Pacific Northwest Seismic Network Research Articles

Recent Expansion of the Cascades Volcano Observatory Geophysical Network at Mount Rainier for Improved Volcano and Lahar Monitoring

Abstract The U.S. Geological Survey Cascades Volcano Observatory (CVO) recently expanded its continuous geophysical monitoring at Mount Rainier, an active stratovolcano in Washington state. CVO monitors volcanoes in Oregon, Washington, and Idaho to characterize volcanic systems and detect unrest. Mount Rainier has a history of large lahar occurrences in the Holocene, including at least one that may not have been associated with volcanic activity. Pierce County, Washington, is one of the areas most at risk from large lahars. In the 1990s, CVO collaborated with Pierce County to install the Rainier lahar detection system (RLDS), an automated system designed to detect large lahars in high-risk drainages and mitigate hazards to heavily populated areas. The system was designed to detect lahars within 5–10 min of their occurrence and alert authorities of the need to evacuate populated low-lying areas before lahar arrival. In addition, CVO and the Pacific Northwest Seismic Network (PNSN) maintained and expanded a network of seismic and geodetic monitoring stations on and near the edifice to provide adequate volcano monitoring capabilities. Since 2016, CVO has worked to upgrade the existing RLDS and to expand its capabilities into other drainages around Mount Rainier. This expansion includes installation of 25 new broadband seismic stations with many including infrasound along high-risk drainages, as well as support for equipment upgrades at existing PNSN and CVO volcano monitoring sites. All stations transmit continuous, near-real-time data with dramatically improved spatial coverage for volcano monitoring and lahar hazard mitigation compared to the previous system.

Curated Pacific Northwest AI-ready Seismic Dataset

The curation of seismic datasets is the cornerstone of seismological research and the starting point of machine-learning applications in seismology. We present a 21-year-long AI-ready dataset of diverse seismic event parameters, instrumentation metadata, and waveforms, as curated by the Pacific Northwest Seismic Network and ourselves. The dataset contains about 190,000 three-component (3C) waveform traces from more than 65,000 earthquake and explosion events, and about 9,200 waveforms from 5,600 exotic events. The magnitude of the events ranges from 0 to 6.4, while the biggest one is 20 December 2022 M6.4 Ferndale Earthquake. We include waveforms from high-gain (EH, BH, and HH channels) and strong-motion (EN channels) seismometers and resample to 100 Hz. We describe the earthquake catalog and the temporal evolution of the data attributes (e.g., event magnitude type, channel type, waveform polarity, and signal-tonoise ratio, phase picks) as the network earthquake monitoring system evolved through time. We propose this AI-ready dataset as a new open-source benchmark dataset.

Cataloging Tectonic Tremor Energy Radiation in the Cascadia Subduction Zone

AbstractFor the past ∼12 years the Pacific Northwest Seismic Network has been automatically detecting and locating tectonic tremor across the Cascadia subduction zone, resulting in a catalog of more than 500,000 tremor epicenters to date, which has served as a valuable resource for tremor and slip research. This manuscript presents an updated methodology for routine tremor detection in Cascadia and a new catalog of over 180,000 tremor epicenters including amplitudes detected along the subduction zone margin from 2017 to 2021. The events are detected via cross‐correlation of continuous vertical envelope data of 128 stations from northern California to northern Vancouver Island. The modified approach results in less scatter and a 55% increase in detected epicenters than previously observed, as well as a newly identified tremor source offset updip from the main tremor and slip region at the southern edge of the subduction zone. Radiated seismic energy in the 1.5–5 Hz band is used to assign epicenters an energy magnitude (MeL), which is calibrated to the ML of local earthquakes. Southern Cascadia is most active, but the highest tremor energy rates occur in northern Cascadia. Tremor in central Cascadia is systematically weaker and less frequent. Individual epicenter magnitudes range from ∼0.5–2 and spatiotemporally cluster into 1,060 swarms with cumulative MeL ranging from ∼0.8 to 3.7. The swarms reflect underlying slow slip events and occur with an earthquake‐like energy distribution with a b value ∼1. Tremor epicenters, however, follow a tapered Gutenberg‐Richter distribution with high b values, suggesting individual tremor bursts and their constituent low‐frequency earthquakes are fault‐dimension limited.

