Monitoring Earthquakes in Hawaii
Scientists at the Hawaiian Volcano Observatory (HVO) monitor, analyze, and report on earthquakes that occur throughout the Hawaiian Islands. HVO is unique among USGS volcano observatories in that it is responsible for earthquake monitoring as it relates to both volcanic and seismic hazards. In this dual role, HVO operates a "Tier 1" regional seismic network as part of the USGS Advanced National Seismic System (ANSS).
Seismic network is operated in partnership.
The seismic monitoring network in the State of Hawaii includes various types of ground-shaking sensors at about 100 sites. These are operated by different partners as one statewide virtual network known as the Hawaii Integrated Seismic Network (HISN). Partners include the NOA
HVO maintains about 60 stations on the Island of Hawai‘i to detect and locate small-magnitude earthquakes beneath the summits, rift zones, and flanks of the most active volcanoes. Since earthquakes can also happen throughout the island chain, sensors operated by HVO's partners help to record and measure potentially damaging earthquakes and warn of tsunamis. For a detailed history of seismic monitoring at HVO since its founding in 1912, see this report.
Various types of seismic instruments record waves of sound and motion.
Different types of sensors called seismometers are needed to record the full ground motions of both small- and large-magnitude earthquakes. HVO uses four main types of seismic instruments as part of its monitoring network.
Short period – HVO's network has historically been dominated by short period instruments. Most sensitive to frequencies of around 1 second, this type of seismometer is good at recording high-frequency signals from local earthquakes. It is especially useful for recording P-wave arrival times and first motions in a cost effective way. In recent years, short period instruments have begun to be phased out by more capable broadband sensors.
Broadband – Broadband seismometers come in many varieties but generally are responsible to seismic signals ranging from about 0.01 – 120 seconds or higher. This allows for recording a broad range of signals at a variety of periods, allowing for more in-depth study of seismic sources and other phenomena. This type of seismometer is significantly more expensive than other options and is more sensitive to local site conditions.
Strong motion – Large earthquakes can shake the ground with accelerations that exceed the force of gravity. This can cause velocity-based seismometers, such as short period and broadband, to go off-scale and "clip." A strong motion accelerometer is a type of seismometer that will stay on scale no matter how strong the shaking. Most broadband instruments are paired with strong motion accelerometers to ensure good recording of signals no matter how strong the shaking. Strong motion instruments are used widely by the engineering community to measure how shaking affects manmade structures. They are also the primary data source behind the USGS ShakeMap products.
Infrasound – Earthquakes, eruptions, explosions, and other phenomena emit sounds into the air as well as into the solid earth. An infrasound sensor is a special type of microphone that measures sound waves in the air. This offers a unique way to monitor explosive volcanic activity and has great promise to aid in rapid eruption detection.
HVO Seismologists analyze Hawai‘i earthquakes.
Seismic data arriving at HVO is processed and analyzed using different tools to track dramatic and subtle changes that occur in seismicity, especially within and beneath the volcanoes. In 2009, HVO became the first regional seismic network in the U.S. to adopt the ANSS Quake Management System (AQMS). AQMS is a database-driven extension to the Earthworm seismic system for both automated real-time and manual post-processing of seismic data. It has helped HVO improve its capability to detect small changes in volcanic processes and better characterize large, damaging earthquakes.
Computers running this software use special algorithms to continuously look for seismic signals from earthquakes and changes in volcanic activity. They alert HVO duty seismologists to significant changes around the clock. Some key parameters that HVO monitors include:
Hypocenter and Magnitude – The mainstay of any seismic network is determining the magnitude and location of individual earthquakes. HVO uses the Hypoinverse package for its routine earthquake processing, both automatic and manual. The types of magnitude computed by HVO include Md (duration) and Ml (amplitude, or local). This information can be displayed on the map of monitoring instruments. Read more in our volcano watch article, How big is that earthquake? Why magnitudes sometimes change.
RSAM – Real-time Seismic Amplitude Measurement (RSAM) uses the average amplitude of signals measured by seismometers in a given area. The RSAM value climbs higher during periods of persistent and stronger ground shaking that occurs during earthquake swarms, tremor, sustained gas emissions, spatter, and lava fountains. Learn more about RSAM at the Alaska Volcano Observatory Web site.
Swarm detection – An earthquake swarm is a sequence of events closely clustered in time and space without a clear main shock. When swarms occur at volcanoes, they can represent subsurface magma movement and be an indicator of an impending or ongoing eruption. HVO alarms on possible swarm activity when the event rate significantly increases and surpasses a preset threshold value in a given geographic area.
Tremor detection – Tremor occurs when fluids, such as magma or gas, move within a conduit, causing it to resonate. Thus, characterizing tremor is important for understanding and monitoring magmatic pathways. Using an automated envelope cross-correlation method, HVO can detect and estimate the location of tremor sources. Learn more in Wech and Thelen 2015.
HVO reports seismic activity after review.
After HVO detects seismic signals with its monitoring network and analyzes those signals with computer tools, it disseminates the information. The AQMS system automatically locates earthquakes and posts them to the internet in real-time. Within hours to days, HVO seismic analysts review, re-compute, and update earthquake locations and magnitudes. This information becomes part of the ANSS Comprehensive Earthquake Catalog (ComCat) for ease of searching by anyone.
Larger earthquakes above magnitude 4.0 are usually widely felt and may be damaging. These trigger a rapid response by HVO duty seismologists and others, who manually review the earthquake and issue a news release within two hours. Additionally, the National Earthquake Information Center (NEIC) will compute a number of enhanced earthquake information products such as Did You Feel It, ShakeMap, PAGER, and moment tensors that are used by USGS and its partners for hazard analysis and decision making.