S41C-1847
New Hypocenter Relocation Results From Volcano-Tectonic Events (1995-2006) at Popocatepetl Volcano, Mexico
Popocatepetl, one of the most active strato-volcanoes in Mexico, started a fumarolic and seismic reactivation in December 1994. New hypocenter relocation results have been calculated for some 1,800 volcano-tectonic (VT) events recorded by the seismic network operating at Popocatepetl during 1995-2006, and previously located by the National Center for Disasters Prevention (CENAPRED). We used two location programs to determine hypocenter relocation. One is a recently developed genetic algorithm program, Disloca, which adjusts the differences in arrival times between the recording seismic stations. The second is HypoDD, which uses the double difference earthquake location algorithm. Disloca allowed evaluation of station corrections, plus location of non-clustered hypocenters, while HypoDD refined the locations of clustered ones. Thus, for a given velocity model, hypocenters of clustered events varied slightly depending on the location program. For both programs, four different crustal velocity models were used, two of which include a low velocity zone (LVZ) below 6 km depth. This LVZ represents the presence of magma, which has been suggested to exist at this depth. The spatial distribution of the relocated hypocenters varies from one model to another, but a carefully considered combination of features common to the four distributions, allows a new characterization of the VT activity at Popocatepetl. The distribution of the relocated hypocenters found in this study differs from that of former investigations at Popocatépetl, and gives new insights into the volcano's structures. Hypocenters occur mainly above 10 km depth, with a horizontal range of about 5 km. Features of the spatial distribution allow a tentative interpretation of several internal volcanic structures. Chief among these are branched dike complexes and different sized zones free of volcano-tectonic events, which are in turn surrounded by zones of magma-rock interaction, as indicated by the presence of relocated hypocenters. These structures are interpreted as magma reservoirs.
S41C-1848
Recent Seismic Activity at the Western end of the Central Mexican Volcanic Belt
Beginning 31 July through 13 August 2006 a series of earthquakes (Mc 3.5-5.9) hit the western end of the Central Mexican Volcanic Belt. The most prominent earthquake (Mc 5.9) occurred on 11 August 2006 at 14:30 local time (19:30 UTC) approximately at 18.32° N, 101°W and 51 km depth. The epicenter was less than 40 km from Huetamo, Michoacan a 41,250 inhabitants city and 60 km from the Infiernillo dam the third largest hydroelectric plant in Mexico. This earthquake was widely felt trough out the region with minor to moderate reported damage. In Mexico City 240 km away from the epicenter the earthquake produced alarm among the population and several buildings were evacuated. First-motion data and regional and teleseismic waveform modeling indicate that the rupture occurred as normal faulting along a fault plane striking roughly east-west. In the present paper a global analysis of the earthquake series is made in an effort to correlate with the known geotectonic features of the region.
S41C-1849
Evidence for Long-period (14-30 Days) and Against Short-period (12-24 Hours) Tidal Modulation of Volcanic Tremor at Arenal Volcano, Costa Rica
Many studies have sought a correlation between the occurrence of earthquakes or volcanic activity and various Earth tide components, which would provide evidence for external tidal modulation of these geophysical phenomena. Several studies of short duration seismic experiments at Arenal Volcano in Costa Rica have found evidence of diurnal and semi-diurnal tidal periodicities in the seismic record. However, studies at other volcanoes, using longer time series, with improved spectral resolution, do not find tidal peaks in the seismic spectrum, but rather solar peaks (at exactly 12 and/or 24 hours), suggesting that the modulation is caused not by tidal stresses, but by weather related parameters - temperature, barometric pressure, rainfall. In contrast, recent studies of nonvolcanic tremor in the subduction zones of Japan and Cascadia do find evidence for tidal modulation of tremor activity with a period of 12.4 hours. Thus, the questions of whether or not earthquakes and volcanoes are triggered by external forces, and if so, whether these forces are related to elastic tides or to weather, are still highly relevant. We examine a continuous, 302-day long recording of ground motion at Arenal Volcano, Costa Rica, for potential solar and lunar periodicities in the volcanic seismicity. No evidence is found for significant energy in the semidiurnal (near 12 hr) or diurnal (near 24 hr) frequency bands, in contrast to previous, lower- resolution studies at Arenal. However, analysis with multi-taper method (MTM) and singular spectrum analysis (SSA) reveals significant low-frequency (f < .005 cycles/hr) energy in the tremor and explosivity series, including 14 and 30-day quasi-periodic components, relative to a red noise hypothesis. We attempt to fit the data to long-period tidal frequencies in order to verify potential tidal modulation of the long-period seismic energy at Arenal.
