Relationship between the seismicity and geologic structure of the Blanco Transform Fault Zone

Morphologic studies of an oceanic transform, the Blanco Transform Fault Zone (BTFZ), have shown it to consist of a series of extensional basins that offset the major strike-slip faults. The largest of the extensional basins, the Cascadia Depression, effectively divides the transform into a northwest segment, composed of several relatively short strike-slip faults, and a southeast segment dominated by fewer, longer faults. The regional seismicity distribution (m _b 4.0) and frequency-magnitude relationships (b-values) of the BTFZ show that the largest magnitude events are located on the southeast segment. Furthermore, estimates of the cumulative seismic moment release and seismic moment release rate along the southeast segment are significantly greater than that of the northwest segment. These observations suggest that slip along the southeast segment is accommodated by a greater number of large magnitude earthquakes. Comparison of the seismic moment rate, derived from empirical estimates, with the seismic moment rate determined from plate motion constraints suggests a difference in the seismic coupling strength between the segments. This difference in coupling may partially explain the disparity in earthquake size distribution. However, the results appear to confirm the relation between earthquake size and fault length, observed along continental strike-slip faults, for this oceanic transform.     

Detection of small earthquakes along the Pacific-Antarctic Ridge from T-waves recorded by abyssal ocean-bottom observatories

A number of seismoacoustic T-wave events were observed between January 2003 and January 2004 by broadband ocean-bottom seismometers installed on the French Polynesia seafloor at depths of 4,000?C5,000?m, well below the conjugate depth of the SOFAR channel. Using T-wave arrival times, we located 89 T-wave events along the Pacific-Antarctic Ridge. Among these, 63 events were not detected by land-based seismic observations of the United States Geological Survey (USGS), which was nearly twice the number of earthquakes reported by the USGS in the area during the observation period. We used a simple method to estimate earthquake magnitude from T-wave energy. The magnitudes of the events unidentified by the USGS ranged from 2.3 to 5.5, whereas magnitudes of the earthquakes reported by the USGS ranged from 4.1 to 6.2. Our study suggests that T-wave observations with abyssal ocean-bottom seismometers can improve the detection of small earthquakes and help our understanding of the seismotectonics of remote oceanic areas.     

Reflection profiling in the deep ocean using a near bottom hydrophone

A 20 km long high resolution seismic reflection profile was carried out approximately 300 km southwest of Bermuda. The data were collected using a small airgun sound source and a single hydrophone receiver towed 100 m above the seafloor at a depth of 5400 m. Comparisons with nearby conventional seismic reflection profiles show the considerable improvement of resolving power provided by this method, particularly of the basement morphology beneath the 700 m thick sediment column. The data were recorded digitally and selected data examples show the enhancement provided by filtering, stacking, source deconvolution and corrections for hydrophone motion. The precise picture of basement topography that results from this data is compared with deep tow bathymetry profiles of the present day mid-Atlantic Ridge flanks, and is seen to be remarkably similar.     

The Kuril Earthquakes and tsunamis of November 15, 2006, and January 13, 2007: Observations,analysis, and numerical modeling

Major earthquakes occurred in the region of the Central Kuril Islands on November 15, 2006 (M _w = 8.3) and January 13, 2007 (M _w = 8.1). These earthquakes generated strong tsunamis recorded throughout the entire Pacific Ocean. The first was the strongest trans-Pacific tsunami of the past 42 years (since the Alaska tsunami in 1964). The high probability of a strong earthquake (M _w ≥ 8.5) and associated destructive tsunami occurring in this region was predicted earlier. The most probable earthquake source region was investigated and possible scenarios for the tsunami generation were modeled. Investigations of the events that occurred on November 15, 2006, and January 13, 2007, enabled us to estimate the validity of the forecast and compare the parameters of the forecasted and observed earthquakes and tsunamis. In this paper, we discuss the concept of “seismic gaps,” which formed the basis for the forecast of these events, and put forward further assumptions about the expected seismic activity in the region. We investigate the efficiency of the tsunami warning services and estimate the statistical parameters for the observed tsunami waves that struck the Far Eastern coast of Russia and Northern Japan. The propagation and transformation of the 2006 and 2007 tsunamis are studied using numerical hydrodynamic modeling. The spatial characteristics of the two events are compared.     

