Apparent stress, fault maturity and seismic hazard for normal-fault earthquakes at subduction zones

Abstract

S U M M A R YThe behavior of apparent stress for normal-fault earthquakes at subduction zones is derived by examining the apparent stress (τ a = µE S /M 0 , where E S is radiated energy and M 0 is seismic moment) of all globally distributed shallow (depth, h < 70 km) earthquakes with normal-fault mechanisms that occurred in or near subduction zones between 1987 and 2001 for which E S and M 0 are available. Accurately determined hypocentres from the Engdahl-HilstBuland (EHB) catalogue establish the fine detail of the Wadati-Benioff zone. In many cases, we can relate trends in apparent stress to specific features within the subduction zone and compare these with trends for interplate-thrust earthquakes in the same subduction zones. There are two depth ranges over which τ a for normal-fault earthquakes shows maxima. The highest and most anomalous values of τ a are found in the deeper depth range from 35 to 70 km. The high-τ a events (up to 5 MPa) are characteristically intraslab and in proximity to zones of intense deformation such as a sharp slab bend or where opposing slabs collide. High-τ a events in the region of the shallower maximum (hypocentres between 10-35 km and τ a > 1 MPa) are also generally intraslab, but occur where the lithosphere has just begun subduction beneath the overriding plate. They usually occur in cold slabs near trenches where the direction of plate motion across the trench is oblique to the trench axis, or where there are local contortions or geometrical complexities of the plate boundary. Lower τ a (<1 MPa) is associated with events occurring at the outer rise (OR) complex (between the OR and the trench axis), as well as with intracrustal events occurring just landward of the trench. The average apparent stress of intraslab-normal-fault earthquakes is considerably higher than the average apparent stress of interplate-thrust-fault earthquakes. In turn, the average τ a of strike-slip earthquakes in intraoceanic environments is considerably higher than that of intraslab-normal-fault earthquakes. The variation of average τ a with focal mechanism and tectonic regime suggests that the level of τ a is related to fault maturity. Lower stress drops are needed to rupture mature faults such as those found at plate interfaces that have been smoothed by large cumulative displacements (from hundreds to thousands of kilometres). In contrast, immature faults, such as those on which intraslab-normal-fault earthquakes generally occur, are found in cold and intact lithosphere in which total fault displacement has been much less (from hundreds of metres to a few kilometres). Also, faults on which high τ a oceanic strike-slip earthquakes occur are predominantly intraplate or at evolving ends of transforms. At subduction zones, earthquakes occurring on immature faults are likely to be more hazardous as they tend to generate higher amounts of radiated energy per unit of moment than earthquakes occurring on mature faults. We have identified earthquake pairs in which an interplate-thrust and an intraslab-normal e...

Global evaluation of large earthquake energy from 1997 through mid-2010

“…At this locale the failed Cocos ridge subducts [e.g., Protti et al, 1994], potentially increasing local high-frequency attenuation. Alternatively, because the rate of convergence gradually increases to the southeast from 70 to approximately 100 mm yr −1 [DeMets, 2001], there may be a modest increase in fault maturity due to increased cumulative slip, thus allowing for reduced t a [Choy and Kirby, 2004;Schorlemmer et al, 2005;Choy and Boatwright, 2009].…”

“…The deficiency in radiated seismic energy in thrust events is potentially due to the preferential occurrence of these events on mature subduction megathrusts [Choy and Kirby, 2004], while increased relative radiated energy in strike-slip events likely occur because of their preferential occurrence on immature continental and oceanic transforms.…”

“…Because events with the same M 0 can have considerable differences in their radiated energy, damage evaluations based solely on M 0 may inaccurately characterize the true damage caused by an earthquake. This is particularly the case for both high stress earthquakes that have disproportionately energetic shaking given their M 0 [Choy and Kirby, 2004], and tsunami earthquakes (TsE hereafter), which can be energetically deficient by more than a factor of ten relative to global averages of E/M 0 , due to their slow rupture [Newman and Okal, 1998]. To better characterize the global distribution of earthquake energy release, we develop a catalog of recent earthquake energies and their energy-to-moment ratio using available broadband seismic recordings at teleseismic distances for events of M 0 ≥ 10 19 Nm (M W ≥ 6.7), and include earthquakes up to 20 times larger in moment release than in previous systematic studies.…”

Seismic scaling relations and aftershock activity from the sequences of the 2004 mid Niigata and the 2005 west off Fukuoka earthquakes (MW 6.6) in Japan

“…The different source duration may be associated with the different fault maturity between the MNPE and WOFE. The difference in E R and V of the mainshocks can be related to the fault maturity model [Choy and Kirby, 2004;Choy et al, 2006]. An earthquake rupturing an immature fault radiates greater energy per unit of moment than an earthquake rupturing a mature fault.…”

Earthquake Swarm Recorded by an Ocean Bottom Seismic Array in Southwest Offshore of Taiwan in October, 2005

“…Prior to that, seismic activity is mainly bounded at the shallow layers within 10 km, but spreads in a wider range from 5 to 30 km afterwards in vertical direction. Previous studies on the subduction seismicity also reported the existence of the vertical delineation of aftershocks (a) (b) that is located at the top of the Wadati-Benioff zone or passes through the hypocenter of the main event (e.g., Hansen and Ratchkovsi 2001;Choy and Kirby 2004). They inferred that intraslab earthquakes could have ruptured into the upper mantle of the subducting lithosphere or, alternatively, that it could stress the upper mantle sufficiently to induce secondary activities.…”