An Earthquake Early Warning System for Southwestern British Columbia

Southwestern British Columbia (BC) is exposed to the highest seismic hazard in Canada. Ocean Networks Canada (ONC) has developed an Earthquake Early Warning (EEW) system for the region. The system successfully utilizes offshore cabled seismic instruments in addition to land-based seismic sensors and integrates displacement data from Global Navigation Satellite Systems (GNSS). The seismic and geodetic data are processed in real-time onsite at 40 different stations along the coast of BC. The processing utilizes P-wave and S-wave detection algorithms for epicentre calculations as well as incorporation of geodetic and seismic displacement data into a Kalman filter to provide magnitude estimates. The system is currently in its commissioning phase and has successfully detected over 60 earthquakes since being deployed in October 2018. To increase the coverage of the EEW system, we are in the process of incorporating detection parameters from neighbouring networks (e.g., the Pacific Northwest Seismic Network (PNSN)) to provide additional information for future event notifications.

The Productivity of Cascadia Aftershock Sequences

ABSTRACTThis study addresses questions about the productivity of Cascadia mainshock–aftershock sequences using earthquake catalogs produced by the Geological Survey of Canada and the Pacific Northwest Seismic Network. Questions concern the likelihood that future moderate to large intermediate depth intraslab earthquakes in Cascadia would have as few detectable aftershocks as those documented since 1949. More broadly, for Cascadia, we consider if aftershock productivities vary spatially, if they are outliers among global subduction zones, and if they are consistent with a physical model in which aftershocks are clock-advanced versions of tectonically driven background seismicity. A practical motivation for this study is to assess the likely accuracy of aftershock forecasts based on productivities derived from global data that are now being issued routinely by the U.S. Geological Survey. For this reason, we estimated productivity following the identical procedures used in those forecasts and described in Page et al. (2016). Results indicate that in Cascadia we can say that the next intermediate depth intraslab earthquake will likely have just a few detectable aftershocks and that aftershock productivity appears to be an outlier among global subduction zones, with rates that on average are lower by more than half, except for mainshocks in the upper plate. Our results are consistent with a clock-advance model; productivities may be related to the proximity of mainshocks to a population of seismogenic fault patches and correlate with background seismicity rates. The latter and a clear correlation between productivities with mainshock depth indicate that both factors may have predictive value for aftershock forecasting.

The Productivity of Cascadia Aftershock Sequences

ABSTRACT This study addresses questions about the productivity of Cascadia mainshock–aftershock sequences using earthquake catalogs produced by the Geological Survey of Canada and the Pacific Northwest Seismic Network. Questions concern the likelihood that future moderate to large intermediate depth intraslab earthquakes in Cascadia would have as few detectable aftershocks as those documented since 1949. More broadly, for Cascadia, we consider if aftershock productivities vary spatially, if they are outliers among global subduction zones, and if they are consistent with a physical model in which aftershocks are clock-advanced versions of tectonically driven background seismicity. A practical motivation for this study is to assess the likely accuracy of aftershock forecasts based on productivities derived from global data that are now being issued routinely by the U.S. Geological Survey. For this reason, we estimated productivity following the identical procedures used in those forecasts and described in Page et al. (2016). Results indicate that in Cascadia we can say that the next intermediate depth intraslab earthquake will likely have just a few detectable aftershocks and that aftershock productivity appears to be an outlier among global subduction zones, with rates that on average are lower by more than half, except for mainshocks in the upper plate. Our results are consistent with a clock-advance model; productivities may be related to the proximity of mainshocks to a population of seismogenic fault patches and correlate with background seismicity rates. The latter and a clear correlation between productivities with mainshock depth indicate that both factors may have predictive value for aftershock forecasting.

