Keith D. Koper

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.

En échelon and orthogonal fault ruptures of the 11 April 2012 great intraplate earthquakes

The Indo-Australian plate is undergoing distributed internal deformation caused by the lateral transition along its northern boundary—from an environment of continental collision to an island arc subduction zone. On 11 April 2012, one of the largest strike-slip earthquakes ever recorded (seismic moment magnitude Mw 8.7) occurred about 100–200 kilometres southwest of the Sumatra subduction zone. Occurrence of great intraplate strike-slip faulting located seaward of a subduction zone is unusual. It results from northwest–southeast compression within the plate caused by the India–Eurasia continental collision to the northwest, together with northeast–southwest extension associated with slab pull stresses as the plate underthrusts Sumatra to the northeast. Here we use seismic wave analyses to reveal that the 11 April 2012 event had an extraordinarily complex four-fault rupture lasting about 160 seconds, and was followed approximately two hours later by a great (Mw 8.2) aftershock. The mainshock rupture initially expanded bilaterally with large slip (20–30 metres) on a right-lateral strike-slip fault trending west-northwest to east-southeast (WNW–ESE), and then bilateral rupture was triggered on an orthogonal left-lateral strike-slip fault trending north-northeast to south-southwest (NNE–SSW) that crosses the first fault. This was followed by westward rupture on a second WNW–ESE strike-slip fault offset about 150 kilometres towards the southwest from the first fault. Finally, rupture was triggered on another en échelon WNW–ESE fault about 330 kilometres west of the epicentre crossing the Ninetyeast ridge. The great aftershock, with an epicentre located 185 kilometres to the SSW of the mainshock epicentre, ruptured bilaterally on a NNE–SSW fault. The complex faulting limits our resolution of the slip distribution. These great ruptures on a lattice of strike-slip faults that extend through the crust and a further 30–40 kilometres into the upper mantle represent large lithospheric deformation that may eventually lead to a localized boundary between the Indian and Australian plates.

Energy Release of the 2013 Mw 8.3 Sea of Okhotsk Earthquake and Deep Slab Stress Heterogeneity

Delineating Deep Faults Most large, damaging earthquakes initiate in Earths crust where friction and brittle fracture control the release of energy. Strong earthquakes can occur in the mantle too, but their rupture dynamics are difficult to determine because higher temperatures and pressures play a more important role. Ye et al. (p. 1380) analyzed seismic P waves generated by the 2013 Mw 8.3 Sea of Okhotsk earthquake—the largest deep earthquake recorded to date—and its associated aftershocks. The earthquake ruptured along a fault over 180-kilometer-long and structural heterogeneity resulted in a massive release of stress from the subducting slab. In a set of complementary laboratory deformation experiments, Schubnel et al. (p. 1377) simulated the nucleation of acoustic emission events that resemble deep earthquakes. These events are caused by an instantaneous phase transition from olivine to spinel, which would occur at the same depth and result in large stress releases observed for other deep earthquakes. Distribution of strong and weak zones in the subducting slab controlled the extent of the largest recorded deep earthquake. Earth’s deepest earthquakes occur in subducting oceanic lithosphere, where temperatures are lower than in ambient mantle. On 24 May 2013, a magnitude 8.3 earthquake ruptured a 180-kilometer-long fault within the subducting Pacific plate about 609 kilometers below the Sea of Okhotsk. Global seismic P wave recordings indicate a radiated seismic energy of ~1.5 × 1017 joules. A rupture velocity of ~4.0 to 4.5 kilometers/second is determined by back-projection of short-period P waves, and the fault width is constrained to give static stress drop estimates (~12 to 15 megapascals) compatible with theoretical radiation efficiency for crack models. A nearby aftershock had a stress drop one to two orders of magnitude higher, indicating large stress heterogeneity in the deep slab, and plausibly within the rupture process of the great event.