S41C-1850
Application of a new Structural Joint Inversion Approach to Teleseismic and Gravity Data from Mt.Vesuvius, Italy
A 3-D joint inversion of seismic and gravimetric data is performed to re-investigate the subsurface structure of Mt. Vesuvius (Italy) utilizing an improved joint inversion method. The aim is to derive models of the 3D distribution of velocity and density perturbations that are consistent with both data sets and with local velocity models. Mt. Vesuvius is a strato volcano located within a graben (Campania Plain) formed in Plio-Pleistocene. Campania Plain is bordered by mostly Mesozoic carbonaceous rocks. Mt. Vesuvius is the southernmost and the youngest of a group of Pleistocene volcanoes, three of which (Ischia, Campi Flegrei and Mt. Vesuvius) have erupted in historical times. The most recent eruption of Mt. Vesuvius occurred in 1944 and since then the volcanic activity has been characterized by moderate low magnitude seismicity and low temperature fumaroles at the summit crater. We modified the coupling mechanism between velocity and density models in the JI-3D optimized joint inversion method (Jordan and Achauer, 1999). This method was designed to provide stable and high resolution results and involves iterative optimized parameterization, 3D ray tracing, and the incorporation of a priori information. The coupling of the velocity and density models, vital to the joint inversion, is based on a cross-gradient approach (e.g. Gallardo and Meju, 2004), which has been proven to work very well in a variety of cases involving seismic, magnetic, CSEM, MT and gravity data sets. We implemented the cross-gradient coupling for our 3-D irregular adaptive grid parameterization. In contrast to conventional joint inversion methods this approach encourages structural similarities in the models and does not rely on predefined relationships between velocity and density parameters. As a consequence, the resulting velocity-density relations are not contaminated by a priori assumptions and can be utilized to derive rock physical parameters. We apply this method to data from the TomoVes project (Gasparini et al. 1998), combining seismics and Bouguer gravity and local high resolution velocity models as a priori information. The starting models for the joint inversion are derived by separate inversions of the individual data sets. We show 3D distributions of velocity perturbations and density variations from the joint inversion of teleseismic relative traveltimes and Bouguer anomaly data with the aim of extracting further information about the physical status of the volcano- tectonic system.
S41C-1851
Crust and Upper Mantle Structure of Northern Iran Across Alborz Mountains and Monitoring of Damvand Volcano
We present the preliminary results of investigating the crust and upper mantle structure along a seismic profile across Alborz mountain range in northern Iran. A temporary deployment of 11 broadband seismographs strengthened by existence of four permanent seismic stations in the area during a 7 month experiment, from October 2007 through June 2008, allowed us to register over 190 teleseismic events, adequate for conducting both a receiver function and tomography imaging along a 170 km seismic profile. Also, in this context, six stations of the network by surrounding Mount Damavand, a dormant volcano, 70 km northeast of the metropolitan Tehran, has enabled the first seismic monitoring of the volcano at local distances. The result of the analysis of this data will show whether Damavand, in addition to obvious fumarolic emissions -which has been intensified since May 2007 and continues up the present time-, and many hot water springs, shows signs of volcanic seismic activity as well.
S41C-1852
Discovery of the Torsional Hum of the Earth
Ten years ago, Nawa et al. (1998) showed that the fundamental spheroidal modes of the Earth (0Sl) are continuously excited, even in times devoid of large earthquakes. Meanwhile, numerous studies tried to identify characteristic properties and the source of these background oscillations between 2 and 7 mHz, often called the "Hum of the Earth". All of these studies were based on data from vertical component seismometers and gravimeters with particularly low noise levels. Horizontal component seismic data were not considered, since they are much noisier than the vertical component at periods T > 30 sec. Thus, the horizontal motions from the spheroidal background oscillations could not be observed, and a much more important question remained unanswered: Is there a permanent excitation of toroidal modes ? Despite the difference in noise levels between vertical and horizontal components, we analyzed horizontal component seismic data from the quietest stations and found indications for the continuous excitation of fundamental toroidal modes (0Tl) between 3 and 7 mHz (Kurrle and Widmer-Schnidrig, 2008). We could identify numerous peaks at both fundamental spheroidal and toroidal mode frequencies in seismic noise spectra. Both kinds of modes exhibit similar amplitudes. There is no clear indication of the excitation of overtones. Furthermore, using a method based on the cross correlation of seismograms with their inversely dispersed counterparts (Ekström, 2001), we detected globe circling Love and Rayleigh waves in the data. Because of the duality between fundamental spheroidal modes and Rayleigh waves and between fundamental toroidal modes and Love waves, this finding confirms that the horizontal hum of the Earth is composed of both fundamental spheroidal and toroidal modes. Regardless of whether the spheroidal and the toroidal modes are excited together or not, new theoretical models will be necessary to explain the torsional hum of the Earth, since pressure forces usually invoked for the excitation of the spheroidal hum cannot explain the excitation of toroidal modes at the observed level.
