Charles J. Ammon

Depth‐varying rupture properties of subduction zone megathrust faults

Subduction zone plate boundary megathrust faults accommodate relative plate motions with spatially varying sliding behavior. The 2004 Sumatra-Andaman (M_w 9.2), 2010 Chile (Mw 8.8), and 2011 Tohoku (M_w 9.0) great earthquakes had similar depth variations in seismic wave radiation across their wide rupture zones – coherent teleseismic short-period radiation preferentially emanated from the deeper portion of the megathrusts whereas the largest fault displacements occurred at shallower depths but produced relatively little coherent short-period radiation. We represent these and other depth-varying seismic characteristics with four distinct failure domains extending along the megathrust from the trench to the downdip edge of the seismogenic zone. We designate the portion of the megathrust less than 15 km below the ocean surface as domain A, the region of tsunami earthquakes. From 15 to ∼35 km deep, large earthquake displacements occur over large-scale regions with only modest coherent short-period radiation, in what we designate as domain B. Rupture of smaller isolated megathrust patches dominate in domain C, which extends from ∼35 to 55 km deep. These isolated patches produce bursts of coherent short-period energy both in great ruptures and in smaller, sometimes repeating, moderate-size events. For the 2011 Tohoku earthquake, the sites of coherent teleseismic short-period radiation are close to areas where local strong ground motions originated. Domain D, found at depths of 30–45 km in subduction zones where relatively young oceanic lithosphere is being underthrust with shallow plate dip, is represented by the occurrence of low-frequency earthquakes, seismic tremor, and slow slip events in a transition zone to stable sliding or ductile flow below the seismogenic zone.

Upper mantle velocity structure beneath the Tibetan Plateau from Pn travel time tomography

We inverted 1510 P arrival times from regional distances (333–1600 km), in and around the Tibetan Plateau to map the lateral velocity variation within the uppermost mantle. Previous studies have placed first-order constraints on upper mantle velocities but relied on data recorded almost exclusively at stations outside of the plateau. We improve resolution by using 40 events recorded at stations within the Tibetan Plateau. We combine these data with observations obtained from the International Seismological Centre (ISC) to extend our coverage by including Pn arrivals from 85 additional plateau events, relocated in previous studies, and recorded at stations in and around the Tibetan Plateau. We use synthetic travel time data to evaluate the resolution of our data set. The observations provide good resolution to about 1° over most of the plateau and surrounding regions. Our results show average Pn velocities that are about 3% lower in the northern plateau relative to the southern plateau. These variations correlate well with major tectonic features and previous geophysical observations. In the Qiangtang terrane of the northern plateau, an area known to be inefficient for Sn propagation, Pn is slow relative to both the plateau south of the Banggong-Nujiang suture and the tectonically stable Tarim basin north of the plateau. This is strong evidence for the existence of partial melt within the uppermost mantle beneath the northern Tibetan Plateau. However, when laboratory estimates of relationships between temperature, velocity, and attenuation are applied, a relatively small temperature variation (240° to 370°C) is required to explain our Pn velocity observations. When combined with geochemical constraints from volcanics in the northern plateau, our results strongly suggest that the mantle lid is intact beneath the northern plateau. This result would preclude tectonic models involving wholesale delamination of the mantle lithosphere in the northern Tibetan Plateau.

Teleseismic inversion for rupture process of the 27 February 2010 Chile (Mw 8.8) earthquake

The 27 February 2010 Chile (M_w 8.8) earthquake is the fifth largest earthquake to strike during the age of seismological instrumentation. The faulting geometry, slip distribution, seismic moment, and moment-rate function are estimated from broadband teleseismic P, SH, and Rayleigh wave signals. We explore some of the trade-offs in the rupture-process estimation due to model parameterizations, limited teleseismic sampling of seismic phase velocities, and uncertainty in fault geometry. The average slip over the ~81,500 km^2 rupture area is about 5 m, with slip concentrations down-dip, up-dip and southwest, and up-dip and north of the hypocenter. Relatively little slip occurred up-dip/offshore of the hypocenter. The average rupture velocity is ~2.0–2.5 km/s.