Massive landslide at Utah copper mine generates wealth of geophysical data

On the evening of 10 April 2013 (MDT) a massive landslide occurred at the Bingham Canyon copper mine near Salt Lake City, Utah, USA. The northeastern wall of the 970-m-deep pit collapsed in two distinct episodes that were each sudden, lasting ~90 seconds, but separated in time by ~1.5 hours. In total, ~65 million cubic meters of material was deposited, making the cumulative event likely the largest non-volcanic landslide to have occurred in North America in modern times. Fortunately, there were no fatalities or injuries. Because of extensive geotechnical surveillance, mine operators were aware of the instability and had previously evacuated the area. The Bingham Canyon mine is located within a dense regional network of seismometers and infrasound sensors, making the 10 April landslide one of the best recorded in history. Seismograms show a complex mixture of shortand long-period energy that is visible throughout the network (6–400 km). Local magnitudes (M L ) for the two slides, which are based on the amplitudes of short-period waves, were estimated at 2.5 and 2.4, while magnitudes based on the duration of seismic energy (m d ) were much larger (>3.5). This magnitude discrepancy, and in particular the relative enhancement of longperiod energy, is characteristic of landslide seismic sources. Interestingly, in the six days following the landslide, 16 additional seismic events were detected and located in the mine area. Seismograms for these events have impulsive arrivals characteristic of tectonic earthquakes. Hence, it appears that in this case the common geological sequence of events was inverted: Instead of a large earthquake triggering landslides, it was a landslide that triggered several small earthquakes.

Supershear rupture of the 5 January 2013 Craig, Alaska (Mw 7.5) earthquake

Supershear rupture, in which a fractures crack tip expansion velocity exceeds the elastic shear wave velocity, has been extensively investigated theoretically and experimentally and previously inferred from seismic wave observations for six continental strike-slip earthquakes. We find extensive evidence of supershear rupture expansion of an oceanic interplate earthquake, the 5 January 2013 Mw = 7.5 Craig, Alaska earthquake. This asymmetric bilateral strike-slip rupture occurred on the Queen Charlotte Fault, offshore of southeastern Alaska. Observations of first-arriving Sn and Sg shear waves originating from positions on the fault closer than the hypocenter for several regional seismic stations, with path calibrations provided by an empirical Greens function approach, indicate a supershear rupture process. Several waveform inversion and modeling techniques were further applied to determine the rupture velocity and space-time distribution of slip using regional seismic and geodetic observations. Both theoretical and empirical Greens functions were used in the analyses, with all results being consistent with a rupture velocity of 5.5 to 6 km/s, exceeding the crustal and upper mantle S wave velocity and approaching the crustal P wave velocity. Supershear rupture occurred along ~100 km of the northern portion of the rupture zone but not along the shorter southern rupture extension. The direction in which supershear rupture developed may be related to the strong material contrast across the continental-oceanic plate boundary, as predicted theoretically and experimentally. The shear and surface wave Mach waves involve strongly enhanced ground motions at azimuths oblique to the rupture direction, emphasizing the enhanced hazard posed by supershear rupture of large strike-slip earthquakes.

Dominant seismic noise sources in the Southern Ocean and West Pacific, 2000–2012, recorded at the Warramunga Seismic Array, Australia

Seismic noise is important in determining Earth structure and also provides an insight into ocean wave patterns and long-term trends in storm activity and global climate. We present a long-duration study of seismic noise focused on the Southern Ocean using recordings from the Warramunga Seismic Array, Northern Territory, Australia. Using high-resolution analysis, we determine the seismic slowness and back azimuth of observed seismic noise, microseisms, at hourly intervals through over a decade (2000–2012). We identify three dominant sources of body wave ( P ) noise in the Southern Ocean which we interpret to originate from a South Atlantic source propagating as PP waves, and Kerguelen Island and Philippine Sea sources propagating as P waves. We also identify surface waves from around the Australian coast. All sources show distinct seasonality and a low, but discernable, interannual variability.

The fine structure of double‐frequency microseisms recorded by seismometers in North America

