Intense interface seismicity triggered by a shallow slow slip event in the Central Ecuador subduction zone

Abstract

International audienceWe document a 1 week long slow slip event (SSE) with an equivalent moment magnitude of 6.0-6.3 which occurred in August 2010 below La Plata Island (Ecuador), south of the rupture area of the 1906 Mw= 8.8 megathrust earthquake. GPS data reveal that the SSE occurred at a depth of about 10 km, within the downdip part of a shallow (<15 km), isolated, locked patch along the subduction interface. The availability of both broadband seismometer and continuous geodetic station located at the La Plata Island, 10 km above the SSE, enables a careful analysis of the relationships between slow and rapid processes of stress release along the subduction interface. During the slow slip sequence, the seismic data show a sharp increase of the local seismicity, with more than 650 earthquakes detected, among which 50 have a moment magnitude between 1.8 and 4.1. However, the cumulative moment released through earthquakes accounts, at most, for 0.2% of the total moment release estimated from GPS displacements. Most of the largest earthquakes are located along or very close to the subduction interface with focal mechanism consistent with the relative plate motion. While the earthquake sizes show a classical distribution (Gutenberg-Richter law with a b-value close to 1), the space-time occurrence presents a specific pattern. First, the largest earthquakes appear to occur randomly during the slow slip sequence, which further evidence that the seismicity is driven by the stress fluctuations related to aseismic slip. Moreover, the seismicity observed during the SSE consists in individual events and families of repeating earthquakes. These observations indicate that the stress increment induced by the episodic aseismic slip may lead both to sudden seismic moment release and to progressive rupture within small locked patches. This study offers an a posteriori interpretation of the seismogenesis in the Central Ecuador subduction zone, where intense seismic swarms have been regularly observed (1977, 1998, 2002, and 2005). These swarms have likely been triggered by large-magnitude slow slip events

Earthquake and tsunami forecasts: Relation of slow slip events to subsequent earthquake rupture

“…However, all recorded events had at least some shallow slip, and four had shallow slip in excess of 30 mm. Shallow SSEs have been reported in only a few subduction zones: New Zealand (20,21), Japan (22,23), and Ecuador (24). We suspect the paucity of such events is at least in part a sampling artifact, as many on-shore geodetic networks lack sensitivity to slip events far offshore (25,26).…”

The frictional, hydrologic, metamorphic and thermal habitat of shallow slow earthquakes

“…In contrast, SSEs at the Hikurangi 6 and Ecuador 17 margins occur within a region that is currently dominated by interseismic creep, but with probable locked, seismogenic patches embedded within the shallow creeping region. Some shallow slow earthquakes, such as those at Nankai and Costa Rica, are characterized by or are intimately associated with tremor or VLFEs [14][15][16]20 , whereas others are associated with bursts of microseismicity (such as at the Boso Peninsula, Hikurangi, Japan Trench and Ecuador) 6,8,12,13,17 .…”

“…The size, duration, association with seismicity and location of these shallow events vary between subduction zones. At northern Hikurangi, Costa Rica, the Boso Peninsula, Ecuador and the Ryukyu trench, moderate to large (equivalent to moment magnitude, M w = 6.0-7.0) slow slip events commonly last a few to several weeks with cumulative slip of a few to tens of centimetres [6][7][8]17,18 . The up-dip limit of slip in these events is not well constrained, and they may propagate all the way to the trench 6,19 .…”

Strain budget of the Ecuador-Colombia subduction zone: a stochastic view

“…Green's functions are computed for a semi-80 infinite stratified elastic medium derived from regional velocity models shown in Fig. S2 81 (Béthoux et al, 2011;Vallee et al, 2013;Nocquet et al, 2017). We account for prediction 82 uncertainties due to inaccuracies in this layered model using the approach of Duputel et al…”

“…The blue line represents the CRUTST2.0 model in the area (http://igppweb.ucsd.edu/~gabi/rem.html). The other models are from (Vallee et al, 2013;Bethoux et al, 2011;Nocquet et al, 2017). Grey histograms are the probability density function representing our confidence level on the elastic properties, as used to build the model prediction error.…”