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USGS 2002

The New England Seismic Network

USGS Award Number 01HQAG0022

PI John E. Ebel

Weston Observatory

Boston College

Department of Geology and Geophysics

381 Concord Rd.

Weston, MA 02493

Tel: 617-552-8300

Fax: 617-552-8388

Email: ebel@bc.edu

URL: http://www.bc.edu/westonobservatory




Project Summary

The operation of a regional seismic network to monitor earthquake activity in New England and vicinity is supported under this project. The purpose of this earthquake monitoring is to compile a complete database of earthquake activity in New England to as low a magnitude as possible in order to understand the causes of the earthquakes in the region, to assess the potential for future damaging earthquakes, and to better constrain the patterns of strong ground motions from earthquakes in the region. The New England Seismic Network (NESN) is cooperatively operated by Weston Observatory of Boston College and the Earth Resources Laboratory of the Massachusetts Institute of Technology (MIT). This is a progress report for the first year of this cooperative agreement.

Regional Seismic Network Status

The New England Seismic Network is operated by Weston Observatory of Boston College in cooperation with the Earth Resources Laboratory at MIT. During the time period of this report, the Weston Observatory component of the network was comprised of 12 seismic stations, although one of the stations (TRY at Troy, New York) was not operational during the reporting period. Station TRY was being fitted with an internet connection to allow direct communication with the site. That link was finally established in mid-2002. However, we have not been able to make available a computer for installation at the site, since new computers are not compatible with the current Weston Observatory digitizing and control system for the remote seismic stations. This problem must be addressed by purchasing new equipment for the seismic stations, and we currently are requesting new equipment to allow us to reestablish this station sometime in 2003.

The configuration of the regional network operated by Weston Observatory did not change during the year 2002. At present Weston Observatory has 10 seismic stations operating in New England, seven of which have signals that are also fed directly to the USGS NEIC in Golden, Colorado via the internet. A station near Quabbin Reservoir in central Massachusetts developed a computer problem in late 2002, and that site must be repaired. Once repaired, that station will also send its signals via the internet directly to the NEIC.

We are continuing to work with U.S. Army Corps of Engineers (USACE) concerning our site at one of their dams at Franklin Falls, New Hampshire. Currently, communications to this site is via a dial-up telephone connection. We wish to convert this site to continuous internet telemetry, but security issues at the USACE have created problems that we have not yet overcome. We are working with USACE staff to solve this problem. We also are negotiating with the USACE to assist them with their program to install digital strong-motion stations at a number of dams in New England, replacing the current analog strong-motion instruments.

There are plans to site a USNSN national backbone station in central Maine sometime during the year 2003. We are currently working with the Maine Geological Survey to identify potential sites in central Maine that would be suitable for a USNSN station.

All of the Weston Observatory stations remain PC-based with on-site recording, three-component broadband sensors, and dial-up telephone telemetry or direct internet links to the central station at Weston Observatory. The sensors are CMG-40T feedback geophones with a flat response to ground velocity between roughly 30 Hz and 30 sec. The digitizers are Nanometrics 16-bit with gain-ranging, yielding effectively 136 db dynamic range. The sensor signals are being digitized at a rate of 100 samples per second per channel. The output from the digitizer is sent to a PC computer using OS/2, a multitasking operating system, at the digitizing site. The software controlling the stations stores the signals from the sensor in a continuous disk loop. eight of the sites (BCX, BRY, EMMW, HNH, PQI, WES, WVL, and YLE) are available via internet connection to Weston Observatory, seven of which are also sending their data to the USGS NEIC in Golden, Colorado.

At each station the signals from the seismometer are recorded on a local hard disk. The datastream from the digitizer is examined by a program that uses a filter and STA/LTA scheme to test for possible events. When the STA/LTA threshold is exceeded, a notation of the time and duration of the exceedence is added to a text file on the recording computer. An analyst at Weston Observatory currently uses this detection file from a station to determine the possible times at which events may be contained on the remote disks. The analyst then uses these times to send requests to the remote stations to send windows of waveform data back to Weston Observatory for analysis. The retrieved waveforms from all stations are analyzed and archived at Weston Observatory.

In early 2001 an Earthworm station at Weston Observatory became operational. Data from the stations in New England being sent to the NEIC along with the waveforms from several regional stations outside New England are currently being received by the Earthworm server. The data from these stations are currently being accessed and investigated on a routine basis for earthquake activity in the region.

In addition to the Weston Observatory NESN stations, the Earth Resources Laboratory (ERL) at MIT has 3 analog-telemetry seismic stations and one 3-component boradband digital seismic station Massachusetts. The data from these stations provide important additional data for locating earthquakes centered within New England. Also, there are two USNSN stations and one cooperating USNSN station operating in New England. Event arrival time readings, waveforms, and hypocentral information are routinely exchanged between the Weston Observatory and MIT. Weston Observatory also obtains data from (and sends data to) the Geological Survey of Canada, Lamont-Doherty Earth Observatory, and the U.S. Geological Survey NEIC as required by the occurrences of earthquakes in the region. MIT and Weston Observatory produce a single, joint quarterly seismic network bulletin for the New England area. That bulletin is produced in html format and is posted on the NESN web pages of each institution.

