically best-instrumented areas on Earth. Accurate quantification of seismic hazard from potential future earthquakes requires accurate shallow velocity structure for ground-motion prediction. Toward this end, we present a shear-wave velocity model developed through the joint inversion of multimode Loveand Rayleigh-wave dispersion curves (DCs) and H/V spectral ratio (HVSR) measurements. We obtain local DCs from azimuthally averaged frequency–time analysis of the cross correlation of the ambient seismic field (ASF) between pairs of stations. HVSR is measured at each station from the directional energy density, that is, the autocorrelation of the ASF for all components. We simultaneously fit these observables at each station of the dense Loppersum array to infer a 1D velocity model from the surface to a depth of ∼900 m. In the frequency range considered (∼1–7 Hz), Rayleigh-wave DCs show high modal complexity, which makes clear identification of the modes challenging and leads us to downweight their contribution to the result. Fundamentaland higher-mode Love-wave dispersion is much clearer. We find good agreement between our model and independently derived models of shallow structure, which validates our approach and supports the value of HVSR analysis as a tool to map subsurface properties. Electronic Supplement: Frequency–time diagrams, theoretical kx , omega diagrams, example joint inversion for site 235587, and example of horizontal-to-vertical (H/V) spectral ratio (HVSR) at station site 235587.

The Ambient Seismic Field at Groningen Gas Field: An Overview from the Surface to Reservoir Depth

The long-term exploitation of the Groningen gas field led to compaction at reservoir depth, subsequent ground subsidence, and recently earthquakes. As part of an ongoing effort to quantify the hazard and risk in the region, several permanent and temporary seismic arrays have been deployed. As a result, the Groningen area is one of the seismologically best-instrumented areas worldwide. In this article, we describe several seismic experiments that were conducted in the region and take advantage of the numerous possibilities they offer to characterize the ambient seismic wavefield at the surface, in the shallow subsurface, and at reservoir depth. By means of beamforming, analysis of cross-correlation functions, surface-wave eigenfunction analysis, and correlations of neighboring frequencies, we are able to determine the main characteristics of the ambient seismic field (ASF), including the predominant propagation modes and phases. We retrieve clear multimode Rayleigh and Love waves, as well as and P waves, from cross correlations of the ASF. At reservoir depth, we show that the wavefield is largely trapped and reflected between geologic boundaries above and below the reservoir. This article reviews the characteristics of ASF observations with the goal of guiding future investigations of shallow structure of the Groningen area. Electronic Supplement: Figure showing cross-correlation envelope functions between the two deep borehole arrays.

Bend Faulting at the Edge of a Flat Slab: The 2017 Mw7.1 Puebla-Morelos, Mexico Earthquake

We present results of a slip model from joint inversion of strong motion and static GPS data for the Mw7.1 Puebla-Morelos earthquake. We find that the earthquake nucleates at the bottom of the oceanic crust or within the oceanic mantle with most of the moment release occurring within the oceanic mantle. Given its location at the edge of the flat slab the earthquake is likely the result of bending stresses occurring at the transition from flat slab subduction to steeply dipping subduction. While the event strikes obliquely to the slab we find a good agreement between the seafloor fabric offshore the source region and the strike of the earthquake. We argue that the event likely reactivated a fault first created during seafloor formation. We hypothesize that large bending related events at the edge of the flat slab are more likely in areas of low misalignment between the seafloor fabric and the slab strike where reactivation of preexisting structures is favored. This hypothesis predicts decreased likelihood of bending related events northwest of the 2017 source region but also suggests that they should be more likely southeast of the 2017 source region.

Comparison of the Seismicity Before and After the 1982 El Chichon Eruption

The seismicity recorded at El Chichon Volcano shows significant differences before and after the 1982 eruptive episodes. The analysis of the seismicity was performed using two well-known laws in seismology: the Gutenberg-Richter law, which describes the frequency-magnitude distribution of seismicity and the Omori law, which applies to the temporal decay of the number of earthquakes . Results of the analysis suggest that large quantities of fluids (hydrothermal fluids, water, and/or magma) were involved in the physical processes that generated the precursory seismicity one month before the onset of the eruption; only shallow (2–8 km depth) hybrid earthquakes were located and long-period and volcanic tremors were recorded. The volcanic signature of the seismicity is better described by two slopes instead of only one as is usually the case for tectonic events. Furthermore, the seismicity did not follow the Omori law. In contrast, for one month after the last eruptive phases of the 1982 eruptive events, the seismicity showed a more ‘tectonic’ signature, as evidenced by the occurrence of five large earthquakes (magnitudes ~3.8) on 4 April 1982. These followed both the Gutenberg-Richter law (with a single slope) and the Omori law. The tectonic signature is confirmed further by the single-slope observed in the distribution of the seismicity recorded during the period 1985–1990. Such behavior suggests the absence of abundant fluid at depth after the last plinian event of 4 April 1982, with a slow return to a regular tectonic response of the volcanic system. Seismic analysis of the 1982 eruptive sequence illustrates clear volcano-tectonic feedback interactions.