Magnitude 5.7 earthquake in Batangas baffles researchers

Monday’s quake occurred, the third moderate earthquake in nine weeks, as tension eased after a strong quake in July. Researchers want to know why.

Written by Mario Aurelio, Director, National Institute of Geosciences, University of the Philippines, Alfredo Mahar Francisco Lagme, Executive Director, Resilience Institute, University of the Philippines – National Operational Hazard Assessment Center (nababaha), John Agustin Escudero, Structural Geology and Tectonic Laboratory of the University of the Philippines, National Institute Geosciences, Sandra Dona Catugas, Laboratory of Structural and Tectonic Geology at the National Institute of Geosciences, University of the Philippines

Citation: Aurelio, M., Lagmay, AFM, Escudero, JA, and Catugas, SD, 2021, TK, Temblor, http://doi.org/10.32858/temblor.211

At 1:12 a.m. Monday (local time), an earthquake measuring 5.7 on the Richter scale hit the Batangas coast of the Philippines. The aftershock was followed 28 minutes later by an aftershock of magnitude 4.5.

The earthquake bulletin issued by the Philippine Institute of Volcanology and Seismology (PHIVOLCS) indicated that infrastructure damage was expected. Residents in Mindoro, Batangas, Laguna and Cavite provinces and even the capital, Manila, 50 miles (80 kilometers) away, felt the shaking (intensity III-V on the PHIVOLCS intensity scale). Structures, especially high-rise residential and commercial complexes, were significantly affected in Tagaytay City in Cavite Province and in Metro Manila, alarming commuters.

The event comes nine weeks after a 6.6-magnitude earthquake struck Batangas on July 24, 2021. Three weeks later, a 5.8-magnitude event hit the same region. These earthquakes are associated with a complex fault movement along the Manila Trench

Over the past 10 years, an average of 2.5 greater than 5.0-magnitude impact events per year have been within 30 miles (50 km) of the July 24 event (Aurelio et al., 2021b). The recent wave of moderate earthquakes has far exceeded this average and indicates that this is a developing sequence.

Stress changes after big earthquakes

Similar to the July and August earthquakes, the location, focal mechanism, and depth of this recent seismic event are all consistent with subduction-related fault movement near the Manila Trench. Here, the South China Sea Plate sinks beneath the Philippine Moving Belt, a region of active deformation that defines the boundary between the Philippine Sea Plate and the Sunda Plate

The 6.6-magnitude quake in July likely transferred pressure to the area where the 5.8-magnitude quake would strike later in August. Interestingly, the Monday event occurred in a region where pressure change modeling at Coulomb (Toda et al., 2011) predicted that pressures had decreased (blue area in Figure 1) as a result of the magnitude 6.6 event.

The event of magnitude 6.6 was followed by an aftershock of magnitude 5.8 less than 10 minutes later. Coulomb stress transfer modeling indicates that the 5.8 magnitude aftershock and the 5.8 magnitude earthquake on August 13 are within the increased stress lobe of the 6.6 magnitude event (Fig. 1). This indicates that these two magnitude 5.8 earthquakes were either reinforced or triggered by a magnitude 6.6 earthquake. In this sense, they can be considered as dependents.

However, it is unusual for more of an aftershock to have a unit magnitude smaller than the main shock. It is also possible that these two earthquakes of magnitude 5.8 were independent of each other and of magnitude 6.6 in July, and they would have hit regardless of nearby earthquakes.

Figure 1. Tectonic earthquakes from four medium-sized and thrust-mechanical earthquakes (shown by beach balls) occur in the same region in Batangas, South Luzon, Philippines, within nine weeks. The last event on September 27, 2021 is shown in red for confirmation. The result of the coulomb stress change model is shown after the July-6.6 maganitude event. References: Jarvis et al., 2008 for SRTM topography; Weatherall et al., 2020 Bathymetry; Toda et al., 2011 for CST modeling; PHIVOLCS for earthquake data. GMT (Wessel and Smith, 1995) was used to create the map. Credit: Aurelio, Lagmay, Escudero, Catugas.

Monday’s event of magnitude 5.7 was produced by rupturing a portion of the slip plate where motion is less expected, based on the Coulomb stress change model using the magnitude 6.6 earthquake on July 24, 2021 as the source fault (Aurelio et al., 2021b). The main shock of the September 27 event was followed 28 minutes later, by an aftershock of less than one unit with a magnitude of less than 4.5 on the Richter scale, strengthening the argument that this earthquake could have been generated independently and not dependent on previous earthquakes.

low cost seismology

Monday’s earthquake was also recorded by the low-cost Raspberry Shake seismometers which are part of a network being developed by the University of the Philippines Diliman (UP Diliman) in Quezon City (Fig. 2). Citizens operate these privately owned sensors on their private property and exchange data with scientists. Although there are few seismographs in the network currently, the seismic parameters computed from these units are similar to those determined by existing international seismographs (Table 1).

These data indicate that low-cost, privately owned and maintained seismographs can complement existing state-owned networks and provide reliable data. A citizen science approach to studying earthquakes enhances community participation and enables communities to build resilience.