S41C-1853
Typhoon-induced Microseisms Recorded Offshore and Onland Western Pacific Regions
Microseisms usually are generated by continuous forcing of winds and ocean waves in shallow water, with the frequency controlled by the fetch area. Here we study microseisms during extreme weather conditions. In 2006 we deployed the first broadband ocean bottom seismometers (OBSs) offshore Taiwan. The OBSs at 1749 meters and 4726 meters water depths recorded strong microseisms signals (0.02 to 1 Hz passband) when Shan-Shan typhoon passed over the OBSs. This provides a unique opportunity to study how the microseisms wavefield evolves with a moving source. We studied the displacement waveforms recorded by OBSs, F-net and GSN onland stations. The spectrograms show primary peak energy between 0.05 to 0.1 Hz, and the stronger secondary peak appeared between 0.1 and 0.2 Hz. The primary peak has longer duration than the secondary peak. The arrival time of the peak typhoon-induced displacement for each station is consistently earlier than the time when the eye of typhoon gets closest to the station. We also see the microseisms ground motion energy migrating from south to north, and its primary energy frequency does not correlate with fetch areas. In sum, we have recorded strong microseisms motions in deep water, and the frequency band of the strong microseisms may not change with different fetch areas. Besides, the moving typhoon lits up the seismic stations along the track, showing clear evidence of a moving microseism source region.
S41C-1854
Global propagation of cyclone-induced seismic wave from the Atlantic detected by the high-sensitivity accelerometers of Hi-net, Japan
A nationwide seismic network in Japan detected long period microtremors from the northern Atlantic region. It is reported that a cyclone generate ocean swells which excite microtremors. If the microtremors have sufficient intensity, the seismic waves propagate far from the source. Such propagation was sometimes observed at the high-sensitivity accelerometers of Hi-net, NIED. In this study, a migration of the source location with a cyclone is estimated by an array analysis technique, combining broadband seismic data of another array. In the middle of March 2007, anomalous seismic waves were continuously arrived from the north direction in Japan. Such waves were automatically detected by the array analysis of Hi-net data. The automated analysis also shows that the seismic wave is originated far from Japan because the propagation is well approximated to plane waves rather than cylindrical waves. The waves are especially predominant at the period of around 20 s. In addition, from a semblance analysis, apparent velocity is estimated to 3.4--3.6 km/s and 3.8--4.0 km/s in radial and transverse components, respectively. This suggests that the observed waves are composed both of Rayleigh and Love waves. To discuss the more accurate direction and the temporal change, we apply a multiple signal classification (MUSIC) method to the data of high-sensitivity accelerometers. The arrival direction rotated to several degrees clockwise from the azimuth of -15 degrees. In addition, we analyze broadband seismic data of the Graefenberg-array (GRF array) in Germany, and also obtain an evident rotation of the arrival direction from - 40 to -5 degrees. The result of array analysis suggests that the source of seismic wave moves to the north direction at the North Sea and the Norwegian Sea. The location of the source is estimated as the intersections of the expected ray paths from two arrays. To calculate a ray path, we assumed the Rayleigh wave velocity at the period of 35 s. The shooting azimuths of the ray at the arrays are given by the polynomially curves fitted to the result of the MUSIC analysis. These locations are consistent to the path of the cyclone during this period and the swells estimated by WAVEWATCH III. Though cyclones do not excite microtremors directly, such migration has a close relationship to the cyclone through the interaction with ocean waves. As shown in our study, dense broadband seismic networks enable us to track the source of microtremors in global scale.
S41C-1855
Coherence and Phase Relationships of Broadband Ambient Seismic Noise in the Pacific Ocean
The persistence and spectral variation of coherence between the pressure and vertical particle velocity of the broadband ambient seismic noise field in the North Pacific reflects both local and remote climate-induced ocean gravity wave variability. The noise spectrum is forced by four ocean gravity wave mechanisms in separate frequency bands: (a) infragravity (IG) waves [< 0.04] Hz, (b) primary microseisms (PM) [0.04, 0.085] Hz, (c) double frequency (DF) microseisms [0.085, ~6-7] Hz, and (d) acoustic noise from breaking waves [> 7] Hz. The Hawaii-2 Observatory (H2O) and the Ocean Seismic Network Pilot Experiment (OSNPE) data each show a distinct and systematic banding pattern in the coherence between pressure and vertical velocity. This banding generally does not correspond to peaks in either the pressure or velocity spectra, which have been associated with sediment shear wave resonances (Scholte modes), a common feature in ambient noise records. The magnitude and phase of this coherence are useful tools for identifying primary (PM) and double frequency (DF) microseism energy that has been generated at distant coastlines by direct loading in shallow water and wave-wave interaction, respectively. Furthermore it is possible that coherence/phase relationships can be used to distinguish DF microseisms generated near coastlines from DF microseisms generated in the open ocean. Time-domain finite-difference modeling can be used to study the phase coherence for complex models involving Rayleigh waves, pseudo-Rayleigh waves, Scholte (Stoneley) waves and higher order modes (resonances). As an example, for an unsedimented seafloor (just an ocean layer over igneous crust, with suitable velocity gradients in each, over a bandwidth of 7-13Hz), model data show only a single interface wave, the fundamental Scholte mode, for which the vertical velocity is 90 degrees out of phase with the pressure. In contrast, adding a 25m thick soft sediment layer introduces a family of higher order modes with a number of the characteristics that much more closely resemble those observed in ocean bottom data: i) the relative contributions of pressure and particle velocities vary substantially between modes, ii) phase shifts change continuously and gradually through strong and sudden changes in magnitude, and iii) phase shifts of 45 and 135 degrees are not uncommon.
