Dariush Motazedian

Region-Specific Key Seismic Parameters for Earthquakes in Northern Iran

Strong-motion accelerograms recorded within northern Iran are used to examine the propagation characteristics of shear wave, including geometric spreading behavior, Q-value, j 0 , and horizontal-to-vertical (H/V) ratio. These region-specific key seismic parameters are estimated from 259 three-component records of 22 earth- quakes with magnitude ranging from M 4.9 to M 7.4 in northern Iran. The geometric spreading follows a trilinear behavior with a strong postcritical reflection from the Moho. The first and second hinges of the trilinear behavior are at 75 and 150 km, respectively. The associated Q-value, based on the vertical component is Q 87 f 1.46 . j0 value for vertical and horizontal components are 0.03 and 0.05, respec- tively. Because of lack of station-specific site information, the H/V ratio is considered to be a rough estimation of generic site amplification. The obtained region-specific parameters are used to estimate the average stress drop based on three stochastic modeling approaches. Stochastic point-source modeling suggests a Brune stress drop of 125 bars, whereas stochastic finite-fault modeling based on static and dynamic corner frequency approaches suggests a stress drop of 68 bars.

Development of an NEHRP map for the Orleans suburb of Ottawa, Ontario

The average shear-wave velocity to a depth of 30 m (Vs30) has been obtained for 73 sites in the Orleans area in the northeast part of the City of Ottawa. Measurements of Vs30 were made using both ground surface reflection and refrac- tion methods. In addition, borehole data was used to estimate Vs versus depth profiles using average Vs values assigned to distinct geological units. High values of Vs (>1500 m/s) were obtained in areas of thin surficial sediments overlying Paleo- zoic bedrock, and low Vs values (<180 m/s) were calculated in areas of thick late-post-glacial clay. The Vs30 values have been used to prepare an NEHRP map for the study area. Much of the suburb of Orleans is classified as NEHRP zone E, whereas the perimeter areas and some isolated central areas are classified as zones ranging from zone D to zone A. The presence of thick unconsolidated late-post-glacial sediments deposited in the Champlain Sea is the main contributing fac- tor to the wide range of average shear-wave velocities in the study area.

Probabilistic Liquefaction Hazard Analysis for Four Canadian Cities

Changes to building codes in the last decade, lowering the probability at which design ground motions for geotechnical applications are defined, have led to an urgent need for a probabilistic approach/tool for liquefaction potential assessment. We propose a consistent approach for probabilistic liquefaction hazard analysis (PLHA) that is based on probabilistic seismic hazard analysis and incorporates a reliability- based liquefaction potential evaluation method based on shear-wave velocity data. The method directly takes the joint probability distribution of peak ground accelera- tion and moment magnitude into account. We demonstrate the method for four Canadian cities, employing our interim updated seismic hazard models for eastern and western Canada. Using the developed method and representative site profiles, PLHA is implemented for four major cities across Canada with the aim of investigating the impact of regional seismic characteristics on liquefaction hazard assessment. Sensitivity analysis indicates that different magnitude ranges of dominant contributing seismic events have significant impact on the extent of liquefaction hazard. More specifically, for a given seismic excitation level, the relatively high hazard contribu- tions from small-to-moderate earthquakes in eastern Canada leads to less significant liquefaction potential, in comparison with similar sites in western Canada.

Seismic Site Response Analysis for Ottawa, Canada: A Comprehensive Study Using Measurements and Numerical Simulations

