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Updated: 33 min 22 sec ago

Tuesday Midday Update on South Napa Earthquake

August 26, 2014 - 16:52
Summary: This release of information serves as an updated summary of U.S. Geological Survey information as it relates to the current understanding of the South Napa earthquake. Yesterday’s more comprehensive news release can be found here.

Contact Information:

Justin  Pressfield ( Phone: 916-335-1211 ); Susan  Garcia ( Phone: 650-329-4668 );



This release of information serves as an updated summary of U.S. Geological Survey information as it relates to the current understanding of the South Napa earthquake. Yesterday’s more comprehensive news release can be found here.

The area surrounding the epicenter of the mainshock is continuing to experience a number of aftershocks. As of Tuesday Aug. 26, 4 PM PDT, there have been more than 80 aftershocks; only four of these have had magnitudes greater than 3. The greater-than-magnitude 3 aftershocks include:

  • M3.0 Tuesday 6:45 AM PDT
  • M3.9 (largest aftershock) Tuesday 5:33 AM PDT
  • M3.6 Sunday 5:47 AM PDT
  • M3.5 (4 minutes after mainshock) Sunday 3:24 AM PDT

There are also updated probabilities of additional aftershocks. These will continue to be updated on the USGS website for this event.

At this time (two days after the mainshock) the probability of a strong and possibly damaging aftershock (M5 or greater) in the next 7 days is approximately 12 percent.

Most likely, the recent mainshock will be the largest in the sequence. However, there is a small chance (approximately 2 percent) of an earthquake equal to or larger than this mainshock in the next 7 days.

In addition, USGS anticipates approximately 1 to 10 small (M3-M5) aftershocks in the next 7 days.

“Scientists from the USGS continue to work day and night to do careful field research in the area of the South Napa earthquake,” said Tom Brocher, Director of the USGS’s Earthquake Science Center. “The flow of new and refined information is allowing us to continue to inform the emergency managers and the public about this incident as well as to grow the knowledge about earthquakes to allow society to better prepare for future occurrences.”

The USGS is continuing to incorporate the new data into existing models to refine our estimates. While USGS publishes prompt approximations of economic losses based on real-time and later-arriving data, the California Governor’s Office of Emergency Services is expected to issue an official economic loss estimation after a comprehensive, and more accurate, damage assessment is completed.

The USGS is interested in finding volunteers willing to host seismic instruments so that scientists can obtain more records from aftershocks and learn more about this sequence of earthquakes.  Those interested, who are in the area of strong shaking, should go to http://earthquake.usgs.gov/monitoring/netquakes/ and complete the "sign up" page.

The Earthquake Early Warning test system functioned as designed in Sunday's earthquake.  Within five seconds of the earthquake it produced a warning (estimated at magnitude 5.7 within three seconds of its occurrence), sufficient to provide warning to Berkeley, San Francisco, and areas farther south. The EEW prototype was developed by the USGS in partnership with the UC Berkeley, California Institute of Technology, University of Washington, and the Gordon and Betty Moore Foundation.

Update on the Magnitude 6 South Napa Earthquake of August 24, 2014

August 25, 2014 - 12:03
Summary: Yesterday at 3:20 AM local time, the northern San Francisco Bay Area was struck by the largest earthquake to impact the Bay Area since the 1989 M6.9 Loma Prieta earthquake

Contact Information:

Justin  Pressfield ( Phone: 916-335-1211 ); Susan  Garcia ( Phone: 650-329-4668 );



Yesterday at 3:20 AM local time, the northern San Francisco Bay Area was struck by the largest earthquake to impact the Bay Area since the 1989 M6.9 Loma Prieta earthquake. Yesterday’s earthquake appears to have ruptured on or just west of mapped traces of the West Napa Fault, the most seismically active of the faults mapped between the longer Rodgers Creek Fault on the west and the Concord-Green Valley Fault to the east. USGS has named the earthquake the “South Napa earthquake.”