Great Tohoku earthquake of March 11, 2011: Preliminary seismological analysis

The results of a preliminary analysis of the source zone of the 2011 Tohoku earthquake (M _w = 9.1) near the east coast of Honshu, Japan, are considered. We traced the seismic history of the release of the cumulative scalar seismic moment for the last 110 years and temporal variations in the ordering index for the last 35 years. These parameters are important characteristics of a seismotectonic deformation process. The foreshock activation stage and the initial phase of the aftershock process are studied based on these characteristics.     

Megaearthquake of March 11, 2011, in Japan: The event magnitude and the character of the aftershock sequence

The great Japanese earthquake (GJE) of March 11, 2011, was a megaevent. The conditions under which such seismic catastrophes occurred are discussed. The regime of the aftershocks of this megaevent is compared with the data on the aftershock sequences which accompanied the Simushir earthquakes (2006 and 2007) and the Andaman earthquake (2004) and with the seismicity behavior in the generalized vicinity of a strong earthquake. The aftershock sequences of the abovementioned strong earthquakes are shown to represent the sets of trend changes in the postshock activity and specific outbursts of seismic activity. Activity outbursts are characterized not only by an increase in the number and energy of events, but also by a decrease in the recurrence plot slope (b value) and the average earthquake depth. Some such outbursts correspond to the occurrence of strong repeated shocks. A possible mechanism for outbursts of seismic activity is proposed. The possibility of a stronger repeated shock in the vicinity of the megaearthquake of March 11, 2011, is discussed.     

An investigation of tsunami source mechanism off the coast of central Peru

The source mechanism of the tsunami generated by the earthquake of 17 October, 1966 off the coast of central Peru was inferred by studying the seismic and oceanic phenomena associated with this event. The seismic mechanism was deduced from geologic structure, seismic intensities, energy releases, spatial distribution of aftershocks, and fault-plane solutions. Using this information and empirical relationships of seismic parameters, the fault length, azimuthal orientation of the tsunamigenic area, and initial tsunami height, were obtained. From the tsunami arrival times at selected stations and from a reverse wave-refraction technique, the limits of the tsunami-generating area were estimated. Using these source dimensions, an estimate of the tsunami energy was obtained. The spatial distribution of aftershocks associated with the main earthquake and the earthquake strain-release pattern correlated well with known seismotectonic trends and the seismic-velocity structure anomalies which are characteristic of thrust fault systems at continent-ocean boundaries. The investigation revealed that the tsunamigenic area was on the continental shelf off Peru, northwest of Lima, in the western part of an active seismic belt between the Andean Mountain block and the Peru-Chile trench. This area is considered to be one of three distinct seismic zones in the Peruvian upper mantle and has been responsible for a number of tsunamigenic earthquakes within recorded history. The aftershock distribution and strain-release patterns suggest that the earthquake fault was a seaward extension of a fault system which has a pronounced surface expression in the Tertiary formations of the area near Ancon, Peru. The limits of the tectonic displacements and the tsunami-generating area were determined by a reverse wave-refraction method, refracting waves from Chimbote, Callao-Lima, San Juan, and Honolulu. The investigation revealed that the tsunami was generated by displacements of crustal blocks with a total area of 13,000 sq. km. Seismic and water motion data indicated that the uplifted portion of the crustal block was on the continental side of the rift. The energy of the main earthquake was estimated to be 1.122·10²³ ergs. The energy of the aftershocks was estimated to be 2.357·10²⁰ ergs. The tsunami energy was calculated to be 6.8·10¹⁹ ergs, or $\text{1}\text{1,650}$ of the earthquake energy.     