The surficial geology of the city of Ottawa primarily consists of soft soil sediments with low shear‐wave velocities (![Graphic][1] ) underlain by hard bedrock with very large shear‐wave velocities (![Graphic][2] ). Earthquake recordings show unusually large seismic amplification values for weak motion. These unusually large seismic amplification factors were reconfirmed with the earthquake spectral ratio method using two stations, the horizontal‐to‐vertical earthquake spectral ratio method using a single station, and the horizontal‐to‐vertical spectral ratio technique using background noise. These findings were the motivation for carrying out an extensive site response analysis, using finite element modeling (FEM), as a part of the seismic microzonation studies for the city of Ottawa. The FEM results confirmed the large amplification ratios for weak‐motion recordings. FEM analysis was also carried out using a selection of strong‐motion time series for the study area. The combined effect of the soil–bedrock acoustic impedance contrast and the level of ground shaking on the variation of soil amplification factors for the fundamental frequency were investigated. The maximum value of the soil amplification factor for the fundamental frequency increased with increasing impedance contrast ratios until the soil/bedrock acoustic impedance contrast ratio reached values that were usually greater than 12; however, the change in peak amplification was much less with subsequent increases in the contrast ratio beyond that value. As expected, the value of the soil amplification factors for the fundamental frequency decreased with increasing peak ground acceleration (PGA) of the input motion due to nonlinear soil damping. Finally, for the Ottawa region, a mathematical model is suggested for soil amplification factors at the fundamental frequency, as a function of the soil/bedrock acoustic impedance contrast ratio and the PGA of the input motion. [1]: /embed/inline-graphic-1.gif [2]: /embed/inline-graphic-2.gif

On the Efficiency of the Multi-Channel Analysis of Surface Wave Method for Shallow and Semi-Deep Loose Soil Layers

The multi-channel analysis of surface waves (MASWs) method was used to obtain the shear wave velocity variations through near surface (depth l 30 m) and semi-deep (30 m l depth l 100 m) soil layers in the city of Ottawa, Canada. Sixteen sites were examined to evaluate the capability of the active and passive MASW methods for cases where the shear wave velocity (𝑉𝑠) contrast between very loose soil (𝑉𝑠 l 200 m/s) and very firm bedrock (𝑉𝑠 g 2,300 m/s) is very large. The MASW velocity results compared with those of other geophysical approaches, such as seismic reflection/refraction methods and borehole data, where available, mostly confirming the capability of the MASW method to distinguish the high shear wave velocity contrast in the study area. We have found that, of the inversion procedures of MASW data, the random search inversion technique provides better results than the analytical generalized inversion method.

Earthquake Magnitude Measurements for Puerto Rico

Reliable determination of earthquake magnitude is a fundamental building block of seismic hazard assessment. The seismicity catalog for Puerto Rico is dominated by small earthquakes ( M < 5), mostly M D (a local magnitude based on duration) and m b (body-wave magnitude). There is considerable uncertainty over the interpretation of M D. To reduce this uncertainty, we evaluate moment magnitude ( M ) and M 1 (1-Hz magnitude) for events within the catalog and develop relationships between these and other magnitude measures. The available seismographic data are mostly short-period records, because broadband instruments in Puerto Rico have been installed only recently. A difficulty with the calculation of moment magnitude is that short-period data do not generally extend to sufficiently low frequencies to reliably obtain the displacement spectrum at low frequencies. Moment magnitudes for small earthquakes in Puerto Rico are thus estimated from a single broadband station and subject to much uncertainty. To get around this difficulty, we used M 1, which closely tracks moment magnitude for small to moderate events (Chen and Atkinson, 2002). M 1 is obtained from the spectral amplitude at 1 Hz and is defined such that it will equal moment magnitude for earthquakes following a Brune point-source model. Unlike moment magnitude, M 1 can be determined from short-period seismograms. Our values of M and M 1 are in close agreement with each other for small to moderate earthquakes. There is a systematic difference between M 1 or M and catalog magnitudes m b or M D, with the catalog magnitude exceeding moment magnitude by about 0.4 units on average. It is recommended that M 1 be used as a regional magnitude scale for earthquakes in Puerto Rico, and as an estimate of M for events of M < 5. Online material : List of earthquakes in Puerto Rico from 1993 through 2002.