Yesterday’s M6.0 earthquake caused significant damage in south Napa County.  It occurred in the broad zone of deformation that accommodates the relative motion of the North American and Pacific Plates.  The 2000 M5.0 Yountville earthquake occurred on the West Napa Fault and also damaged Napa.  The 1898 M6.3 Mare Island earthquake occurred in the vicinity of yesterday’s earthquake.

“USGS scientists are working around the clock to understand the earthquake and relay information to emergency managers and the public,” stated Tom Brocher, Director of the USGS’s Earthquake Science Center.  “In less than a day we made tremendous strides in understanding what happened and have crews of scientists continuing to investigate this event.”

Damage is localized in the region surrounding Napa due to the rupture directivity to the north-west. River valley sediments in Napa Valley likely contributed to the amplification of shaking around Napa.

Yesterday, USGS and California Geological Survey (CGS) geologists mapped surface rupture produced by the earthquake from the epicenter NNW at least 10 km (6 miles) on a previously mapped strand of the West Napa Fault.  At that point the surface rupture may have jumped eastward about half a mile toward Napa and extended NNW another few miles along a previously unmapped strand of the West Napa Fault.  USGS and CGS geologist continue to conduct field reconnaissance to refine these interpretations and to look for additional surface rupture.  The surface ruptures show a northward shift west of the West Napa fault of about two inches.

GPS receivers operated by the USGS and others also measured a shift of the earth of a few inches caused by the earthquake.  Yesterday, USGS geophysicists made additional measurements of the earth’s movement that will refine models for the earthquake movement.

USGS analysis of the seismic recordings indicates the earthquake rupture propagated to the NNW and upward, directing the brunt of the earthquake energy to the NNW towards Napa.  The dozens of aftershocks that have been recorded to date are also aligned on this NNW trend.  At this time (one day after the mainshock) the probability of a strong and possibly damaging aftershock in the next seven days is approximately 1 in 4.

Today, USGS technicians will be retrieving additional seismic data from several seismic stations that either do not automatically communicate their data to us or failed to do so.  They will also be deploying additional recorders in Napa.  These data should help refine the ShakeMap showing the intensity of shaking throughout the Bay Area and better understand the strong shaking experienced in Napa.

The Earthquake Early Warning test system functioned as designed in yesterday’s earthquake.  Within five seconds of the earthquake it produced a warning (estimated at magnitude 5.7 within three seconds of its occurrence), sufficient to provide warning to Berkeley, San Francisco, and areas farther south.  No warning would have been possible within 20 miles of the earthquake.  EEW prototype was developed by the USGS in partnership with the UC Berkeley, California Institute of Technology, University of Washington, and the Gordon and Betty Moore Foundation.

Natural Methane Seepage on U.S. Atlantic Ocean Margin Widespread

August 25, 2014 - 06:10
Summary: Natural methane leakage from the seafloor is far more widespread on the U.S. Atlantic margin than previously thought, according to a study by researchers from Mississippi State University, the U.S. Geological Survey, and other institutions

Contact Information:

Carolyn Ruppel ( Phone: 617-806-6768 ); Adam  Skarke ( Phone: 662-268-1032 ext. 258 ); Hannah Hamilton ( Phone: 703-648-4356 );



Natural methane leakage from the seafloor is far more widespread on the U.S. Atlantic margin than previously thought, according to a study by researchers from Mississippi State University, the U.S. Geological Survey, and other institutions.

Methane plumes identified in the water column between Cape Hatteras, North Carolina and Georges Bank, Massachusetts, are emanating from at least 570 seafloor cold seeps on the outer continental shelf and the continental slope.  Taken together, these areas, which lie between the coastline and the deep ocean, constitute the continental margin.  Prior to this study, only three seep areas had been identified beyond the edge of the continental shelf, which occurs at approximately 180 meters (590 feet) water depth between Florida and Maine on the U.S. Atlantic seafloor.

