Slip Patches Research Articles

Source Complexity of an Injection Induced Event: The 2016Mw5.1 Fairview, Oklahoma Earthquake

AbstractComplex rupture processes are occasionally resolved for weak earthquakes and can reveal a dominant direction of the rupture propagation and the presence and geometry of main slip patches. Finding and characterizing such properties could be important for understanding the nucleation and growth of induced earthquakes. One of the largest earthquakes linked to wastewater injection, the 2016 Mw5.1 Fairview, Oklahoma earthquake, is analyzed using empirical Green's function techniques to reveal its source complexity. Two subevents are clearly identified and located using a new approach based on relative hypocenter‐centroid location. The first subevent has a magnitude of Mw5.0 and shows the main rupture propagated toward the NE, in the direction of higher pore pressure perturbations due to wastewater injection. The second subevent appears as an early aftershock with lower magnitude, Mw4.7. It is located SW of the mainshock in a region of increased Coulomb stress, where most aftershocks relocated.

Complex Fault Geometry and Rupture Dynamics of the MW 6.5, 30 October 2016, Central Italy Earthquake

AbstractWe study the 30 October 2016 Norcia earthquake (MW 6.5) to retrieve the rupture history by jointly inverting seismograms and coseismic Global Positioning System displacements obtained by dense local networks. The adopted fault geometry consists of a main normal fault striking N155° and dipping 47° belonging to the Mt. Vettore‐Mt. Bove fault system (VBFS) and a secondary fault plane striking N210° and dipping 36° to the NW. The coseismic rupture initiated on the VBFS and propagated with similar rupture velocity on both fault planes. Updip from the nucleation point, two main slip patches have been imaged on these fault segments, both characterized by similar peak‐slip values (~3 m) and rupture times (~3 s). After the breakage of the two main slip patches, coseismic rupture further propagated southeastward along the VBFS, rupturing again the same fault portion that slipped during the 24 August earthquake. The retrieved coseismic slip distribution is consistent with the observed surface breakages and the deformation pattern inferred from interferometric synthetic aperture radar measurements. Our results show that three different fault systems were activated during the 30 October earthquake. The composite rupture model inferred in this study provides evidences that also a deep portion of the NNE trending section of the Olevano‐Antrodoco‐Sibillini thrust broke coseismically, implying the kinematic inversion of a thrust ramp. The obtained rupture history indicates that in this sector of the Apennines, compressional structures inherited from past tectonics can alternatively segment boundaries of NW trending active normal faults or break coseismically during moderate‐to‐large magnitude earthquakes.

Fault slip and identification of the second fault plane in the Varzeghan earthquake doublet

An intraplate earthquake doublet, with 11-min delay between the events, devastated the city of Varzeghan in northwestern Iran on August 11, 2012. The first Mw 6.5 strike-slip earthquake, which occurred after more than 200 years of low seismicity, was followed by an Mw 6.4 oblique thrust event at an epicentral separation of about 6 km. While the first event can be associated with a distinct surface rupture, the absence of a surface fault trace and no clear aftershock signature makes it challenging to identify the fault plane of the second event. We use teleseismic body wave inversion to deduce the slip distribution in the first event. Using both P and SH waves stabilize the inversion and we further constrain the result with the surface rupture extent and the aftershock distribution. The obtained slip pattern shows two distinct slip patches with dissimilar slip directions where aftershocks avoid high-slip areas. Using the estimated slip for the first event, we calculate the induced Coulomb stress change on the nodal planes of the second event and find a preference for higher Coulomb stress on the N-S nodal plane. Assuming a simple slip model for the second event, we estimate the combined Coulomb stress changes from the two events on the focal planes of the largest aftershocks. We find that 90% of the aftershocks show increased Coulomb stress on one of their nodal planes when the N-S plane of the second event is assumed to be the correct fault plane.