The 2009 Samoa–Tonga great earthquake triggered doublet

Great earthquakes (having seismic magnitudes of at least 8) usually involve abrupt sliding of rock masses at a boundary between tectonic plates. Such interplate ruptures produce dynamic and static stress changes that can activate nearby intraplate aftershocks, as is commonly observed in the trench-slope region seaward of a great subduction zone thrust event. The earthquake sequence addressed here involves a rare instance in which a great trench-slope intraplate earthquake triggered extensive interplate faulting, reversing the typical pattern and broadly expanding the seismic and tsunami hazard. On 29 September 2009, within two minutes of the initiation of a normal faulting event with moment magnitude 8.1 in the outer trench-slope at the northern end of the Tonga subduction zone, two major interplate underthrusting subevents (both with moment magnitude 7.8), with total moment equal to a second great earthquake of moment magnitude 8.0, ruptured the nearby subduction zone megathrust. The collective faulting produced tsunami waves with localized regions of about 12 metres run-up that claimed 192 lives in Samoa, American Samoa and Tonga. Overlap of the seismic signals obscured the fact that distinct faults separated by more than 50 km had ruptured with different geometries, with the triggered thrust faulting only being revealed by detailed seismic wave analyses. Extensive interplate and intraplate aftershock activity was activated over a large region of the northern Tonga subduction zone.

A great earthquake doublet and seismic stress transfer cycle in the central Kuril islands

Temporal variations of the frictional resistance on subduction-zone plate boundary faults associated with the stick–slip cycle of large interplate earthquakes are thought to modulate the stress regime and earthquake activity within the subducting oceanic plate. Here we report on two great earthquakes that occurred near the Kuril islands, which shed light on this process and demonstrate the enhanced seismic hazard accompanying triggered faulting. On 15 November 2006, an event of moment magnitude 8.3 ruptured the shallow-dipping plate boundary along which the Pacific plate descends beneath the central Kuril arc. The thrust ruptured a seismic gap that previously had uncertain seismogenic potential, although the earlier occurrence of outer-rise compressional events had suggested the presence of frictional resistance. Within minutes of this large underthrusting event, intraplate extensional earthquakes commenced in the outer rise region seaward of the Kuril trench, and on 13 January 2007, an event of moment magnitude 8.1 ruptured a normal fault extending through the upper portion of the Pacific plate, producing one of the largest recorded shallow extensional earthquakes. This energetic earthquake sequence demonstrates the stress transfer process within the subducting lithosphere, and the distinct rupture characteristics of these great earthquakes illuminate differences in seismogenic properties and seismic hazard of such interplate and intraplate faults.

The 2006-2007 Kuril Islands great earthquake sequence

The southwestern half of a ∼500 km long seismic gap in the central Kuril Island arc subduction zone experienced two great earthquakes with extensive preshock and aftershock sequences in late 2006 to early 2007. The nature of seismic coupling in the gap had been uncertain due to the limited historical record of prior large events and the presence of distinctive upper plate, trench and outer rise structures relative to adjacent regions along the arc that have experienced repeated great interplate earthquakes in the last few centuries. The intraplate region seaward of the seismic gap had several shallow compressional events during the preceding decades (notably an M_S 7.2 event on 16 March 1963), leading to speculation that the interplate fault was seismically coupled. This issue was partly resolved by failure of the shallow portion of the interplate megathrust in an M_W = 8.3 thrust event on 15 November 2006. This event ruptured ∼250 km along the seismic gap, just northeast of the great 1963 Kuril Island (M_w = 8.5) earthquake rupture zone. Within minutes of the thrust event, intense earthquake activity commenced beneath the outer wall of the trench seaward of the interplate rupture, with the larger events having normal-faulting mechanisms. An unusual double band of interplate and intraplate aftershocks developed. On 13 January 2007, an M_W = 8.1 extensional earthquake ruptured within the Pacific plate beneath the seaward edge of the Kuril trench. This event is the third largest normal-faulting earthquake seaward of a subduction zone on record, and its rupture zone extended to at least 33 km depth and paralleled most of the length of the 2006 rupture. The 13 January 2007 event produced stronger shaking in Japan than the larger thrust event, as a consequence of higher short-period energy radiation from the source. The great event aftershock sequences were dominated by the expected faulting geometries; thrust faulting for the 2006 rupture zone, and normal faulting for the 2007 rupture zone. A large intraplate compressional event occurred on 15 January 2009 (M_w = 7.4) near 45 km depth, below the rupture zone of the 2007 event and in the vicinity of the 16 March 1963 compressional event. The fault geometry, rupture process and slip distributions of the two great events are estimated using very broadband teleseismic body and surface wave observations. The occurrence of the thrust event in the shallowest portion of the interplate fault in a region with a paucity of large thrust events at greater depths suggests that the event removed most of the slip deficit on this portion of the interplate fault. This great earthquake doublet demonstrates the heightened seismic hazard posed by induced intraplate faulting following large interplate thrust events. Future seismic failure of the remainder of the seismic gap appears viable, with the northeastern region that has also experienced compressional activity seaward of the megathrust warranting particular attention.

