The Sanriku-Oki low-seismicity region on the northern margin of the great 2011 Tohoku-Oki earthquake rupture

We examine a region of the megathrust fault offshore of northeastern Honshu (38.75°–40.25°N, 141.5°–143.25°E) that we designate as the Sanriku-Oki low-seismicity region (SLSR). The SLSR, located near the northern termination of the 2011 Tohoku-Oki (M_w 9.0) rupture, lacks historical great earthquake ruptures and has relatively low levels of moderate-size (M_j ≥ 5.0) earthquakes, with subregions having many small events (M_j 2.5–5.0) in the Japan Meteorological Agency unified catalog. The SLSR is located downdip along the megathrust from the rupture zone of the great 1896 Sanriku tsunami earthquake and the great 1933 Sanriku outer trench slope normal-faulting event; weak seismic coupling of the SLSR had been deduced based on the occurrences of those unusual events. Relatively low slip deficit on the SLSR megathrust was estimated based on GPS deformations prior to 2011 compared with adjacent areas with strong inferred locking to the south and north. The southern portion of the SLSR appears to have had, at most, modest levels (<5 m) of coseismic slip during the 2011 event. Some thrust-faulting aftershocks did occur in the SLSR, primarily at depths near 40 km where there had previously been small (M_j ~5.0) repeating earthquakes (e.g., the Kamaishi repeater). An M_w ~ 7.4 underthrusting aftershock occurred near the northeastern edge of the SLSR ~22.5 min after the great 2011 event. Postseismic convergence along the megathrust is peaked in the SLSR. The collective observations indicate that the SLSR is primarily undergoing quasi-static aseismic convergence, and the lack of regional strain accumulation likely delimited the northern extent of the great 2011 rupture as well as the downdip extent of the 1896 rupture. The triggering of the M_w 7.4 aftershock and heightened activity in the downdip repeater regions suggest that high postseismic strain rates drove the region to have ephemerally increased seismic failure, but it appears unlikely that a great earthquake will nucleate or rupture through this region. Similar properties may exist on the megathrust near the southern end of the 2011 rupture

Structure of the core‐mantle transition zone

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95181/1/eost7681.pd

Estimating explosion yield by analytical waveform comparison

The yields of 28 underground nuclear explosions at NTS (25 on Pahute Mesa) are estimated by applying a relative waveform analysis called intercorrelation to 1256 teleseismic short-period P -waves recorded at 74 WWSSN and CSN stations. Corrections for the effects of pP interference and yield-scaling of the explosion source functions are determined and applied to the waveforms, enabling analytical comparison of signals from events with different yields and burial depths. The procedure accounts for common receiver and propagation effects. Relative explosion source strengths in the 0.5–2.0 Hz frequency band are determined, with results of near-field modelling of strong ground motions establishing the absolute source spectral levels. Four events with detailed near-field models are used as master events in the intercorrelation process, and it is demonstrated that the relative source strengths are better resolved than the absolute values. Events with announced yields are used to determine empirical relations between yield and source strength, which in turn predict the yields of the other events. These yield estimates are shown to be comparable with those obtained by standard m b and relative amplitude analysis. The analytical waveform comparisons also provide estimates of the pP parameters for each event, and criteria for identifying anomalous events, such as PIPKIN and MUENSTER, for which the waveforms differ from those of other events in the test site. Possible mechanisms affecting the anomalous events are considered. Pahute Mesa is shown to be a distinct subsite within NTS, with different teleseismic amplitude and waveform variations than observed at other subsites.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/73925/1/j.1365-246X.1985.tb05126.x.pd

Geometric effects of global lateral hetergeneity on long-period surface wave propagation