The 12 June 2017 Mw 6.3 Lesvos Island (Aegean Sea) earthquake: Slip model and directivity estimated with finite-fault inversion

On 12 June 2017 (UTC 12:28:38.26) a magnitude Mw 6.3 earthquake occurred offshore Lesvos Island in SE Aegean Sea, which was widely felt, caused 1 fatality, and partially ruined the village of Vrisa on the south-eastern coast of the island. I invert broad band and strong motion waveforms from regional stations to obtain the source model and the distribution of slip onto the fault plane. The hypocentre is located at a depth of 7km in the upper crust. The mainshock ruptured a WNW-ESE striking, SW dipping, normal fault, projecting offshore and bounding the Lesvos Basin. The strongest and most aftershocks clustered away from the hypocentre, at the eastern edge of the activated area. This cluster indicates the activation of a different fault segment, exhibiting sinistral strike-slip motions, along a plane striking WNW-ESE. The slip of the mainshock is confined in a single large asperity, WNW from the hypocentre, with dimensions 20km×10km along fault strike and dip, respectively. The average slip of the asperity is ~50cm and the peak slip is ~1m. The rupture propagated unilaterally towards WNW to the coastline of Lesvos island at a relatively high speed (~3.1km/s). The imaged slip model and forward modelling was used to calculate peak ground velocities (PGVs) in the near-field. The damage pattern produced by this earthquake, especially in the village of Vrisa is compatible with the combined effect of rupture directivity, proximity to the slip patch and the fault edge, spectral content of motions, and local site conditions.

Slip distribution and source parameters of the 20 July 2017 Bodrum-Kos earthquake (Mw6.6) from GPS observations

Greek-Turkish boundary near the cities Kos and Bodrum has been shaken on July 20, 2017 by a Mw6.6 earthquake. The mainshock is located offshore and did not generate an on-land surface rupture. Analyzing pre- and post-earthquake continuous/survey-type static GPS observations, we investigated co-seismic surface displacements at 20 sites to characterize source parameters and slip-distribution of the mainshock. Fault plane solutions as well as co-seismic slip distribution have been acquired through the inversion of co-seismic GPS displacements modeling the event as elastic dislocations in a half space. Fault plane solution shows a southward dipping normal-type fault segment extending a depth down to ~12 km, which remains within the brittle upper crust. Results from the distributed slip inversion show that the mainshock activated a ~65 km fault section, which has three high slip patches, namely western, central and eastern patches, where the coseismic slips reach up to 13, 26, and 5 cm, respectively. This slip pattern indicates that the pre-earthquake coupling, which is storing the slip deficit, occurred on these three patches.

Source Parameters of the 2016–2017 Central Italy Earthquake Sequence from the Sentinel-1, ALOS-2 and GPS Data

In this study, joint inversions of Synthetic Aperture Radar (SAR) and Global Position System (GPS) measurements are used to investigate the source parameters of four Mw > 5 events of the 2016–2017 Central Italy earthquake sequence. The results show that the four events are all associated with a normal fault striking northwest–southeast and dipping southwest. The observations, in all cases, are consistent with slip on a rupture plane, with strike in the range of 157° to 164° and dip in the range of 39° to 44° that penetrates the uppermost crust to a depth of 0 to 8 km. The primary characteristics of these four events are that the 24 August 2016 Mw 6.2 Amatrice earthquake had pronounced heterogeneity of the slip distribution marked by two main slip patches, the 26 October 2016 Mw 6.1 Visso earthquake had a concentrated slip at 3–6 km, and the predominant slip of the 30 October 2016 Mw 6.6 Norcia earthquake occurred on the fault with a peak magnitude of 2.5 m at a depth of 0–6 km, suggesting that the rupture may have reached the surface, and the 18 January 2017 Mw 5.7 Campotosto earthquake had a large area of sliding at depth 3–9 km. The positive static stress changes on the fault planes of the latter three events demonstrate that the 24 August 2016 Amatrice earthquake may have triggered a cascading failure of earthquakes along the complex normal fault system in Central Italy.

On the existence of minimisers for strain‐gradient single‐crystal plasticity

AbstractWe prove the existence of minimisers for a family of models related to the single‐slip‐to‐single‐plane relaxation of single‐crystal, strain‐gradient elastoplasticity with ‐hardening penalty. In these relaxed models, where only one slip‐plane normal can be activated at each material point, the main challenge is to show that the energy of geometrically necessary dislocations is lower‐semicontinuous along bounded‐energy sequences which satisfy the single‐plane condition, meaning precisely that this side condition should be preserved in the weak ‐limit. This is done with the aid of an ‘exclusion’ lemma of Conti & Ortiz, which essentially allows one to put a lower bound on the dislocation energy at interfaces of (single‐plane) slip patches, thus precluding fine phase‐mixing in the limit. Furthermore, using div‐curl techniques in the spirit of Mielke & Müller, we are able to show that the usual multiplicative decomposition of the deformation gradient into plastic and elastic parts interacts with weak convergence and the single‐plane constraint in such a way as to guarantee lower‐semicontinuity of the (polyconvex) elastic energy, and hence the total elasto‐plastic energy, given sufficient () hardening, thus delivering the desired result.

