Simulation of Pulse-Like Ground Motions during the 2015 Mw 8.3 Illapel Earthquake with a New Source Model Using Corrected Empirical Green’s Functions

Pulse-like near-source ground motions were observed by the local network during the 2015 Mw 8.3 Illapel, Chile earthquake. Such ground motions can be quite damaging to a wide range of infrastructures. The primary objective of this study is to provide a source model that can explain such ground motions. The isolated nature of the pulses indicated that the rupture of some small isolated region on the fault contributed to the generation of the pulse. Therefore, we considered such regions and termed them as Strong Motion Pulse Generation Areas (SPGAs). We used the corrected empirical Green's function (EGF) method because this method has been successfully applied to near-source pulse-like ground motions in previous studies. We simulated synthetic waveforms using the frequency dependent quality factor Q=239f0.71 and empirical site amplification factors, which we obtained by applying a generalized inversion technique to local weak-motion data. The result indicated that the observed ground motions from the Mw 8.3 Illapel earthquake can readily be explained with a source model that involves two SPGAs with dimensions of several kilometers in spite of the huge rupture zone of the earthquake. The source model can reproduce velocity waveforms, acceleration Fourier amplitude spectra (FAS) and pseudoacceleration response spectra. It also reproduces the duration of strong ground motions quite accurately. No significant bias was found with respect to distance and frequency. In conclusion, the corrected EGF method is a very efficient tool to simulate near-source ground motions of a subduction earthquake when it is combined with higher stress-drop subevents whose sizes are adjusted to the observed pulse widths.

Transient Deformation and Stress Patterns Induced by the 2010 Maule Earthquake in the Illapel Segment

Evaluating the transfer of stresses from megathrust earthquakes to adjacent segments is fundamental to assess seismic hazard. Here, we use a 3D forward model as well as GPS and seismic data to investigate the transient deformation and Coulomb Failure Stresses (CFS) changes induced by the 2010 Maule earthquake in its northern segment, where the Mw 8.3 Illapel earthquake occurred in 2015. The 3D model incorporates the coseismically instantaneous, elastic response, and time-dependent afterslip and viscoelastic relaxation processes in the postseismic period. We particularly examine the impact of linear and power-law rheology on the resulting postseismic deformation and CFS changes that may have triggered the Illapel earthquake. At the Illapel hypocenter, our model results in CFS changes of ∼0.06 bar due to the coseismic and postseismic deformation, where the coseismic deformation accounts for ∼85% of the total CFS changes. This is below the assumed triggering threshold of 0.1 bar and, compared to the annual loading rate of the plate interface, represents a clock advance of approximately only 2 months. However, we find that sixteen events with Mw ≥ 5 in the southern region occurred in regions of CFS changes > 0.1 bar, indicating a potential triggering by the Maule event. Interestingly, while the power-law rheology model increases the positive coseismic CFS changes, the linear rheology reduces them. This is due to the opposite polarity of the postseismic displacements resulting from the rheology model choice. The power-law rheology model generates surface displacements that fit better to the GPS-observed landward displacement pattern.

The delayed aftershocks of the Illapel Earthquake Mw 8.3, 2015, Chile

<p>The delayed aftershocks 2018 Mw 6.2 on April 10 and Mw 5.8 on Sept 1 and 2019 Mw 6.7 on January 20, Mw 6.4 on June 14, and Mw 6.2 on November 4, associated with the Mw 8.3 2015 Illapel Earthquake occurred in the ​​central Chile. The seismic source of this earthquake has been studied with the GPS, InSAR and tide gauge network. Although there are several studies performed to characterize the robust aftershocks and the variations in the field of deformation induced by the megathrust, but there are still aspects to be elucidated of the relationship between the transfer of stresses from the interface between plates towards delayed aftershocks with the crustal structures with seismogenic potential. Therefore, the principal objective of this study is to understand how the stress transfer induced by the 2015 Illapel earthquake of the heterogeneous rupture mechanism to intermediate-deep or crustal earthquakes. For this, coulomb stress changes from  finite fault model of the Illapel earthquake and with the biggest aftershocks in year 2015 are used. These cumulative stress pattern provides substantial evidences for the delayed aftershocks in this region. The subducting Challenger Fault Zone and Juan Fernandez Ridge heterogeneity are existing feature, which releases the accumulated coulomb stress changes and provide delayed aftershocks.  Therefore along with stress induced by a large earthquake such as Mw 8.3 from Illapel 2015 along with biggest aftershocks, have a direct mechanism that may activate the  delayed aftershocks. Our study suggests  the activation of crustal faults in this research as a risk assessment factor for the evaluating in the seismic context of the region and useful for another subduction zone.</p>

Efforts toward automatic aftershock sequences processing at the International Data Centre