Long-period Rayleigh waves from Iranian earthquakes have large amplitude asymmetries between minor arc and major arc arrivals (e.g., R_2 and R_3) at digital stations in the azimuth range N20°W to N60°E. These asymmetries are as large as a factor of 2 at a period of 256 s and persist to periods greater than 300 s. In some cases the entire Rayleigh wave group arrival spanning periods from 100 to 300 s is either uniformly enhanced in amplitude or diminished to such a degree that the group arrival appears to be missing. The amplitude anomalies are generally not accompanied by significant phase anomalies. The irregular azimuthal distribution of the amplitude asymmetries and their occurrence for events with different focal mechanisms and epicentral separations of several hundred kilometers preclude an explanation of these observations by source complexity. Events in the Mediterranean and Nepal do not produce similar amplitude asymmetries at the same stations. The anomalies are thus most likely due to focusing and defocusing propagation effects. As a preliminary investigation of the effects of lateral heterogeneity of upper mantle velocity structure on long-period surface wave amplitudes, surface wave ray-tracing calculations are performed using recently proposed global phase velocity distributions. Dramatic deviations from great circle paths are predicted for long propagation paths (e.g., R_3). The particular spatial distribution of lateral velocity gradients around a given source location determines whether substantial amplitude asymmetries will be observed between minor arc and major arc arrivals and whether these will persist for sequential great circle orbits. The 200-s period amplitude asymmetry observed at KIP for the Iranian source region (R_(2,4) ≪ R_3) is well predicted by the ray-tracing results. The absence of this anomaly for the other source regions is also predicted. Other observed anomalies are not all well predicted, but it is clear that geometric effects can contribute significantly to the observed variations of Rayleigh and Love wave amplitudes. This is the probable explanation for the instability of Q estimates made from surface waves. Other source regions producing large surface wave amplitude anomalies include Japan and southeastern Alaska

An asperity model of large earthquake sequences

The variation in maximum rupture extent of large shallow earthquakes in circum-Pacific subduction zones is interpreted in the context of the asperity model of stress distribution on the fault plane. Comparison of the historic record of large earthquakes in different zones indicates that four fundamental categories of behavior are observed. These are: (1) the Chile-type regular occurrence of great ruptures spanning more than 500 km; (2) the Aleutians-type variation in rupture extent with occasional ruptures up to 500 km long, and temporal clustering of large events; (3) the Kurile-type repeated failure over a limited zone of 100–300 km length in isolated events; and (4) the Marianas-type absence of large earthquakes. Southern Chile, Alaska, Southern Kamchatka, and possibly the Central Aleutians are grouped in the first category. The Rat Island portion of the Aleutians, Colombia, Southwest Japan, and the Solomon Islands zones demonstrate the temporal variation of rupture length and multiple earthquake sequences that characterize category 2. The New Hebrides and Middle America have earthquake clustering on a more moderate scale, and are intermediate between categories 2 and 3. Category 3 includes the Kurile Islands, Northeast Japan, Peru and Central Chile. Zones lacking large earthquakes (category 4) include the Marianas, Izu-Bonin, and large portions of Tonga-Kermadec. By loosely grouping each subduction zone into these categories and comparing the general range in behavior with a simple fault model, which is used in a numerical simulation, the parameters governing large earthquake development are clarified. Interpretation of the four categories in terms of asperity distribution and interaction permits some inferences of the nature of stress distribution in particular zones. Two factors appear to dominate in the development of large earthquake failure zones; the nature and degree of coupling on the fault contact, and the extent of lateral segmentation of the subduction zone by transverse stress barriers. Strong coupling and uniform stress distribution on the fault plane produces larger events, whereas more heterogeneous stress distributions produce smaller ruptures and temporal variation in rupture length. Segmentation of the subduction zone that may result in stress barriers affecting rupture length is produced by subduction of transverse structures such as aseismic ridges, and is reflected by submarine canyons and geometric variations in trench configuration

Insights from the great 2011 Japan earthquake

The diverse set of waves generated in Earth’s interior, oceans, and atmosphere during the devastating Tohoku-oki earthquake reveal some extraordinary geophysics

Measuring complex spectra of long‐period surface waves for earthquake source analysis

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94599/1/grl4266.pd

Body-wave amplitude and travel-time correlations across North America

Relationships between travel-time and amplitude station anomalies are examined for short- and long-period SH waves and short-period P waves recorded at North American WWSSN and Canadian Seismic Network stations. Data for two azimuths of approach to North America are analyzed. To facilitate intercomparison of the data, the S-wave travel times and amplitudes are measured from the same records, and the amplitude data processing is similar for both P and S waves. Short-period P- and S-wave amplitudes have similar regional variations, being relatively low in the western tectonic region and enhanced in the shield and mid-continental regions. The east coast has intermediate amplitude anomalies and systematic, large azimuthal travel-time variations. There is a general correlation between diminished short-period amplitudes and late S-wave arrival times, and enhanced amplitudes and early arrivals. However, this correlation is not obvious within the eastern and western provinces separately, and the data are consistent with a step-like shift in amplitude level across the Rocky Mountain front. Long-period S waves show no overall correlation between amplitude and travel-time anomalies

Waveform complexity in teleseismic broadband SH displacements: Slab diffractions or deep mantle reflections?

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95458/1/grl4356.pd

Comparison of long‐period surface wave amplitude and phase anomalies for two models of global lateral heterogeneity

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95459/1/grl2883.pd