2017 Valparaíso earthquake sequence and the megathrust patchwork of central Chile

AbstractIn April 2017, a sequence of earthquakes offshore Valparaíso, Chile, raised concerns of a potential megathrust earthquake in the near future. The largest event in the 2017 sequence was a M6.9 on 24 April, seemingly colocated with the last great‐sized earthquake in the region—a M8.0 in March 1985. The history of large earthquakes in this region shows significant variation in rupture size and extent, typically highlighted by a juxtaposition of large ruptures interspersed with smaller magnitude sequences. We show that the 2017 sequence ruptured an area between the two main slip patches during the 1985 earthquake, rerupturing a patch that had previously slipped during the October 1973 M6.5 earthquake sequence. A significant gap in historic ruptures exists directly to the south of the 2017 sequence, with large enough moment deficit to host a great‐sized earthquake in the near future, if it is locked.

Near-source high-rate GPS, strong motion and InSAR observations to image the 2015 Lefkada (Greece) Earthquake rupture history

The 2015/11/17 Lefkada (Greece) earthquake ruptured a segment of the Cephalonia Transform Fault (CTF) where probably the penultimate major event was in 1948. Using near-source strong motion and high sampling rate GPS data and Sentinel-1A SAR images on two tracks, we performed the inversion for the geometry, slip distribution and rupture history of the causative fault with a three-step self-consistent procedure, in which every step provided input parameters for the next one. Our preferred model results in a ~70° ESE-dipping and ~13° N-striking fault plane, with a strike-slip mechanism (rake ~169°) in agreement with the CTF tectonic regime. This model shows a bilateral propagation spanning ~9 s with the activation of three main slip patches, characterized by rise time and peak slip velocity in the ranges 2.5–3.5 s and 1.4–2.4 m/s, respectively, corresponding to 1.2–1.8 m of slip which is mainly concentrated in the shallower (<10 km) southern half of the causative fault. The inferred slip distribution and the resulting seismic moment (M0 = 1.05 × 1019 N m) suggest a magnitude of Mw 6.6. Our best solution suggests that the occurrence of large (Mw > 6) earthquakes to the northern and to the southern boundaries of the 2015 causative fault cannot be excluded.

Joint inversion of GPS, InSAR and teleseismic data sets for the rupture process of the 2015 Gorkha, Nepal, earthquake using a generalized ABIC method

Various observations are made to invert for the source model of the 2015 Gorkha earthquake. Previously published slip models involve different slip distributions and slip amplitudes. To obtain a robust model, we jointly invert for the rupture process of the Gorkha earthquake using three types of observations: Global Positioning System (GPS), Interferometric Synthetic Aperture Radar (InSAR), and teleseismic P wave data. Three constraints (i.e., spatial and temporal smoothness and minimum moment constraints) are applied to regulate the rupture process. The weight ratios between data sets and prior constraints are determined from a new developed generalized Akaike’s Bayesian Information Criterion (gABIC) weight determination method. This gABIC method can simultaneously and objectively weigh various kinds of observations and prior constraints, rather than being limited to the two types in the traditional ABIC method. The inverted slip model reveals the seismic moment of the earthquake is 9.40×1020Nm released primarily in the first 60s, equivalent to Mw 7.92. The rupture front propagates unilaterally along the strike direction and spreads in both the strike and dip directions. The slip distribution shows one large slip patch with two peaks along the strike direction with a maximum slip amplitude of approximately 7.6m and identifies large slips (> 2m) that occur in the updip section of the Main Himalayan Thrust (MHT) fault plane. Our results indicate that the released slip amplitude is larger than the accumulated slip amplitude since 1833, so the 2015 earthquake likely does not overlap with the 1833 earthquake.

Large and primarily updip afterslip following the 2012 Mw 7.6 Nicoya, Costa Rica, earthquake

AbstractWe present detailed surface measurements of the first 3.5 years of postseismic deformation following the 5 September 2012 moment magnitude (Mw) 7.6 Nicoya, Costa Rica, earthquake. The dominant signal in the first 2.5 years is uniform horizontal trenchward motion totaling 7–26 cm across 40 stations. Trenchward velocity is strongly diminished by mid‐2014 and appears by 2016 to have begun reversing. We invert the first 2.5 years to determine the corresponding afterslip on a detailed 3‐D interface. Results show significant afterslip both updip and downdip of the main coseismic rupture zone, with as much as 1.7 m of offset in two patches at 15–20 km depth and immediately updip of the maximum coseismic slip. This updip slip represents an important mechanism to address unrelieved interseismic locking, although sufficient strain energy remains to generate up to a Mw 7.1 event near the coastline. The afterslip patches are anticorrelated with strongly clustered aftershocks at the same depth, which is indicative of varying frictional behavior along strike. An additional patch of slip is colocated with reoccurring slow‐slip events beneath the Gulf of Nicoya. The magnitude of the observed slip, however, cannot be sufficiently explained by the known slow‐slip events. Ongoing measurements will be crucial to understanding the relocking process in Nicoya.