Cold seeps are areas where gases and fluids leak into the overlying water from the sediments.  They are designated as cold to distinguish them from hydrothermal vents, which are sites where new oceanic crust is being formed and hot fluids are being emitted at the seafloor.  Cold seeps can occur in a much broader range of environments than hydrothermal vents.

“Widespread seepage had not been expected on the Atlantic margin. It is not near a plate tectonic boundary like the U.S. Pacific coast, nor associated with a petroleum basin like the northern Gulf of Mexico,” said Adam Skarke, the study’s lead author and a professor at Mississippi State University.

The gas being emitted by the seeps has not yet been sampled, but researchers believe that most of the leaking methane is produced by microbial processes in shallow sediments.  This interpretation is based primarily on the locations of the seeps and knowledge of the underlying geology.  Microbial methane is not the type found in deep-seated reservoirs and often tapped as a natural gas resource. 

Most of the newly discovered methane seeps lie at depths close to the shallowest conditions at which deepwater marine gas hydrate can exist on the continental slope.  Gas hydrate is a naturally occurring, ice-like combination of methane and water, and forms at temperature and pressure conditions commonly found in waters deeper than approximately 500 meters (1640 feet). 

“Warming of ocean temperatures on seasonal, decadal or much longer time scales can cause gas hydrate to release its methane, which may then be emitted at seep sites,” said Carolyn Ruppel, study co-author and chief of the USGS Gas Hydrates Project.  “Such continental slope seeps have previously been recognized in the Arctic, but not at mid-latitudes.  So this is a first.”

Most seeps described in the new study are too deep for the methane to directly reach the atmosphere, but the methane that remains in the water column can be oxidized to carbon dioxide. This in turn increases the acidity of ocean waters and reduces oxygen levels. 

Shallow-water seeps that may be related to offshore groundwater discharge were detected at the edge of the shelf and in the upper part of Hudson Canyon, an undersea gorge that represents the offshore extension of the Hudson River. Methane from these seeps could directly reach the atmosphere, contributing to increased concentrations of this potent greenhouse gas.  More extensive shallow-water surveys than described in this study will be required to document the extent of such seeps.

Some of the new methane seeps were discovered in 2012.  In summer 2013 a Brown University undergraduate and National Oceanic and Atmospheric Administration Hollings Scholar Mali’o Kodis worked with Skarke to analyze about 94,000 square kilometers (about 36,000 square miles) of water column imaging data to map the methane plumes.  The data had been collected by the vessel Okeanos Explorer between 2011 and 2013.  The Okeanos Explorer and the Deep Discoverer remotely operated vehicle, which has photographed the seafloor at some of the methane seeps, are managed by NOAA’s Office of Ocean Exploration and Research.

"This study continues the tradition of advancing U.S. marine science research through partnerships between federal agencies and the involvement of academic researchers,” said John Haines, coordinator of the USGS Coastal and Marine Geology Program “NOAA's Ocean Exploration program acquired state-of-the-art data at the scale of the entire margin, while academic and USGS scientists teamed to interpret these data in the context of a research problem of global significance."

The study, Widespread methane leakage from the sea floor on the northern US Atlantic Margin, by A, Skarke, C. Ruppel, M, Kodis, D. Brothers and E. Lobecker in Nature Geoscience is available on line.

USGS Gas Hydrates Project

The USGS has a globally recognized research effort studying natural gas hydrates in deepwater and permafrost settings worldwide.  USGS researchers focus on the potential of gas hydrates as an energy resource, the impact of climate change on gas hydrates, and seafloor stability issues.

For more information about the U.S. Geological Survey’s Gas Hydrates Project, visit the Woods Hole Coastal and Marine Science Center, U.S. Geological Survey Gas Hydrates Project website.

For more information, visit the Mississippi State University website.

Map of the northern US Atlantic margin showing the locations of newly-discovered methane seeps mapped by researchers from Mississippi State University, the US Geological Survey, and other partners. None of the seeps shown here was known to researchers before 2012. (High resolution image)

 

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