S41C-1856
Investigating the Nature of Short-Period Ambient Seismic Noise in Taiwan
We investigate the nature of ambient seismic noise using the continuous vertical-component ground vibrations recorded from 71 short-period seismograph stations operated by the Central Weather Bureau (CWB) of Taiwan. The impulse response for each station pair is retrieved from cross correlation of one-bit normalized seismic data in daily lengths filtered in the frequency bands of 0.2-3 Hz. The signal-to-noise ratios (SNR) of stacked correlation functions decay with increasing station-to-station distances, probably because the azimuths of noise sources that would generate the constructively-interfered signals decrease for longer path pairs (Harmon et al., 2007). The amplitudes of Rayleigh wave responses between the causal and acausal portions yield both symmetric and asymmetric properties, indicating the regional variability and inhomogeneous directivity of the noise sources across Taiwan. The ratios of the higher to lower SNR values from the positive and negative time lags display a skewed distribution with the coming directions of stronger noises toward the Taiwan Strait in the west. We compare the average ratios of all the correlation pairs for each day with the water wave heights measured from 11 wave stations deployed along the shorelines of Taiwan by the CWB. They exhibit excellent correlations in both the quietscent and noisy periods, suggesting that the short-period ambient seismic noises in Taiwan are primarily excited by waves striking the shallow- water coasts. Moreover, the seismic noise and wave height records show strong peaks at the days when six typhoons hit Taiwan between May and September in 2006.
S41C-1857
Non Conventional Seismic Events Along the Himalayan Arc Detected in the Hi-Climb Dataset
From September 2002 to August 2005, more than 200 broadband seismic stations were operated across the Himalayan arc and the southern Tibetan plateau in the framework of the Hi-Climb project. Here, we take advantage of the high density of stations along the main profile to look for coherent seismic wave arrivals that can not be attributed to ordinary tectonic events. An automatic detection algorithm is applied to the continuous data streams filtered between 1 and 10 Hz, followed by a visual inspection of all detections. We discovered about one hundred coherent signals that cannot be attributed to local, regional or teleseismic earthquakes and which are characterized by emergent arrivals and long durations ranging from one minute to several hours. Most of these non conventional seismic events have a low signal to noise ratio and are thus only observed above 1 Hz in the frequency band where the seismic noise is the lowest. However, a small subset of them are strong enough to be observed in a larger frequency band and show an enhancement of long periods compared to standard earthquakes. Based on the analysis of the relative amplitude measured at each station or, when possible, on the correlation of the low frequency part of the signals, most of these events appear to be located along the High Himalayan range. But, because of their emergent character and the main orientation of the seismic profile, their longitude and depth remain poorly constrained. The origin of these non conventional seismic events is still unsealed but their seismic signature shares several characteristics with non volcanic tremors, glacial earthquakes and/or debris avalanches. All these phenomena may occur along the Himalayan range but were not seismically detected before. Here we discuss the pros and cons for each of these postulated candidates based on the analysis of the recorded waveforms and slip models.
S41C-1858
Regularly Spaced (in time) Seismic Events Originating From Beneath David Glacier in the Transantarctic Mountain Range of Antarctica
Highly regular seismicity associated with the flow of David Glacier in the Transantarctic Mountains of Antarctica has been detected and analyzed. We used data from the Transantarctic Mountain Seismic Experiment (TAMSEIS) network, which consisted of 42 broadband seismometers deployed from November 2000 through December 2003. The seismic events recur regularly (approximately 20 min). Travel time measurements determined that the events originated from the base of David Glacier (approximately 2.3 km deep in this area). Low angle faulting determined from P-wave first motions indicate a fault strike of 185 degrees, which is normal to the flow of David Glacier. The events are likely caused by an asperity beneath David Glacier that regularly releases stress, which is accumulated as David glacier flows over the asperity. The regularity of the events is steady due to the constant and homogenous driving stress of the overlying ice as well as the weakness of the bed. Models of earthquake source regions that include a few asperities within a weak active fault are thought to display this behavior. The regularly recurring ruptures beneath David Glacier provide a field based study of stick slip faulting, on a time scale in which large numbers of events can be recorded within relatively short monitoring period.
S41C-1859
Investigation of Finite Sources through Time Reversal
Under certain conditions time reversal is a promising method to determine earthquake source characteristics without any a-priori information (except the earth model and the data). It consists of injecting flipped-in-time records from seismic stations within the model to create an approximate reverse movie of wave propagation from which the location of the source point and other information might be inferred. In this study, the backward propagation is performed numerically using a spectral element code. We investigate the potential of time reversal to recover finite source characteristics (e.g., size of ruptured area, location of asperities, rupture velocity etc.). We use synthetic data from the SPICE kinematic source inversion blind test initiated to investigate the performance of current kinematic source inversion approaches (http://www.spice- rtn.org/library/valid). The synthetic data set attempts to reproduce the 2000 Tottori earthquake with 33 records close to the fault. We discuss the influence of relaxing the ignorance to prior source information (e.g., origin time, hypocenter, fault location, etc.) on the results of the time reversal process.