Ridge event detection: T-phase signals from the Juan de Fuca spreading center

An analysis of T-phase source locations determined in the mid-1960s for an area of the northeast Pacific Ocean encompassing the Juan de Fuca spreading center reveals that most of the source locations are associated with regions where seamount chains intersect the spreading center and with edifices both along and near the spreading center. The T-phase source locations also tend to cluster on, or near, areas of the most concentrated and vigorous hydrothermal venting along the Juan de Fuca Ridge. Of the 58 T-phase source locations determined for a period from October 1964 through December 1966, only one was found to be associated with an earthquake detected by the National Geophysical Data Center/National Earthquake Information Service because of the characteristic small magnitude of spreading-center seismic events. Monitoring T-phase activity originating along the 80 000 km-long global seafloor spreading-center system offers a practical and unique opportunity to better understand the dynamics and oceanic effects of episodic spreading-center tectonic, volcanic, and hydrothermal processes.     

Seismic activity beneath the Nankai trough revealed by DONET ocean-bottom observations

We conducted a detailed investigation of seismic activity from January 2011 to February 2013 along the Nankai trough off the Kii Peninsula, central Japan, by using data obtained from the DONET ocean-bottom observation network. The hypocenters are mostly within the subducting Philippine Sea (PHS) plate, although a few are along the plate boundary or in the sedimentary wedge below the Kumano forearc basin. The seismic activity can be separated into events above and below 20 km depth, which corresponds approximately to the Moho. The hypocenter distributions are distinctly different for these groups. The seismic activity in the oceanic crust can be further separated into three clusters. Most of the seismic activity recorded in our data represents aftershocks of the 2004 off the Kii Peninsula earthquakes (M _JMA = 7.1, 7.4, and 6.5), which occurred in the PHS plate. The hypocenter distribution in the oceanic crust correlates well with the location of the Paleo-Zenisu ridge, which is formed by a chain of seamounts that is subducting beneath the forearc basin. The hypocenters in the uppermost mantle are aligned on a plane dipping to the southeast, consistent with the existence of a thrust fault cutting through the lithosphere of the oceanic plate. The focal mechanisms of the earthquakes show that the axis of compressive stress in the PHS plate is oriented N–S, almost perpendicular to the direction of plate convergence, indicating a complex tectonic regime in this region. These results suggest that intraplate shortening may be occurring in the subducting oceanic plate.     

Relocation of two earthquakes in the Southwest Indian Ridge area combining land seismic stations’ with OBSs’ data

Two earthquakes were recorded by 20 ocean bottom seismometers (OBS) deployed in the Southwest Indian Ridge (SWIR) area during a three-dimensional seismic survey in 2010. Their magnitudes (both M _b = 4.4) and hypocenters have been determined by National Earthquake Information Center (NEIC) only using land seismic stations onset times. After the frequency analysis and the band-pass filtering of the OBSs’ data, 7 and 13 P-phase onset times from OBSs were successfully picked for these two events, respectively. Then these two events were relocated by HYPOSAT program with onset times together from OBSs and land seismic stations using different velocity models. These relocation experiments confirm both the importance of adding OBSs’ onset data and the need to apply a local oceanic velocity model for the location of these two events happened on the SWIR. This research has accumulated a wealth of experience for earthquakes observation and research using OBSs in the ocean.     

