Mark Meremonte

Site response for Seattle and source parameters of earthquakes in the Puget Sound Region

We analyzed seismograms from 21 earthquakes ( ML 2.0-4.9) recorded by digital seismographs we deployed in urban Seattle to determine site response and earthquake stress drops. The seismometers were situated on a wide variety of geologic units, including artificial fill (e.g., Kingdome, Harbor Island), Pleistocene age soils (glacial till and outwash deposits of Seattles hills), modified land (downtown Seattle, Space Needle), and Tertiary sedimentary rock. Two mainshock-aftershock sequences were recorded: the June 1997 Bremerton sequence (mainshock ML 4.9) and the February 1997 South Seattle sequence (mainshock ML 3.5), along with other events in the Puget Sound region. We developed a new inversion procedure to estimate site response, source corner frequencies, and seismic moments from the S -wave spectra. This inversion uses corner frequencies determined from spectral ratios of mainshock-aftershock pairs as constraints. The site responses found from the inversion are not relative to the rock site but are relative to an idealized site with a flat frequency response. The response of the rock site is also found from the inversion. The inversion results show high response for the sites on artificial fill, more moderate amplification for most sites on stiff Pleistocene soils or modified land, and low response for the rock site. Some sites display resonances, such as a strong 2-Hz resonance at our site near the Kingdome, which is caused by the surficial layers of fill and younger alluvium. The sites in West Seattle exhibit high amplification, even though they are on relatively stiff soils of glacial outwash. This may be partly caused by basin surface waves produced by conversion of incident S waves. This high response in West Seattle is consistent with damage reports from the 1949 ( mb 7.1) and 1965 ( mb 6.5) earthquakes. Stress-drop estimates for the events we recorded were generally low, between 0.4 and 25 bars, although some of the events may have had higher stress drops that could not be resolved because of the limited passband. We calculated a stress drop of 24 bars for the Bremerton mainshock and 10 bars for the South Seattle mainshock.

Frequency‐Dependent Seismic Attenuation within the Hispaniola Island Region of the Caribbean Sea

Wedetermine frequency-dependentattenuation1=Qffor theHispanio- la region using direct S and Lg waves over five distinct passbands from 0.5 to 16 Hz. Data consist of 832 high-quality vertical and horizontal component waveforms recordedonshort-period andbroadbandseismometersfromthedevastating12January 2010 M 7.0 Haiti earthquake and the rich sequence of aftershocks. For the distance range 250-700 km, we estimate an average frequency-dependent Qf �� 224�� 27� f 0:64�� 0:073� using horizontal components of motion and note that Qf� estimated with Lg at regional distances is very consistent across vertical and horizontal components. We also determine a Qf �� 142�� 21� f 0:71�� 0:11� for direct S waves at local distances, ≤100 km. The strong attenuation observed on both vertical and horizontal components of motion is consistent with expectations for a tectonically active region. Online Material: Figures of filtered and broadband data, Lg- and S-wave ampli- tudes, and apparent frequency-dependent Q, and tables of earthquake and station parameters.

Correlation of 1- to 10-Hz earthquake resonances with surface measurements of S-wave reflections and refractions in the upper 50 m