Lithospheric structure of the Arabian Shield from the joint inversion of receiver functions and surface-wave group velocities

Abstract We estimate lithospheric velocity structure for the Arabian Shield by jointly modeling receiver functions and fundamental-mode group velocities from events recorded by the 1995–1997 Saudi Arabian Portable Broadband Deployment. Receiver functions are primarily sensitive to shear-wave velocity contrasts and vertical travel times, and surface-wave dispersion measurements are sensitive to vertical shear-wave velocity averages, so that their combination bridge resolution gaps associated with each individual data set. Our resulting models correlate well with the observed surface geology; the Asir terrane to the West consists of a 10-km-thick upper crust of 3.3 km/s overlying a lower crust of 3.7–3.8 km/s; in the Afif terrane to the East, the upper crust is 20 km thick and has an average velocity of 3.6 km/s, and the lower crust is about 3.8 km/s; separating the terranes, the Nabitah mobile belt is made of a gradational upper crust up to 3.6 km/s at 15 km overlying an also gradational lower crust up to 4.0 km/s. The crust–mantle transition is found to be sharp in terranes of continental affinity (east) and gradual in terranes of oceanic affinity (west). The upper mantle shear velocities range from 4.3 to 4.6 km/s. Temperatures around 1000 °C are obtained from our velocity models for a thin upper mantle lid observed beneath station TAIF, and suggest that the lithosphere could be as thin as 50–60 km under this station.

The 17 July 2006 Java tsunami earthquake

The 17 July 2006 Java earthquake involved thrust faulting in the Java trench and excited a deadly tsunami (∼5–8 m) that inundated the southern coast of Java. The earthquakes size estimates vary significantly with seismic wave period: very long-period signals (300–500+ s) indicate a seismic moment of 6.7 × 10^(20) Nm (M_w = 7.8), M_S (∼20 s) = 7.2, m_b (∼1 s) = 6.2, while shaking intensities (3–10 Hz) were ≤ MMIV. The large tsunami relative to MS characterizes this event as a tsunami earthquake. Like previous tsunami earthquakes, the Java event had an unusually low rupture speed of 1.0–1.5 km/s, and occurred near the up-dip edge of the subduction zone thrust fault. Most large aftershocks involved normal faulting. The rupture propagated ∼200 km along the trench, with several pulses of shorter period seismic radiation superimposed on a smooth background rupture with an overall duration of ∼185 s.

Imaging the Upper Crust of the Korean Peninsula by Surface-Wave Tomography

Cross correlation of seismic-background motions (Campillo and Paul, 2003; Shapiro et al. , 2005) is applied to observations from the Korean Meteorological Administration seismic network to estimate the short-period Rayleigh and Love wave dispersion characteristics of the region. Standard processing procedures are applied to the cross correlation, except that signal whitening is used in place of one-bit sampling to equalize power in signals from different times. Multiple-filter analysis is used to extract the group velocities from the estimated Green’s functions, which are then used to image the spatially varying dispersion at periods between 0.5 and 20 sec. The tomographic inversion technique used inverts all periods simultaneously to provide a smooth dispersion curve as a function of period in addition to the usual smooth spatial image for a given period. The Gyeongsang Basin in the southeastern part of the peninsula is clearly resolved with lower group velocities.

Rapid estimation of rupture directivity: Application to the 1992 Landers (MS = 7.4) and Cape Mendocino (MS = 7.2), California earthquakes

Using empirical Green functions with regional and teleseismic surface waves, it is possible to resolve fault finiteness effects, in many cases uniquely defining the fault plane for relatively large earthquakes. The technique requires very little data processing and can be applied in near-real time with the current distribution of seismic stations. The Landers strike-slip earthquake was dominated by two sub-events with predominantly north-northwestward rupture. The second sub-event was 1.5 times larger and rotated in strike by 12° counterclockwise relative to the first. The Cape Mendocino thrust event had a relatively smooth rupture that propagated to the southwest on a shallow dipping fault.