Real-Time and Retrospective Analyses of Volcanic Earthquakes

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Real-Time and Retrospective Analyses of Volcanic EarthquakesClifford H. Thurber, University of Wisconsin-MadisonEAR- 040929 This project involves a collaboration between University of Wisconsin-Madison researchers and U.S. Geological Survey scientists at the Alaska Volcano Observatory (AVO) and in the Volcano Disaster Assistance Program (VDAP) on retrospective and real-time analysis of seismic data from a number of volcanoes in Alaska and worldwide. The main scientific goals are improvement in understanding the nature of the background and eruption-related seismicity at these active volcanoes, the development of new or improved models of their internal structure, and the development of an improved eruption warning and monitoring methods. There are four main types of analysis tools that being applied in the project: (1) waveform alignment methods, (2) event clustering and auto-picking, (3) double-difference location and tomography, and (4) real-time event location processing. Tools (1) and (3) are well-developed methods, whereas the development of (2) and (4) will require additional work as part of this project. AVO is currently responsible for the seismic monitoring of 25 volcanoes. With an eruption on average every 1 to 1.5 years and thousands of earthquakes to analyze annually, the opportunities for valuable seismic research are enormous. Four targets have been identified for analysis in this project: the Katmai Group, Shishaldin, Mount Spurr, and Makushin. Each of these targets is seismically active and has an adequate number of seismic stations to allow detailed relocation work, and in some cases tomography as well. During the last 8 years, VDAP has been deeply involved in several major volcanic crises, including Guagua Pichincha, Tungurahua, and Cotopaxi (Ecuador), Popocatepetl and Colima (Mexico), Rabaul and Pago (New Guinea), Soufriere Hills (Montserrat), Cerro Negro (Nicaragua), and Anatahan (Commonwealth of the Northern Marianas Islands). Because VDAP operates extensively in 3rd world countries and predominantly during crises, seismic networks must be installed or expanded hurriedly, and generally consist of no more than four to seven 1-Hz vertical sensors and often one 3-component station. Considerable time is generally lost during the initial week or so trying to devise reasonable seismic velocity models for the volcanoes owing to absence of previous work on the volcano's structure. Events are located using P and S phase first motions picked by human analysts. Owing to the small networks, simple first arrival picking, and probable velocity model errors, location errors are probably several kilometers for all axes. Automating a location scheme and integrating simple (1D) modeling to produce a velocity model quickly would save considerable valuable time during the critical onset of a crisis, producing locations with about an order of magnitude greater precision in the process. Most VDAP and Alaskan volcanoes are monitored by ~6 short period seismic stations. Earthquakes are located by analysts' picks, and location errors are often 1-2 km. Thus VDAP and AVO share difficulties in locating earthquakes, especially during crisis times when the analysts cannot keep up with seismicity rates. We plan to test our system at AVO, which uses the same Earthworm real-time processing system as VDAP, to determine its effectiveness in a larger-scale monitoring environment. The proposed work will build many years of experience by UW-Madison researchers on the investigation of the seismicity and structure of active volcanoes. Accurate earthquake locations are essential for characterizing magmatic pathways, identifying magma migration, and for obtaining meaningful results in analyses (such as spatial b-value studies) that rely on locations. An enhanced real-time location system would contribute to improved eruption warning capability directly by providing high-quality locations rapidly and indirectly by reducing the network operator's work load, allowing for more time to evaluate the seismicity patterns and their changes with time. Monitoring the spatio-temporal development of earthquake families will help us understand a volcano's magma conduit system and help identify when a volcano may shifting from a relatively steady-state system to a more dangerous condition.