Model of the 2011 Great East Japan earthquake (M = 9.0)

Intraplate earthquakes are described by a model of a thrust fault in continuous or cracked media. Such a model can also be used to describe interplate earthquakes, in particular, strong earthquakes in subduction zones. However, new seismic, tectonic, and GPS data for this strong Japanese earthquake demand a more detailed model. One possible model can be a model of the elastic island plate coupled with a dipping oceanic plate with submarine mountings. These mountings, sitting on the dipping oceanic plate, hinder its motion due to coupling with asperities on the bottom of the island plate. When coupling ends, the bottom of the plate can be cut as if by a plough and an earthquake can take place. The decoupling of a mountain leads to a weaker interpolate earthquake, a forshock, and an aftershock. The main earthquake is a result of the effect of a basaltic plateau or a large mountain, which leads to the avalanching decoupling of all mountains on a large area of coupled plates. In the first approximation we can consider that, despite its deformation, an oceanic plate is constantly moving with a nearly constant velocity all times both during earthquakes and in between them. An island plate behaves similarly to an elastic plate, which permanently bends due to torque acting on its junction with a dipping oceanic plate. After the earthquake, the bending plate becomes straight. This leads to it thrusting on the oceanic plate with displacement toward the ocean, an uplift of its oceanic part, and the sinking of its island part by the following tsunami.     

Regional long-period magnitude scales and their capabilities for tsunami warning

The tsunami warning system in the Russian Far East employs the medium-period magnitude MS (BB) by Vaniek–Soloviev. However, its use may lead to inadequacies and underestimates for the tsunamigenic potential of an earthquake. Specifically, this can happen in the case of a so-called tsunami–earthquake. This kind of earthquakes with a nonstandard spectrum was revealed by H. Kanamori in 1972. This problem can be overcome by using a magnitude scale that deals with longer period seismic waves. This study develops a technique for determining the magnitudes at regional distances (from 70 to 4500 km) using the amplitudes of surface seismic waves of periods of 40 and 80 s. At distances of 70–250 km, the amplitude of the joint group of shear and surface waves is used. For the new magnitudes designated M _S(40) and M _S(80), experimental calibration curves are constructed using more than 1250 three-component records at 12 stations of the region. The magnitudes are calibrated so as to produce an unbiased estimate of the moment magnitude M _w in the critical range 7.5–8.8. The rms error of the single-station estimate M _w is around 0.27. At distances below 250 km and M _w ≥ 8.3, the estimate of M _w obtained by the proposed technique becomes saturated at the level of M _w ~ 8.3, which is acceptable for operative analysis because no missed alarms arise. The technique can be used in operational tsunami warning based on seismological data. This can markedly decrease the number of false alarms.     

An Ocean Bottom Hydrophone instrument for seismic refraction experiments in the deep ocean

Tests of a new Ocean Bottom Hydrophone (obh) instrument have recently been completed at Woods Hole Oceanographic Institution. This instrument is designed to float 3 m above the seafloor at depths of up to 6100 m for periods of up to 10 days and continuously records the output of a single hydrophone on a four-channel 0.064 cm/s (1/40 in./s) analog magnetic tape recorder. This instrument has an acoustic transponder and release system and is designed primarily for multiple deployments as a fixed ocean bottom receiver for seismic refraction work.Contribution No. 4174 of the Woods Hole Oceanographic Institution.     

A case study: travel time inversion for P-wave velocity using OBS data of South China Sea

It is very important for converting the seismic data from the time domain to the depth domain. Here we discuss the approaches of inverse modeling of travel times for determination of the P-wave velocity (Vp). The migration section of the single channel seismic data is used to define the model horizons and help to control their geometry. Wide angle hydrophone data of OBS are used to determine P-wave travel times. The picked travel times from various shots are inverted for P-wave interval velocities using RayInvr, which calculated theoretical travel times via ray tracing. Damped least squares optimization is performed to fine tune the fits between observed and calculated travel times. In the end, the Vp curve is achieved and the results are compared with that derived from the conventional hyperbolic curve velocity analysis method, the shape of the two curves are similar, and the velocity increases in the layer where gas hydrates are present.     

