A STUDY OF NEW ENGLAND SEISMICITY

Quarterly Earthquake Report



January - March 1998

New England

Seismic Network

Weston Observatory 381 Concord Road Weston, MA 02493	Earth Resources Lab Massachusetts Institute of Technology 42 Carleton Street Cambridge, MA 02142

NEW ENGLAND SEISMIC NETWORK

Prepared by Charles Doll, Jr.

Email: doll@erl.mit.edu

and Susan O'Connor

Email: dannolfo@bcvms.bc.edu

October 1998



for

United States Geological Survey

905 National Center

12201 Sunrise Valley Drive

Reston, Virginia 20192

Notice

Network operation supported by the U.S. Geological Survey (USGS), Department of the Interior, under USGS award number 1434-HQ-98-AG-01943 and award number 1434-HQ-98-AG-01926. The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the U.S. Government.

Quarterly Earthquake Report

January - March 1998

Table of Contents

• Introduction
• Current Network Operation and Status
• Seismicity
• Data Management
• Tables
  • Explanation of Tables
  • Table 1 Project Personnel
  • Table 2 Seismic Stations
  • Table 3 Earthquake Hypocenter List
  • Table 4 Earthquake Phase Data List
  • Table 5 Microearthquakes and Other Non-locatable Events
• Figures
  • NESN Station Map
  • NESN Strong-Motion Station Map
  • NESN Quarterly Seismicity Map
  • NESN Cumulative Seismicity Map
• Acknowledgments
• References

Introduction

The New England Seismic Network (NESN) is operated collaboratively by the Weston Observatory (WES) of Boston College and the Earth Resources Lab (ERL) of the Massachusetts Institute of Technology. The mission of the NESN is to operate and maintain a regional seismic network with digital recording of seismic ground motions for the following purposes: 1) to determine the location and magnitude of earthquakes in and adjacent to New England and report felt events to public safety agencies, 2) to define the crust and upper mantle structure of the northeastern United States, 3) to derive the source parameters of New England earthquakes, and 4) to estimate the seismic hazard in the area.

This report summarizes the work of the NESN for the period January - March 1998. It includes a brief summary of the network's equipment and operation, and a short discussion of data management procedures. A list of participating personnel is given in Table 1. There were 2 earthquakes that occurred within or near the network during this reporting period. Phase information for these earthquakes is included in this report.

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Current Network Operation and Status

The New England Seismic Network currently consists of 12 broadband three-component, 4 short-period vertical, and 8 strong-motion stations. The coordinates of the stations are given in Table 2, and a map of the network is shown in Figure 1.

WES operates 11 stations with broadband instruments consisting of Guralp CMG-40T three-component sensors. Ground motions recorded by these sensors are digitized at 100 sps with 16-bit resolution. Additional gain-ranging provides 126 dB dynamic range. These stations are operated in dialup mode with waveform segments of suspected events transmitted in digital mode to Weston Observatory for analysis and archiving. WES is continuing to upgrade its recording stations with 4 more broadband instruments scheduled for installation in 1999. WES also maintains 8 SMA-1 strong-motion instruments in New England.

ERL at MIT currently operates 1 broadband and 4 short-period stations, all located within 100 km of Boston. The short- period instruments have 1.0 Hz L4C vertical seismometers. Data recorded by these seismometers is transmitted continuously in analog mode to ERL and digitized (12-bit) into a PC at 50 sps. Station WFM has a CMG-40T sensor and transmits 3-channel, 24-bit data continuously to a central processor (Pentium PC) at ERL where waveform windows of suspected events are analyzed and archived. WES and ERL record some stations in analog format on helicorders to provide additional data for analysis.

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Seismicity

There were 2 earthquakes that occurred in or adjacent to the NESN during this reporting period. A summary of the location data is given in Table 3. Figure 2 shows the locations of these events. Figure 3 shows the locations of all events since the beginning of network operation in October, 1975.

Table 4 gives the station phase data and detailed hypocenter data for each event listed in Table 3. Final locations for this section were computed using the program HYPO78. For regional events (those too far from the NESN to obtain accurate locations and magnitudes) phase data are given for NESN stations, but the header lists the hypocenter and geographic location information adopted from the authoritative network. Travel-time information has been computed for these regional events by fixing the hypocenter obtained from the authoritative network and then running HYPO78.

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Data Management

Recent event locations are available via FTP at: SEISMOEAGLE.BC.EDU. Waveform data are saved in Nanometrics, ASCII, and SEED formats and are available via SEISMOEAGLE.BC.EDU or through personal contact. Earthquake lists can be fingered at QUAKE@SEISMOEAGLE.BC.EDU. Weston Observatory maintains two web pages with information about local earthquakes: http://www.bc.edu:80/bc_org/avp/cas/wesobs/ and http://seismoeagle.bc.edu/. The latter page is still under contruction. Currently available on the seismoeagle web page is the full catalog of northeastern earthquake activity to 1990. This will be updated as new Northeastern U.S. Seismic Network Bulletins are produced.