Submarine landslides triggered by destabilization of high‐saturation hydrate anomalies

AbstractSubmarine landslides occur along continental margins at depths that often intersect the gas hydrate stability zone, prompting suggestions that slope stability may be affected by perturbations that arise from changes in hydrate stability. Here we develop a numerical model to identify the conditions under which the destabilization of hydrates results in slope failure. Specifically, we focus on high‐saturation hydrate anomalies at fine‐grained to coarse‐grained stratigraphic boundaries that can transmit bridging stresses that decrease the effective stress at sediment contacts and disrupt normal sediment consolidation. We evaluate slope stability before and after hydrate destabilization. Hydrate anomalies act to significantly increase the overall slope stability due to large increases in effective cohesion. However, when hydrate anomalies destabilize there is a loss of cohesion and increase in effective stress that causes the sediment grains to rapidly consolidate and generate pore pressures that can either trigger immediate slope failure or weaken the surrounding sediment until the pore pressure diffuses away. In cases where failure does not occur, the sediment can remain weakened for months. In cases where failure does occur, we quantify landslide dynamics using a rate and state frictional model and find that landslides can display either slow or dynamic (i.e., catastrophic) motion depending on the rate‐dependent properties, size of the stress perturbation, and the size of the slip patch relative to a critical nucleation length scale. Our results illustrate the fundamental mechanisms through which the destabilization of gas hydrates can pose a significant geohazard.

A Comparative Study of Source Complexity of Two Moderate-Sized Earthquakes in Southern Taiwan: 2016 Mw6.4 and 2010 Mw6.2 Earthquakes

We study the complex rupture history of the 2016 Mw6.4 Meinong earthquake and the 2010 Mw6.2 Jiaxian earthquake in southern Taiwan by simultaneously inverting near-field strong-motion records, local and teleseismic broadband waveforms along with long-period surface waves. The focal mechanism results reveal that both earthquakes may rupture two different blind faults. The rupture of the 2016 event is dominated by the strike-slip motion with minor thrust faulting, while the 2010 event is a relatively high-angle thrust earthquake. Our preferred finite-fault source models suggest that the coseismic rupture history of the 2016 event should be relatively more complex than the 2010 event. During the 2016 event, two main patches of slip characterize the coseismic slip history. The cumulative seismic moment within 12 s of rupture is about 5.36 × 1018 N m, which is 1.9 times of the moment of the 2010 event. The slip of the 2016 event is associated with relatively larger perk slip of 0.7 m at shallower depth (~18–19 km), higher average slip of 0.3 m and faster slip rate with the maximum value of 3.4 m/s. Our kinematic models can shed some light on the cause of the difference in seismic hazard between these two earthquakes, and will be further applied to analyze the deep blind fault structure in southern Taiwan.

Fault geometry of 2015, Mw7.2 Murghab, Tajikistan earthquake controls rupture propagation: Insights from InSAR and seismological data

Combining space-based geodetic and array seismology observations can provide detailed information about earthquake ruptures in remote regions. Here we use Landsat-8 imagery and ALOS-2 and Sentinel-1 radar interferometry data combined with data from the European seismology network to describe the source of the December 7, 2015, Mw7.2 Murghab (Tajikistan) earthquake. The earthquake reactivated a ∼79 km-long section of the Sarez–Karakul Fault, a NE oriented sinistral, trans-tensional fault in northern Pamir. Pixel offset data delineate the geometry of the surface break and line of sight ground shifts from two descending and three ascending interferograms constrain the fault dip and slip solution. Two right-stepping, NE-striking segments connected by a more easterly oriented segment, sub-vertical or steeply dipping to the west were involved. The solution shows two main patches of slip with up to 3.5 m of left lateral slip on the southern and central fault segments. The northern segment has a left-lateral and normal oblique slip of up to a meter. Back-projection of high-frequency seismic waves recorded by the European network, processed using the Multitaper-MUSIC approach, focuses sharply along the surface break. The time progression of the high-frequency radiators shows that, after a 10 second initiation phase at slow speed, the rupture progresses in 2 phases at super-shear velocity (∼4.3–5 km/s) separated by a 3 second interval of slower propagation corresponding to the passage through the restraining bend. The intensity of the high-frequency radiation reaches maxima during the initial and middle phases of slow propagation and is reduced by ∼50% during the super-shear phases of the propagation. These findings are consistent with studies of other strike-slip earthquakes in continental domain, showing the importance of fault geometric complexities in controlling the speed of fault propagation and related spatiotemporal pattern of the high-frequency radiation.

