Broadband Waveforms Research Articles

Active backstop faults in the Mentawai region of Sumatra, Indonesia, revealed by teleseismic broadband waveform modeling

Two earthquake sequences that affected the Mentawai islands offshore of central Sumatra in 2005 (Mw 6.9) and 2009 (Mw 6.7) have been highlighted as evidence for active backthrusting of the Sumatran accretionary wedge. However, the geometry of the activated fault planes is not well resolved due to large uncertainties in the locations of the mainshocks and aftershocks. We refine the locations and focal mechanisms of medium size events (Mw > 4.5) of these two earthquake sequences through broadband waveform modeling. In addition to modeling the depth-phases for accurate centroid depths, we use teleseismic surface wave cross-correlation to precisely relocate the relative horizontal locations of the earthquakes. The refined catalog shows that the 2005 and 2009 “backthrust” sequences in Mentawai region actually occurred on steeply (∼60 degrees) landward-dipping faults (Masilo Fault Zone) that intersect the Sunda megathrust beneath the deepest part of the forearc basin, contradicting previous studies that inferred slip on a shallowly seaward-dipping backthrust. Static slip inversion on the newly-proposed fault fits the coseismic GPS offsets for the 2009 mainshock equally well as previous studies, but with a slip distribution more consistent with the mainshock centroid depth (∼20 km) constrained from teleseismic waveform inversion. Rupture of such steeply dipping reverse faults within the forearc crust is rare along the Sumatra–Java margin. We interpret these earthquakes as ‘unsticking’ of the Sumatran accretionary wedge along a backstop fault separating imbricated material from the stronger Sunda lithosphere. Alternatively, the reverse faults may have originated as pre-Miocene normal faults of the extended continental crust of the western Sunda margin. Our waveform modeling approach can be used to further refine global earthquake catalogs in order to clarify the geometries of active faults.

Earthquake Focal Mechanism Studies in Koyna-Warna Region in the Last Five Decades — Current Understanding on Tectonics and Seismogenesis

ABSTRACT On 10th December 1967, the world’s largest reservoir triggered seismic (RTS) event of magnitude 6.3 shook the Koyna region, the prime site of RTS globally. Ever since, several studies have attempted to infer the seismotectonics and to comprehend the actual causative mechanism of triggered seismicity in this region. Initial studies, including those of the 1967 Koyna main shock and its aftershocks, were based on the conventional P wave polarity or the first motion approach. These studies provided the first ever understanding of a predominantly strike-slip environment in the Koyna region, concurrent with the direction of ambient stress field due to the Indian plate motion. Subsequent studies pointed to a normal faulting environment in the Warna region further south, subsequent to impoundment later in 1985. A few studies did report solutions based on composite focal mechanisms, which however, only represent the average picture of the region. More recent studies based on modelling of seismic broadband waveform data provided more accurate focal mechanisms with unprecedented location accuracies including focal depths. A catalog of 50 focal mechanism solutions is now available for the earthquakes of magnitude ∼4 and larger that occurred during the last 50 years, which has paved way for a clear understanding of the stress field and the causative model of seismogenesis in this active intra-plate seismic RTS zone in western India. Based on stress inversion using this catalog, a new tectonic model depicting a periodically varying stress field and hence faulting mechanism has been inferred.

Strong SH‐to‐Love Wave Scattering off the Southern California Continental Borderland

AbstractSeismic scattering is commonly observed and results from wave propagation in heterogeneous medium. Yet deterministic characterization of scatterers associated with lateral heterogeneities remains challenging. In this study, we analyze broadband waveforms recorded by the Southern California Seismic Network and observe strongly scattered Love waves following the arrival of teleseismic SH wave. These scattered Love waves travel approximately in the same (azimuthal) direction as the incident SH wave at a dominant period of ~10 s but at an apparent velocity of ~3.6 km/s as compared to the ~11 km/s for the SH wave. Back projection suggests that this strong scattering is associated with pronounced bathymetric relief in the Southern California Continental Borderland, in particular the Patton Escarpment. Finite‐difference simulations using a simplified 2‐D bathymetric and crustal model are able to predict the arrival times and amplitudes of major scatterers. The modeling suggests a relatively low shear wave velocity in the Continental Borderland.