We performed a frequency-dependent polarization analysis on ambient seismic energy recorded by 1768 USArray Transportable Array (TA) seismometers for the time period of 1 April 2004 through 31 October 2014. The seismic energy has strong seasonal variations in power and polarization at essentially all stations; however, the annual variation is much smaller. One year of data is sufficient to determine the average properties of the ambient seismic wavefield at a particular site. The average power and dominant period in the double-frequency (DF) microseism band, defined here as periods of 2–10 s, vary significantly and coherently across North America. Proximity to a coastline generally leads to increased DF microseism amplitude, but site geology is much more important, with sedimentary basins having especially large DF amplitudes. The western U.S. as a whole has longer dominant DF periods than the central and eastern U.S., with the southeastern U.S. having the shortest dominant DF periods. Power spectral density estimates at many TA stations show a splitting of the DF microseism peak into two distinct subpeaks. This has been observed previously in data recorded by ocean bottom seismometers, with the shorter-period DF peak attributed to the local sea and the longer-period DF peak attributed to more distant, coastally generated microseisms. In the case of the land-based TA data analyzed here, the DF splitting arises from simultaneous microseism generation at various source areas (Pacific Ocean, Atlantic Ocean, and Gulf of Mexico) with distinct, preferred excitation frequencies. DF microseism source properties derived from global models of ocean wave interaction support this interpretation.

The frequency dependence and locations of short period microseisms generated in the Southern Ocean and west Pacific

The origin of the microseismic wavefield is associated with deep ocean and coastal regions where, under certain conditions, ocean waves can excite seismic waves that propagate as surface and body waves. Given that the characteristics of seismic signals generally vary with frequency, here we explore the frequency and azimuth dependent properties of microseisms recorded at a medium aperture (25 km) array in Australia. We examine the frequency dependent properties of the wavefield, and its temporal variation, over two decades (1991–2012), with a focus on relatively high-frequency microseisms (0.325–0.725 Hz) recorded at the Warramunga Array (WRA), which has good slowness resolution capabilities in this frequency range. The analysis is carried out using the Incoherently Averaged Signal (IAS) Capon beamforming, which gives robust estimates of slowness and backazimuth, and is able to resolve multiple wave arrivals within a single time window. For surface waves, we find that fundamental mode Rayleigh waves (Rg) dominate for lower frequencies ( 0.55 Hz) show a transition to higher mode surface waves (Lg). For body waves, source locations are identified in deep ocean regions for lower frequencies and in shallow waters for higher frequencies. We further examine the association between surface wave arrivals and a WAVEWATCH III ocean wave hindcast. Correlations with the ocean wave hindcast show that secondary microseisms in the lower frequency band are generated mainly by ocean swell, while higher frequency bands are generated by the wind sea, i.e. local wind conditions.

Along‐dip seismic radiation segmentation during the 2007 Mw 8.0 Pisco, Peru earthquake

[1] The short-period (0.5–2 s) seismic radiation properties of the August 15 (23:40:57 UTC) 2007 Mw 8.0 Pisco, Peru earthquake are imaged by back-projecting P waves recorded at 374 elements of USArray deployed in western North America at distances of 54–74 from the source region. The coherent short-period seismic energy release has two main intervals similar to moment-rate functions determined by inversion of longer-period teleseismic body waves; however, the spatial locations of the coherent bursts of short-period energyrelease are located northanddown-dip of the region of major slip. The contrast between short- and long-period seismic radiation properties of the Pisco earthquake is more subtle than for the 2011 Mw 9.0 Tohoku earthquake, but provides further support for the idea of depth-dependent changes in sliding behavior during megathrust ruptures. Citation: Sufri, O., K. D. Koper, and T. Lay (2012), Along-dip seismic radiation segmentation during the 2007 Mw 8.0 Pisco, Peru earthquake, Geophys. Res. Lett., 39, L08311, doi:10.1029/2012GL051316.

High‐resolution probing of inner core structure with seismic interferometry

Increasing complexity of Earths inner core has been revealed in recent decades as the global distribution of seismic stations has improved. The uneven distribution of earthquakes, however, still causes a biased geographical sampling of the inner core. Recent developments in seismic interferometry, which allow for the retrieval of core-sensitive body waves propagating between two receivers, can significantly improve ray path coverage of the inner core. In this study, we apply such earthquake coda interferometry to 1846 USArray stations deployed across the U.S. from 2004 through 2013. Clear inner core phases PKIKP^2 and PKIIKP^2 are observed across the entire array. Spatial analysis of the differential travel time residuals between the two phases reveals significant short-wavelength variation and implies the existence of strong structural variability in the deep Earth. A linear N-S trending anomaly across the middle of the U.S. may reflect an asymmetric quasi-hemispherical structure deep within the inner core with boundaries of 99°W and 88°E.