Detailed spatiotemporal evolution of microseismicity and repeating earthquakes following the 2012 Mw 7.6 Nicoya earthquake

AbstractWe apply a waveform matching technique to obtain a detailed earthquake catalog around the rupture zone of the 5 September 2012 moment magnitude 7.6 Nicoya earthquake, with emphasis on its aftershock sequence. Starting from a preliminary catalog, we relocate ~7900 events using TomoDD to better quantify their spatiotemporal behavior. Relocated aftershocks are mostly clustered in two groups. The first is immediately above the major coseismic slip patch, partially overlapping with shallow afterslip. The second one is 50 km SE to the main shock nucleation point and near the terminus of coseismic rupture, in a zone that exhibited little resolvable afterslip. Using the relocated events as templates, we scan through the continuous recording from 29 June 2012 to 30 December 2012, detecting approximately 17 times more than template events. We find 190 aftershocks in the first half hour following the main shock, mostly along the plate interface. Later events become more scattered in location, showing moderate expansion in both along‐trench and downdip directions. From the detected catalog we identify 53 repeating aftershock clusters with mean cross‐correlation values larger than 0.9, and indistinguishably intracluster event locations, suggesting slip on the same fault patch. Most repeating clusters occurred within the first major aftershock group. Very few repeating clusters were found in the aftershock grouping along the southern edge of the Peninsula, which is not associated with substantial afterslip. Our observations suggest that loading from nearby afterslip along the plate interface drives spatiotemporal evolution of aftershocks just above the main shock rupture patch, while aftershocks in the SE group are to the SE of the observed updip afterslip and poorly constrained.

A Review of Source Models of the 2015 Illapel, Chile Earthquake and Insights from Tsunami Data

The 16 September 2015 Illapel, Chile, earthquake and associated tsunami have been studied by many researchers from various aspects. This paper reviews studies on the source model of the earthquake and examines tsunami data. The Illapel earthquake occurred in the source region of previous earthquakes in 1943 and 1880. The earthquake source was studied using various geophysical data, such as near-field seismograms, teleseismic waveform and backprojection, GPS and InSAR data, and tsunami waveforms. Most seismological analyses show a duration of ~100 s with a peak at ~50 s. The spatial distribution has some variety, but they all have the largest slip varying from 5 to 16 m located at 31°S, 72°W, which is ~70 km NW of the epicenter. The shallow slip seems to be extended to the trench axis. A deeper slip patch was proposed from high-frequency seismic data. A tsunami earthquake model with a total duration of 250 s and a third asperity south of the epicenter is also proposed, but we show that the tsunami data do not support this model.

Application and evaluation of a rapid response earthquake-triggered landslide model to the 25 April 2015 Mw 7.8 Gorkha earthquake, Nepal

The 25 April 2015 Mw 7.8 Gorkha earthquake produced strong ground motions across an approximately 250km by 100km swath in central Nepal. To assist disaster response activities, we modified an existing earthquake-triggered landslide model based on a Newmark sliding block analysis to estimate the extent and intensity of landsliding and landslide dam hazard. Landslide hazard maps were produced using Shuttle Radar Topography Mission (SRTM) digital topography, peak ground acceleration (PGA) information from the U.S. Geological Survey (USGS) ShakeMap program, and assumptions about the regional rock strength based on end-member values from previous studies. The instrumental record of seismicity in Nepal is poor, so PGA estimates were based on empirical Ground Motion Prediction Equations (GMPEs) constrained by teleseismic data and felt reports. We demonstrate a non-linear dependence of modeled landsliding on aggregate rock strength, where the number of landslides decreases exponentially with increasing rock strength. Model estimates are less sensitive to PGA at steep slopes (>60°) compared to moderate slopes (30–60°). We compare forward model results to an inventory of landslides triggered by the Gorkha earthquake. We show that moderate rock strength inputs over estimate landsliding in regions beyond the main slip patch, which may in part be related to poorly constrained PGA estimates for this event at far distances from the source area. Directly above the main slip patch, however, the moderate strength model accurately estimates the total number of landslides within the resolution of the model (landslides≥0.0162km2; observed n=2214, modeled n=2987), but the pattern of landsliding differs from observations. This discrepancy is likely due to the unaccounted for effects of variable material strength and local topographic amplification of strong ground motion, as well as other simplifying assumptions about source characteristics and their relationship to landsliding.