Cascade of blind deconvolution and array invariant for robust source-range estimation

The array invariant proposed for robust source localization in shallow water is based on the dispersion characteristics in ideal waveguides. It involves conventional plane-wave beamforming using a vertical array, exploiting multiple arrivals separated in beam angle and travel time, i.e., beam-time migration. The approach typically requires either a short pulse emitted by a source or the Green's function that can be estimated from a probe signal to resolve distinct multipath arrivals. In this letter, the array invariant method is extended to unknown source waveforms by extracting the Green's function via blind deconvolution. The cascade of blind deconvolution and array invariant for robust source-range estimation is demonstrated using a 16-element, 56-m long vertical array at various ranges (1.5–3.5 km) from a towed source broadcasting broadband communication waveforms (0.5–2 kHz) in approximately 100-m deep shallow water.

Double-ramp on the Main Himalayan Thrust revealed by broadband waveform modeling of the 2015 Gorkha earthquake sequence

The 2015Mw7.8 Gorkha earthquake sequence that unzipped the lower edge of the Main Himalayan Thrust (MHT) in central Nepal provides an exceptional opportunity to understand the fault geometry in this region. However, the limited number of focal mechanisms and the poor horizontal locations and depths of earthquakes in the global catalog impede us from clearly imaging the ruptured MHT. In this study, we generalized the Amplitude Amplification Factor (AAF) method to teleseismic distance that allows us to model the teleseismic P-waves up to 1.5 Hz. We used well-constrained medium-sized earthquakes to establish AAF corrections for teleseismic stations that were later used to invert the high-frequency waveforms of other nearby events. This new approach enables us to invert the focal mechanisms of some early aftershocks, which is challenging by using other long-period methods. With this method, we obtained 12 focal mechanisms more than that in the GCMT catalog. We also modeled the high-frequency teleseismic P-waves and the surface reflection phases (pP and sP) to precisely constrain the depths of the earthquakes. Our results indicate that the uncertainty of the depth estimation is as small as 1–2 km. Finally, we refined the horizontal locations of these aftershocks using carefully hand-picked arrivals. The refined aftershock mechanisms and locations delineate a clear double-ramp geometry of the MHT, with an almost flat décollement sandwiched in between. The flat (dip ∼7 degrees) portion of the MHT is consistent with the coseismic rupture of the mainshock, which has a well-constrained slip distribution. The fault morphology suggests that the ramps, both along the up-dip and down-dip directions, play a significant role in stopping the rupture of the 2015 Gorkha earthquake. Our method can be applied to general subduction zone earthquakes and fault geometry studies.

THE NATURE OF THE LITHOSPHERE‐ASTHENOSPHERE BOUNDARY BENEATH THE CENTRAL SOUTH AMERICA AREA FROM THE STACKING OF sP PRECURSORS

AbstractThe lithosphere‐asthenosphere boundary (LAB) is a seismic discontinuity with negative velocity contrast in the upper mantle. Seismic detections of the LAB in subduction zone region are helpful to understand the interaction between the lithosphere and asthenosphere and the geodynamic process associated with the slab subduction. In this paper, the vertical broadband waveforms are collected from four deep earthquakes occurred from 2006 to 2012 beneath the central South America area. The seismic waveforms are processed with the linear slant stack method to get the vespagrams in the relative travel‐time vs. slowness domain, and the sP precursors reflected at the bottom of the LAB (sLABP) are successfully extracted. Based on the one‐dimensional modified velocity model (IASP91‐SA), the horizontal distribution for the six sLABP reflecting points is obtained and divided into the western part (I) and the eastern part (II). In the part I, the LAB depth ranges between 60 km and 63 km, with the average depth of 61 km and the topography of 3 km; in the part II, the LAB depth ranges between 78 km and 82 km, with the average depth of 80 km and the topography of 4 km. Our results reveal the increasing LAB depths from west to east in the central South America area, and the trend may possibly represent the reformation differences of the continental lithosphere. We infer that near the trench, the continental lithosphere may be subjected to the stronger erosion for the higher degree of partial melting and more fertile melts in the asthenosphere; while far from the trench, the continental lithosphere may be subjected to the weaker erosion for the lower degree of partial melting and less fertile melts in the asthenosphere.