<div> <p>The number of aftershocks after a large main shock may increase the daily number of seismic events by an order of magnitude for a few days or even weeks. The large number of incoming arrivals reduces the effectiveness of automatic bulletin generation and significantly increases the work of the analysts. In the verification context such aftershocks may delay the production of the CTBTO Reviewed Event Bulletin, as well as mask clandestine nuclear tests. Consequently, the CTBTO has been investigating ways to improve the performance of the automatic processing during aftershock sequences.  </p> </div><div> <p>In line with this investigation, the PTS launched a project with the objective to evaluate three algorithms that could address this issue, namely the Empirical Matched Field developed at NORSAR, the SeisCorr developed at Sandia National Labs and XSEL developed at the IDC. In this abstract we present comparisons on the performance of the three methods on the aftershock sequences of four very strong earthquakes: the Tohoku earthquake in Japan (March 2011), the Gorkha earthquake in Nepal (April 2015), the  Illapel earthquake off the coast of Chile (September 2015) and the devastating earthquake in Papua New Guinea (February 2018).</p> </div>

The Mw 8.3 2015 Illapel afterslip imaged through a time-dependent inversion of continuous and survey GPS data

<div> <div> <div> <p>The Mw 8.3 2015 Illapel earthquake ruptured a 190 km long segment of the Chilean subduction zone. In the past, this area ruptured several times through large and great earthquakes, the most recent event before 2015 being a Mw 7.9 earthquake in 1943. Here, we combine continuous and survey GPS ground displacements to perform a kinematic inversion of the two-months afterslip following the mainshock. We show that the postseismic slip developed South and North of the coseismic rupture, but also overlaps the deeper part of it. We estimate that two months after the large mainshock, the postseismic moment released represents 13% of the coseismic moment (the mainshock released 3.16x10<sup>21</sup> N.m whereas the afterslip released 3.98x10<sup>20</sup> N.m). At a first order, seismicity and areas experiencing afterslip match together and are concentrated at the edges of the coseismic rupture between 25 and 45 km depth. One interesting feature is the occurrence of two moderate size aftershocks on November, 11<sup>th</sup> at shallow depth North of the rupture. We investigate the relationship between the evolution of afterslip and these aftershocks. Finally, we interpret the result in the light of past earthquakes history and calculate the moment balance through the last centuries.</p> </div> </div> </div>

Spatial–temporal properties of afterslip associated with the 2015 Mw 8.3 Illapel earthquake, Chile

In order to characterize the spatial–temporal properties of postseismic slip motions associated with the 2015 Illapel earthquake, the daily position time series of 13 GNSS sites situated at the near-field region are utilized. Firstly, a scheme of postseismic signal extraction and modeling is introduced, which can effectively extract the postseismic signal with consideration of background tectonic movement. Based on the extracted postseismic signal, the spatial–temporal distribution of afterslip is inverted under the layered medium model. Compared with coseismic slip distribution, the afterslip is extended to both deep and two sides, and two peak slip patches are formed on the north and south sides. The afterslip is mainly cumulated at the depth of 10–50 km, and the maximum slip reaches 1.46 m, which is situated at latitude of − 30.50°, longitude of − 71.78°, and depth of 18.94 m. Moreover, the postseismic slip during the time period of 0–30 days after this earthquake is the largest, and the maximum of fault slip and corresponding slip rate reaches 0.62 m and 20.6 mm/day. Whereas, the maximum of fault slip rate during the time period of 180–365 days is only around 1 mm/day. The spatial–temporal evolution of postseismic slip motions suggests that large postseismic slip mainly occurs in the early stage after this earthquake, and the fault tend to be stable as time goes on. Meanwhile, the Coulomb stress change demonstrate that the postseismic slip motions after the Illapel earthquake may be triggered by the stress increase in the deep region induced by coseismic rupture.

Tsunami Detection by GPS-derived Ionospheric Total Electron Content

To unravel the relationship between earthquake and tsunami using ionospheric total electron content (TEC) changes, we analyzed two Chilean tsunamigenic subduction earthquakes: the 2014 Pisagua Mw 8.1 and the 2015 Illapel Mw 8.3. During the Pisagua earthquake, the TEC changes were detected at the GPS sites located to the north and south of the earthquake epicenter, whereas during the Illapel earthquake, we registered the changes only in the northward direction. Tide-gauge sites mimicked the propagation direction of tsunami waves similar to the TEC change pattern during both earthquakes. The TEC changes were represented by two signals. The initial weaker signal correlated well with seismic Rayleigh waves, while the following stronger perturbation was interpreted to be caused by acoustic and gravity waves induced by earthquakes and subsequent tsunamis. As a result, TEC changes can be utilized to evaluate earthquake occurrence and tsunami propagation within a framework of multi-parameter early warning systems.