S41C-1860
Deconvolution of Teleseismic P-Waves Using the SV Autocorrelation Method with Application to the P-Wave Structure Beneath the Hi-CLIMB Array in Tibet
The analysis of seismic receiver functions has become an effective approach for determining crust and upper mantle structure. In the traditional receiver function method, the vertical component is used to deconvolve the radial component, where the vertical component is assumed to be equivalent to the source wavelet. However, in the approach of Dasgupta and Nowack (2006), the deconvolution of three-component teleseismic P-waves is performed by using the autocorrelation of P to SV scattered waves (SVA method). In this approach, three-component seismic data (in the vertical-radial-transverse frame) is transformed into P- SV-SH frame (Kennett, 1991) with the effects of the free surface also taken into account. The P to SV scattering is assumed to be random and white so that the autocorrelation of the SV component is equivalent to the autocorrelation of the source wavelet. This is similar to the assumption used for predictive deconvolution using P to P scattering in exploration geophysics. A minimum-phase source wavelet is estimated from the autocorrelation of the SV component and this wavelet can be used to deconvolve the teleseismic P-wave (unrotated radial and vertical components). However, the radial and vertical components must be transformed to minimum phase before the deconvolution. This can be done since under conditions typical of a teleseismic incident wave, the P-wave impulse response of receiver-side stratification will be minimum phase (Bostock, 2004). In this way we can get both deconvolved P to P and P to SV scattered components. The approach is first tested with synthetic data in which random velocity fluctuations are added to a 1-D deterministic crust and upper mantle structure. The approach is then applied to deconvolve teleseismic P-wave components (radial and vertical) for data from stations of the Hi-CLIMB seismic array which were deployed across the Himalayan-Tibetan collision zone. The SV-autocorrelation method is used for different earthquake events for each station and then the results of a group of events are stacked. The Moho depths obtained for the PpPmp phase on the deconvolved vertical component are then compared to those obtained from the Ps phases for selected stations beneath the Hi-CLIMB array.
S41C-1861
Improved Sub-Basalt Seismic Imagery. Can Outcrops in East Greenland Suggest a Solution?
The Hold-with-Hope project has the primary aim to investigate the possibility of imaging sedimentary structure beneath plateau basalts. Fieldwork in the Hold-with-Hope region of north-east Greenland revealed uplifted sediments that provided field examples analogous to the volcanic margins of north-west Europe. Mapping of eleven distinct strata, ranging from Carboniferous basement to Paleocene basalts, characterised the region with N-S and NE-SW trending normal faults and beds dipping 20 degrees to the WSW. The entire succession is intruded by primary basaltic dykes, with potential targets for hydrocarbon exploration in the Jurassic and early Cretaceous sandstones. The mapped surfaces were then converted into a 35×15×14 km3 volume sampled on a 20 m grid with rock properties assigned from borehole data drilled in the Faroe Shetland trough. All units were considered homogeneous except the basalt succession, which was determined as a random velocity distribution with distinct correlation lengths in each orthogonal direction. This method, derived from the von- Kàrmàn power spectrum, provided a Gaussian velocity range which was weighted to match velocity distributions from previously drilled basalt sequences. Realistic plateau basalts were generated as a single volume with autocorrelation functions and power spectra that matched the results derived from drilled data. Finally the phase-screen forward modelling technique was used to generate a synthetic seismic reflection data on a scale comparable to real industrial acquisition. The data, acquired with a deep-tow low-frequency source was processed to image the intra- and sub-basalt reflectors. Final stacked sections are presented alongside exploding reflector data, vertical seismic profiles and gravity plots from the same region.
S41C-1862
A Bayesian Approach to Determining and Parameterising Earthquake Focal Mechanisms
We develop a new probabilistic (Bayesian) method for estimating the distribution of focal mechanism parameters based on seismic wave polarity data. We investigate the use of generalised Matrix Fisher distributions for parameterising focal mechanism uncertainties. The advantages of our approach are that it (1) models the data generation process and incorporates observational errors, particularly those arising from imperfectly known earthquake locations; (2) allows exploration of the entire parameter space; (3) leads to natural point estimates of focal mechanism parameters; (4) allows the inclusion of a priori information about the focal mechanism parameters; and (5) that the resulting posterior probability density function (PDF) can be well approximated by generalised Matrix Fisher distributions. We present here the results of our method in two situations. We first consider the case in which the seismic velocity of the region of interest (described by a velocity model) is presumed to be precisely known, with application to seismic data from the Raukumara Peninsula, New Zealand. We then consider the case in which the velocity model is imperfectly known, with application to data from the Kawerau region, New Zealand. We find that our estimated focal mechanism solutions are for the most part consistent with all available polarity data, and correspond closely to solutions obtained using established methods. Further, the generalised Matrix Fisher distributions we examine provide a good fit to our Bayesian posterior PDF of the focal mechanism parameters. Finally, we demonstrate how informative prior distributions on focal mechanism parameters can be incorporated into our model.