Accomplishments During the Report Period


Seismic Monitoring

The Weston Observatory NESN seismic stations detected a number of earthquakes from New England and vicinity from February 1, 2002 to December 30, 2002. A total of 22 local earthquakes from New England and vicinity with magnitudes from 1.6 to 5.1 were detected and located by the network, some of which were felt. In addition to these events, some microearthquakes and suspected events, too small to be located, were detected by the network. The number of earthquakes during this reporting period is somewhat great than that from recent years.

The most significant earthquake detected during this reporting period was the moment-magnitude M 5.0 earthquake that was centered at Au Sable Forks, NY on April 20, 2002. This earthquake, which was the largest earthquake centered in the northeastern U.S. since 1983, was felt throughout all of the New England states. The earthquake caused some minor damage in the epicentral area. Fortunately, no damage was reported anywhere in New England from this shock.

The Au Sable Forks earthquake and its aftershocks provided some important data both on the seismotectonics of northern New York state and on the propagation and attenuation of seismic energy in the region. A preliminary seismotectonic analysis of the earthquake and its aftershocks was presented in Seeber et al. (2002) and Kim (2002). Preliminary studies of the strong ground-motion attenuation for this event were presented by Atkinson and Sonley (2002) and Ebel (2002). Further studies on the data from this earthquake are ongoing.

Continuing a practice started in late 2002, Weston Observatory has a web site offering weekly estimates of the probability of a felt earthquake in New England. The temporal probability is based on the work of Ebel and Kafka (2002), while the spatial probability is based on research published by Kafka and Levin (2000) and Kafka (2002). A link called Weekly Probability of Felt Earthquakes in New England on the Weston Observatory web page (www.bc.edu/westonobservatory) shows the probability of a felt earthquake in New England for each upcoming 7-day period. Also shown on this web page is a map of those areas in New England that have about a "67%" probability of being the epicenter of an earthquake of MLg >= 2.7 during the 7-day period. The number of hits on this web page indicates that there is a steady interest in this information.

Data Dissemination

Weston Observatory and MIT continue to archive independently the waveform data for the seismic stations which they are operating. Weston Observatory has the capability to convert the waveforms, routinely stored in Nanometrics format, to either ASCII, SAC or SEED format for external distribution. An ftp account can be set up to allow users from outside Weston Observatory to access waveforms recorded by the network. Weston Observatory has not yet completed the process of developing the capabilities to deliver SEED waveforms of local events to the IRIS DMC. In addition, Weston Observatory plans to begin the process of contributing hypocentral data to the CNSS composite catalog on a routine basis.

Weston Observatory maintains the following web page with information about local earthquakes:

http://www.bc.edu/westonobservatory

Currently available on the web page is the full catalog of northeastern earthquake activity to 1991 along with recent quarterly reports (joint with MIT) of the seismicity detected by the NESN. Weston Observatory attempts to regularly maintain and update these web pages with the latest information on earthquakes in the region.

Rapid Data and Communication Systems

Weston Observatory is running a computer with the Earthworm system on it. A number of seismic stations from New England and vicinity are being received by the Earthworm computer. Unfortunately, up to now we have been unable to devote time to working the Earthworm datastream into our routine data handling and processing scheme other than to use it as a consultative reference. We are particularly interested in either acquiring or developing an automated system to compute earthquake locations and magnitudes using the Earthworm datastream. We intend to make use of resources from other regional networks, where applicable, to accomplish this goal. We are also interested in using the wavelet-transform event detector and identifier that has been developed at Weston Observatory for computing event locations and magnitudes for sparse networks like the NESN. Unfortunately, the current low staffing level of the NESN effort at Weston Observatory combined with the work needed to keep the current network running have precluded any serious attempts to work on these important development projects.

Financial Report

Year 2 funding was spent in accordance with the arrangements agreed upon in the cooperative agreement between Boston College and the USGS. For the current year of funding, less than "1%" of the funding provided by the USGS has not been spent, and that is expected to be spent by the end of this year of the agreement period of work.

References

Atkinson, G.M. and E. Sonley (2002). Ground Motions from the M5.0 2002 Au Sable Forks, New York Earthquake, Seism. Res. Lett.73, in press.

Ebel, J.E. (2002). Ground Motions in New England from the 20 April 2002 Au Sable Forks, NY Earthquake, Seism. Res. Lett.73, in press.

Ebel, J.E. and A.L. Kafka (2002). A Non-Poissonian Element in the Seismicity of the Northeastern United States, Bull. Seism. Soc. Am.92, 2040-2046.

Kafka, A.L. (2002). Statistical Analysis of the Hypothesis that Seismicity Delineates Areas Where Future Large Earthquakes Are Likely to Occur in the Central and Eastern United States, Seism. Res. Lett.73, 992-1003.

Kafka, A.L. and S.Z. Levin (2000). Does the spatial distribution of smaller earthquakes delineate areas where larger earthquakes are likely to occur?, Bull. Seism. Soc. Am., 90, 724-738.

Kim, W.-Y. (2002). Source Process of the Au Sable Forks, New York, Earthquake Sequence from Local Aftershock Monitoring Network Data, Seism. Res. Lett.73, in press.

Seeber, L., W.-Y. Kim, J.G. Armbruster, W.-X. Du, A. Lerner-Lam, and P. Friberg (2002). The 20 April 2002 Mw 5.0 Earthquake near Au Sable Forks, Adirondacks, New York: A First Glance at a New Sequence, Seism. Res. Lett.73, 480-489.