Large intermediate-depth earthquakes and the subduction process

Abstract

This study provides an overview of intermediate-depth earthquake phenomena, placing emphasis on the larger, tectonically significant events, and exploring the relation of intermediate-depth earthquakes to shallower seismicity. Especially, we examine whether intermediate-depth events reflect the state of interplate coupling at subduction zones. and whether this activity exhibits temporal changes associated with the occurrence of large underthrusting earthquakes. Historic record of large intraplate earthquakes (m B 7.0) in this century shows that the New Hebrides and Tonga subduction zones have the largest number of large intraplate events. Regions associated with bends in the subducted lithosphere also have many large events (e.g. Altiplano and New Ireland). We compiled a catalog of focal mechanisms for events that occurred between 1960 and 1984 with M> 6 and depth between 40 and 200 km. The final catalog includes 335 events with 47 new focal mechanisms, and is probably complete for earthquakes with mB 6.5. For events with M 6.5, nearly 48% of the events had no aftershocks and only 15% of the events had more than five aftershocks within one week of the mainshock. Events with more than ten aftershocks are located in regions associated with bends in the subducted slab. Focal mechanism solutions for intermediate-depth earthquakes with M> 6.8 can be grouped into four categories: (1) Normal-fault events (44%), and (2) reverse-fault events (33%), both with a strike nearly parallel to the trench axis. (3) Normal or reverse-fault events with a strike significantly oblique to the trench axis (10%), and (4) tear-faulting events (13%). The focal mechanisms of type I events occur mainly along strongly or moderately coupled subduction zones where a down-dip extensional stress prevails in a gently dipping plate. In contrast, along decoupled subduction zones great normal-fault earthquakes occur at shallow depths (e.g., the 1977 Sumbawa earthquake in Indonesia). Type 2 events, with strike subparallel to the subduction zone, and most of them with a near vertical tension axis, occur mainly in regions that have partially coupled or uncoupled subduction zones and the observed continuous seismicity is deeper than 300 km. The increased dip of the downgoing slab associated with weakly coupled subduction zones and the weight of the slab may be responsible for the near vertical tensional stress at intermediate depth and, consequently, the change in focal mechanism from type 1 to type 2 events. Events of type 3 occur where the trench axis bends sharply causing horizontal (parallel to the trench strike) extensional or compressional intraplate stress. Type 4 are hinge-faulting events. For strongly coupled zones we observed temporal changes of intermediate-depth earthquake activity associated with the occurrence of a large underthrusting event. After the occurrence of a large underthrusting event, the stress axis orientation of intermediate-depth earthquakes changes from down-dip tensional to down-dip compressional (e.g., 1960 Chile, 1974 Per...

Preliminary re-evaluation of probabilistic seismic hazard assessment in Chile: from Arica to Taitao Peninsula

“…Within this context, several researchers have put in evidence two main kind of seismogenic sources: interplate, thrust events, with epicentres located along the coast, depths ranging between Correspondence to: F. Leyton (fleyton@utalca.cl) 15 and 50 km (for a review, see Barrientos, 2007), and a second group of earthquakes located inside the subducting Nazca plate with continental epicentres of intermediate depth (greater than 50 km), known as intraplate or inslab events (Kausel and Campos, 1992;Barrientos, 2007). Similar classification has also been found in Mexico's subduction zone (Singh et al, 2000;García et al, 2005) and other subduction zones world-wide (for a review, see Astiz et al, 1988). Each one of these seismogenic sources have particular properties, producing a different effect on structures; hence, each one requires a special analysis (Saragoni et al, 2004;Astroza et al, 2002Astroza et al, , 2005Leyton et al, 2009b).…”

Rupture Process During the Mw 8.1 2017 Chiapas Mexico Earthquake: Shallow Intraplate Normal Faulting by Slab Bending

“…As shown in Figures 4e and 4f, the upper and lower edges of the rupture area moved deeper as the rupture front propagated northwest, and the down-dip width of the main rupture was limited to 30 km. If the slab was bending at the source region, the down-dip stress should be extensional in the upper part of the slab and compressional in the lower part (e.g., Astiz et al, 1988). This change in stress may account for the main rupture area being dominantly in the upper part of the plate, with its down-dip width and down-dip edge defined by the transition of the stress regime from extension to compression.…”

“…The SSN assigned it a moment magnitude of M W 8.2, making it one of the largest instrumentally recorded earthquakes in Mexico (Ramírez-Herrera et al, 2011;Singh et al, 1981;SSN UNAM Special Report, 2017). A cross section of the source area is shown in Figure 2a, summarizing the focal depths of the centroid moment tensor (CMT) solutions fetched from database of the Japan Meteorological Agency (JMA; http://www.data.jma.go.jp/svd/eqev/data/mech/world_cmt/fig/cmt20170908044921.html), the Intraplate normal faulting in a subducting plate can occur in a tensile stress field due to plate bending at shallow depths and near the trench, to slab pull from the negative buoyancy of the subducting plate, and to unbending of the subducting plate beneath a strongly coupled interface at intermediate depths beneath the continental plate (Astiz et al, 1988;Fujita & Kanamori, 1981;Isacks et al, 1968;Spence, 1986). The epicenter determined by the SSN is 55 km landwards of the trench and 115 km southwest of Pijijiapan on the coast ( Figures.…”

“…The epicenter determined by the SSN is 55 km landwards of the trench and 115 km southwest of Pijijiapan on the coast ( Figures. 1 and 2a), a position between the typical locations of large intraplate normal-faulting earthquakes, which are either near the trench or under the continent (e.g., Astiz et al, 1988). According to the thermal structural modeling of the region (Manea & Manea, 2008), at the SSN epicenter the 300°-700° isotherms are at a depth range of 23-38 km, and the mean depth of the CMT solutions (53.1 km; Figure 2a) would be beyond the 1000°C isotherm.…”

A strong-motion database from the Peru–Chile subduction zone

“…Tables 2 and 3 also summarise all geological and geotechnical information collected for the sites contributing data to this study. Site conditions assigned to the stations in Central Chile were based on information collected from a number of references including descriptions of the surface geology (EERI, 1986;Çelebi, 1987, 1988Campbell et al, 1989Campbell et al, , 1990Midorikawa et al, 1991;Midorikawa, 1992), the site categories following the Chilean seismic design code assigned by Riddell (1995) and NEHRP site classes assigned by Atkinson & Boore (2003) to the Chilean sites whose data were included in the regression database for subduction-zone events. Shear-wave velocity (V S ) profiles obtained by Araneda & Saragoni (1994), Midorikawa et al (1991) and Midorikawa (1992) 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 Site conditions assigned to the stations in Central and Southern Peru were based on descriptions of the surface geology (EERI, 2007;Bernal & Tavera, 2007a, 2007b Besides the site information collected, the spectral shapes of the records were considered by normalising the response spectra by their PGA value (for all records) and by dividing the spectra recorded at soil stations by the spectrum obtained on roc...…”