Local earthquake tomography of the Central Oregon forearc using a large-N, short duration, nodal array

The crustal structure of the overriding plate in a subduction zone can have significant effects on subduction processes and therefore hazard along the margin. To investigate the controls of the overriding North American crust on the Cascadia Subduction Zone, a dense, trench perpendicular line of nodal seismometers was deployed in the Central Oregon forearc. The location of this deployment was chosen to fulfill two main goals (1) to examine the utility of large-N, short duration, nodal deployments in passive source seismic imaging (2) and to gain a better understanding of the structure of the Central Oregon forearc. With ∼40 days of continuous data recorded on 174, 3-component, 5 Hz instruments in a 130 km line with 500 m station spacing, 116 earthquakes and explosions are detected and located, a ∼3.5-fold increase from the number reported by the regional Pacific Northwest Seismic Network (PNSN) catalog. P, S, and PmP travel times from earthquakes located near the line of stations are used in a local earthquake tomography study to model Vp/Vs in the upper/middle crust and Vp throughout the crust of central Oregon down to 40 km depth. In conjunction, these models show a clear compositional discontinuity between the upper and lower Siletzia Terrane at ∼10 km depth as well as a zone of low Vp (6.5–6.8 km/s) in the lower crust that is attributed to slab derived fluids. This region of low Vp is juxtaposed with high Vp (6.9–7.2 km/s) to the east, interpreted as a strong, unfractured, block of Siletzia. High pore fluid pressures in the lower crust have been imaged to varying degrees across the Cascadia subduction zone. These features may be related to both the occurrence of tremor near the plate interface and the reduction of upper plate crustal earthquakes.

Basin Amplification Effects in the Puget Lowland, Washington, from Strong-Motion Recordings and 3D Simulations

ABSTRACTSedimentary basins in the Puget Sound region, Washington State, increase ground-motion intensity and duration of shaking during local earthquakes. We analyze Pacific Northwest Seismic Network and U.S. Geological Survey strong-motion recordings of five local earthquakes (M 3.9–6.8), including the 2001 Nisqually earthquake, to characterize sedimentary basin effects within the Seattle and Tacoma basins. We observe basin-edge generated surface waves at sites within the Seattle basin for most ray paths that cross the Seattle fault zone. We also note previously undocumented basin-edge surface waves in the Tacoma basin during one of the local earthquakes. To place quantitative constraints on basin amplification, we determine amplification factors by computing the spectral ratios of inside-basin sites to outside-basin sites at 1, 2, 3, and 5 s periods. Ground shaking is amplified in the Seattle basin for all the earthquakes analyzed and for a subset of events in the Tacoma basin. We find that the largest amplification factors in the Seattle basin are produced by a shallow earthquake located to the southwest of the basin. Our observation suggests that future shallow crustal and megathrust earthquakes rupturing west of the Puget Lowland will produce greater amplification within the Seattle basin than has been seen for intraslab events. We also perform ground-motion simulations using a finite-difference method to validate a 3D Cascadia velocity model (CVM) by comparing properties of observed and synthetic waveforms up to a frequency of 1 Hz. Basin-edge effects are well reproduced in the Seattle basin, but are less well resolved in the Tacoma basin. Continued study of basin effects in the Tacoma basin would improve the CVM.

Open-Source ANSS Quake Monitoring System Software

AbstractANSS stands for the Advanced National Seismic System of the U.S.A., and ANSS Quake Monitoring System (AQMS) is the earthquake management system (EMS) that most of its member regional seismic networks (RSNs) use. AQMS is based on Earthworm, but instead of storing files on disk, it uses a relational database with replication capability to store pick, amplitude, waveform, and event parameters. The replicated database and other features of AQMS make it a fully redundant system. A graphical user interface written in Java, Jiggle, is used to review automatically generated picks and event solutions, relocate events, and recalculate magnitudes. Add-on mechanisms to produce various postearthquake products such as ShakeMaps and focal mechanisms are available as well. It provides a configurable automatic alarming and notification system. The Pacific Northwest Seismic Network, one of the Tier 1 ANSS RSNs, has modified AQMS to be compatible with a freely available, capable, open-source database system, PostgreSQL, and is running this version successfully in production. The AQMS Software Working Group has moved the software from a subversion repository server hosted at the California Institute of Technology to a public repository at gitlab.com. The drawback of AQMS as a whole is that it is complex to fully configure and comprehend. Nevertheless, the fact that it is very capable, documented, and now free to use, might make it an attractive EMS choice for many seismic networks.