S41C-1863
Source Mechanisms of Mine-Related Seismicity, Savuka Mine, South Africa
Seismic monitoring of mine-related activity at Savuka gold mine, South Africa, is routinely carried through a 3D seismic network consisting of 20 three-component, short-period geophones deployed as deep as 3.5 km within the mine. This in-mine network is complemented by a small surface array of 4 broadband stations interspaced ~10 km apart, and by a number of AfricaArray stations located at regional distances (50- 1000 km). Sources of seismic events related to mine activity include mine blasts, pillar collapses, stress fracturing, and faulting events, and accurate understanding of the corresponding source mechanisms is critical for improving our understanding of rockmass response to mining. We report full moment tensor solutions for a selection of over 100 seismic events recorded by the in-mine network with good azimuthal and depth coverage. Our approach has consisted of inverting P-, SV-, and SH-wave spectral amplitudes with visually assigned polarity for the 6 independent moment tensor components through a Singular Value Decomposition (SVD) of the forward problem for a propagating medium of constant wavespeed. Our results reveal that the majority of the inverted focal mechanisms have comparable implosive and deviatoric contributions and best-fitting double couple solutions are of normal fault type, consistent with the stress field expected in a mining environment.
S41C-1864
Comparison of the Cut-and-Paste and Full Moment Tensor Methods for Estimating Earthquake Source Parameters
Earthquake source parameters (seismic moment, focal mechanism and depth) are now routinely reported by various institutions and network operators. These parameters are important for seismotectonic and earthquake ground motion studies as well as calibration of moment magnitude scales and model-based earthquake-explosion discrimination. Source parameters are often estimated from long-period three- component waveforms at regional distances using waveform modeling techniques with Green's functions computed for an average plane-layered models. One widely used method is waveform inversion for the full moment tensor (Dreger and Helmberger, 1993). This method (TDMT) solves for the moment tensor elements by performing a linearized inversion in the time-domain that minimizes the difference between the observed and synthetic waveforms. Errors in the seismic velocity structure inevitably arise due to either differences in the true average plane-layered structure or laterally varying structure. The TDMT method can account for errors in the velocity model by applying a single time shift at each station to the observed waveforms to best match the synthetics. Another method for estimating source parameters is the Cut-and-Paste (CAP) method. This method breaks the three-component regional waveforms into five windows: vertical and radial component Pnl; vertical and radial component Rayleigh wave; and transverse component Love waves. The CAP method performs a grid search over double-couple mechanisms and allows the synthetic waveforms for each phase (Pnl, Rayleigh and Love) to shift in time to account for errors in the Green's functions. Different filtering and weighting of the Pnl segment relative to surface wave segments enhances sensitivity to source parameters, however, some bias may be introduced. This study will compare the TDMT and CAP methods in two different regions in order to better understand the advantages and limitations of each method. Firstly, we will consider the northeastern China/Korean Peninsula region where average plane-layered structure is well known and relatively laterally homogenous. Secondly, we will consider the Middle East where crustal and upper mantle structure is laterally heterogeneous due to recent and ongoing tectonism. If time allows we will investigate the efficacy of each method for retrieving source parameters from synthetic data generated using a three-dimensional model of seismic structure of the Middle East, where phase delays are known to arise from path-dependent structure.
S41C-1865
Seismic moment tensor for anisotropic media: implication for Non-double-couple earthquakes
It is often found that the inversion results of seismic moment tensor from real seismic recorded data show the trace of seismic moment tensor M is not zero, a phenomenon called non-double-couple earthquake sources mechanism. Recently we have derived the analytical expressions of M in transversely isotropic media with the titled axis of symmetry and the results shows even only pure shear-motion of fault can lead to the implosive components determined by several combined anisotropic elastic constants. Many non-double-couple earthquakes from observations often appear in volcanic and geothermal areas (Julian, 1998), where there exist a mount of stress-aligned fluid-saturated parallel vertical micro-cracks identical to transversely isotropic media (Crampin, 2008), this stress-aligned crack will modify the seismic moment tensor. In another word, non-double-couple earthquakes don't mean to have a seismic failure movement perpendicular to the fault plane, while traditional research of seismic moment tensor focus on the case of isotropy, which cannot provide correct interpretation of seismic source mechanism. Reference: Julian, B.R., Miller, A.D. and Foulger, G.R., 1998. Non-double-couple earthquakes,1. Theory, Rev. Geophys., 36, 525¨C549. Crampin,S., Peacock,S., 2008, A review of the current understanding of seismic shear-wave splitting in the Earth's crust and common fallacies in interpretation, wave motion, 45,675-722
S41C-1866
Simulation of vector-wave envelopes in 3-D random elastic media for non-spherical radiation source based on the stochastic ray path method
