Piero Poli

The Mw 8.3 Illapel earthquake (Chile): Preseismic and postseismic activity associated with hydrated slab structures

The accumulated stress in subduction zones is discharged with earthquake and aseismic activity; the latter is hosted in rheological complex regions, characterized by high pore fluid pressure, and is often accompanied by repeated earthquakes and earthquake swarms. The spatiotemporal analysis of seismic activity can reveal the presence of aseismic transients associated with large earthquakes. Here we study 20 years of seismicity prior to and after the M w 8.3 earthquake that occurred in A.D. 2015 in central Chile. We identified several earthquake swarms before the main shock and repeating aftershocks at the border of the main slip area. Spatial clustering of the seismic activity shares similar orientation with the main fracture zones observed on the outer rise of the subducting Nazca plate. Our findings suggest that the fracture zones enclosing the rupture are playing a major role in accommodating the pre and post–main shock stress evolution. We further recognize how fracture regions have acted as barriers to the propagation of large earthquakes in the region.

Earthquake rupture below the brittle-ductile transition in continental lithospheric mantle

The slow and inefficient deep Wyoming earthquake ruptured in the ductile regime of the upper mantle. Earthquakes deep in the continental lithosphere are rare and hard to interpret in our current understanding of temperature control on brittle failure. The recent lithospheric mantle earthquake with a moment magnitude of 4.8 at a depth of ~75 km in the Wyoming Craton was exceptionally well recorded and thus enabled us to probe the cause of these unusual earthquakes. On the basis of complete earthquake energy balance estimates using broadband waveforms and temperature estimates using surface heat flow and shear wave velocities, we argue that this earthquake occurred in response to ductile deformation at temperatures above 750°C. The high stress drop, low rupture velocity, and low radiation efficiency are all consistent with a dissipative mechanism. Our results imply that earthquake nucleation in the lithospheric mantle is not exclusively limited to the brittle regime; weakening mechanisms in the ductile regime can allow earthquakes to initiate and propagate. This finding has significant implications for understanding deep earthquake rupture mechanics and rheology of the continental lithosphere.

Earthquakes initiation and thermal shear instability in the Hindu‐Kush intermediate‐depth nest.

Intermediate depth earthquakes often occur along subducting lithosphere, but despite their ubiquity the physical mechanism responsible for promoting brittle or brittle-like failure is not well constrained. Large concentrations of intermediate depth earthquakes have been found to be related to slab break-off, slab drip, and slab tears. The intermediate depth Hindu Kush nest is one of the most seismically active regions in the world and shows the correlation of a weak region associated with ongoing slab detachment process. Here we study relocated seismicity in the nest to constraint the geometry of the shear zone at the top of the detached slab. The analysis of the rupture process of the Mw 7.5 Afghanistan 2015 earthquake and other several well-recorded events over the past 25 years shows an initially slow, highly dissipative rupture, followed by a dramatic dynamic frictional stress reduction and corresponding large energy radiation. These properties are typical of thermal driven rupture processes. We infer that thermal shear instabilities are a leading mechanism for the generation of intermediated-depth earthquakes especially in presence of weak zone subjected to large strain accumulation, due to ongoing detachment process.

Reawakening of large earthquakes in south central Chile: The 2016 Mw 7.6 Chiloé event

On 25 December 2016, the Mw 7.6 Chiloe earthquake broke a plate-boundary asperity in South- Central Chile near the center of the rupture zone of the Mw 9.5 Valdivia earthquake of 1960. To gain insight on decadal-scale deformation trends and their relation with the Chiloe earthquake, we combine geodetic, teleseismic and regional seismological data. GPS velocities increased at continental scale after the 2010 Maule earthquake, probably due to a readjustment in the mantle flow and an apparently abrupt end of the viscoelastic mantle relaxation following the 1960 Valdivia earthquake. It also produced an increase in the degree of plate locking. The Chiloe earthquake occurred within the region of increased locking, breaking a circular patch of ~15 km radius at ~30 km depth, located near the bottom of the seismogenic zone. We propose that the Chiloe earthquake is a first sign of the seismic reawakening of the Valdivia segment, in response to the interaction between postseismic viscoelastic relaxation and changes of interseismic locking between Nazca and South-America.

Global and along‐strike variations of source duration and scaling for intermediate‐depth and deep‐focus earthquakes

The systematic behavior of earthquake rupture as a function of earthquake magnitude and/or tectonic setting is a key in our understanding of the physical mechanisms involved during earthquake rupture. Geophysical evidence suggests that although deep earthquakes—including intermediate-depth and deep—are similar to shallow ones, the mechanism involved during deep earthquakes is different from that of shallow ones. In particular, the magnitude and depth dependence of scaled duration, a measure of earthquake rupture duration, has led to controversy of what controls deep earthquake behavior. Here we estimate scaled source durations for 600 intermediate-depth and deep-focus earthquakes recorded at teleseismic distances and show deviation from self-similar scaling. No depth dependence is observed which we interpret as due to little differences between intermediate-depth and deep-focus earthquake mechanisms. The data show no correlation between durations and plate age or thermal parameters, suggesting that the thermal properties of the plate have little effect on source durations. We nevertheless report differences in average source duration and scaling between subduction zones and along-strike variations of source durations that more closely resemble the geometry of subduction (flat or steep subduction) rather than plate age.