Performance of High‐Rate GPS Waveforms at Long Periods: Moment Tensor Inversion of the 2003 M w 8.3 Tokachi‐Oki Earthquake

Abstract We present a comparison of long‐period (30 T M w 8.3) with both very broadband seismograms and synthetic waveforms. We show that GPS provides valuable waveform data between 40 and 160 s periods, especially in the near field of the earthquake where seismograms are saturated. There are no long‐period broadband seismic data available in the near field of large earthquakes (∼100 km for earthquakes M w >7 and ∼500 km for earthquakes M w >8). The comparison with synthetic seismic displacement waveforms shows that the GPS waveforms are performing well at periods of 40–80 s at epicentral distances between 150 and 600 km and at periods of 80–160 s at epicentral distances between 150 and 400 km. At longer periods and larger epicentral distances, the amplitudes do not always exceed the noise level (∼2–3 mm), but we still observe at selected GPS sites that the phase and the amplitude of the surface waves are of good quality, up to ∼1200 km. We conclude that GPS waveforms in the near field of a large event can be used for seismological applications. In this article, we recover the focal mechanism of the Tokachi‐Oki event by inverting the GPS data recorded within 300 km of the epicenter.

Integrated geophysical characteristics of the 2015 Illapel, Chile, earthquake

AbstractOn 16 September 2015, a Mw 8.3 earthquake ruptured the subduction zone offshore of Illapel, Chile, generating an aftershock sequence with 14 Mw 6.0–7.0 events. A double source W phase moment tensor inversion consists of a Mw 7.2 subevent and the main Mw 8.2 phase. We determine two slip models for the mainshock, one using teleseismic broadband waveforms and the other using static GPS and InSAR surface displacements, which indicate high slip north of the epicenter and west‐northwest of the epicenter near the oceanic trench. These models and slip distributions published in other studies suggest spatial slip uncertainties of ~25 km and have peak slip values that vary by a factor of 2. We relocate aftershock hypocenters using a Bayesian multiple‐event relocation algorithm, revealing a cluster of aftershocks under the Chilean coast associated with deep (20–45 km depth) mainshock slip. Less vigorous aftershock activity also occurred near the trench and along strike of the main aftershock region. Most aftershocks are thrust‐faulting events, except for normal‐faulting events near the trench. Coulomb failure stress change amplitudes and signs are uncertain for aftershocks collocated with deeper mainshock slip; other aftershocks are more clearly associated with loading from the mainshock. These observations reveal a frictionally heterogeneous interface that ruptured in patches at seismogenic depths (associated with many aftershocks) and with homogeneous slip (and few aftershocks) up to the trench. This event likely triggered seismicity separate from the main slip region, including along‐strike events on the megathrust and intraplate extensional events.

Gaussian beam tracing for calculating the broadband field in three-dimensional environments

Ray tracing methods have a long history in underwater acoustics going back to a paper by H. Lichte in 1919 which also predicted the SOFAR channel. They remain extremely valuable today, partly because they present an intuitive view of sound propagation that readily allows for many extensions. For instance, targets and boundaries with complicated scattering can be included in a natural way; similarly, motion of boundaries, sources, receivers, and the ocean itself are easily treated. This talk will focus on the 3D extension with particular emphasis on broadband waveforms such as chirps or waveforms due to acoustic modems.