In high frequency (>1 Hz) seismograms of local earthquakes, the apparent duration is much broader than the source duration expected from the scaling law, and the excitation of the transverse components is apparent for P-waves. These phenomena are caused by scattering of seismic waves in randomly heterogeneous lithosphere. In the case that the wavelength is shorter than the correlation distance of the random media, forward scattering dominates over large angle scattering and conversion scattering becomes negligible. In such conditions, the Markov approximation is very effective to describe wave envelopes near the direct wave arrival. Synthesis of vector-wave envelopes has been precisely studied for an isotropic radiation source. Here, we propose a method to synthesize vector-wave envelopes for a non-spherical radiation source by using the stochastic ray path method, which treats the travel time of ray paths as a stochastic random process by a Monte-Carlo simulation. We imagine an ensemble of random velocity inhomogeneity media, which is characterized by the von-Karman type power spectral density function. We first study the bending process of seismic rays radiated to one direction from a point source at the origin, where the medium is divided into many spherical layers with thickness dr. We introduce the mutual coherence function (MCF) as an ensemble average of cross- correlation of the wavefield at different locations on the transverse plane which is orthogonal to the global ray direction. Neglecting backward scattering and using causality, we can derive the master equation for MCF. The Fourier transform of MCF on the transverse plane gives the angular spectral function (ASF), which is equivalent to the distribution of ray angles. For an increment of small distance dr from a spherical shell to the next spherical shell, each seismic ray is bent due to velocity inhomogeneity following a stochastic random process. The ASF is calculated for the increment dr by using the Monte-Carlo method, where the probability density function of the scattering angle distribution is obtained from the power spectral density function of the random velocity heterogeneity. Calculating the accumulated travel time of each seismic ray from the source to the outermost spherical shell, we obtain the distribution of travel times and location angles measured from the original ray direction. Making convolution of the distribution of location angles with the non-spherical radiation source pattern, and taking the projection of the oscillation directions of ray particles to three component unit base-vectors, we obtain three-component vector-wave envelopes at different azimuths from the source. In the synthesized wave envelopes, we find an excitation of amplitude on the nodal planes for both P- and S- waves, which can not be predicted for the case of homogeneous media. The azimuthal dependence of the vector-wave envelope amplitudes well reflects the source radiation pattern near the direct arrival; however, the azimuthal dependence gradually diminishes as the lapse time increases. The azimuthal dependence diminishes as the frequency increases. These simulations give a positive insight into the observed fact that the later tail of seismograms is rather insensitive to radiation pattern especially for high-frequency seismograms.
S41C-1867
Ultrasonic Modeling for Borehole Studies
Elastic wave propagation in and around wells is of interest in a variety of instances. It is relevant in petroleum and geothermal studies as the seismic signals in carefully designed experiments can be useful to characterize the reservoir rocks and design optimal operation. Numerical modeling of field data is usually the chosen strategy. However, the complexity of realistic settings sometimes is beyond analytical techniques and experiments may be helpful to visualize wave propagation. In this work both experiments and numerical simulations are presented to explore the possibilities of using ultrasonic scaled models. A laser ultrasonic experimental setup is used to study wave propagation around scaled wells. Two simulation techniques, the discrete wave number method and a finite differences, are used to understand the physics, to compute comparisons with the experimental signals, and to explore the advantages of scaled model. The main difference with the real scale is the mechanism of attenuation. Two simple and well-known attenuation models are used at the ultrasonic scale: the Kelvin-Voigt model for the solid and the Maxwell one into the fluid. Both are part of the Zener model usually used in the petroleum industry. According to the results quality, the model matches perfectly with the experience. Scaled models seem to be a new useful way to study elastic wave propagation in complex media. Acknowledgements. Partial support from DGAPA-UNAM, Project IN114706, Mexico is greatly appreciated.
S41C-1868 [WITHDRAWN]
On effective elasticity tensors
We consider the problem of obtaining the effective orthotropic tensor that corresponds to a given generally anisotropic one; by "effective", we mean the closest in the sense of the Euclidean or log-Euclidean distance. It is difficult to find the absolute minimum of the distance function, since the minimization process is nonlinear, exhibiting several local minima. In general, the minimization process must be performed on a three-dimensional manifold SO(3). In the case of monoclinic and transversely isotropic tensors, it can be performed on a two-dimensional sphere, which lends itself to an insightful plot that allows us to guide a numerical scheme. We use the orientation of the natural coordinates of the effective monoclinic and transversely isotropic tensors to guide the numerical scheme and obtain the effective orthotropic tensor — a two-step process.