Global rupture parameters for deep and intermediate depth earthquakes

We investigate the global rupture parameters for deep and intermediate-depth earthquakes. From measurements of rupture duration and radiated seismic energy we estimate stress drop, apparent stress and radiation efficiency, and obtain a detailed earthquake energy budget. From scaling of the source parameters we highlight differences between crustal and deep seismicity, with the latter showing larger fracture energies. The observed increase of radiation efficiency with depth suggests that rupture mechanism for deep and intermediate depth events differs. In agreement with previous studies we observe along strike variability of rupture properties for deep and intermediate-depth earthquakes, correlating with slab morphology, plate age or presence of volcanic structures.

Imaging the D″ reflector with noise correlations

The lowermost mantle of the Earth is characterized by seismic structures that range from a few tens to thousands of kilometers. At present, it is difficult to test hypotheses put forward to explain seismic observations due to poor seismic coverage, as particular earthquake-station geometries are needed. We demonstrate here that seismic noise correlations can be used to robustly image deep-mantle reflections with larger stacked amplitudes of reflected waves compared with earthquake data. In a comparison between noise and earthquake data, we find that the arrival times and the slowness of reflected waves, both sampling a region beneath Siberia, agree with those for a reflector at 2530 km depth, and the small amplitude reflections are sufficiently clear in the noise correlations to compare them reliably with synthetic data. Our data open exciting prospects for illuminating new target zones in the deep mantle to further constrain the dynamics and mineralogy of the deep Earth.

Nucleation Phase and Dynamic Inversion of the Mw 6.9 Valparaíso 2017 Earthquake in Central Chile

The Valparaiso 2017 sequence occurred in the Central Chile mega-thrust, an active zone where the last mega-earthquake occurred in 1730. Intense seismicity started 2 days before the Mw 6.9 main-shock, a slow trench-ward movement was observed in the coastal GPS antennas and was accompanied by foreshocks and repeater-type seismicity. To characterize the rupture process of the main-shock, we perform a dynamic inversion using the strong-motion records and an elliptical patch approach. We suggest that a slow slip event preceded and triggered the Mw 6.9 earthquake, which ruptured an elliptical asperity (semi-axis of 10 km and 5 km, with a sub-shear rupture, stress drop of 11.71 MPa, yield stress of 17.21 MPa, slip weakening of 0.65 m and kappa value of 1.98). This earthquake could be the beginning of a long-term nucleation phase to a major rupture, within the highly coupled Central Chile zone where a mega-thrust earthquake like 1730 is expected.

Depth variations of 410 km and 660 km discontinuities in eastern North China Craton revealed by ambient noise interferometry

Recent studies have demonstrated that body waves between pairs of stations can be successfully retrieved from ambient noise cross-correlation at both regional and global scales, although surface waves are the dominant signals. However, it is still difficult to use these retrieved body wave signals to map lateral depth variations of main structural discontinuities or velocity contrasts because of its low signal to noise ratio (SNR). In this research, based on a dense seismic array in eastern North China Craton, reflected P-wave signals from 410-km and 660-km discontinuities can be successfully recovered from ambient noise cross-correlation. To improve SNR, the cross-correlations are stacked within each bin with the phase-weighted stack (PWS) method. The retrieved P410P and P660P phases from stacked correlations reveal lateral variations of both depths and sharpness of the 410-km and 660-km discontinuities along two profiles, which may be related with hot material upwelling and the effect of stagnant Pacific plate in the transition zone beneath North China Craton. The imaging results are generally consistent with the results from teleseismic receiver functions, which demonstrate the possibility of mapping high-resolution topography and sharpness of deep internal discontinuities without earthquake-station geometric limitations.

Earthquake Directivity, Orientation, and Stress Drop Within the Subducting Plate at the Hikurangi Margin, New Zealand: Directivity of New Zealand Earthquakes

We develop an approach to calculate earthquake source directivity and rupture velocity for small earthquakes, using the whole source time function rather than just an estimate of the duration. We apply themethod to an aftershock sequence within the subducting plate beneath North Island, New Zealand, and investigate its resolution. We use closely located, highly correlated empirical Green’s function (EGF) events to obtain source time functions (STFs) for this well-recorded sequence. We stack the STFs from multiple EGFs at each station, to improve the stability of the STFs. Eleven earthquakes (M 3.3–4.5) have sufficient azimuthal coverage, and both P and S STFs, to investigate directivity. The time axis of each STF in turn is stretched to find the maximum correlation between all pairs of stations. We then invert for the orientation and rupture velocity of both unilateral and bilateral line sources that best match the observations. We determine whether they are distinguishable and investigate the effects of limited frequency bandwidth. Rupture orientations are resolvable for eight earthquakes, seven of which are predominantly unilateral, and all are consistent with rupture on planes similar to the main shock fault plane. Purely unilateral rupture is rarely distinguishable from asymmetric bilateral rupture, despite a good station distribution. Synthetic testing shows that rupture velocity is the least well-resolved parameter; estimates decrease with loss of high-frequency energy, and measurements are best considered minimum values. We see no correlation between rupture velocity and stress drop, and spatial stress drop variation cannot be explained as an artifact of varying rupture velocity.