Cascade of blind deconvolution and array invariant for robust source-range estimation.

The array invariant proposed for robust source localization in shallow water is based on the dispersion characteristics in ideal waveguides. It involves conventional plane-wave beamforming using a vertical array, exploiting multiple arrivals separated in beam angle and travel time, i.e., beam-time migration. The approach typically requires either a short pulse emitted by a source or the Green's function that can be estimated from a probe signal to resolve distinct multipath arrivals. In this letter, the array invariant method is extended to unknown source waveforms by extracting the Green's function via blind deconvolution. The cascade of blind deconvolution and array invariant for robust source-range estimation is demonstrated using a 16-element, 56-m long vertical array at various ranges (1.5-3.5 km) from a towed source broadcasting broadband communication waveforms (0.5-2 kHz) in approximately 100-m deep shallow water.

Basic ultrasonic field measurement: Overview of standardized methods and expected developments

Basic ultrasonic field measurement methods to support the acoustic output characterization of medical ultrasound equipment are standardized in IEC 61161 for ultrasonic power by means of radiation force balance measurements and in IEC 62127-1,2,3 for ultrasonic pressure using hydrophones. Descriptions and requirements for ultrasonic power determination seem to be broadly elaborated even, for instance, for extended frequency ranges of high frequency diagnostic systems or for very high output powers of high intensity therapeutic ultrasound devices. In contrast, hydrophone measurements often are accompanied by several challenges still not fully addressed by current standards. Important deficiencies to overcome in future editions are the frequency range of calibrations currently limited to 40 MHz and being insufficient for broadband waveform deconvolution applications, and the descriptions of deconvolution and of the corresponding measurement uncertainty determination themselves. Improvements of these technical items according to recently developed calibration and data evaluation procedures are expected to result in better quality and reliability of acoustic output parameter determination as required, for instance, for diagnostic ultrasound machines within the output declaration and output display according to IEC 60601-2-37 and IEC 62359. In addition, new projects on standards in the high intensity therapeutic area may also take advantage of such improvements.

Coherent matched-filter reflection loss from a moving rough surface as a function of pulse duration and ensonified area

The loss incurred when matched filtering a sonar pulse after reflection from a rough surface depends on the ensonified area on the surface and the motion of the surface throughout the pulse. Simple solutions exist for the loss in the coherently reflected component when the ensonified area is either very large or very small. The large ensonified area (LEA) result is generally accurate for narrowband pulses while a small ensonified area (SEA) requires broadband waveforms. By assuming the reflection comprises a finite number of statistically independent surface heights and that the surface is Gaussian distributed with a narrowband spectrum, the loss can be determined as a function of both the pulse duration relative to the wave period and effective ensonified area (which depends on pulse bandwidth) using the spatial correlation function of the surface. The result is shown to be a convex combination of the LEA and SEA results. A one-dimensional example illustrates how surface reflection loss increases with pu...

Rupture characteristics of the 2016 Meinong earthquake revealed by the back projection and directivity analysis of teleseismic broadband waveforms

AbstractThe 2016 Mw 6.4 Meinong earthquake struck a previously unrecognized fault zone in midcrust beneath south Taiwan and inflicted heavy causalities in the populated Tainan City about 30 km northwest of the epicenter. Because of its relatively short rupture duration and P wave trains contaminated by large‐amplitude depth phases and reverberations generated in the source region, accurate characterization of the rupture process and source properties for such a shallow strong earthquake remains challenging. Here we present a first high‐resolution MUltiple SIgnal Classification back projection source image by using both P and depth‐phase sP waves recorded at two large and dense arrays to understand the source behavior and consequent hazards of this peculiar catastrophic event. The results further corroborated by the directivity analysis indicate a unilateral rupture propagating northwestward and slightly downward on the shallow NE‐dipping fault plane. The source radiation process is primarily characterized by one single peak, ~7 s duration, with a total rupture length of ~17 km and average rupture speed of 2.4 km/s. The rupture terminated immediately east of the prominent off‐fault aftershock cluster about 20 km northwest of the hypocenter. Synergistic amplification of ground shaking by the directivity and strong excitation of sP and reverberations mainly caused the destruction concentrated in the area further to the northwest away from the rupture zone.