The Komandor seismic gap: Earthquake prediction and tsunami computation

The “seismic silence” period in the seismic gap in the region of the Komandor Islands (hereinafter, the Komandor seismic gap) is close to the duration of the maximal recurrence interval for the strongest earthquakes of the Aleutian Islands. This indicates the possibility of a strong earthquake occurring here in the nearest time. In the present work, the results of simulation for a tsunami from such an earthquake are presented. The scheme successfully used by the authors for the nearest analog—the 2004 Sumatra-Andaman earthquake—is applied. The magnitude of the supposed earthquake is assumed to be 9.0; the tsunamigenic source is about 650 km long and consists of 9 blocks. The parameters of the tsunami propagation in the Pacific Ocean and the characteristics of the waves on the coasts are computed for several possible scenarios of blocks’ motion. The spectral analysis of the obtained wave characteristics is made and the effects of the wave front interference are found. Simulation has shown that the wave heights at some coastal sites can reach 9 m and, thus, may cause considerable destruction and deaths.     

A real-time observation network of ocean-bottom-seismometers deployed at the Sagami trough subduction zone,central Japan

We installed a real-time operating regional observation network of Ocean-Bottom-Seismometers, connected to an electro-optical fiber communication cable, at the Sagami trough subduction zone, just south of the Tokyo metropolitan area, central Japan. The network, called ETMC, has six seismic observation sites at approximately 20 km spacing. In addition, there are three tsunami observation sites along the ETMC network to monitor the propagation process of tsunamis around the Sagami trough region.The on-line data from the ETMC has been improving the detection capability of smaller-magnitude earthquakes even at areas close to the margin of the trough. The ETMC data analyzing system, which has a function of real-time digital filtering for each seismic channel, can read the arrival times of P- and S-waves precisely, constraining well the automatic on-line hypocenter locations. The network has been providing useful information regarding the bending and downgoing process of the Philippine sea plate at the Sagami trough subduction zone.The pressure sensors of the installed network have a detection capability of tsunami wave trains with an amplitude of less than 1 cm. For example, the sensors recorded the full time history of tsunami wave trains, with mm order resolution, originating from a tsunami earthquake with 5.7 M_W and the tsunami magnitude of 7.5 occurred near Tori Shima (Tori Is.) of the Izu-Bonin Is. arc on September 4, 1996. The maximum amplitude of the tsunami signals on the trough-floor was approximately 1 cm (P-P), in contrast with approximately 20 cm (0-P) at a coastal site on Izu-Oshima, near the trough. Also, the pressure sensors observed tsunamis due to a large tsunami earthquake (7.1 M_W) at the northern New Guinea, on July 17, 1998.     

Retrospective analysis of seismic regime features before the Taiwan earthquakes of 1999 and 2002

The features of the seismic regime before the strongest earthquakes of Taiwan in the late 20th (Chi-Chi on September 21, 1999, M_w = 7.6) and the early 21st century (March 31, 2002, M_w = 7.4) are analyzed. Based on 1990–1999 and 1994–2002 data, respectively, retrospective analysis of three seismic regime parameters are studied: the total annual number of earthquakes N_Σ in the range of M_L = 2.5–5.5 and M_w = 3.0–7.0; the total annual quantity of released seismic energy ΣE, J; and angular coefficient b of earthquake recurrence graphs. Two explicit subperiods are revealed in the course of the seismic regime: quiescence in 1990–1996 before the Chi-Chi earthquake and in 1994–1997 before the March 2002 earthquake; in 1997–1999 and 1998–2002, respectively, seismic activation is observed. Due to the predominance of weak earthquakes during the Chi-Chi earthquake preparation, factor b appeared relatively higher (–1.16 on average); in contrast, before the March 2002 earthquake, due to the occurrence of foreshocks with M_w = 6.8–7.0, the factor b values appeared relatively lower (–0.55 and–0.74 for the quiescence and activation subperiods, respectively). Despite the fundamental difference in the seismotectonic situation between the domains where two mainshocks occurred and significantly difference energy ranges of the initial seismic events, the analysis results are similar for both earthquakes. In both cases, the mainshock occurred at the peak of released energy, which can be considered a coincidence. Solid verification of this positive tendency requires the accumulation of seismological statistics.     