Peak Tremor Rates Lead Peak Slip Rates During Propagation of Two Large Slow Earthquakes in Cascadia

AbstractWe explore the evolution of slow slip on the Cascadia megathrust during two large episodic tremor and slip events and compare stress changes to the spatial evolution of tremor from Pacific Northwest Seismic Network tremor locations. We used displacement time series from ~72 GPS stations, along with the Extended Network Inversion Filter to solve for the time‐dependent fault slip. The 2010 (Mw 6.8) and 2012 (Mw 6.8) events propagated northward and southward, respectively, allowing us to assess directional effects on slip behavior. We observed that tremor occurs on the leading edge of propagating slipping regions, well ahead of the highest slip rates, independent of the along‐strike propagation direction. Resolution tests using the actual tremor distributions to generate synthetic data show that our result of peak tremor rates leading peak slip rates is not due to biases introduced by temporal smoothing. Calculated stress changes due to the time‐dependent slip distributions imply that tremor is sensitive to kilopascals of stress, consistent with studies of tidally triggered tremor. Within the resolution of our model, our results are consistent with the hypothesis that significant tremor is triggered by stresses ahead of the highest slip rates. We also observe ongoing slip continuing several days after tremor has passed. Our observations are consistent with some numerical models of tremor patches that suggest that this behavior can be explained by densely packed asperities resulting in somewhat crack‐like propagation rather than a slip pulse that is as concentrated as the tremor activity.

Automated detection and location of microseismicity at Mount St. Helens with a large‐N geophone array

AbstractIn the summer of 2014 a dense array of 904 geophones was deployed at Mount St. Helens along the road and trail system within 15 km distance of the summit crater. The array recorded continuous data for approximately 2 weeks and presents an unprecedented seismic observation of an active volcano. A reverse‐time imaging method is applied to short‐term‐average over long‐term‐average time series data to automatically detect and locate microseismicity. These efforts resulted in an order of magnitude increase in earthquake detections over the normal monitoring operations of the Pacific Northwest Seismic Network. Earthquake locations resolve a narrow, ≤1 km wide, vertical lineament of seismicity which extends from the surface to 4 km depth directly beneath the summit crater. This feature is interpreted as a fracture network that acts as a conduit connecting an underlying magma chamber to the surface.

How to Recognize a "Beast Quake" and a "Dance Quake"

Any good seismologist–carryball (i.e., American football) fan remembers the “beast quake” of 2011. Seahawks fans’ exuberant reactions to a Marshawn Lynch (in beast mode) run caused the stadium to shake enough to be well recorded on a strong‐motion seismograph a block away (Vidale, 2011). With the Seahawks at home for the National Football Conference (NFC) Championship playoffs, both last year and this year, the Pacific Northwest Seismic Network (PNSN) received permission to install some seismometers in the stadium. Motivations for the experiment included testing the field and telemetry equipment, examining analysis and display techniques, and practicing quick reactions to mobilize field experiments. With PNSN students and staff who are also Seahawks fans, it was not hard to get volunteers to do the installations and monitor the on‐field actions and the resulting seismograms. Last year, additional beast quakes were recorded, although none were as high on the beast scale as the 2011 original event. Rapid data acquisition and web‐based, real‐time seismogram displays allowed a viewer to anticipate a successful play before it would be seen on live television (which was delayed about 15 s from real time). This was all fun stuff for many, with the PNSN webservers getting a good test in providing high‐traffic service. Public interest in the real‐time seismograms was so great that the …

A comparative study of low frequency earthquake templates in northern Cascadia

Using network correlation methods, we generate low frequency earthquake templates for a set of 4 composite arrays on Vancouver Island and Washington state that employ data from EarthScope (Transportable and Flexible Arrays, Plate Boundary Observatory), POLARIS and permanent network (Canadian National Seismograph Network, Pacific Northwest Seismic Network) sources. On the basis of empirical and semi-analytical arguments, the templates can be viewed as Green's function sections corresponding to moment tensor point sources with step-function time dependence in displacement. Low frequency earthquake hypocentres follow the general epicentral distribution of tremor and occur along tightly defined surfaces in depth with Washington locations averaging slightly deeper than those on Vancouver Island. We invert template waveforms for moment tensor mechanisms and find that data are well modelled by double couple sources. For southern Vancouver Island, with the highest quality templates, the majority of mechanisms are consistent with shallow thrusting in the direction of plate motion. The three other data sets with lower signal to noise levels show predominantly thrust mechanisms with more variable orientations. Taken together with other constraints, our observations support the hypothesis that low frequency earthquakes manifest shear slip on a relatively thin plate boundary.