A Comparative Study of Source Complexity of Two Moderate-Sized Earthquakes in Southern Taiwan: 2016 Mw6.4 and 2010 Mw6.2 Earthquakes

We study the complex rupture history of the 2016 Mw6.4 Meinong earthquake and the 2010 Mw6.2 Jiaxian earthquake in southern Taiwan by simultaneously inverting near-field strong-motion records, local and teleseismic broadband waveforms along with long-period surface waves. The focal mechanism results reveal that both earthquakes may rupture two different blind faults. The rupture of the 2016 event is dominated by the strike-slip motion with minor thrust faulting, while the 2010 event is a relatively high-angle thrust earthquake. Our preferred finite-fault source models suggest that the coseismic rupture history of the 2016 event should be relatively more complex than the 2010 event. During the 2016 event, two main patches of slip characterize the coseismic slip history. The cumulative seismic moment within 12 s of rupture is about 5.36 × 1018 N m, which is 1.9 times of the moment of the 2010 event. The slip of the 2016 event is associated with relatively larger perk slip of 0.7 m at shallower depth (~18–19 km), higher average slip of 0.3 m and faster slip rate with the maximum value of 3.4 m/s. Our kinematic models can shed some light on the cause of the difference in seismic hazard between these two earthquakes, and will be further applied to analyze the deep blind fault structure in southern Taiwan.

Modeling of Source Parameters of the 15 December 2015 Deogarh Earthquake of Mw 4.0

ABSTRACT We herein present source parameters and focal mechanism of a rare cratonic upper crustal earthquake of Mw4.0, which occurred at 8 km depth (centroid depth) below a region near Deogarh, Jharkhand. For our study, we used broadband waveform data from a seismic network of 15 three-component seismographs in the eastern Indian craton. The average seismic moment, moment magnitude and source radius are estimated to be 1.1 x 1015 N-m, 4.0 and 180.6 m, respectively. The high average stress drop of 14.27 MPa could be attributed to its lower-crustal origin. The mean corner frequency is calculated to be 4.1 Hz. To study the source mechanism, we perform a deviatoric constrained full waveform moment tensor inversion of multiple point sources on the band-passed (0.06 - 0.14 Hz) broadband displacement data of the Deogarh event, using ISOLA software. The best fit is obtained for the source at 8 km centroid depth, with a moment magnitude 3.7, and a right-lateral strike-slip mechanism with strike 162°, dip 72° and rake 169°. The P-axis orients N24°E, which is parallel to the direction of the absolute plate motion direction of Indian plate, while T-axis orients E-W, which is parallel to the strike of the pre-existing Damodar Graben (DG) of Gondwana age. The occurrence of this earthquake is attributed to the neotectonic reactivation of a fault associated with the E-W trending DG shear zone.

Seismic evidence of the lithosphere-asthenosphere boundary beneath Izu-Bonin area