Ground-Motion Amplitudes for Earthquakes in Puerto Rico

We explore ground-motion amplitudes for earthquakes in Puerto Rico using the Referenced Empirical approach. The technique is based on the use of residual analysis to model discrepancies between ground-motion observations for Puerto Rico and a reference ground-motion prediction equation (GMPE). The refer- ence GMPE is that of Boore and Atkinson (2008) for shallow crustal earthquakes in active tectonic regions (as modified by Atkinson and Boore (2011) to improve the fit for small-to-moderate events). Amplitudes are examined for both shallow (depth < 34 km) and deep (35-200 km) earthquakes in Puerto Rico. Overall, we conclude that ground motions in Puerto Rico are consistent in attenuation rates with those in other active regions, for shallow events. Amplitude levels appear to be lower than those predicted by Boore and Atkinson (2008), but the discrepancy is magnitude dependent, decreasing to a near-zero bias near M 6. We thus postulate that amplitude levels may be considered equivalent for the magnitude-distance range of engineering interest, allowing GMPEs for other active regions to be used for Puerto Rico. Furthermore, deeper events have amplitudes that also appear consistent with typical GMPEs for shallow events, if the greater distance of the events due to their depth is considered.

Crustal Shear-Wave Velocity Models Retrieved from Rayleigh-Wave Dispersion Data in Northeastern North America

On 23 June 2010, a moderate earthquake with Mw 5.2 occurred near the town of Val-des-Bois, Quebec, Canada, ∼60 km northeast of Ottawa, Ontario. The earthquake generated excellent crustal Rayleigh-wave records. We divided the 54 seismic stations that recorded clear Rayleigh-wave trains into 14 groups by station azimuth. In each group, we measured the Rayleigh-wave dispersion data station by station and formed one dispersion data file for the inversion. In this way, we ob- tained 14 crustal velocity models around the epicenter. We compared all 14 models and found that there are low-velocity layers in the top 10 km on the north side of the Ottawa-Bonnechere graben. Based on model similarity, we formed one model for the north side by averaging the north-side models and another model for the south side by averaging the south-side models. The separation of the north-side and south-side mod- els appears to follow the Ottawa-Bonnechere graben. In the top 10 km, the velocities in the south model are obviously slower than those in the north model.

Source Parameter Studies on the 8 January 2017 Mw 6.1 Resolute, Nunavut, Canada, Earthquake

On 8 January 2017, a strong earthquake with Mw 6.1 occurred 8° north of the Arctic Circle and 90 km southeast of Resolute, Nunavut, Canada. Because the epicenter was in a very remote region, the seismic station coverage was not good. As such, the errors in the source parameters determined using conventional procedures were not small. In this article, some results obtained on the source parameters by processing waveform records and modeling procedures are introduced. The teleseismic depth phase sP was used to determine the focal depths of the mainshock and the two principal aftershocks, because the nearest seismic station was at a distance of ∼90 km. The selected mantle Rayleigh-wave records surrounding the epicenter were used to invert for the moment tensor of the mainshock, and the nodal plane corresponding to the rupture plane was selected by analyzing the arrival-time differences between the Sg and Pg or Sn and Pn phases at the three close stations. Based on the focal depth values, it was found that the mainshock and its two principal aftershocks occurred within the lower crust. The fault plane was inferred to strike southeast and dip to the southwest, from the relocated hypocenters of the mainshock and the two principal aftershocks. A procedure using the calculated sP-P time duration to quickly determine focal depth was established. An average crustal model surrounding the epicenter was retrieved using Rayleigh-wave dispersion data.

Some Applications of Near Surface Geophysics to Earthquake Geohazards Investigations: Examples from Eastern Ontario, Canada

The nature of seismic shaking is dependent on source characteristics, travel path and near-surface site conditions. Many years of observations of earthquake damage have indicated that the presence of thick soil is a major contributing factor to the shaking response of structures. As well, seismic-induced changes in soil parameters can lead to other effects such as loss of resistance to shear (liquefaction) and landsliding. In the last several years, many national building codes have recognized the importance of soil effects, including shear strength, damping, amplification and resonance. Many of these insitu geotechnical parameters can now be measured or estimated using modern near-surface geophysical techniques. Indeed, current building codes indicate that the preferred measurement technique for seismic zonation is based on shear wave velocity structure of soil and bedrock. Other active and passive, surface or invasive techniques, also contribute valuable ancillary data leading to assessment of soil strength and liquefaction potential in granular materials.