S41C-1869
Response of 3D Free Rigid Objects under Seismic Excitations
Previous studies of precariously balanced structures in seismically active regions provide important information for aseismatic engineering and theoretical seismology. They are almost founded on an oversimplified assumption: any 3D practical structure with special symmetry could be regarded as a 2D finite object in light of the corresponding symmetry. The complex and troublesome problem of 3D rotation, in mathematics, can be reduced to a tractable one of 1D rotation but a distorted description of the real motion in physics. To gain an actual evolution of precariously balanced structures subjected to various levels of ground accelerations, a 3D investigation should be performed. In this study the responses of a cylinder under a set of half- and full-sine-wave excitations with different frequencies, related to seismic ground motion, are discussed in virtue of some reasonable works from a number of mechanicians. The three resultant second-order ordinary differential equations determine the evolution of 3D rotation following angular momentum balance about the contact point. A computer program for numerical solution of these equations is also developed possibly to simulate the rocking and rolling response of axisymmetric precariously balanced structures under various levels of ground accelerations. It is shown that the 2D and 3D estimates on the minimum overturning acceleration of a cylinder under the same condition are almost consistent except at several frequency bonds, such as 0.5-3Hz, >5.5Hz of circular sine waves.
S41C-1870
New Computational Approach to Nonplanar Elastodynamic Ruptures
We present a new approach to modeling dynamic ruptures on nonplanar faults. A fundamental challenge for modeling rupture dynamics on complicated fault networks using current techniques is dealing with the computational mesh. Generation of a mesh that is both faithful to the underlying fault structure and suitable for efficient computation is an open problem. Here, we test the possibility of using an extended finite element method, XFEM, (e.g. [Dolbow, Moes, and Belytschko, 2001]) for problems of repeated rupture. This method is mesh-independent -- the fault need not lie on mesh edges -- drastically reducing the requirements for suitable computational meshes. We extend the method to include elastodynamics, and demonstrate the feasibility of XFEM by modeling long-time series of ruptures on complicated, two-dimensional fault networks. While the problems and geometries we solve are feasible with existing methods, this demonstration indicates that XFEM should prove useful for the solution of problems limited by mesh generation. Using XFEM, sequences of elastodynamic earthquake events on networks of faults, including branching, are generated. Efficient solution via XFEM enables the study of statistics of populations of events and the effects of variation of geometry. As varying geometry is handled easily by the method, we study the resulting variation in event populations. Distributions of event rupture length, magnitude, epicenter location, and other statistical measures are presented and compared as a function of geometry. Results for flat fault are shown to be consistent with previous results on flat faults using other computational approaches. New results for complicated geometries are presented, and compared with those for flat faults.
S41C-1871
Multivariate Statistics Applied to Seismic Phase Picking
The initial effort of the Seismogram Picking Error from Analyst Review (SPEAR) project has been to establish
a common set of seismograms to be picked by the seismological community. Currently we have 13 analysts
from 4 institutions that have provided picks on the set of 26 seismograms. In comparing the picks thus far, we
have identified consistent biases between picks from different institutions; effects of the experience of
analysts; and the impact of signal-to-noise on picks. The institutional bias in picks brings up the important
concern that picks will not be the same between different catalogs. This difference means less precision and
accuracy when combing picks from multiple institutions. We also note that depending on the experience level
of the analyst making picks for a catalog the error could fluctuate dramatically. However, the experience level
is based off of number of years in picking seismograms and this may not be an appropriate criterion for
determining an analyst's precision. The common data set of seismograms provides a means to test an
analyst's level of precision and biases. The analyst is also limited by the quality of the signal and we show
that the signal-to-noise ratio and pick error are correlated to the location, size and distance of the event. This
makes the standard estimate of picking error based on SNR more complex because additional constraints
are needed to accurately constrain the measurement error. We propose to extend the current measurement
of error by adding the additional constraints of institutional bias and event characteristics to the standard
SNR measurement. We use multivariate statistics to model the data and provide constraints to accurately
assess earthquake location and measurement errors.
http://www.geo.utep.edu/pub/SPEAR/SPEAR.html
S41C-1872
Improvements of the Simplex Optimum Method for Seismic Location
Precise location of seismic hypocenters continues to be a significant challenge in seismology. Because the focal depths are usually much smaller than the epicentral distance, we improved the simplex optimum method for seismic location from the commonly-used tetrahedral optimum to triangular optimum. Tests using synthetic data of various noise-levels and real data suggest that the triangular optimum approach is more effective than the tetrahedral optimum approach, as reflected by the higher accuracy and the capability of automatically identifying seismic records with unreasonably large errors in arrival time picks. For instance, for a seismic network with 6 stations evenly distributed on a circle and one at the center of the circle with a radius of 25 km, tests using noise-free synthetic data show that the maximum error of the epicenter location for events inside the network is as small as 0.046 km, and the located depths and origin times are almost perfectly accurate; and for events that are up to 150 km from the center of the network, the maximum errors for depth, origin time and epicenter location are 7 km, 0.6 s and 0.89 km, respectively. When arrival times have uniformly distributed random errors of (-0.1 s, 0.1 s), for events inside the network, the maximum errors for depth, origin time and epicenter location are 2 km, 0.3 s and 1.345 km, respectively; and for the location of earthquakes up to 150 km from the center of the network, the maximum errors for depth, origin time and epicenter location are 8 km, 2.3 s and 10.106 km, respectively. Results from other network sizes and noise- levels and the comparisons with commonly-used event location methods will be presented.