The lithosphere-asthenosphere boundary (LAB) is the seismic discontinuity with the negative velocity contrast in the upper mantle. The plate tectonic theory describes that the rigid lithosphere translate coherently over the ductile asthenosphere, and the LAB implies the decoupling between the two layers. The seismic velocity, viscosity, electrical resistivity and other physical parameters change with the depths across the boundary. The LAB has become a focus of seismology studies for its significance in understanding the plate motions, the mantle convection and the lithospheric evolution processes. Seismic detections on the LAB in subduction zone areas are helpful to understand the interactions between the lithosphere and asthenosphere layers and the geodynamic processes related with the slab subductions. For the dense permanent networks and transportable arrays of the USArray, the abundant waveforms of the deep earthquake events beneath the Izu-Bonin area can be recorded and retrieved. In this study, the vertical broadband waveforms are collected from three earthquake events occurring from 2002 to 2014 with the focal depths of 400–600 km. The source parameters are taken from the Preliminary Determinations of Epicenters catalog of US Geological Survey, and the focal fault solutions are taken from the global Centroid-Moment-Tensor project. The seismic waveforms are manually selected with two criteria: (1) simple sP arrivals with clear peaks, and (2) high signal-to-noise ratios and without abnormal spikes. In order to enhance the weak unknown precursors, the linear slant stack method, which is the special form of the N -th root slant stack method and phase-weighted stack method, is applied to scan the differential slowness and stack the seismograms in the domain of differential travel-time and differential slowness. The selected waveform data is processed to obtain the vespagrams and the stacked waveforms. The sP precursors reflected on the LAB ( s LAB P ), which have the opposite polarities with the amplitude ratios of 0.17–0.21 relative to the sP phases, are successfully extracted in our results. To reduce the possible effects of the crustal structures on the propagations of sP phases, the one-dimensional modified velocity model (IASP91-IB) is established by replacing the crust part of the IASP91 model with the CRUST1.0 model. On the basis of the modified model, we obtain the distributions for six reflected points of the s LAB P phases near the source area. Our results reveal that the LAB depths range between 58 and 65 km beneath the Izu-Bonin Arc, with the average depth of 62 km and the small topography of 7 km. Based on the half-space cooling model for the oceanic lithosphere, the lithosphere beneath the Izu-Bonin Arc is about located at the isotherm of 950°C, and the lithosphere beneath the West Philippine Basin (WPB) and Parece Vela Basin (PVB) is about located at the isotherm of 1100°C. Compared with the results of the tectonic stable areas (WPB and PVB) in the Philippine Sea, the lithosphere beneath the Izu-Bonin Arc shows the obvious thinning phenomenon. The numerical modeling experiments have revealed the strong erosions of the convecting asthenosphere in the mantle wedge, resulting in the destruction of the craton. We infer that the lithospheric thinning beneath the Izu-Bonin Arc is closely related with the partial melting, which is caused by the volatiles continuously released from the subducted western Pacific slab, and the strong erosions of the small-scale mantle convections in the back-arc mantle wedge.

Role of the Kopili Fault in Deformation Tectonics of the Indo‐Burmese Arc Inferred from the Rupture Process of the 3 January 2016Mw 6.7 Imphal Earthquake

The Burmese arc is a peculiar plate boundary where there exists an eastward subducted Indian slab, but the current tectonics of the region are predominantly governed by a northward strike‐slip motion of the Indian plate past the Burmese plate, as evidenced by seismic and Global Positioning System data. The shallower portion of the dipping Indian slab is characterized mostly by earthquakes with strike‐slip type focal mechanisms with roughly northwest‐ and northeast‐trending fault planes, although there is no clear evidence for the choice of rupture planes. In the present study, we use teleseismic broadband waveforms to model the rupture propagation of an M w 6.7 strike‐slip earthquake of 3 January 2016 that occurred in the Indian city of Imphal, and infer the fault‐plane orientation. The result indicates a right‐lateral strike‐slip motion along a N21°W plane, which is confirmed by both waveform fitting and distribution of aftershocks. Interestingly, this earthquake occurred at the intersection of three major faults in this region—the Churachandpur Mao fault bounding the India and Burma plates, the Dauki fault south of the Shillong plateau, and the Kopili fault separating the Shillong plateau from the Brahmaputra valley. This raises various possibilities for interpreting this event. However, based on the finite‐fault inversion, we infer that this earthquake occurred on the Kopili fault running northwest–southeast from Bhutan Himalaya down to the Burmese arc, and manifested as a right‐lateral strike‐slip motion in response to the northward motion of the Indian plate and a counterclockwise rotation of the Brahmaputra valley. We further interpret that the overall deformation in the Burmese arc region is accommodated by northward slivering of the eastward subducted Indian lithospheric slab along a series of northwest‐ to north–south‐trending right‐lateral strike‐slip faults from west to east as the Indian plate moves northward past the Burma and Sunda blocks.