Resonances observed in earthquake seismograms recorded in Seattle, Washington, the central United States and Sherman Oaks, California, are correlated with each sites respective near-surface seismic velocity profile and reflectivity determined from shallow seismic-reflection/refraction surveys. In all of these cases the resonance accounts for the highest amplitude shaking at the site above 1 Hz. These results show that imaging near-surface reflections from the ground surface can locate impedance structures that are important contributors to earthquake ground shaking. A high-amplitude S -wave reflection, recorded 250-m northeast and 300-m east of the Seattle Kingdome earthquake-recording station, with a two-way travel time of about 0.23 to 0.27 sec (about 18- to 22-m depth) marks the boundary between overlying alluvium ( V S < 180 m/sec) and a higher velocity material ( V S about 400 m/sec). This reflector probably causes a strong 2-Hz resonance that is observed in the earthquake data for the site near the Kingdome. In the central United States, S -wave reflections from a high-impedance boundary (an S -wave velocity increase from about 200 m/sec to 2000 m/sec) at about 40-m depth corresponds to a strong fundamental resonance at about 1.5 Hz. In Sherman Oaks, strong resonances at about 1.0 and 4 Hz are consistently observed on earthquake seismograms. A strong S -wave reflector at about 40-m depth may cause the 1.0 Hz resonance. The 4.0-Hz resonance is possibly explained by constructive interference between the first overtone of the 1.0-Hz resonance and a 3.25- to 3.9-Hz resonance calculated from an areally consistent impedance boundary at about 10-m depth as determined by S -wave refraction data.

Crustal Structure and Fault Geometry of the 2010 Haiti Earthquake from Temporary Seismometer Deployments

Haiti has been the locus of a number of large and damaging historical earthquakes. The recent 12 January 2010 Mw 7.0 earthquake affected cities that were largely unprepared, which resulted in tremendous losses. It was initially assumed that the earthquake ruptured the Enriquillo Plantain Garden fault (EPGF), a major active structure in southern Haiti, known from geodetic measurements and its geomorphic expression to be capable of producing M 7 or larger earthquakes. Global Positioning Systems (GPS) and Interferometric Synthetic Aperture Radar (InSAR) data, however, showed that the event ruptured a previously unmapped fault, the Leogâne fault, a north‐dipping oblique transpressional fault located immediately north of the EPGF. Following the earthquake, several groups installed temporary seismic stations to record aftershocks, including ocean‐bottom seismometers on either side of the EPGF. We use data from the complete set of stations deployed after the event, on land and offshore, to relocate all aftershocks from 10 February to 24 June 2010, determine a 1D regional crustal velocity model, and calculate focal mechanisms. The aftershock locations from the combined dataset clearly delineate the Leogâne fault, with a geometry close to that inferred from geodetic data. Its strike and dip closely agree with the global centroid moment tensor solution of the mainshock but with a steeper dip than inferred from previous finite fault inversions. The aftershocks also delineate a structure with shallower southward dip offshore and to the west of the rupture zone, which could indicate triggered seismicity on the offshore Trois Baies reverse fault. We use first‐motion focal mechanisms to clarify the relationship of the fault geometry to the triggered aftershocks.

Site Effects in Avcilar, West of Istanbul, Turkey, from Strong- and Weak-Motion Data

Approximately 1000 people were killed in the collapse of buildings in Istanbul, Turkey, during the 17 August 1999 Izmit earthquake, whose epicenter was roughly 90 km east of the city. Most of the fatalities and damage occurred in the suburb of Avcilar that is 20 km further west of the epicenter than the city proper. To investigate this pattern of damage, the U.S. Geological Survey, in cooperation with the Kandilli Observatory & Earthquake Research Institute (KOERI), deployed portable digital seismographs at seven free-field sites in western Istanbul, to record aftershocks during the period from 24 August to 2 September. The primary objective of this deployment was to study the site effects by comparing the aftershock ground motions recorded at sites inside and outside the damaged area, and to correlate site effects with the distribution of the damaged buildings. In addition to using weak-motion data, mainshock and aftershock acceleration records from the KOERI permanent strong-motion array were also used in estimating the site effects. Site effects were estimated using S waves from both types of records. For the weak-motion data set, 22 events were selected according to the criteria of signal-to-noise ratio (S/N ratio) and the number of stations recording the same event. The magnitudes of these events ranged from 3.0 to 5.2. The acceleration data set consisted of 12 events with magnitudes ranging from 4.3 to 5.8 and included two mainshock events. Results show that the amplifying frequency band is, in general, less than 4 Hz, and the physical properties of the geologic materials are capable of amplifying the motions by a factor of 5-10. In this frequency band, there is a good agreement among the spectral ratios obtained from the two mainshocks and their aftershocks. The damage pattern for the 17 August Izmit earthquake is determined by several factors. However, our study suggests that the site effects in Avcilar played an important role in contributing to the damage. Manuscript received 30 August 2000.

