Episodic Tremor in the Alaska/Aleutian Subduction Zone Chloe Peterson, Douglas Christensen, and...
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Episodic Tremor in the Alaska/Aleutian Episodic Tremor in the Alaska/Aleutian Subduction Zone Subduction Zone Chloe Peterson, Douglas Christensen, and Steve McNutt Geophysical Institute, University of Alaska Fairbanks ABSTRACT—Episodic tremor and slip (ETS) has been observed in the Cascadia subduction zone [1] and in the Nankai trough [2]. Low frequency tremor-like seismic signals, which are located at depths of 25-40 km, are often associated with slow slip events on the downdip edge of the interplate coupled zone. These slow slip events have been recognized in data from continuously recording Global Positioning System (GPS) networks. Here we present evidence for similar seismic signals in the Alaska/Aleutian subduction zone. Broadband seismic stations located in southcentral Alaska between 1999-2001 (BEAAR Passcal experiment) were used to identify several instances of seismic tremor. The seismic signals observed consist of a series of bursts lasting between 10 and 15 minutes with frequencies ranging from 1-6 Hz. These signals were recorded over a broad area (>100km) roughly centered above the 40-50 km contour of the subducting Pacific plate. This region was the site of a slow slip event, which lasted for several years (1998-2001) [3] and may be related to the tremor episodes. Several possible tremor episodes have also been recorded in the eastern Aleutians on stations of the Makushin and Akutan volcanic networks. These events were identified on spectrograms used to monitor volcanic activity, however, the geographic distribution of stations suggest that the source of these signals does not stem from the volcanoes themselves. Similar to the previously observed tremor episodes, the tremor episodes recorded on the volcanic networks consist of a series of bursts with energy concentrated in the band 1-6 Hz. These episodes last up to six minutes. Although the non-volcanic tremors observed in the Alaska/Aleutian subduction zone are similar to the ETS observed in the Cascadia subduction zone and the Nanakai trough, identifying slow slip associated with tremor in the Alaska/Aleutian subduction zone is difficult because there is no continuously recording GPS network in the region. (Revised) Objective Objective Station Coverage Station Coverage Signal Signal Processing Processing Examples of Tremor Examples of Tremor Locations Locations Results Results References References Future Future Work Work Identify and locate several episodes of non-volcanic tremor using data from the BEAAR PASSCAL Experiment. Example of non-volcanic tremor occurring August 3, 2000 at approximately 10:36 UTC. BEAAR station coverage with selected AEIC stations used to help constrain locations. 1.Extract a single pulse of tremor (1-2 minutes). 2.Broadband filter (1- 6 Hz). 3.Smoothing and envelope creation using a Hilbert transform. 4.Low Pass filter (0.5–2 Hz). Envelopes resulting from filtering process are cross-correlated and relative time offsets determined, as shown in the above figures. Locations are calculated using a generic velocity model for southern Alaska and tremor signals are assumed to propagate at S-wave velocities. Five episodes of tremor located slightly updip of the 50 km contour of the subducting Pacific plate and just north of 1998- 2001 slow-slip event [3]. Location errors are on the order of 10-15 km. Depth constraints are difficult. This Study The events used in this study occurred in August 2000, in the midst of the 1998-2001 slip event (right). Many more events have been identified during this time period. Future work includes locating these events as well as identifying and locating tremor prior to and after the slip-event. Future research also includes exploring other regions of the subduction zone for slab tremor. For example, a tremor- like signal was identified on two volcanic networks in the Aleutian arc that are approx. 60 km apart (left). This signal is consistent with the tremor signals seen in the BEAAR PASSCAL data. 1. Rogers, G. and Draget, 2003. Episodic tremor and slip on the Cascadia subduction zone: The chatter of silent Slip, Science, 300, 1942-1943 2. Obara, K., 2002. Nonvolcanic deep tremor associated with subduction in southwest Japan, Science, 296, 1679-1681 3. Ohta, Y., J.T. Freymueller, S. Hreinsdottir, and H. Suito, 2005. A large slow slip event and the depth of the seismogenic zone in the south central Alaska subduction zone, Earth Planet. Sci. Lett., submitted.