SOURCE PARAMETERS OF MODERATE AND STRONG EARTHQUAKES IN THE BROADER AREA OF ZAKYNTHOS ISLAND (W. GREECE) FROM REGIONAL AND TELESEISMIC DIGITAL RECORDINGS

The source parameters of all the moderate and strong events that occurred in the broader area of Zakynthos Island for the period 1997–2009 are determined using two different techniques depending on epicentral distance. For the strong events we employed teleseismic body-wave modeling between 30° and 90°, while for moderate events a regional moment tensor inversion approach was used. In both cases we used broadband waveform data with a good signal-to-noise ratio. The calculated focal mechanisms are representative of the displacement and strain fields of the broader area. Those associated with the Cephalonia transform fault are consistent with dextral strike-slip motion. The area between the island of Zakynthos and the Western coast of Peloponnese is also characterized by strike–slip faulting, while reverse faulting is mainly observed south of Zakynthos Island. Using both techniques, we also retrieved the source parameters of the biggest events of the seismic sequence of April 2006 near the southern coast of Zakynthos Island. The depths of the located events of the sequence vary mainly between 10 and 25 km. The fault plane solutions revealed thrust type faulting in all cases, something which can be related with upward motions in the southern part of the Island observed by DGPS measurements.

MODERATE MAGNITUDE EARTHQUAKE SEQUENCES IN CENTRAL GREECE (FOR THE YEAR 2008)

The small-to-moderate magnitude earthquake sequences offshore NE Evia island (Mantoudi, October 2008) and in central Greece, to the NW of the town of Amfiklia (December 2008) are studied in terms of their source parameters and strong shaking in nearby populated areas. Moment tensors of the strongest events of the sequences, their seismogenic faults and the distribution of slip on them are sought through the inversion of regional broadband waveforms. We then apply a forward modelling technique, which incorporates synthetic Green’s functions calculated for a 1D velocity model, previously proposed for the broader studied area, to get a first-order evaluation of the distribution of strong ground shaking around the hypocenters. The Mantoudi earthquake sequence is found to be associated to an offshore NESW trending normal fault that dips to the NW, with a considerable dextral strike-slip component. The Amfiklia earthquake sequence is related to a pure normal, E-W trending fault which dips to the south. This conclusion is drawn by both the distribution of aftershocks and the inversion process for the computation of slip on the seismogenic fault of the strongest event of the sequence (Mw5.1). Forward modelling of strong ground shaking based on event-specific moment tensor and slip distribution indicates low levels of expected ground motion in accordance with the limited observed damage.

Some effects of ocean surface roughness on high and low duty cycle, broadband waveforms

In this paper, data from TREX13 are used to compare the impact of the environment on High Duty Cycle (HDC) sonar and Pulsed Active Sonar (PAS). This paper presents the results of an examination of short-range single surface-reflection echoes, and longer-range target echoes from an air hose. Measurements showed that for an 18 s HDC pulse, the mean (coherent) component of the specular arrival decreased with increasing rms roughness whereas 0.5 s PAS pulse echoes showed no correlation with roughness. The standard deviations of the mean echo levels are used to examine the incoherent (scattered) component of the specular arrivals. The incoherent component of the specular arrival increased with the product of the surface correlation length and the square of the rms roughness, for both HDC and PAS, with the PAS data having a 1 dB higher standard deviation. A normal mode propagation model and a rough surface scattering model used in conjunction with a simple model that accounts for motion-induced coherence loss from the matched filter, are used to interpret the results. [Work sponsored by ONR Code 32.]