Our entire earthquake database can be accessed through the World Wide Web using the address "http://www-erl.mit.edu/NESN/homepage.html". For extraction of waveforms and hypocenter data, use our database search engine. Link to "Seismic Event Server at MIT ERL (SESAME)" and then click on "Interactive query form" under the heading "Custom Materials". Alternatively, the most recent local earthquake data, recorded by the M.I.T. Seismic Network, may be accessed by logging in to our anonymous FTP directory ("ftp erl-dialup.mit.edu"). The waveform files are in SAC format at both sites. Waveforms are downloaded as a Unix-compressed tar volume from our web-site and as individual Unix-compressed files from our FTP site.

For more information on matters discussed in this report or general earthquake information (reports, maps, catalogs, etc.) consult our web-sites www-erl.mit.edu/NESN and www.bc.edu:80/bc_org/avp/cas/wesobs/ or contact:

Voice: 617-253-7863 / FAX: 617-253-6385 / Email: doll@erl.mit.edu

Voice: 617-552-8319 / FAX: 617-552-8388 / Email: ebel@bcvms.bc.edu

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Explanation of Tables

Table 1: List of personnel operating the NESN

Table 2: List of Seismic and Strong Motion Stations

1. Code = station name
2. Lat = station latitude, degrees north
3. Long = station longitude, degrees west
4. Elev = station elevation in meters
5. Location = geographic location
6. Operator = network operator

Table 3: Earthquake Hypocenter List

1. Date = date event occurred, Yr (year)/Mo (month)/Dy (day)
2. Time = origin time of event, Hr (hour):Mn (minute):Sec (second)
3. Lat = event location, latitude north in degrees
4. Long = event location, longitude west in degrees
5. Depth = event depth in kilometers
6. Mag = event magnitude
7. Int = event epicentral intensity
8. Location = event geographic location

Table 4: Earthquake detailed hypocenter and phase data list

Table Header: detailed hypocenter data

1. Geographic location
2. DATE = date event occurred, yr/mo/dy (year/month/day)
3. ORIGIN = event origin time in hours, minutes, and seconds
4. LAT N = latitude north in degrees and minutes
5. LONG W = longitude west in degrees and minutes
6. DEPTH = event depth in kilometers
7. MN = Nuttli Lg phase magnitude with amplitude divided by period
8. MC = signal duration (coda) magnitude

   WES: 2.23 Log(FMP) + 0.12Log(Dist) - 2.36 (Rosario, 1979)

   MIT: 2.21 Log(FMP) - 1.7 (Chaplin et al., 1980)

9. ML = local magnitude

   WES: calculated from Wood-Anderson seismograms (Ebel, 1982)

   GSC: Richter Lg magnitude

10. GAP = largest azimuthal separation, in degrees, between stations
11. RMS = root mean square error of travel time residual in seconds
12. ERH = standard error of epicenter in kilometers
13. ERZ = standard error of event depth in kilometers
14. Q = solution quality of hypocenter

    A = excellent

    B = good

    C = fair

    D = poor

Table Body: earthquake phase data

1. STN = station name
2. DIST = epicentral distance in kilometers
3. AZM = azimuthal angle in degrees measured clockwise between true north and vector pointing from epicenter to station
4. Description of onset of phase arrival

   I = impulsive

   E = emergent

5. R = phase

   P = first P arrival

   S = first S arrival

6. M = first motion direction of phase arrival

   U = up or compression

   D = down or dilatation

7. K = weight of arrival

   0 = full weight (1.0)

   1 = 0.75 weight

   1 = 0.50 weight

   1 = 0.25 weight

   1 = no weight (0.0)

8. HRMN = hour and minute of phase arrival
9. SEC = second of phase arrival
10. TCAL = calculated travel time of phase in seconds
11. RES = travel time residual (error) of phase arrival
12. WT = weight of phase used in hypocentral solution
13. AMX = peak-to-peak ground motion, in millimicrons, of the maximum envelope amplitude of vertical-component signal, corrected for system response
14. PRX = period in seconds of the signal from which amplitude was measured
15. XMAG = Nuttli magnitude recorded at station
16. FMP = signal duration (coda), in seconds, measured from first P arrival
17. FMAG = coda magnitude recorded at station

Table 5: Microearthquakes and other non-locatable events

1. Date = date event occurred, Yr (year)/Mo (month)/Dy (day)
2. Sta = nearest station recording event
3. Arrival Time = phase arrival time, Hr (hour):Mn (minute):Sec (second)

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Acknowledgments

We would like to thank the Undergraduate Research Opportunities Program (UROP) of M.I.T. for its support to the network. Some maps in this report were made using the "mapgen" program provided by the U.S. Census Bureau at http://tiger.census.gov.

References

Chaplin, M.P., Taylor, S.R., and Toksöz, M.N. (1980), A coda length magnitude scale for New England, Earthquake Notes, 51, 15-22.

Ebel, J.E. (1982), M_L measurements for northeastern United States Earthquakes, Bull. Seism. Soc. Am., 72, 1367-1378.

Rosario, M. (1979), A coda duration magnitude scale for the New England Seismic Network, Master's Thesis, Boston College, 82 pp.

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