Early aftershocks and afterslip surrounding the 2015 Mw 8.4 Illapel rupture

On 16 September 2015, the Mw 8.4 Illapel earthquake ruptured a section of the subduction thrust on the west coast of central Chile. The mainshock was followed by numerous aftershocks including some normal-faulting events near the trench. We apply a template matching approach to improve the completeness of early aftershocks within one month of the mainshock. To constrain the distribution of afterslip, we utilize repeating earthquakes among the aftershocks and perform a joint slip inversion of postseismic GPS and InSAR data. The results show that the aftershock zone abruptly expands to the south ∼14 h after the mainshock while growing relatively continuously to the north within the first day. The repeating earthquakes accompanying the early expansion suggest that aseismic afterslip on the subduction thrust surrounding the coseismic rupture is an important triggering mechanism of aftershocks in addition to stress transfer or poroelastic effects. An energetic earthquake sequence near the trench initiated with a M 4.6 event ∼3.5 h after the mainshock, suggesting delayed triggering by the static or dynamic stress changes induced by the mainshock. The spatial distribution of repeating earthquakes and the geodetic-inverted afterslip are consistent and appear to wrap around the large coseismic slip patch. Both data sets suggest that the largest cumulative afterslip is located at ∼30.5°S to the north of the mainshock rupture zone. The estimated postseismic moment released in the first ∼24 days of afterslip is equivalent to an earthquake of Mw 7.5. The afterslip illuminates the velocity strengthening sections of the plate interface that surround the mainshock rupture, consistent with plate coupling models inferred from interseismic GPS velocities.

Slip heterogeneity and directivity of the ML 6.0, 2016, Amatrice earthquake estimated with rapid finite‐fault inversion

AbstractOn 24 August 2016 a magnitude ML 6.0 occurred in the Central Apennines (Italy) between Amatrice and Norcia causing nearly 300 fatalities. The main shock ruptured a NNW‐SSE striking, WSW dipping normal fault. We invert waveforms from 26 three‐component strong motion accelerometers, filtered between 0.02 and 0.5 Hz, within 45 km from the fault. The inferred slip distribution is heterogeneous and characterized by two shallow slip patches updip and NW from the hypocenter, respectively. The rupture history shows bilateral propagation and a relatively high rupture velocity (3.1 km/s). The imaged rupture history produced evident directivity effects both N‐NW and SE of the hypocenter, explaining near‐source peak ground motions. Fault dimensions and peak slip values are large for a moderate‐magnitude earthquake. The retrieved rupture model fits the recorded ground velocities up to 1 Hz, corroborating the effects of rupture directivity and slip heterogeneity on ground shaking and damage pattern.

The 16 April 2016, MW 7.8 (MS 7.5) Ecuador earthquake: A quasi-repeat of the 1942 MS 7.5 earthquake and partial re-rupture of the 1906 MS 8.6 Colombia–Ecuador earthquake

The 2016 Ecuador MW7.8 earthquake ruptured the subduction zone boundary between the Nazca and South American plates. Joint modeling of seismic and tsunami observations indicates an ∼120 km long rupture area beneath the coastline north of the 1998 MW7.2 rupture. The slip distribution reveals two discrete asperities near the hypocenter and around the equator. Their locations and the patchy pattern are consistent with the prior interseismic geodetic strain, which showed highly locked patches also beneath the coastline. Aftershocks cluster along two streaks, one aligned nearly parallel to the plate convergence direction up-dip of the main slip patches, and the other on a trench-perpendicular lineation south of the 1958 rupture zone. Comparisons of seismic waveforms and magnitudes show that the 2016 event and 1942 earthquakes have similar surface wave magnitude (MS7.5), overlapping rupture areas, and similar main pulses of moment rate. The same area ruptured as the southernmost portion of the larger earthquake of 1906 (MW8.6, MS8.6). The seismic behavior reflects persistent heterogeneous frictional properties of the Colombia–Ecuador megathrust.