Watching the Wind: Seismic Data Contamination at Long Periods due to Atmospheric Pressure-Field-Induced Tilting

Abstract Following the installation of a new broadband seismic station near the Pacific coast of the Olympic Peninsula, Washington, the Pacific Northwest Seismic Network recorded elevated background seismic noise at periods exceeding 30 s. We observed power spectral density variations of about 15–20 dB in the horizontal‐component seismograms between times of high and low noise, and differences of up to about 25 dB between the horizontal and vertical components during noisy intervals. The amplitude of the signal at long periods exhibited clear correlations with cyclic daytime fluctuations of air temperature and wind speed. Our observations provided measurable evidence that the variations of the local atmospheric pressure field associated with local wind systems represents an effective source of ground displacements and rotations. The rotational component of the pressure‐field‐induced motion is recorded by horizontal‐component seismometers that, below their low‐frequency corner, are sensitive to surface tilts. The response of seismometers to ground rotations is characterized by a transfer function, which we use to estimate the amplitude of tilt. We measured a pressure‐field‐induced surface tilt of the order of several tenths of μ rad. Based on simple modeling, tilt contamination is expected to attenuate rapidly with depth and to become negligible just a few hundreds of meters below the surface. Removal of this unwanted effect from seismograms remains a challenging task as the details of tilting depend on the nonstationary character of the pressure field, the compliance of the near surface to pressure variations, and the design and construction of the seismic station. Online Material: Animated time‐sequenced plots of daily seismic noise PDFs.

Failure Mechanics of the Nile Valley Landslide, Yakima County, Washington

Research Article| November 01, 2011 Failure Mechanics of the Nile Valley Landslide, Yakima County, Washington THOMAS C BADGER; THOMAS C BADGER Washington State Department of Transportation, Geotechnical Division, P.O. Box 47365, Olympia, WA 98504-7365 Search for other works by this author on: GSW Google Scholar ERIC L SMITH; ERIC L SMITH Washington State Department of Transportation, Geotechnical Division, P.O. Box 47365, Olympia, WA 98504-7365 Search for other works by this author on: GSW Google Scholar STEVE M LOWELL STEVE M LOWELL Washington State Department of Transportation, Geotechnical Division, P.O. Box 47365, Olympia, WA 98504-7365 Search for other works by this author on: GSW Google Scholar Environmental & Engineering Geoscience (2011) 17 (4): 353–376. https://doi.org/10.2113/gseegeosci.17.4.353 Article history first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation THOMAS C BADGER, ERIC L SMITH, STEVE M LOWELL; Failure Mechanics of the Nile Valley Landslide, Yakima County, Washington. Environmental & Engineering Geoscience 2011;; 17 (4): 353–376. doi: https://doi.org/10.2113/gseegeosci.17.4.353 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyEnvironmental & Engineering Geoscience Search Advanced Search Abstract On October 11, 2009, a very large translational landslide, referred to as the Nile Valley Landslide (NVL), destroyed more than 2500 ft (750 m) of State Route 410 near the community of Nile, located about 25 mi northwest of Yakima, Washington. Based on eyewitness accounts, Pacific Northwest Seismic Network records, and an intensive subsurface investigation, we infer two translational failure mechanisms: an initial shallow failure within the unconsolidated surficial deposits, followed by failure within a deep, adversely dipping, claystone interbed between two Grand Ronde Basalt flows. The underlying basalt flow contains a highly pressurized aquifer that significantly reduces the effective stress within the inferred deep failure zone and its stability. Recent inclinometer data showing slope movement within both the shallow and deep failure zones support this interpretation. The NVL is located at the base of Cleman Mountain, an anticlinal ridge that hosts the enormous and currently dormant Sanford Pasture landslide complex (SPLC). Geochemical analyses of basalts encountered in the numerous borings and outcrops within and around the NVL suggest that the basal failure zones of the NVL and SPLC are stratigraphically different, and that the NVL is likely not a reactivation of the SPLC. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Basin Shear-Wave Velocities beneath Seattle, Washington, from Noise-Correlation Rayleigh Waves