Modeling of 500-year tsunamis for probabilistic design of coastal infrastructure in the Pacific Northwest

This paper describes the development of tsunami scenarios from the National Seismic Hazard Maps for design of coastal infrastructure in the Pacific Northwest. The logic tree of Cascadia earthquakes provides four 500-year rupture configurations at moment magnitude 8.8, 9.0, and 9.2 for development of probabilistic design criteria. A planar fault model describes the rupture configurations and determines the earth surface deformation for tsunami modeling. A case study of four bridge sites at Siletz Bay, Oregon illustrates the challenges in modeling of tsunamis on the Pacific Northwest coast. A nonlinear shallow-water model with a shock-capturing scheme describes tsunami propagation across the northeastern Pacific as well as barrier beach overtopping, bore formation, and detailed flow conditions at Siletz Bay. The results show strong correlation with geological evidence from the six paleotsunamis during the last 2800 years. The proposed approach allows determination of tsunami loads that are consistent with the seismic loads currently in use for design of buildings and structures.     

High resolution velocity analysis of ocean bottom seismometer data by theτ-p method

A method of high resolution seismic velocity analysis for ocean bottom seismometer (OBS) records is applied to the study of the shallow oceanic crust, especially sedimentary and basement layers. This method is based on the direct-p mapping and the-sum inversion. We use data obtained from a 1989 airgun-OBS experiment in the northern Yamato Basin, Japan Sea and derive P- and S-wave velocity functions that can be compared with the seismic reflection profiles. Using split-spread profile records, we obtain interface dips and true interval velocities from the OBS data. These results show good agreement with the reflection profile records, the acoustic velocities of core samples, and sonic log profiles. We also present a method for estimating errors in the derived velocity functions by calculating covariance of the derived layers' thicknesses. The estimated depth errors are about 150 m at shallow depths, which is close to the seismic wavelength used. The high resolution of this method relies on accurate determination of shot positions by GPS, spatially dense seismic observations, and the use of unsaturated reflected waves arriving after the direct water wave that are observed on low-gain component records.     

Characterization of earthquake sources in the Gulf of Alaska

Abstract

The characterization of earthquake sources in the Gulf of Alaska and the relative significance of earthquake sources for establishing seismic design inputs at a typical site for engineering purposes are discussed. Earthquake sources in the complex tectonic environment can be divided into two groups: (a) a subduction zone that underlies the entire region (maximum magnitude M = 8.5); and (b) individual thrust and strike‐slip faults associated with the plate motions (maximum magnitude M = 6 to 7.5). The sources of either group and individual earthquake events can be represented as planar surfaces for consistency with the physical process and a mathematically tractable computational scheme.

Although the area is very active seismically, the degree of activity of individual sources varies significantly. Therefore, even for sources with the same maximum earthquakes, different magnitudes may apply for a selected design return period. The area is considered to be a “seismic gap.”; No great earthquakes have occurred in nearly 80 years. Estimates based on a temporally varying seismic function such as the semi‐Markov model indicate that the probability of occurrence of a great earthquake in the near future is significantly higher than the average probability inferred from a statistical analysis of historical seismicity data of the entire region.

Separate attenuation relationships should be used for calculating ground motions due to earthquakes on the dipping subduction zone in the northern portion of the gulf. The dominant earthquake source for almost the entire Gulf of Alaska region is the subduction zone that contributes over 80 percent of the seismic exposure at a typical site. The dominant magnitude range is M_s = 6.5 to 7.5. “Gap filling”; earthquakes (M_s = 7.5 to 8.25) contribute a little over a third of the seismic exposure at a typical site. Deterministic assessments of ground motion values using the maximum earthquake on the subduction zone at the closest distance yield values significantly higher than those calculated for even 500‐year return periods. Estimated 100‐year return period accelerations in the area range from 180 to 340 cm/sec².