Figure 2. Scientific seismograms of citizens from the September 27, 2021 earthquake, generated by seismographs deployed in Quezon City, and in Batangas, Laguna and Cavite provinces in southern Luzon. Credit: Aurelio, Lagmay, Escudero, Catugas

Table 1 Some parameters (magnitude and focal depth) of the September 27 event computed by various international seismic observatories, compared to a low-cost seismic network based on UP Diliman

references

Aurelio, M., Lagmay, M., Escudero, JA, and Catugas, S. (2021a). Latest earthquake in the Philippines reveals tectonic complexity, Temblor, doi.org/10.32858/temblor.191

Aurelio, M., Lagmay, M., Escudero, JA, and Catugas, S. (2021b). Philippine fault rocks Batangas again, with 5.8-magnitude earthquake, Temblor, doi.org/10.32858/temblor.198

German GEOFON Research Center for Geosciences. Available at: www.geofon.gfz-potsdam.de

Jarvis, A.; Hay; Reuter, A. Nelson, E. Guevara (2008). SRTM is full of hole-globules version 4, available from the CGIAR-CSI SRTM 90m database (http://srtm.csi.cgiar.org).

gyroscope monitor. Available at: http://geoscope.ipgp.fr/index.php/en/

Philippine Institute of Volcanology and Seismology (PHIVOLCS). Available at: www.phivolcs.dost.gov.ph

Toda, Shinji, Stein, R. S., Sevilgen, Volkan, and Lin, J. (2011). Coulomb 3.3 Graphic Rich Deformation and Strain Change Software for Earthquake, Tectonic, and Volcano Research and Teaching – User’s Guide: US Geological Survey Open-File Report 2011-1060, 63 pages, Available at https://pubs.usgs. gov / of / 2011/1060 /

USGS-National Earthquake Information Center (USGS-NEIC). Available at: www.earthquake.usgs.gov

Weatherall P., Tozer B., Arndt JE, Bazhenova E., Bringensparr C., Castro CF, Dorschel B., Ferrini V., Hehemann L., Jakobsson M., Johnson P., Ketter T., Mackay K., Martin TV, Mayer LA, McMichael-Phillips J., Mohammad R., Nitsche FO, Sandwell DT, Snaith H., Viquerat S. (2020). GEBCO_2020 Network – Continuous Terrain Model of the Global Oceans and Lands. British Ocean Data Centre, National Oceanographic Centre, NERC, UK. doi: 10.5285/a29c5465-b138-234d-e053-6c86abc040b9

Wessel, P. and Smith, WHF, (1995). A new version of public mapping tools has been released. EOS Trans. I be. Geophysics. Union 76, 329.

What Are The Main Benefits Of Comparing Car Insurance Quotes Online

LOS ANGELES, CA / ACCESSWIRE / June 24, 2020, / Compare-autoinsurance.Org has launched a new blog post that presents the main benefits of comparing multiple car insurance quotes. For more info and free online quotes, please visit https://compare-autoinsurance.Org/the-advantages-of-comparing-prices-with-car-insurance-quotes-online/ The modern society has numerous technological advantages. One important advantage is the speed at which information is sent and received. With the help of the internet, the shopping habits of many persons have drastically changed. The car insurance industry hasn't remained untouched by these changes. On the internet, drivers can compare insurance prices and find out which sellers have the best offers. View photos The advantages of comparing online car insurance quotes are the following: Online quotes can be obtained from anywhere and at any time. Unlike physical insurance agencies, websites don't have a specific schedule and they are available at any time. Drivers that have busy working schedules, can compare quotes from anywhere and at any time, even at midnight. Multiple choices. Almost all insurance providers, no matter if they are well-known brands or just local insurers, have an online presence. Online quotes will allow policyholders the chance to discover multiple insurance companies and check their prices. Drivers are no longer required to get quotes from just a few known insurance companies. Also, local and regional insurers can provide lower insurance rates for the same services. Accurate insurance estimates. Online quotes can only be accurate if the customers provide accurate and real info about their car models and driving history. Lying about past driving incidents can make the price estimates to be lower, but when dealing with an insurance company lying to them is useless. Usually, insurance companies will do research about a potential customer before granting him coverage. Online quotes can be sorted easily. Although drivers are recommended to not choose a policy just based on its price, drivers can easily sort quotes by insurance price. Using brokerage websites will allow drivers to get quotes from multiple insurers, thus making the comparison faster and easier. For additional info, money-saving tips, and free car insurance quotes, visit https://compare-autoinsurance.Org/ Compare-autoinsurance.Org is an online provider of life, home, health, and auto insurance quotes. This website is unique because it does not simply stick to one kind of insurance provider, but brings the clients the best deals from many different online insurance carriers. In this way, clients have access to offers from multiple carriers all in one place: this website. On this site, customers have access to quotes for insurance plans from various agencies, such as local or nationwide agencies, brand names insurance companies, etc. "Online quotes can easily help drivers obtain better car insurance deals. All they have to do is to complete an online form with accurate and real info, then compare prices", said Russell Rabichev, Marketing Director of Internet Marketing Company. CONTACT: Company Name: Internet Marketing CompanyPerson for contact Name: Gurgu CPhone Number: (818) 359-3898Email: cgurgu@internetmarketingcompany.BizWebsite: https://compare-autoinsurance.Org/ SOURCE: Compare-autoinsurance.Org View source version on accesswire.Com:https://www.Accesswire.Com/595055/What-Are-The-Main-Benefits-Of-Comparing-Car-Insurance-Quotes-Online View photos