Tomography with short-period Rayleigh waves, extracted using noise interferometry, can refine S -wave velocity ( V S ) models in urban areas with dense arrays of short-period and broadband instruments. We apply this technique to the Seattle area to develop a new shallow V S model for use in seismic-hazard assessment. Continuous data from the Seismic Hazards in Puget Sound (SHIPS) array and local broadband stations have interstation distances of 90 km or less. This spacing allows us to extract Rayleigh waves with periods between 2 and 10 s that are sensitive to shallow-basin structure. This new V S model for the Seattle basin is constructed using direct observations rather than using P -wave velocity ( V P ) observations and a V P / V S ratio as all previous 3D models at this scale have been constructed. Our results reveal greater detail in the upper 3.5 km than previous models. Earthquake simulations calculated using our new model better predict peak ground velocities (PGV) at periods between 1 and 2 s for two local earthquakes than the previous model used to calculate Seattle’s seismic-hazard map (Frankel et al. , 2007). We collected data from two local earthquakes and ran finite-difference simulations using our new velocity model as well as the previous velocity model used in development of the Seattle seismic-hazard maps to assess how well our model predicts ground motions relative to the previous model. With a recent deployment of Netquakes strong-motion stations by the Pacific Northwest Seismic Network (PNSN) and the U.S. Geological Survey, we are now able to make more comprehensive assessments of the predictions for recent events.

Deep long-period earthquakes beneath Washington and Oregon volcanoes

Deep long-period (DLP) earthquakes are an enigmatic type of seismicity occurring near or beneath volcanoes. They are commonly associated with the presence of magma, and found in some cases to correlate with eruptive activity. To more thoroughly understand and characterize DLP occurrence near volcanoes in Washington and Oregon, we systematically searched the Pacific Northwest Seismic Network (PNSN) triggered earthquake catalog for DLPs occurring between 1980 (when PNSN began collecting digital data) and October 2009. Through our analysis we identified 60 DLPs beneath six Cascade volcanic centers. No DLPs were associated with volcanic activity, including the 1980–1986 and 2004–2008 eruptions at Mount St. Helens. More than half of the events occurred near Mount Baker, where the background flux of magmatic gases is greatest among Washington and Oregon volcanoes. The six volcanoes with DLPs (counts in parentheses) are Mount Baker (31), Glacier Peak (9), Mount Rainier (9), Mount St. Helens (9), Three Sisters (1), and Crater Lake (1). No DLPs were identified beneath Mount Adams, Mount Hood, Mount Jefferson, or Newberry Volcano, although (except at Hood) that may be due in part to poorer network coverage. In cases where the DLPs do not occur directly beneath the volcanic edifice, the locations coincide with large structural faults that extend into the deep crust. Our observations suggest the occurrence of DLPs in these areas could represent fluid and/or magma transport along pre-existing tectonic structures in the middle crust.

High-Frequency Ground-Motion Scaling in the Pacific Northwest and in Northern and Central California

Abstract Large data sets of vertical and horizontal seismograms from the Pacific Northwest Seismic Network, Northern California Seismic Network, and Berkeley Digital Seismic Network are used to study the high-frequency (0.25–16 Hz) ground-motion scaling characteristics in Washington–Oregon, northern California (39° N to 42° N and 119° W to 124° W), and central California (35° N to 39° N and 118° W to 123° W). We used peak filtered ground velocities to characterize the propagation, excitation, and site terms. The regression results for propagation were modeled using a geometrical spreading function, g ( r ), and a frequency-dependent attenuation, Q ( f )= Q 0 f η . For the Pacific Northwest, the best Q model that fits the observation is expressed by Q ( f )=280 f 0.55 . The eastern central California and western central California results are parameterized with Q ( f )=280 f 0.50 and Q ( f )=240 f 0.35 , respectively. The northern California results are not easy to model and require using a frequency-dependent η and a frequency-dependent geometrical spreading. The geometrical spreading effect for the frequencies higher than 5 Hz is very strong in that region. The excitation terms for the small events studied were modeled using a Brune’s source model. An average stress drop of 30 bars was obtained for the Pacific Northwest. The northern California average value for stress drop is 90 bars. The observations of eastern central California were modeled with a stress drop of 49 bars, while the best fit for the western part required Δσ =80 bars. The range of the values obtained for Q , g ( r ), and Δσ indicates that the ground-motion parameters for one region should not be used for another. As an example, our results show that the ground-motion amplitude due to an M w 5.0 earthquake at a distance of 50 km is different by a factor of 1.5 between the Pacific Northwest and eastern central California.