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Transcript of Episodic Tremor in the Alaska/Aleutian Subduction Zone Chloe Peterson, Douglas Christensen, and...
Episodic Tremor in the Alaska/Aleutian Subduction ZoneEpisodic Tremor in the Alaska/Aleutian Subduction ZoneChloe Peterson, Douglas Christensen, and Steve McNutt
Geophysical Institute, University of Alaska Fairbanks
ABSTRACT—Episodic tremor and slip (ETS) has been observed in the Cascadia subduction zone [1] and in the Nankai trough [2]. Low frequency tremor-like seismic signals, which are located at depths of 25-40 km, are often associated with slow slip events on the downdip edge of the interplate coupled zone. These slow slip events have been recognized in data from continuously recording Global Positioning System (GPS) networks. Here we present evidence for similar seismic signals in the Alaska/Aleutian subduction zone. Broadband seismic stations located in southcentral Alaska between 1999-2001 (BEAAR Passcal experiment) were used to identify several instances of seismic tremor. The seismic signals observed consist of a series of bursts lasting between 10 and 15 minutes with frequencies ranging from 1-6 Hz. These signals were recorded over a broad area (>100km) roughly centered above the 40-50 km contour of the subducting Pacific plate. This region was the site of a slow slip event, which lasted for several years (1998-2001) [3] and may be related to the tremor episodes. Several possible tremor episodes have also been recorded in the eastern Aleutians on stations of the Makushin and Akutan volcanic networks. These events were identified on spectrograms used to monitor volcanic activity, however, the geographic distribution of stations suggest that the source of these signals does not stem from the volcanoes themselves. Similar to the previously observed tremor episodes, the tremor episodes recorded on the volcanic networks consist of a series of bursts with energy concentrated in the band 1-6 Hz. These episodes last up to six minutes. Although the non-volcanic tremors observed in the Alaska/Aleutian subduction zone are similar to the ETS observed in the Cascadia subduction zone and the Nanakai trough, identifying slow slip associated with tremor in the Alaska/Aleutian subduction zone is difficult because there is no continuously recording GPS network in the region. (Revised)
ObjectiveObjective
Station CoverageStation Coverage
Signal ProcessingSignal Processing
Examples of Tremor LocationsExamples of Tremor Locations
ResultsResults
ReferencesReferences
Future WorkFuture Work
Identify and locate several episodes of non-volcanic tremor using data from the BEAAR PASSCAL Experiment.
Example of non-volcanic tremor occurring August 3, 2000 at approximately 10:36 UTC.
BEAAR station coverage with selected AEIC stations used to help constrain locations.
1. Extract a single pulse of tremor (1-2 minutes).
2. Broadband filter (1-6 Hz).3. Smoothing and envelope
creation using a Hilbert transform.
4. Low Pass filter (0.5–2 Hz).
Envelopes resulting from filtering process are cross-correlated and relative time offsets determined, as shown in the above figures. Locations are calculated using a generic velocity model for southern Alaska and tremor signals are assumed to propagate at S-wave velocities.
Five episodes of tremor located slightly updip of the 50 km contour of the subducting Pacific plate and just north of 1998-2001 slow-slip event [3]. Location errors are on the order of 10-15 km. Depth constraints are difficult.
This StudyThe events used in this study occurred in August 2000, in the midst of the 1998-2001 slip event (right). Many more events have been identified during this time period. Future work includes locating these events as well as identifying and locating tremor prior to and after the slip-event.
Future research also includes exploring other regions of the subduction zone for slab tremor. For example, a tremor-like signal was identified on two volcanic networks in the Aleutian arc that are approx. 60 km apart (left). This signal is consistent with the tremor signals seen in the BEAAR PASSCAL data.
1. Rogers, G. and Draget, 2003. Episodic tremor and slip on the Cascadia subduction zone: The chatter of silent Slip, Science, 300, 1942-1943
2. Obara, K., 2002. Nonvolcanic deep tremor associated with subduction in southwest Japan, Science, 296, 1679-1681
3. Ohta, Y., J.T. Freymueller, S. Hreinsdottir, and H. Suito, 2005. A large slow slip event and the depth of the seismogenic zone in the south central Alaska subduction zone, Earth Planet. Sci. Lett., submitted.