Surface seismic refraction/reflection measurement determinations of potential site resonances and the areal uniformity of NEHRP site class D in Memphis, Tennessee

We determined S-wave velocities (Vs) to about 40-m depth at 65 locations in the Memphis-Shelby County, Tennessee, area. The Vs measurements were made using high-resolution seismic refraction and reflection methods on the ground surface. We find a clear difference in the Vs profiles between sites located on the Mississippi River flood plain and those located to the east, mostly covered by loess, in the urban areas of Memphis. The average Vs to 30-m depth at 19 sites on the modern Mississippi River floodplain averages 197 m/s (±15 m/s) and places 17 of these sites at the low end of NEHRP soil profile category type D (average Vs 180-360 m/s). The two remaining sites are type E. Vs to 30-m depth at 46 sites in the urban areas east of the modern floodplain are more variable and generally higher than the floodplain sites, averaging about 262 m/s (±45 m/s), still within category D. We often observed the base of the loess as a prominent S-wave reflection and as an increase in Vs to about 500 m/s. Based on the two-way travel time of this reflection, during an earthquake the impedance boundary at the loess base may generate resonances in the 3- to 6-Hz range over many areas of Memphis. Amplitude spectra from four local earthquakes recorded at one site located on loess indicate consistent resonance peaks in the 4.5- to 6.5-Hz range.

Ground motion in the presence of complex topography

To study the influence of topography on ground motion, eight seismic recorders were deployed for a period of one year over Poverty Ridge on the east side of the San Francisco Bay Area, California. This location is desirable because of its proximity to local earthquake sources and the significant topographic relief of the array (439 m). Topographic amplification is evaluated as a function offrequency using a variety of methods, including reference-site-based spectral ratios and single-station horizontal-to-vertical spectral ratios using both shear waves from earthquakes and ambient noise. Field observations are compared with the predicted ground motion from an accurate digital model of the topography and a 3D local velocity model. Amplification factors from the theoretical calculations are consistent with observa- tions. The fundamental resonance of the ridge is prominently observed in the spectra of data and synthetics; however, higher-frequency peaks are also seen primarily for sources in line with the major axis of the ridge, perhaps indicating higher resonant modes. Excitations of lateral ribs off of the main ridge are also seen at frequencies consistent with their dimensions. The favored directions of resonance are shown to be transverse to the major axes of the topographic features.

Frequency‐Dependent Seismic Attenuation within the Hispaniola Island Region of the Caribbean SeaFrequency‐Dependent Seismic Attenuation within the Hispaniola Island Region of the Caribbean Sea

Wedetermine frequency-dependentattenuation1=Qffor theHispanio- la region using direct S and Lg waves over five distinct passbands from 0.5 to 16 Hz. Data consist of 832 high-quality vertical and horizontal component waveforms recordedonshort-period andbroadbandseismometersfromthedevastating12January 2010 M 7.0 Haiti earthquake and the rich sequence of aftershocks. For the distance range 250-700 km, we estimate an average frequency-dependent Qf �� 224�� 27� f 0:64�� 0:073� using horizontal components of motion and note that Qf� estimated with Lg at regional distances is very consistent across vertical and horizontal components. We also determine a Qf �� 142�� 21� f 0:71�� 0:11� for direct S waves at local distances, ≤100 km. The strong attenuation observed on both vertical and horizontal components of motion is consistent with expectations for a tectonically active region. Online Material: Figures of filtered and broadband data, Lg- and S-wave ampli- tudes, and apparent frequency-dependent Q, and tables of earthquake and station parameters.