The 8 October 2006 md 4.5 Cowlitz Chimneys Earthquake in Mount Rainier National Park

An Md 4.5 earthquake located ∼12 km east of Mount Rainier occurred on 8 October 2006 at 02:48 UTC (figure 1). Although not large enough to be damaging or of major tectonic significance, a summary description of the earthquake is warranted because of its proximity to Mount Rainier, and because earthquakes of Md ≥ 4.5 are relatively rare in this region. Previous events of Md ≥ 4.5 have occurred approximately once a decade within a radius of ∼60 km from this mainshock, with the closest and most recent prior earthquake being an Md 4.9 event located 46 km to the southwest in 1989. Magnitudes in this paper refer to the coda duration magnitude determined by the Pacific Northwest Seismic Network (PNSN) (Crosson 1972). We refer to the 2006 event as the “Cowlitz Chimneys” earthquake because it occurred beneath the Cowlitz Chimneys, a prominent ridge in Mount Rainier National Park. Figure 1. Hillshade relief map derived from Digital Elevation Model (DEM; GTOPO3 interpolated to 1 arcsec) showing PNSN catalog locations of all earthquakes with Md > 0.0 from 1 January 1969 up to 8 October 2006. The white star indicates the location of the Md = 4.5 mainshock of 8 October 2006. Also labeled are the western Rainier seismic zone (WRSZ); the St. Helens seismic zone (SHSZ); volcanoes Mount Rainier, Mt. St. Helens, and Mt. Adams; the Goat Rocks volcanic center; and temporary seismic station CAYP. White triangles represent seismic stations. This paper describes the mainshock's focal mechanism and its aftershock distribution. The inferred source mechanism, its ordinary aftershock sequence, and the lack of significant triggered seismicity near the volcanic edifice lead us to conclude that this was a regular tectonic crustal earthquake rather than one related to volcanic processes. The Cowlitz Chimneys epicentral region lies within …

Testing Small-Aperture Array Analysis on Well-Located Earthquakes, and Application to the Location of Deep Tremor

We have here analyzed local and regional earthquakes using array tech- niques with the double aim of quantifying the errors associated with the estimation of propagation parameters of seismic signals and testing the suitability of a probabilistic location method for the analysis of nonimpulsive signals. We have applied the zero-lag cross-correlation method to earthquakes recorded by three dense arrays in Puget Sound and Vancouver Island to estimate the slowness and back azimuth of direct P waves and S waves. The results are compared with the slowness and back azimuth computed from the source location obtained by the analysis of data recorded by the Pacific Northwest seismic network (PNSN). This comparison has allowed a quanti- fication of the errors associated with the estimation of slowness and back azimuth obtained through the analysis of array data. The statistical analysis gives σBP � 10° and σBS � 8° as standard deviations for the back azimuth and σSP � 0:021 sec= km and σSS � 0:033 sec=km for the slowness results of the P and S phases, respec- tively. These values are consistent with the theoretical relationship between slowness and back azimuth and their uncertainties. We have tested a probabilistic source location method on the local earthquakes based on the use of the slowness estimated for two or three arrays without taking into account travel-time information. Then we applied the probabilistic method to the deep, nonvolcanic tremor recorded by the arrays during July 2004. The results of the tremor location using the probabilistic method are in good agreement with those obtained by other techniques. Thewide depth range, of between 10 and 70 km, and the source migration with time are evident in our results. The method is useful for locating the source of signals characterized by the absence of pickable seismic phases.