Build seismic cycle balance deformation with InSAR in Northen Chile

<p>Lauriane Baylé (1), Romain Jolivet (2), Nadaya Cubas (1) and Laetitia Le</p><p>Pourhiet (1)</p><p>(1) Institut des Sciences de la Terre de Paris, UMR 7193, UPMC UniversitéParis 6, CNRS, Paris,</p><p>France</p><p>(2) Laboratoire de Géologie, Département de Géosciences, École Normale Supérieure, CNRS UMR 8538,</p><p>PSL ResearchUniversity, Paris, France</p><p>Recent studies have pointed out to a discrepancy between the short- and long-</p><p>term deformation of overriding plates in subduction zones. This led to debates</p><p>about when and how permanent deformation is acquired. This contradiction</p><p>has notably been observed along the Central Andes Subduction Zone, where</p><p>the coast subsides during and shortly after major earthquakes while a coastal</p><p>uplift with rates ranging between 0.1 and 0.3 mm/yr has been inferred the</p><p>last 4000 ky. For instance, during the 15th September 2015 Mw 8.3 Illapel</p><p>earthquake the geodetics (GPS and InSAR) data show a coastal subsidence</p><p>along the line-of-sight of 20 cm in InSAR.</p><p>To reconcile the seemingly contradictory observations, we here propose to</p><p>provide a seismic cycle uplift balance by constrainning inter-, co- and post-</p><p>seismic vertical velocities from InSAR time series. The study focuses on La</p><p>Serena peninsula (71.3°W, 30°S, Chile) along which the Illapel earthquake</p><p>occurred and for which long-term uplift rates have been provided by previous</p><p>geomorphological studies.</p><p>To build this seismic cycle balance, we use InSAR data (Sentinel-1) acqui-</p><p>red between the September 15, 2015 and January 19, 2019. The time series</p><p>for the ascendant orbite is calculated and the accumulated vertical displace-</p><p>ment extracted providing co- and post-seismic displacement. The co-seismic</p><p>displacement are similar to those previously obtain. To constrain the displa-</p><p>cement during the inter-seismic period, data on both sides of the peninsula</p><p>are used. In that respect, we aim determining when, during the seismic cycle,</p><p>and where, along the coast, the uplift occurs.</p><p>The deduced time series will then be confronted to numerical modelling</p><p>to provide the short- and long-term mechanics reproducing the short- and</p><p>long-term observations.</p>

Comparing permanent deformation and seismic asperities in the 2015 Illapel earthquake rupture zone

<p><strong>Abstract:</strong></p><p>Giant subduction earthquakes (M<sub>W</sub> 8 to 9) are usually characterized by heterogeneous slip distributions, including regions of very pronounced slip that are commonly known as asperities. However, it is a matter of ongoing debate whether asperities constitute persistent geologic features or if they rather represent transient features related to the release of elastic strain accumulated in areas of seismic gaps. Recent giant earthquakes along the coast of north-central Chile, such as the 2010 Maule (M8.8), 2015 Illapel (M8.3), and 2014 Iquique (M8.2) events, were all associated with the rupture of single or multiple seismic asperities. Here we compare permanent deformation and seismic-cycle deformation patterns and rates along the 2015 Illapel earthquake rupture zone (~30° to 32°S) spanning orbital to decadal time scales. To decipher permanent deformation features manifested in the upper plate of the subduction system we identified and correlated the elevations of Late Pleistocene marine terraces using TanDEM-X digital topography and previously published terrace ages. We focused on terraces related to the Marine Isotope Stages (MIS) 5 and 9 (~124 ka and ~320 ka) due to their excellent preservation and lateral continuity. We furthermore compared deformation rates based on these uplifted terraces and compared them with published co-seismic slip and interseismic locking models of the Illapel earthquake. Uplift rates derived from the MIS-5 marine terraces range between 0.08 and 0.35 m/ka, while uplift rates based on MIS-9 terraces range between 0.38 to 0.96 m/ka. The higher uplift rates are found at the northern part of the Illapel rupture and these areas correlate to crustal structures (e.g. Puerto Aldea Fault). We observed a direct correlation between MIS-5 and MIS-9 uplift rates and co-seismic slip in the northern parts of the rupture while there was no clear correlation in the south at the central and southern parts of the rupture zone. The comparison between the spatial distribution of locked areas and uplift rates provided only a weak correlation for the MIS-9 terraces at the southern part of the rupture. Our results suggest that the northern part of the IIIapel rupture zone may accumulate permanent deformation during megathrust earthquakes. In contrast, accumulation of deformation at the southern part of the rupture may be controlled by activity in the neighboring seismotectonic segment. Broad warping patterns of marine terraces might reflect changes in boundary conditions at interplate depths, such as subduction of seamounts or other oceanic bathymetric features. This analysis highlights the temporal and spatial variability of deformation at convergent plate margins over multiple time scales.</p>